Patent Publication Number: US-6987741-B2

Title: System and method for managing bandwidth in a two-way satellite system

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application is related to, and claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 60/197,246, filed Apr. 14, 2000, entitled “System and Method for Providing Control of a Two-way Satellite System,” the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a satellite communication system, and is more particularly related to a two-way satellite communication system providing access to a packet switched network. 
     2. Discussion of the Background 
     Modem satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems. As the popularity of the Internet continues to grow in unparalleled fashion, the communication industry has focused on providing universal access to this vast knowledge base. Satellite based Internet service addresses the problem of providing universal Internet access in that satellite coverage areas are not hindered by traditional terrestrial infrastructure obstacles. 
     The Internet has profoundly altered the manner society conducts business, communicates, learns, and entertains. New business models have emerged, resulting in the creation of numerous global businesses with minimal capital outlay. Traditional business organizations have adopted the Internet as an extension to current business practices; for example, users can learn of new products and services that a business has to offer as well as order these products by simply accessing the business&#39;s website. Users can communicate freely using a wide variety of Internet applications, such as email, voice over IP (VoIP), computer telephony, and video conferencing, without geographic boundaries and at nominal costs. Moreover, a host of applications within the Internet exist to provide information as well as entertainment. 
     Satellite communication systems have emerged to provide access to the Internet. However, these traditional satellite-based Internet access systems support unidirectional traffic over the satellite. That is, a user can receive traffic from the Internet over a satellite link, but cannot transmit over the satellite link. The conventional satellite system employs a terrestrial link, such as a phone line, to send data to the Internet. For example, a user, who seeks to access a particular website, enters a URL (Universal Resource Locator) at the user station (e.g., PC); the URL data is transmitted over a phone connection to an Internet Service Provider (ISP). Upon receiving the request from the remote host computer where the particular website resides, the ISP relays the website information over the satellite link. 
     The above traditional satellite systems have a number of drawbacks. Because a phone line is used as the return channel, the user has to tie up an existing phone line or acquire an additional phone line. The user experiences temporary suspension of telephone service during the Internet communication session. Another drawback is that the set-top box has to be located reasonably close to a phone jack, which may be inconvenient. Further, additional costs are incurred by the user. 
     Based on the foregoing, there is a clear need for improved approaches for providing access to the Internet over a satellite communication system There is a need to minimize costs to the user to thereby stimulate market acceptance. There is also a need to permit existing one-way satellite system users to upgrade cost-effectively. There is also a need to eliminate use of a terrestrial link. Therefore, an approach for providing access to a packet switched network, such as the Internet, over a two-way satellite communication system is highly desirable. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a method is provided for managing return channel bandwidth in a two-way satellite communication system. The method includes receiving backlog information from a plurality of transceivers, wherein the backlog information specifies an amount of queued traffic for the respective transceivers. The method also includes allocating a minimal amount of return channel bandwidth to each of the plurality of transceivers, and selectively setting a bandwidth level associated with the return channel bandwidth that is available based upon one of a plurality of predetermined bandwidth levels according to a predetermined criteria. The method further includes selectively allocating additional return channel bandwidth based upon the bandwidth level to the transceivers that require additional return channel bandwidth in excess of the allocated minimal amounts. The above arrangement advantageously minimizes costs to the user, thereby stimulating market acceptance. 
     According to another aspect of the invention, a system is provided for managing return channel bandwidth in a two-way satellite communication network. A plurality of transceivers are configured to transmit backlog information over a return channel via a satellite. The backlog information specify an amount of queued traffic for the respective transceivers. A hub is configured to receive the backlog information and to allocate a minimal amount of return channel bandwidth to each of the plurality of transceivers. The hub selectively sets a bandwidth level associated with the return channel bandwidth that is available based upon one of a plurality of predetermined bandwidth levels according to a predetermined criteria. The hub selectively allocates additional return channel bandwidth based upon the bandwidth level to the transceivers that require additional return channel bandwidth in excess of the allocated minimal amounts This approach permits existing one-way satellite system users to upgrade cost-effectively. 
     According to one aspect of the invention, a system is provided for managing return channel bandwidth in a two-way satellite communication network. The system includes means for receiving backlog information from a plurality of transceivers. The backlog information specify an amount of queued traffic for the respective transceivers. The system also includes means for allocating a minimal amount of return channel bandwidth to each of the plurality of transceivers, and means for selectively setting a bandwidth level associated with the return channel bandwidth that is available based upon one of a plurality of predetermined bandwidth levels according to a predetermined criteria. Further, the system includes means for selectively allocating additional return channel bandwidth based upon the bandwidth level to the transceivers that require additional return channel bandwidth in excess of the allocated minimal amounts. The above arrangement advantageously provides compatibility with existing equipment. 
     In yet another aspect of the invention, a computer-readable medium carrying one or more sequences of one or more instructions for managing return channel bandwidth in a two-way satellite communication system is disclosed. The one or more sequences of one or more instructions include instructions which, when executed by one or more processors, cause the one or more processors to perform the step of receiving backlog information from a plurality of transceivers. The backlog information specify an amount of queued traffic for the respective transceivers. Other steps include allocating a minimal amount of return channel bandwidth to each of the plurality of transceivers, and selectively setting a bandwidth level associated with the return channel bandwidth that is available based upon one of a plurality of predetermined bandwidth levels according to a predetermined criteria. Yet another step includes selectively allocating additional return channel bandwidth based upon the bandwidth level to the transceivers that require additional return channel bandwidth in excess of the allocated minimal amounts. This approach advantageously eliminates use of a terrestrial link, thereby providing a convenient and cost-effective mechanism to access the Internet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of a two-way satellite communication system configured to provide access to a packet switched network (PSN), according to an embodiment of the present invention; 
         FIG. 2  is a diagram of the return channel interfaces employed in the system of  FIG. 1 ; 
         FIG. 3  is a diagram of the transceiver components utilized in the system of  FIG. 1 ; 
         FIG. 4  is a diagram of the architecture of a network operations center (NOC) in the system of  FIG. 1 ; 
         FIGS. 5   a  and  5   b  show a diagram of the system interfaces and packet formats, respectively, that used in the system of  FIG. 1 ; 
         FIGS. 6A–6P  are diagrams of the structures of exemplary packets used in the system of  FIG. 1 ; 
         FIG. 7  is a flow chart of the return channel bandwidth limiting process utilized in the system of  FIG. 1 ; 
         FIG. 8  is a flow chart of the auto-commissioning process utilized in the system of  FIG. 1 ; 
         FIG. 9  is a flow chart of the antenna pointing operation associated with the auto-commission process of  FIG. 8 ; 
         FIG. 10  is a diagram showing the scalable architecture of the system of  FIG. 1 ; and 
         FIG. 11  is a diagram of a computer system that can support the interfaces for two-way satellite communication, in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for the purpose of explanation, specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. In some instances, well-known structures and devices are depicted in block diagram form in order to avoid unnecessarily obscuring the invention. 
     The present invention provides a two-way satellite system that eliminates the requirement for a phone line to support two-way applications and provides the ability to use dedicated high-speed return channels. The high-speed satellite broadcast system supports a Universal Serial Bus (USB) ready transceiver (i.e., adapter) that may be attached to a personal computer (PC) transmit data and to receive the satellite broadcast through a single antenna. 
     Although the present invention is discussed with respect to protocols and interfaces to support communication with the Internet, the present invention has applicability to any protocols and interfaces to support a packet switched network, in general. 
       FIG. 1  shows a two-way satellite communication system that is configured to provide access to a packet switched network (PSN), according to an embodiment of the present invention. A two-way satellite communication system  100  permits a user terminal, such as a PC  101 , to access one or more packet switched networks  103  and  105  via a satellite  107 . One of ordinary skill in the art would recognize that any number of user terminals with appropriate functionalities can be utilized; e.g., personal digital assistants (PDAs), set-top boxes, cellular phones, laptop computing devices, etc. According to an exemplary embodiment the packet switched networks, as shown, may include the public Internet  105 , as well as a private Intranet  103 . The PC  101  connects to a transceiver  109 , which includes an indoor receiver unit (IRU)  109   a , an indoor transmitter unit (ITU)  109   b , and a single antenna  111 , to transmit and receive data from a network hub  113 —denoted as a network operations center (NOC). As will be explained in greater detail with respect to  FIG. 4 , the hub  113  may include numerous networks and components to provide two-way satellite access to PSNs  103  and  105 . The user terminal  101  can transmit data to the NOC  113  with an uplink speed of up to 128 kbps, for example, and receive data on the downlink channel with speeds of up to 45 Mbps. As shown in the figure, the NOC  113  has connectivity to Intranet  103  and the Internet  105 , and supports a multitude of applications (e.g., software distribution, news retrieval, document exchange, real-time audio and video applications, etc.), which may be supplied directly from a content provider or via the Internet  105 . 
     Essentially, the system  100  provides bidirectional satellite transmission channels. The down-link channel from NOC  113  to the transceiver  109  may be a DVB (Digital Video Broadcast)-compliant transport stream The transport stream may operate at symbol rates up to 30 megasymbols per second; that is, the transport stream operates at bit rates up to 45 Mbps. Within the transport stream, the IP traffic is structured using multiprotocol encapsulation (MPE). One or more MPEG PIDs (Program IDs) are used to identify the IP (Internet Protocol) traffic. In addition, another PID is used for the framing and timing information. 
     The uplink channel from the transceiver  109  to the NOC  113  includes multiple carriers, each operating at speeds of 64 kbps, 128 kbps, or 256 kbps, for example. Each of these carriers is a TDMA (Time Division Multiple Access) stream, which employs several transmission schemes. Upon first use of user equipment, tools may be employed to provide initial access and to request further bandwidth as required. The specific bandwidth allocation scheme may be designed to ensure maximum bandwidth efficiency (i.e., minimal waste due to unused allocated bandwidth), and minimum delay of return channel data Further, the scheme is be tunable, according to the mixture, frequency, and size of user traffic. 
     The two-way satellite system  100  can be implemented, according to an exemplary embodiment, based upon an existing one-way broadcast system. The conventional one-way broadcast system utilizes a terrestrial link for a return channel. In contrast, the two-way satellite system  100  obviates this requirement. However, the user terminal  101  may optionally retain the dial-up connection as a back-up connection to the Internet  105 . 
     According to one embodiment of the present invention, the two-way satellite system  100  offers the following services to the user terminal  101 : digital package multicast delivery, multimedia services, and Internet access. Under the digital package delivery service, the system  100  offers a multicast file transfer mechanism that allows any collection of PC files to be reliably transferred to a collection of transceivers. The IP multicast service carries applications, such as video, audio, financial and news feed data, etc., for broadcast to the transceivers (e.g.,  109 ). As already discussed, the system  100  provides high-speed, cost-effective Internet access. 
     To receive the broadcast from system  100 , PC  101  may be equipped with a standard USB (Universal Serial Bus) adapter (not shown) and a 21-inch elliptical antenna  111 . The system  100 , according to one embodiment, uses a Ku- (or Ka-) band transponder to provide up to a 45 Mbps DVB-compliant broadcast channel from the NOC  113 . Further, data encryption standard (DES) encryption-based conditional access can be utilized to ensure that the PC  101  may only access data that the PC  101  is authorized to receive. 
     In accordance with an embodiment of the present invention, the USB adapter may be attached to IRU  109   a , which is connect to ITU  109   b . The data is passed from the PC  101  to the USB adapter of the PC  101 , which formats the data for transmission and provides both the control and data for the ITU  109   a . The ITU  109   a  sends the data to an outdoor unit (ODU), which includes antenna  111 , at the appropriate time for the data to be transmitted in TDMA bursts to equipment at the NOC  113 . In this example, when averaged across a year, each two-way transceiver is expected to have a bit-error rate less than 10 −10  more than 99.5% of the time whereby a single bit error causes the loss of an entire frame. The transceiver is more fully described later with respect to  FIG. 3 . 
       FIG. 2  shows the return channel interfaces that are employed in the system of  FIG. 1 . The architecture of the two-way system  100  is an open architecture, which advantageously affords information providers control over their content. Specifically, the two-way system  100  provides interfaces to information providers at the NOC  113  and standard Application Programing Interfaces (APIs) on the host PC  101 . The user terminal  101  is loaded with host software and drivers to interface with the transceiver  109  and to control antenna  111 . The PC  101 , in an exemplary embodiment, runs the following operating systems: Microsoft® Win98 Second Edition and Windows 2000. The PC software may provide instruction and support for installation and antenna pointing (including automatic registration and configuration), package delivery, and drivers that are used by the native TCP/IP (Transmission Control Protocol/Internet Protocol) stack to support standard applications—including Winsock API with multicast extensions and web browsers. 
     The two-way system  100  supports the exchange of digital packages to one or more receiving PCs. The term “package”, as used herein, refers to any data (including electronic documents, multimedia data, software packages, video, audio, etc.) which can take the form of a group of PC files. Package delivery is used by an information provider to send packages to receiving PCs; for example, the delivery of digitized advertisements to radio and TV stations. 
     To prepare a package for transmission, a publisher (i.e., content provider) may merge the package&#39;s files into a single file using the appropriate utility (e.g., PKZIP), and subsequently load the package into the NOC  113  using an off-the-shelf file transfer mechanism (e.g., TCP/IP&#39;s file transfer protocol (FTP)). The publisher may control the following parameters associated with the package: addresses of the destination PCs, and delivery assurance. The low bit error rate and high availability of the two-way system  100  ensures that packages are delivered in one transmission (that is, without the need to retransmit). 
     With respect to ensuring proper delivery and reporting delivery status of the digital packages, the publisher possesses a number of functionalities. The PC  101  may issue retransmission requests, as needed, if segments of the package is loss or received with errors. The PC  101  may request retransmission of only the loss or corrupt portions of the digital package via the satellite return channel, or optionally, a dial-out modem It should be noted that the multicasting capability of the system  100  advantageously permits the one time retransmission of missing/corrupt data even though the missing/corrupt data may affect multiple PCs. The system  100  also supports delivery confirmation. A PC  101 , after successfully receiving a package, may send a confirmation to a package delivery server (not shown) within the NOC  113 . These confirmations are tabulated and provided in the form of reports to the publisher. 
     Further, the system  100  may provide a best effort service. Under this scenario, if frames are lost on the first transmission, the receiving PCs fill in the gaps on subsequent transmissions. This mechanism helps ensure high probability of delivery without requiring use of a return link for retransmission requests. 
     According to an exemplary embodiment, the digital packages contain the following fields: a transmission rate field that is configurable per package at speeds up to 4 Mbps through the IRU; a forward error correction (FEC) rate for providing correction of sporadic packet loss; a priority field for specifying low, medium, or high priority; and optional topic, descriptive name, and description fields that are used by the user interface of the receiver PC to present the package to the user. The package delivery service of the two-system  100  supports the simultaneous transmission of several packages and the preemption of lower priority packages to ensure the timely delivery of higher priority packages. 
     The system  100  also supplies multimedia services, which provide one-way IP multicast transport. The NOC  113  relays a configurable set of IP multicast addresses over the downlink channel. An information provider may pass IP multicast packets to the NOC  113 , either via a terrestrial line or via the return channel. The receiving PCs may receive the IP multicast through the standard Winsock with IP Multicast extensions API. To prevent unauthorized access, each IP multicast address may be cryptographically protected. Thus, PC  101  may only have access to an address if it has been authorized by the NOC  113 . Hardware filtering in the Indoor Receive Unit (IRU)  109   a  allows the reception of any number of different IP Multicast addresses. 
     The NOC  113 , which provides network management functions, allocates to each multimedia information provider a committed information rate (CIR), and one or more IP multicast addresses. The CIR specifies the fraction of the broadcast channel bandwidth that is guaranteed to the data feed provider. Each IP Multicast address operates as a separate data stream that is multiplexed on the one broadcast channel. 
     As previously mentioned, the two-way system  100  provides high-speed Internet access, in which the PC  101  can connect to the Internet  105 . In one embodiment of the present invention, the access is asymmetric, whereby the downlink channel from the NOC  113  to the user terminal  101  can be an order of magnitude greater that the uplink (or return channel). 
     An NDIS (Network Device Interface Specification) device driver within the PC  101  operates with the native TCP/IP stack for Windows. When the ITU  109   b  is active and enabled, the NDIS software sends the return channel data to the IRU  109   a , which in turn supplies the data to the ITU  109   b . However, when the ITU  109   b  is inactive, the packets may be alternatively sent to a dial-up interface. The two-way system  100  allows operation of the standard Internet applications; for example, Netscape® browser, Microsoft® Internet Explorer browser, email, NNTP Usenet News, FTP, GOPHER, etc. 
       FIG. 3  shows the transceiver components utilized in the system of  FIG. 1 . The transceiver  109  encompasses a number of hardware and software components. A PC host software, which is resident in PC  101  and supports the satellite return channel. The transceiver  109  includes IRU  109   a , ITU  109   b , a power supply  109   c , and connects to an Outdoor Unit (ODU)  307 . The ODU  307  contains a low noise block (LNB)  305 , antenna  111 , and a radio (not shown). The IRU  109   a  operates in the receive-only mode and controls the ITU  109   b.    
     As previously indicated, the IRU  109   a  may have a Universal Serial Bus (USB) interface, which is a standard interface to PC  101  to provide IRU control and data. The IRU  109   a  may be attached to the PC  101  dynamically, and may be loaded with operational software and initialized by PC driver software. Received traffic is forwarded to the PC  101  through the USB connection  301 . The PC driver communicates with the IRU  109   a  for control over the USB channel. By way of example, the receive chain F-connector on an RG-6 cable is connected to the IRU  109   a  to communicate to the LNB  305 . The IRU  109   a  contains an interface that may be used to transfer data to control the transmit unit and to actually provide the transmit data to the ITU  109   b . A clock is received on this channel to ensure that transmit frame timing and transmit symbol clocks are synchronized. 
     The ITU  109   b  may be a standalone component that externally may appear very similar to the IRU  109   a . According to one embodiment of the present invention, the housings of the IRU  109   a  and ITU  109   b  are in a stackable form factor. The ITU  109   b  has an IFL interface (not shown) that attaches to the ODU  307  via an RG-6 interface (not shown). Control information and data from the ITU  109   b  are multiplexed onto the IFL cables  303  to the ODU  307 . One IFL cable  303  may handle the receive patch and the other may handle the transmit path. 
     The ITU  109   b  also includes an ITU control interface for data transfer. In addition, a pulse is received over the ITU control interface to ensure that transmit frame timing and transmit symbol clocks are properly synchronized. The ITU  109   b  may contain an RF transmitter, low phase noise VC-TCXO, and serial data transceiver. ITU  109   b  modulates and transmits, in burst mode, the in-bound carrier at 64 kbps or 128 kbps to a Return Channel Equipment ( FIG. 4 ). The ITU  109   b  may be designed to operate with and to be controlled by the IRU  109   a . Although IRU  109   a  and ITU  109   b  are shown as distinct components, IRU  109   a  and ITU  109   b  may be integrated, according to an embodiment of the present invention. By way of example, a single DB-25 connector on the rear panel provides power, ground and a serial data link via which control of the transmitter is exercised. The ITU  109   b  may be considered a peripheral to the IRU  109   a . Configuration parameters and inbound data from the IRU  109   a  may be input to the serial port (not shown); in addition, transmitter status information to the IRU  109   a  may output from the serial port. 
     The IRU  109   a  and ITU  109   b  utilize dual IFL cables  303  to connect to LNB  305  for receiving signals from the satellite  107 . Each cable  303  may carry the necessary power, data, and control signals from the IRU  109   a  and ITU  109   b  to the LNB  305 , which is mounted on the antenna  111 . According to one embodiment, the antenna  111  is a standard 66 cm elliptical antenna, with dimensions of 97 cm×52 cm (yielding an overall size of approximately 72 cm). Antenna  111  may include mounting equipment to support an FSS feed, BSS feeds, and a feed bracket. 
     The transceiver  109  supports a variety of features that enhance the flexibility and efficiency of the two-way system  100 . Transceiver  109  can be implemented as a receive-only unit that can be later upgraded to support a two-way configuration. In other words, the transceiver  109  may be configured either as a receive-only package or a transmit upgrade package. The transceiver  109  may be designed to be an add-on capability to a standard receive-only transceiver. Thus, in actual implementation, a user can either purchase an upgrade to a transceiver  109  to support a satellite-based return channel or can operate a receiver with no transmit portion for communication over the satellite  107 . Such a receive-only system may employ a terrestrial return channel (e.g., phone line) for two-way IP traffic. 
     In addition, the transceiver  109  supports multiple rate, high speed, receive channel. The transceiver  109  can support for high speed TCP/IP applications using, for example, Turbo Internet™ TCP spoofing. In an exemplary embodiment, a standard USB interface to PC  101  is used to connect the PC  101  with the IRU  109   a ; however, it is recognized that any type of interface can be utilized (e.g., serial, parallel, PCM/CIA, SCSI, etc.). The transceiver  109  supports TCP/IP applications (e.g., web browsing, electronic mail and FTP) and multimedia broadcast and multicast applications using IP Multicast (e.g. MPEG-1 and MPEG-2 digital video, digital audio and file broadcast) to PC  101  per the USB adapter connection  301 . The transceiver  109  can also support IP multicast applications (e.g., MPEG video and package delivery). Further, the transceiver  109  can provide compression of receive and return channel traffic to enhance bandwidth efficiency. 
     The transceiver  109  integrates the capabilities of the broadband receiver via satellite with the capability for a satellite return channel through the use of IRU  109   a  and ITU  109   b . The IRU  109   a  is powered by power supply  109   c . As indicated previously, the received channel to the transceiver  109  may be a DVB transport stream that contains multiprotocol-encapsulated IP traffic. A group of multiple transmit channels may be shared among several DVB transport streams. 
     Further, the transceiver  109 , unlike conventional satellite systems, is controlled at the system level by the NOC  113 . Particularly, the NOC  113  has the capability to enable and disable the operation of the ITU  109   b , thereby making it difficult for an authorized user to access the satellite system  100 . Neither the transceiver  109  nor the connected PC-based host  101  has the capability to override commands from NOC  113 , even in the case in which the equipment is powered down and restarted. Once disabled, the ITU  109   b  can only be enabled by the NOC  113 . That is, the user cannot “re-enable” a disabled ITU  109   b , even through a power reset. Additionally, the NOC  113  may instruct the ITU  109   b  to transmit a test pattern at a pre-determined frequency. This process may not be overridden by the user, who has no capability to cause the generation of the test pattern. The user has no control over the frequency that the test pattern is sent. Thus, the above system-level control of the ITU  109   b  by the NOC  113  prevents users from utilizing the resources of the satellite system  100 . 
       FIG. 4  shows the architecture of a network operations center (NOC) in the system of  FIG. 1 . A NOC  113  provides various management functions in support of the return channel from the user terminal  101 . Specifically, the NOC  113  provides the high-speed receive channel to the transceiver  109  of user terminal  101 . The NOC  113  also provides interfaces to either private Intranets  103  or the public Internet  105 , as directed by user terminal  101 . The NOC  113  can support multiple receive channels (referred to as outroutes) and multiple return channels; however, the NOC  113  can be configured to provide no return channels, depending on the application. Further, a single return channel may be shared by multiple receive channels. Multiple return channels within a single set of Return Channel Equipment (RCE)  411  can operate in conjunction to serve a single receive channel. 
     Within NOC  113 , a Radio Frequency Terminal (RFI)  401  is responsible for retrieving an IF (intermediate frequency) output of a System IF Distribution module  403 , up-converting the IF output signal to RF (radio frequency) for transmission to the satellite  107 . Additionally, the RFT  401  receives from the satellite  107  an RF echo of the transmitted signal, along with the RF input for the return channels; the RFT  401  down-converts these signals to IF and forwards the down-converted signals to the System IF Distribution module  403 . 
     The System IF Distribution module  403  receives as input an output signal from outroute modulators  405  via outroute redundancy equipment  407 . In response to this input signal, the System IF Distribution module  403  sends a signal to the RFT  401  and a Timing Support Equipment module  409 . The System IF Distribution module  403  receives an IF output from the RFT  401 , and distributes the received IF signal to the Timing Support Equipment module  409  and the Return Channel IF Distribution module  411   c.    
     The modulator  405  encodes and modulates the DVB transport stream from a satellite gateway  413 . In an exemplary embodiment, at least two modulators  405  are used for each uplink for redundancy; i.e., support 1-for-1 satellite gateway redundancy. The modulator  405 , which may be, for example, a Radyne® 3030DVB modulator or a NewTec® NTC/2080/Z modulator, is responsible for taking the outroute bit stream received from the satellite gateway and encoding it and modulating it before forwarding it towards the RFT  401 . 
     The satellite gateway  413  multiplexes traffic to be transmitted on the uplink. The multiplexed traffic includes user traffic that is forwarded from standard LAN gateways  415  supporting TCP/IP Multicast traffic. The multiplexed traffic also includes traffic that is forwarded from the return channel components  411 , which include a Network Control Cluster (NCC)  411   a . The NCC  411   a  is a server-class PC running Windows, along with DVB satellite gateway software that supports multiple PIDs. 
     The outroute redundancy component  407  supports a configuration that allows critical traffic components to fail without causing a system outage; this is supported on the IF data following the modulator  405 . If equipment on one transmit chain fails, the lack of a data signal is detected and a switch (not shown) automatically switches to another transmit chain. In this example, 1-for-1 redundancy of the satellite gateway  413  and modulators  405  is supported. 
     Within the outroute redundancy component  407 , a gateway common equipment (GCE) (not shown) accepts input signals from two modulators  405 , in which each serves one of two redundant chains for a return channel of system  100 . The GCE provides an output interface to the system IF distribution module  403  for the currently online modulator  405 . The GCE also has a control interface that can be used to switchover the modulator chain. By way of example, the GCE may have a “baseball switch” that can be used for manual switching. In an exemplary embodiment, the GCE may be a standard off-the-shelf GCE component per uplink. Optionally, a DVB GCE may be used if a single modulator  405  is be used instead of two per uplink. 
     The timing support equipment  409  includes multiple gateway up-link modules (GUMs)  409   a  and  409   b . The GUMs  409   a  and  409   b  provide a translation of IF signals to L-band so the signals can be received on a receive-only unit, which controls a GCE switch (not shown) and on a timing unit  409   c . The GUMs  409   a  and  409   b  receive a signal from the GCE and provides the L-Band signal either directly to a Quality Monitor PC (QMPC) (not shown) or through a splitter (not shown) to multiple receivers; one of these is connected to the system IF distribution module  403  for the uplink signal. The QMPC may be a standard receive-only version of the transceiver  109  with a relay card that controls the RCU. The QMPC, according to one embodiment of the present invention, may include a PC with the Windows operating system The QMPC can operate with the IRU  409   d , thereby permitting the IRU  409   d  to be used in the QMPC. The IRU  409   d  may be able to support more channels because the data is not forwarded to the host and more MAC addresses are used. According to one embodiment, the addressing scheme for messages supports up to 16 million adapters (i.e., transceivers); extending beyond the private class “A” IP address. Accordingly, MAC addressing supports a greater number of adapters that IP addressing. The high order nibble of the byte, which is currently set to “0Ah” (10), may be used to give 16 fold improvement to 256 million adapters. 
     A Redundancy Control Unit (RCU) (not shown) within the outroute redundancy component  407  controls the GCE switch. The RCU interfaces to the QMPC, which provides a control channel that triggers the switching of the GCE. The RCU also includes an interface to the GCE for controlling the switch. Further, the RCU has serial interfaces that interface to the satellite gateway  413  to indicate which satellite gateway is currently online, thereby ensuring that only the online satellite gateway provides flow control to the gateways. 
     Several local area networks (LANs)  421  and  423  may be used to connect the various NOC components together. A Mux LAN  421  is used to multiplex traffic that is to be sent to the satellite gateway  413  for a specific outroute. A Traffic LAN  423  transports customer traffic that are received from the return channel and traffic from the Intranet  103  and Internet  105 . 
     The NOC  113  can maintain several standard gateways  415 ,  417 , and  419  that may forward data to the user terminal  101  over LAN  421 . These gateways  415 ,  417 , and  419  may operate on server-class PCs running Microsoft® Windows-NT. A PDMC (Package Delivery and IP Multicast) Gateway  417  forwards package delivery traffic and IP multicast traffic to the satellite gateway  413 . The gateway  417  uses key material provided by the conditional access controller (CAC) server  425  to instruct the satellite gateway  413  whether to encrypt the traffic as well as the key to be used for encryption. 
     A Hybrid Gateway (HGW)  419  processes two-way TCP traffic to the users. The HGW  419  provides uplink traffic, handles flow control to respond to satellite channel overload, and also acts as a proxy for return channel traffic. For user terminals  101  that generate TCP traffic for transmission over the return channel, the HGW  419  interacts with the public Internet  105  or private Intranet  103  to relay the received user traffic. The software of the HGW  419  may be modified to support the networking functionalities associated with a satellite-based return channel. The software supports variable round-trip times in the throughput limiter calculations; e.g., either a CIR-based or more intelligent round-trip-time based algorithm may be deployed. TCP Selective acknowledgement may also be supported by the software to minimize retransmission data requirements. Other functionalities of the software include TCP Delayed ACK, larger transmission windows, and HMP overhead reduction. Further, the software support return channel units that are “always on”. In addition, the software is backwards compatible. 
     A Dedicated LAN Gateway (LGW)  415  includes the functionality of both the PDMC  417  and HGW  419 . The LGW  415  is used for customers that require a dedicated amount of bandwidth, in which the customers are permitted to share the bandwidth among their different applications. 
     A Conditional Access Controller (CAC) server  425  contains the key material for all of the transceivers  109 . According to one embodiment of the present invention, uplink traffic is encrypted using keys from this server  425 . Alternatively, the receive channel may be unencrypted. The return channel traffic could also be encrypted with the transceiver&#39;s individual key for privacy of data. Multicast traffic is encrypted with a generated key. The CAC server  425  ensures that the key material is provided to the transceivers  109  that are authorized to receive any broadcasts. In addition, the server  425  provides the individual transceiver keys to the gateways  415 ,  417 , and  419 . The CAC server  425  operates on a server-class PC running Windows NT. 
     The NOC  113  also contains a Return Channel Equipment module (RCE)  411 , which manages the return channels associated with NOC  113 . That is, the RCE  411  is responsible for managing return channel bandwidth and for receiving the return channel traffic from the transceivers  109 . The RCE  411  may include Network Control Clusters (NCCs)  411   a , one or more Burst Channel Demodulators (BCDs)  411   b , and are responsible for managing the return channel bandwidth and the BCDs  411   b . According to an exemplary embodiment, each RCE  411  has a limit on the number of BCDs  411   b  which an RCE  411  can support. For example, given a 1-for-7 redundancy scheme, up to 28 return channels can be supported. By way of example, multiple RCEs  411  may be deployed to support more than 32 BCDs  411   b  worth of return channels. As will be discussed later with respect to  FIG. 10 , this approach provides a scalable configuration. 
     The NCC  411   a  may be configured to control several RCEs  411 . The site may be assigned to the NCC  411   a  at ranging time. “Ranging” is a process which configures a site on a NCC  411   a  and adjusts timing of the NCC  411   a  without user intervention. Sites may periodically either be moved to another NCC  411   a , which supports a different set of return channels or may be completely decommissioned from the NOC  113 . For instance, a site may be moved to another NCC  411   a , as needed, for load balancing. The system  100  is capable of communicating site moves between NCCs  411   a  so the sites are no longer enabled on the prior NCC  411   a . In addition, a de-commission of the site from the CAC server  425  may disable the site at the NCC  411   a . According to one embodiment of the present invention, the NCC  411   a  can access the same database (not shown) as that are used by the conditional access and auto-commissioning systems. 
     The RCE  411  further includes Burst Channel Demodulators (BCDs)  411   b , which demodulates return channel transmissions from the transceivers  109  and forwards the received packets to the NCC  411   a . Redundancy of the IF subsystem is supported in the BCDs  411 . These BCDs  411   b  are one for N redundant with automatic switchover in the event of a failure. According to an exemplary embodiment, up to 32 BCDs may be supported by a single NCC  411   a ; the RCE  411  may handle up to 32 BCDs (i.e., up to 31 return channels). 
     The RCE  411  also contains a Return Channel IF Distribution module  411   c . The return channel IF Distribution module  411   c  receives the IF output signal from the System IF Distribution module  403  and forwards the output signal to the BCDs  411   b . The sites may be “polled” to ensure that the BCDs  411   b  stay active, thereby proactively detecting failed sites. 
     As noted above, NCC  411   a  is responsible for managing the bandwidth of a set of up to 32 BCDs  411   b . NCC  411   a  also provides configuration data to the BCDs  411   b . NCC  411   a  also reassembles packets received from the return channels (by way of the BCDs  411   b ) back into IP packets and forwards the IP packets to the appropriate gateway. The NCC  411   a  is internally 1-for-1 redundant between the two NCCs  411   a  by exchanging messages. 
     When a frame is received from a receiver, the first byte of data may indicate the Gateway ID for this serial number. The received frame may be mapped to an IP address by the NCC  411   a  and stored for the particular individual receiver. Accordingly, other packets can be received by this receiver without the 1-byte overhead for the gateway on every packet. The NCC  411  forwards the packet to the appropriate gateway after building an IP-in-IP packet that is compatible with the UDP tunneled packets sent to the gateways. 
     According to one embodiment, the NCC  411   a  may utilize the Microsoft® Windows operating system The NCC  411   a  need not processes or transmit frame timing messages. The NCC  411   a  may support changing the format of outbound messages to include new MAC addresses as well as different return channel headers. In addition, NCC  113  tracks return channel gateway address to IP mapping; this information is periodically provided to receivers. NCC  411   a  may also update and effect BCD configuration files, which can be locally stored and managed, without software restart. NCC  411   a  can support a large number of transceivers  109  (e.g., at least 100,000 transceivers). 
     As indicated previously, the NCC  411   a  manages the return channel bandwidth and forwards inbound traffic to the gateways. The NCC  411   a  may send a timing pulse to its associated timing units  409   c  once every “super frame” before the NCC  411   a  pulses the BCDs  411   b  to receive the frame. These pulses are provided to the timing units on the return channel frame boundary. 
     NCC  411   a  further maintains a transceiver-last-packet-time in a large memory-based sorted array for polling. The polling algorithm poll sites that are not recently transmitting or, as needed, to poll known “good” sites to keep BCDs  411   b  active. That is, the NCC  411   a  performs remote polling of idle remotes on a periodic basis to keep BCDs  411   b  active. The polling message specifies the return channel number to respond on. The remote status assumed to be good if the remote has transmitted packets. Only the least-recent responders are polled. NCC  411   a  can disable transmission from sites with particular serial numbers through its broadcast. 
     The Timing Support Equipment (TSE)  409  provides return channel timing support for each outroute. TSE  409  may employ a pair of PCs (not shown); each PC runs Microsoft® Windows and are connected to two IRUs  409   d . According to one embodiment of the present invention, a NCC  411   a  is allocated to one of the outroutes to ensure a 1-to-1 relationship between NCC  411   a  and timing support equipment  409 . For each outroute pairing, the TSE  409  may include a pair of Gateway Upconverter Modules (GUMs)  409   a  and  409   b , and a timing unit  409   c . The GUMs  409   a  and  409   b  translate the uplink and downlink IF signal to an L-band signal. The uplink signal is sent to a pair of local timing units  409   c  as well as the outroute redundancy equipment  407 . The downlink signal is sent to a pair of echo timing units. The timing unit  409   c  determines both the variable satellite gateway delay for the transmit signal and the NOC satellite delay, and transmits frame timing information to the transceivers  109 . 
     The timing units  409   c  are the portion of the NOC  113  that support network timing. In an exemplary embodiment, a timing unit  409   c  may be a PC with two attached indoor receive units (IRUs)  409   d , both which are configured to support timing. When the timing unit  409   c  receives the local timing, timing unit  409   c  may generate a “frame timing” message with the prior super frame satellite delay and the current super frame delay. The timing unit  409   c  transmits the message to the satellite gateway  413  in an appropriated formatted Traffic Token Ring (TTR) message. Software in the PC may be used to configure the IRUs  409   d  in this mode; a special version of firmware may also be provided to the IRU  409   d . One of the IRUs  409   d  may provide a time difference from the pulse to the local super frame header, while the other IRU  409   d  may provide the difference from the pulse to the super frame after the IRU  409   d  is sent to the satellite  107  and received back at the NOC  113 . Further, one IRU  409   d  receives the transport stream for the outroute prior to transmission to the satellite  107 . The other IRU  409   d  receives the transport stream after the transport stream is transmitted to and received back from the satellite by way of an L-Band output from the downlink GUM  409   b.    
     IRUs  409   a  may include hardware to support network timing. The software of the timing unit  409   c  may use this hardware to perform the necessary timing unit functions. A timing support task may be included in the embedded software, which operates in the IRU  409   d  portion of the Timing Unit  409   c . The host software may receive timing information from the firmware and may use the information to format frame timing messages. The frame timing messages may be sent to the satellite gateway  413  through the MUX LAN  421  using a TTR. 
     The system  100  also measures and reports usage information on the channels. This information may be supplied on a periodic basis to billing, and/or made available on a real-time basis to management nodes in the NOC  113  for troubleshooting and monitoring purposes. 
       FIG. 5   a  shows the system interfaces that are involved with the round trip flow of user traffic through the system of  FIG. 1 . The system interfaces permit transceiver  109  to operate without requiring configuration information from the host  101 . According to one embodiment of the present invention, NOC  113  sends transceiver  109  the necessary information to control and manage the transceiver  109 . In this example, user traffic originates from a gateway  419 , which is a hybrid gateway, to IRU  109   a . The traffic is sent to the host PC  101 , which can initiate traffic through IRU  109   a , ITU  109   b , and then ODU  307  for transmission over the return channel. The user traffic is received by the NOC  113  via BCD  411   b . The BCD  411   b  forwards the traffic to NCC  411   a  to the Internet  105  or Intranet  103  via gateway  419 . 
     The communication among the components  419 ,  109   a ,  101 ,  109   b ,  307 ,  411   b , and  411   a  is facilitated by the following interfaces: NOC to IRU Interface  501 , IRU to PC Interface  503 , IRU to ITU Interface  505 , ITU to ODU Interface  507 , ODU to BCD Interface  509 , BCD to NCC Interface  511 , and NCC to Gateway Interface  513 . The NOC to IRU interface  501  is layered to include DVB, PIDS, and MAC addresses. The IRU to PC Interface  503  uses USB super frames to send a large amount of data in a USB burst to the host PC  101 . The payloads of the super frames are IP datagrams with the IP header. A new format header may be used for each message to provide timing and other information to the host PC  101 . In the IRU to ITU interface  505 , the IRU  109  may break the IP datagram into bursts to transmit to the NOC  113 . The IRU  109  may send a frame format message for each frame if there is data to transmit. 
     The internal NOC interface, IRU to BCD interface, is layered to include the burst structure, the return channel frame format, and the message structure for NCC  411   a  messages. The NCC  411   a  may forward traffic to the appropriate gateway  419  (e.g., dedicated gateway or hybrid gateway) in the NOC  113 . The data forwarded to the gateway  419  may be reformatted in a UDP datagram to allow the NOC  113  to receive the traffic as if it were received over a UDP return channel. 
     The NOC to IRU interface  501  may utilize a multi-layer protocol, which includes the following layers: a DVB transport stream, which can support multiple multiprotocol encapsulation messages, for example, in a single MPEG frame per the implementation and includes fixed-size 204 byte MPEG packets (which contain 188 bytes of user traffic and 16 bytes of FEC data); a DVB PID, which the receiver may filter traffic based on PIDs; and a DVB MPE, which the receiver may filter traffic based on MAC Address and may process MPE headers for user traffic. The receiver may also process service tables for PAT and PMT; data following the MPE header has been added to support encrypted traffic. The multi-layer protocol of the NOC to IRU interface  501  may include an IP Payload (the payload of the MPE is expected to be an IP packet including IP headers) and RCE Messages. It should be noted that specific MAC addresses may be used for return channel messages, which may originate from the NCC  411   a  or from a timing unit  409   c.    
     With respect to the DVB transport stream, the DVB standard multiprotocol encapsulation standard over data piping is employed. The multiprotocol header includes the following fields used by system  100 : a MAC Address field (e.g., 6 bytes in length); an encryption field (e.g., a 1 bit field that can be set if the packet is encrypted); and a length field for specifying the length of the packet header. If encryption is disabled for the packet, the IP header and payload immediately follow the MPE header. If encryption is enabled, then the first 8 bytes contain the initialization vector for packet decryption. This vector includes a packet sequence number used to detect out-of-sequence packets. The satellite gateway  413  removes packets from the TTR buffers and transmit them on an outroute. The payload and padding are transmitted following an appropriately formatted MPE header and the initialization vector (for encrypted packets). The payload of the multiprotocol encapsulation frame is determined by the encryption value in the MPE header. If encryption is enabled for the packet, then the first 8 bytes contain an initialization key that also acts as the sequence number. If encryption is disabled, the packet is the IP payload, which is DVB compliant. 
     As indicated above, the NOC to IRU interface  501  may use DVB compliant MPEG-2 formatting. The header of each frame contains a PID, which is filtered by the receiver hardware. The receiver is capable of receiving several of the PID addresses. The receiver may be configured with the PID addresses it is to use, including the one to be used for its NCC  411   c . Each NCC  411   c  may be allocated its own private PID to ensure that receivers only receive traffic for their allocated NCC  411   c . A TTR buffer may be used by the gateways, the NCC  411   a , the Local Timing Unit, and the CAC Server to send messages to the satellite gateway for transmission on the outroute. 
     As shown in  FIG. 5   b , a TTR buffer  521  is carried as the data field of a multicast UDP/IP packet  523 , which includes a multicast IP header  525  and a UPD header  527 . The TTR buffer  521  includes the following fields: a Gateway ID field  529  (8 bits) for specifying the sending gateway ID; a Number of Packets field  531  (8 bits) for indicating the number of packets in this TTR buffer; and a TTR Sequence Number field  533  (16 bits) for specifying the sequence number. The TTR Sequence Number field  533  is used by the satellite gateway  413  (in conjunction with the Gateway ID) to detect TTR buffers lost on the backbone LAN. The TTR Sequence Number field  533  is sent least significant byte first; a value of 0 is always considered to be in sequence. The TTR buffer  521  also includes N packets  535 . Within each packet  535  are the following fields: a DES Key field  537 , two MAC Address fields  539 , a Length field  541 , a Sequence Number field  543 , a Payload field  545 , a Padding field  547 , and an Alignment field  549 . The DES Key field  537 , which is 8 bytes in length, specifies the encryption key to be used by the satellite gateway  413  to encrypt the packet  523 . When no encryption is required (e.g., for NCC  411   a  packets), zero is placed in this field  537 . Two copies of the MAC Addresses (each have a 6-byte length) are stored in field  539 . The first copy is the spacelink MAC address placed in the DVB Header. The second copy of MAC Address is supplied for backward compatibility. The Length field  541  (2 bytes) indicates the length of the packet  535  (least significant byte first). The Sequence Number field  543  indicates the packet number of this Next TTR frame. In an exemplary embodiment, the Payload field  545  has a variable length from 1 to 8209 bytes and stores the message that is to be sent on the outroute (e.g., an IP packet). The length of the Payload field  545  may be limited to the maximum Ethernet frame size, for example. The Padding field  547 , which may vary from 0 to 3 bytes, makes the packet  535  a multiple of long words when transmitted on the outroute; this is required for proper DES encryption. The Alignment field  549  is a 2 byte field and provides filler between packets, ensuring that the next packet starts on a 4 byte boundary. The Padding field  547 , in an embodiment of the present invention, leaves the packet  535  2 bytes short of the proper boundary to optimize satellite gateway  413  processing of the TTR buffer  521 . 
     The total size of a TTR buffer is only limited by the maximum data field size of the UDP packet  523 . Typically, a maximum UDP packet size of 8192 or 16234 is used on the backbone LAN. Gateways need to forward data at high speed and typically send large TTR buffers with multiple IP packets in them. The CAC Server  425  does not need to send at high speed but does send multiple packets in TTR buffers for efficiency. NCCs  411   a  and the Local Timing Unit send messages at a much lower rate than the IP Gateways and typically may only send one message in each TR buffer in order to reduce latency and jitter. 
     Each sender of outroute messages in the NOC  113  may be assigned a unique Gateway ID for each of the traffic streams it may forward to the satellite gateway  413 . The NCC  411   a , Local Timing Unit  409   c , and the CAC Server  425  are each assigned a single Gateway ID. Gateways handling unicast traffic may be assigned two Gateway IDs for their unicast traffic to support prioritization of interactive traffic ahead of bulk transfers. 
     The satellite gateway  413  may use the Gateway ID to map an incoming TTR buffer  521  to the correct priority input queue. satellite gateway  413  can support up to 256 senders. The NCC  411   a , Local Timing Unit  409   c , and CAC Server  425  traffic should be prioritized ahead of all user traffic. This is necessary to ensure minimal propagation delays and also because these traffic types have very low throughput. The NCC  411   a  should be prioritized ahead of all other traffic to ensure that the super frame header is transmitted as soon as possible to ensure that the return channel timing is received in time at the transceivers. 
     The following types of addresses may be used within a Return Channel of system  100 : Ethernet MAC addresses; IP unicast addresses; and IP multicast addresses. For most IP based communication, UDP is used on top of IP. All references to communication using IP (unicast or multicast) addresses, also imply the use of an appropriate (configurable) UDP port number. In some cases, for example, the conditional access IP multicast address and the flow control IP multicast address, the same specific IP address may be used with different UDP port numbers. 
     Each LAN port in the NOC  113  has an Ethernet MAC address assigned to it. The Ethernet MAC address of a LAN port is simply the burned in IEEE MAC address of the NIC (Network Interface Card) that is used to implement the LAN port. The PC may also use Ethernet MAC addressing if a NIC is attached to the PC for forwarding traffic onto a LAN. 
     System  100  also makes use of multicast Ethernet MAC addresses for carrying multicast IP traffic and the broadcast Ethernet MAC address for carrying broadcast IP traffic. All communication at the NOC  113  (and most of the communication within system  100  in general) is IP based. Every NOC component has (at least) one IP unicast address for each of its LAN ports. These addresses are local to the subnet to which the LAN port is attached. 
     Specific receivers are assigned an IP Unicast address that may be used for all unicast traffic to and from the transceiver. This address is allocated to the site at auto-commissioning time and is bound to the TCP protocol for the USB adapter on the user equipment. At the same time, a specific gateway is configured with the serial number/IP address mapping for that transceiver. These unicast addresses may be private addresses since the interface to the internet in both directions may be through NOC equipment that can translate to a public IP address. 
     In addition to its Satellite Card IP unicast addresses, Transceiver  109  uses a private class-A IP address based on the serial number for its CAC individual traffic. IP multicast addresses are used (for efficiency) for all communication on the MUX LAN  421  where there are potentially multiple receivers, including cases where the multiple receivers only exist because of redundancy. There are at least four types of IP multicast addresses used in system  100 : (1) the satellite gateway IP multicast address; (2) conditional access IP multicast addresses; (3) the flow control IP multicast address; and (4) User traffic IP Multicast addresses. The first three address types are private to the MUX LAN  421 ; the fourth address type is public and used for the traffic LAN  423 . 
     The addresses may be selected by the hub operator and configured into the appropriate components. The satellite gateway IP multicast address is used to forward messages to the satellite gateway  413  to be transmitted onto the outroute. All of the senders of traffic (the Gateways, the NCC  411 A, the CAC, and the Local Timing Unit) send to this same address. Messages are sent to the satellite gateway  413  in TTR buffers. TTR buffers are UDP/IP multicast packets with a specific format for the UDP data field. satellite gateway handling of TTR buffers, as previously described. 
     A conditional access IP multicast address may be used by the CAC Server  425  to send conditional access messages to all of the gateways. Two conditional access IP multicast addresses may be used: one for sending key information for unicast traffic, and one for sending key information for multicast traffic. Separate addresses may be defined for this purpose to minimize key handling load on gateways that do not need to process a large number of individual keys. 
     The flow control IP multicast address is used by the satellite gateway  413  to send flow control messages to all of the Gateways. The NCC  411   a  may be configured with the IP Multicast addresses it is allowed to forward to the traffic LAN. Each gateway may be configured with the set of IP multicast addresses that it may forward to the outroute. If messages appear on the Traffic LAN which match an address in the gateway, the gateway formats the data into TTR buffers and uses the key provided by the CAC server  425  for the multicast address. 
     System messages are messages generated and used internally by the NOC subsystem. The system messages include conditional access messages, flow control messages; and redundancy messages. All message formats defined by the return channel may be little endian. Existing messages which are reused for the return channel may retain the big or little endian orientation they currently have. 
     Conditional access messages may be sent by the CAC Server  425  to deliver conditional access information, e.g. keys. There are at least two types of conditional access messages: gateway conditional access messages, and transceiver conditional access messages. Conditional access messages may be unidirectional. That is, messages are only sent from the CAC Server  425 , not to the CAC Server  425 . 
     The CAC Server  425  sends encryption keys to the gateways. All of the unicast encryption keys for every enabled serial number are sent to all of the gateways. The gateways may store the received keys in a table. The CAC Server  425  also sends encryption keys to the gateways for multicast service elements. The gateways may store the received keys in a table and use the table to extract multicast encryption keys for forwarding multicast IP packets. The CAC Server  425  sends encryption keys, using the backbone LAN, to the conditional access IP multicast addresses. The rate at which these conditional access messages are sent is controlled by parameters in the CAC Server  425 . The messages are sent to support relatively quick notification in the event of a key change and/or the addition of a new transceiver and to support new and restarted Gateways. 
     The CAC Server  425  sends decryption keys to the transceivers  109 . Unicast keys may be sent in Periodic Adapter Conditional Access Update (PACAU) messages, addressed to the specific transceiver&#39;s unicast conditional access spacelink MAC address. The PACAUs also may contain multicast keys for the multicast service elements for which the transceiver  109  has been enabled. The mapping of service elements to actual multicast addresses may be sent by the CAC Server  425  in Periodic (Data Feed) Element Broadcast (PEB) messages. These messages may be sent to the broadcast conditional access spacelink MAC address. All of the transceivers  109  receive the PEB messages. The transceiver  109  also supports the reception of the extended PEB format, which allows a virtually unlimited number of IP multicast addresses by providing the capability to segment the PEB. 
     Flow control messages may be sent by the satellite gateway  413  to the access gateways. The satellite gateway  413  measures the average queue latency in the satellite gateway  413  for each of the priority queues. This information may then be sent to the gateways, mapped to the gateway IDs. The gateways may use this information to increase and decrease the amount of TCP spoofed traffic being accepted and forwarded from IP hosts at the hub. Flow control messages are unidirectional, i.e. they are only sent from the satellite gateway  413  toward the IP gateways. 
     Outbound multicast user traffic, (e.g. file broadcast or MPEG-2 video), is received by an access gateway. The access gateway may be configured with the list of IP multicast addresses that it should forward and receives encryption keys for these IP multicast addresses from the CAC Server  425 . If the gateway receives an IP packet with a multicast address that has not been enabled, the packet is discarded. The IP gateway forwards an IP packet for a multicast address that has been enabled, along with the appropriate spacelink MAC address and encryption key, as a packet payload in a TTR buffer. The satellite gateway  413  may extract the IP packet from the TTR buffer, encrypts it and forwards it to the outroute. 
     An application on the PC  101  opens an IP multicast when it wants to receive the Outbound Multicast stream. The driver may calculate the appropriate MAC address and configures the IRU  109   a  to receive traffic on the MAC address. The PC driver may forward IP packets based on the multicast address to the applications that have opened the address. 
     IP Multicast traffic need not be sourced over the return channel. Where inroute bandwidth can be allocated to users, it could be sourced over the return channel by enabling the transceiver  109  to send IP Multicast per the service plan of the transceiver  109 . TCP traffic may be spoofed at the NOC  113  to allow for higher speed throughputs even with satellite delay. The Access gateway software may buffer additional traffic for transmission through the satellite and locally acknowledge Internet traffic. 
     Base upon the user service plan selections, connections may be initiated through the Internet  105  to a specific transceiver  109  by using the IP address associated with the transceiver. If the transceiver  109  is using Network Address Translation (NAT) to the Internet  105 , Internet-initiated connections may not be possible since the public Internet address is not associated with a specific private address associated with the transceiver until a connection is initiated from within the NOC  113 . 
     The TCP User traffic, when initiated at the PC  101 , may be passed through the system  101  as follows. PC  101  sends an IP Packet to IRU  109   a ; in turn, the IRU  109   a  transmits IP packets (possibly in multiple bursts) to the NOC  113 . The NCC  411   a  reassembles and forwards the IP packet to the gateway. The gateway communicates with the destination host and receives the response. The gateway sends the IP packets to the IRU  109   a . A NCC  411 A may receive return channel packets from the return channels. Each packet may be a subset or a complete IP packet. When the packet is a partial IP packet, the complete IP packet may be reassembled prior to passing the IP packet to an access gateway. First and last bits and a sequence number may be used in each return channel frame to provide the necessary information for the NCC  411   a  to rebuild the message. The NCC  411   a  may be able to rebuild packets from many transceivers at once. In addition, multiple data streams may be supported from the same transceiver to support prioritization of traffic. 
     Within the system  100 , packets are formatted using multiprotocol encapsulation. Therefore, all packets include a DVB-standard header that includes a MAC address. For different types of traffic, the MAC address is set differently. The following types of MAC addresses exist: Unicast traffic; Multicast traffic; Unicast conditional access; Multicast conditional access; Return Channel Broadcast messages; and Return Channel Group messages. 
     Table 1, below, lists exemplary MAC addresses, according to an embodiment of the present invention. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Field 
                 Size 
                 Scope 
                 Description 
               
               
                   
               
             
            
               
                 Serial Number 
                 24 Bits 
                 Unicast 
                 Serial number burned into 
               
               
                   
                   
                   
                 the IRU 
               
               
                 IP Multicast Address 
                 20 Bits 
                 Multicast 
                 IP Multicast addresses are 
               
               
                   
                   
                   
                 32 bit addresses with 
               
               
                   
                   
                   
                 format a.b.c.d, where octet 
               
               
                   
                   
                   
                 “a” may be 224–239. 
               
               
                 Type Indicator 
                  2 Bits 
                 All 
                 Indicates type of address: 
               
               
                   
                   
                   
                 1 - Multicast 
               
               
                   
                   
                   
                 2 - Unicast 
               
               
                   
                   
                   
                 3 - Internal multicast 
               
               
                   
               
            
           
         
       
     
     Table 2, below, lists the MAC addresses associated with the various traffic types that are supported by the system  100 . 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Address Type 
                 Value 
                 MAC Address (Hex) 
               
               
                   
               
             
            
               
                 Unicast User Traffic 
                 Serial Number 1 
                 02 00 0A 00 00 01 
               
               
                   
                 Serial Number 2 
                 02 00 0A 00 00 02 
               
               
                   
                 Serial Number 256 
                 02 00 0A 00 01 00 
               
               
                 IP Multicast Traffic 
                 225.2.3.4 
                 01 00 6E 52 03 04 
               
               
                   
                 239.221.204.1 
                 01 00 6E 5D CC 01 
               
               
                 Unicast Cond. Access 
                 Serial Number 1 
                 02 00 0A 00 00 01 
               
               
                   
                 Serial Number 2 
                 02 00 0A 00 00 02 
               
               
                   
                 Serial Number 256 
                 02 00 0A 00 01 00 
               
               
                 Multicast Cond. Access 
                 Broadcast 
                 03 00 00 00 00 00 
               
               
                 Return Channel Messages 
                 Broadcast 
                 03 00 00 00 00 01 
               
               
                 RC Group Messages 
                 Broadcast - RCE1 
                 03 00 01 00 00 01 
               
               
                   
                 Broadcast - RCE2 
                 03 00 01 00 00 02 
               
               
                   
               
            
           
         
       
     
     A unicast traffic MAC address may be used for traffic that is sent over the outroute to a specific receiver. The MAC address is determined by the serial number of the IRU  109   a ; the same MAC address is also used for CAC individual traffic. The IP Multicast address is determined from the IP multicast address using the TCP standard. This standard only maps the last two octets of the IP address and part of the second octet of the IP address. Therefore, addresses should be configured to ensure that multiple IP addresses that map to the same MAC address are not used. 
     The transceiver  109  periodically receives a list of keys for multicast traffic. If the transceiver  109  is enabled to receive the multicast address, then the IRU  109   a  may enable reception of the appropriate MAC address when an application uses standard Winsock calls to receive from an IP multicast address. Part of enabling the address may be the retrieval of the relevant encryption key and passing that key to the IRU  109   a.    
     The Unicast Conditional Access MAC address is used by the CAC Server  425  to send unicast conditional access messages to a specific transceiver. The address is the same as its unicast traffic MAC. Information about a site&#39;s access to different multicast streams and whether it is enabled are periodically transmitted to a site over this address. 
     The Multicast Conditional Access is used by the CAC Server  425  to broadcast global conditional access information to all transceivers  109 . The list of multicast addresses and their keys are periodically provided to all receivers  109 . These messages are transmitted unencrypted. 
     The Return Channel Messages address is used for messages that may be received by all adapters  109  on specific transponders, including those messages required for the commissioning process. Theses messages received on this address are processed directly in the IRU  109   a , so the IP header is not used at the receiver and should be ignored. The IP datagram includes the following packet types: a Super-frame Numbering Packet (SFNP), which provides a timing reference and identification for the transponder; and an Inroute Group Definition Packet (IFDP), which defines available return channel groups and resources available on each group. 
     The Return Channel Group Messages address is used for messages sent on a specific return channel group to transceivers  109 , which are assigned to the particular group. The grouping is implemented to provide a scalable approach to transmitting information so that a single site does not need to process 300 return channels. The messages received in this address are processed by the IRU  109   a , so the IP header is not used by the receiver and should be ignored. The IP datagram may include the following packet types: Bandwidth Allocation Packet (BAP), Inroute Acknowledgement Packet (IAP), and Inroute Command/Ack Packet (ICAP). The BAP contains the bandwidth allocation structure and the allocation of the bursts to each site on the group. The IAP contains a list of the bursts for a specific frame and a bitmask indicating if the frame was successfully received at the NOC  113 . The ICAP contains a list of commands to be sent to IRUs  109   a  from the NCC  411   a.    
     Exemplary packets are sent for local processing in the IRU  109   a  to support the Return channel. Because these packets can be identified based on the MAC address, they need not be encrypted; consequently, these MAC Addresses can be dynamically added and removed by the IRU  109   a . All of these packets that are intended to be processed from the IRU  109   a  may have UDP/IP headers on them, but these headers may be ignored and assumed to be correct from the IRU  109   a;  an exception is that since there may be padding on the Outroute for word alignment, the length of these packets may be taken from the UDP Header. 
     To ensure these messages are processed in the proper order within the IRU  109   a , these messages may all be transmitted on the same PID. It should be noted that no assumption is made about the order of messages that are sent from different NCCs  411   a , largely because of the possible NOC side network delays. 
     All the fields in the return channel packets may be encoded using a Big Endian (Network Byte Order) format. Specifically, the structure of the bits for these packets may start with bit  7  of byte  0 , and after reaching bit  0  in each byte, they may wrap into bit  7  of the next byte. When a field has bits crossing over the byte boundary, the lower numbered bytes may have the higher place value. For example if a 13 bit field started on bit  2  of byte  7 , then the 3 most significant bits ( 12 : 10 ) would come from byte  7  bits  2 : 0 , the 8 next most significant bits ( 9 : 2 ) would come from byte  8 , and the 2 least significant bits ( 1 : 0 ) would come from byte  9  bits  7 : 6 . 
     According to an embodiment of the present invention, the bandwidth associated with these packets is 700 Kbps, of which only 225 Kbps may be processed by a given IRU  109   a . This is equivalent to just under 168 MPEG packets per super frame, although the total usable bandwidth may depend on the MPEG Packet packing. This bandwidth may require for each outroute. Although the SFNP may have to be distinct for each outroute, the other packets can be identical for all outroutes that share the common Return channels. All of these frames may be sent with very high priority by the appropriate satellite gateway and the Super Frame Numbering Packets may require the highest priority in the system. Encoding of these packets is especially crucial, as incorrect information, and malformed packets can cause IRU misoperation, including transmitting on incorrect frequencies. These messages may all be UDP datagrams, which may include the following packet types: superframe numbering packet (SFNP), Inroute Group Definition Packet (IGDP), Bandwidth Allocation Packet (BAP), Inroute Acknowledgement Packet (IAP), and Inroute Command/Acknowledgement Packet (ICAP). The structures of these packets are discussed below with respect to  FIGS. 6A–6O . 
       FIGS. 6A–6O  are diagrams of the structures of exemplary packets used in the system of  FIG. 1 . The SFNP packet is used to lock network timing for the return channels and as a beacon to identify the proper network. A super frame numbering packet (SFNP)  601 , as seen in  FIG. 6A , includes an 8-bit frame type field  601   a , which has a value of 1 to specify that the packet  601  is a SFNP. A Timing Source field  601   b  has a length of 1 bit and is used to distinguish the particular timing unit that generated the SFNP. This field  601   b  may be used to resolve confusion during switchover between redundant timing references in the NOC  113 . A 7-bit Version field  601   c  is used to indicate the return channel protocol version. If an adapter  109  does not recognize a protocol version as specified in this field  601   c , then the adapter  109  does not transmit or use any of the incoming packets that are related to the return channels. According to one embodiment of the present invention, this protocol may only append additional information onto the packet  601 , without changes to these existing fields. In this manner, a beacon function for dish pointing can be maintained, irrespective of version. 
     The SFNP  601  includes a Frame Number field  601   d , which is 16 bits in length and is incremented by 8 each super frame, and is used to identify global timing; the Frame Number field  601   d  may wrap every 49 minutes. A 32-bit Local Delay field  601   e  captures elapsed time, as obtained from a timing unit, between a previous super frame pulse and the reception of the SFNP through the local equipment. The value of 0 for this field  601   e  may be used to indicate that the value is unknown for the super frame. The IRU  109   a  may need to receive 2 consecutive SFNP to be able to interpret this field  601   e . Additionally, a 32-bit Echo Delay field  601   f  indicates the elapsed time between two prior super frame pulses and the reception of the SFNP  601  through the satellite  107 . As with the Local Delay field  601   e , the value of 0 indicates that the value is unknown for the super frame. The IRU  109   a  may need to receive three consecutive SFNP  601  to be able to interpret this field  601   f . A SFNP Interval field  601   g , which is 32 bits in length, specifies the elapsed time between the current super frame pulse and a previous frame pulse. This may allow the IRU  109   a  to adjust for any differences between the local measurement clock (nominal 8.192 MHz), and the clock used by the timing units, which may be different. The value of 0 may be used to indicate that the value is unknown for the previous super frame. Because of the high accuracy of the timing units, the IRU  109   a  may only need to receive three consecutive SFNPs  601  to interpret this field  601   g . A Space Timing Offset field  601   h  is a 32 bit field that specifies a timing offset value. A Reserved field  601   i , which is 2 bits in length, has a 0 value when transmitted; this field  601   i  can provide a mechanism to confirm whether the correct satellite network is being monitored. Further, a 15-bit Frequency field  601   j  specifies the frequency of the outroute satellite transponder, in units of 100 kHz. A Longitude field  601   k , which is 15 bits long, indicates the longitude of the Outroute Satellite, in which bit  14  is the West/East — indicator, bits  13 : 6  are the degrees, and bits  5 : 0  are the minutes. 
     The SFNP uses 1 packet per Super Frame, or 2 Kbps of bandwidth, and is transmitted on the beacon multicast address. The processing of these packets are as follows. If the FLL (frequency lock loop) Lock is lost, then no timing can be derived from the SFNP, and network timing is declared as out of Sync. Both timing source may be monitored, if present, but a change in selection may only be made after receiving 3 consecutive SFNP from the same source when no network timing source is selected. In addition, network timing is declared as in Sync, only after receiving 3 consecutive SFNP from the selected timing source, and having the local timing match within a given number of clocks. This may typically require 4 super frame times. Network timing is declared as out of Sync, after receiving 2 consecutive SFNP from the selected timing source, and having the local timing being off by more than a given number of clocks. Additionally, network timing is declared as out of Sync, and the network timing source becomes unselected, after not having received any SFNP for 3 super frame times. Further, network timing is declared as out of Sync, and the network timing source becomes unselected, after not receiving 2 consecutive SFNP for a given number of super frame times. In addition, network timing is declared as out of Sync, and the network timing source becomes unselected, after not receiving 3 consecutive SFNP for a given number of super frame times. 
     The Inroute Group Definition Packet (IGDP) packet may be used to define the Return channels on a return channel group, and to allow selection of return channel groups for Aloha and Non-allocated ranging. Return channel groups are used to allow for load sharing between a number of return channels, and to minimize the outroute bandwidth required to control the return channel bandwidth allocation. They also may limit the amount of information that needs to be cached or processed by the IRU  109   a.    
     As seen in  FIG. 6   b , an inroute group definition packet  603  includes the following fields: a Frame Type field  603   a , an Inroute Group ID (identification)  603   b , a Reserved field  603   c , a Return Channel Type field  603   d , an Aloha Metric field  603 , a Ranging Metric field  603   f , and a Frequency Table field  603   g . For the inroute group definition packet  603 , the 8-bit Frame Type field  603   a  has a value of 2. The Inroute Group ID field  7  is 7 bits long and identifies a particular inroute group. The 13-bit Reserved field  603   c  has a 0 value and is ignored during reception. The Return Channel Type field  603   d  use 4 bits to indicate the type of return channels that are defined in the inroute group; e.g., the value of 0 is defined as 64 Kbps with convolutional encoding. The Aloha Metric field  603  (a 16 bit field) is used for random weighted selection of a return channel group when going active, and is based on the number of Aloha bursts that are defined and the collision rate on those bursts. The metric value also accounts for loading on the NCC  411 A, or the Return channel Group. For example, a value of 0 indicates that Aloha is not currently available on this Return channel Group. The Ranging Metric field  603   f , which is 16-bits, is used for random weighted selection of a Return channel Group when performing Nonallocated Ranging. The ranging metric value is based on the number of Nonallocated Ranging bursts that are defined and associated collision rate on those bursts. For example, a value of 0 indicates that Nonallocated Ranging is not currently available on this Return channel Group. Lastly, the packet  603  has a variable length (N×24 bits) Frequency Table field  603   g , which is used to transmit on each of the return channels in the group. Changing the Frequency for a return channel must be carefully coordinated to avoid interruptions of network operation, or transmission on the wrong return channel frequency around the switch over point. According to one embodiment, there is an upper bound of no more than 4K return channels between all return channel groups for an outroute. The upper bound for the number of return channels in each return channel group depends on the limit of the number of Burst Allocations in the Bandwidth Allocation Packet ( FIG. 6   c ). The value of N is derived from the length of the IP Datagram; this uses 1 packet per Return channel Group per Super Frame, or 26 Kbps of bandwidth for 75 Return channels per Group, and 300 return channels. The packet  603  is transmitted on the all IRU Multicast address. 
     Each IRU  109   a  may be expected to monitor all Inroute Group Definition Packets. The IRU  109   a  filters out Return channel Types that the IRU  109   a  is not configured to support, and age out the definition if not received for 3 Super Frame times. The table that is created in each IRU  109   a  from all of these packets should be almost static, with the exception of the Metrics. This is to minimize the overhead in the IRU  109   a  for reorganizing the Inroute Group Table, and because these changes may disrupt network operation. 
     When an IRU  109   a  is active, the IRU  109   a  may monitor its current Inroute Group, as well as a second Inroute Group around the time the IRU  109   a  is moved among Inroute Groups. To limit latency when an adapter needs to go active, all inactive adapters with valid Ranging information may use the following procedures. Every 4 th  frame time in the Super Frame, the IRU  109   a  may make a random weighted selection between all the Inroute Group&#39;s that advertise a non-zero Aloha Metric, and may start to monitor that Inroute Group. The previous Inroute Group may need to be monitored until all previous Bandwidth Allocation Packets have been received, or lost. 
     For every frame time, the IRU  109   a  may randomly select one of the Aloha bursts from the Bandwidth Allocation Packet for the Inroute Group that is selected for that frame time. When the IRU  109   a  goes active and has no outstanding Aloha packets, the IRU  109   a  may select a random number of frames (from 1 to 8), ignoring any frame times that had no Bandwidth available, it may transmit a single burst during the randomly selected frame time, and wait to be acknowledged. If the IRU  109   a  has not received an acknowledgement (e.g., the acknowledgement is lost), the IRU  109   a  may resend the Aloha packet. After a number of retries indicated in the SFNP, the adapter should classify the ITU  109   b  as non-functional, and wait for user intervention. While the Aloha packet is outstanding, the IRU  109   a  may monitor up to 3 Inroute Groups: (1) one for the Aloha Acknowledgement, (2) one for the new Inroute Group to try, and (3) one for the previous Inroute Group. 
     In order to limit latency when an adapter needs to go active, all inactive adapters with invalid Ranging info may use a similar procedure for Nonallocated Ranging bursts. The approach may be augmented to include a default Power Level for the first Nonallocated Ranging burst. Further, this power level may be increased until the Ranging Acknowledgement is received by the IRU  109   a.    
     A bandwidth allocation packet (BAP), shown in  FIG. 6C , is used to define the current bandwidth allocation for all inroutes connected to an Inroute Group. The packet  605  includes an 8-bit Frame Type field  605   a  (which has a value of 3 to indicate a BAP), and a 16-bit Frame Number field  605   b , which indicates the Frame Number that is allocated in this packet  605 , and may be larger than the current Frame Number. The difference between the frame numbers is a fixed offset to allow the IRU  109   a  sufficient time to respond to changes in bandwidth allocation. A Burst Allocation field  605   c  has a length of N×24 bits and specifies all the burst allocations for each Inroute. The field  605   c  may order all the bursts in a Frame, and may repeat a Frame for each Inroute in the Group; the field  605   c  is limited to no more than 489 entries, since IP Datagrams are limited to 1500 bytes. This feature enables the IRU  109   a  to perform a linear search. An incorrect Burst Allocation Table can cause improper operation of the network, as there is limited error checking on this field  605   c . The value of N is derived from the length of the IP Datagram 
       FIG. 6   c  shows an exemplary burst allocation field of the packet  605  in  FIG. 6C . The Burst Allocation field  607  includes an Assign ID field  607   a , a Ranging field  607   b , a Reserved field  607   c , and a Burst Size field  609   d . The Assign ID field  607   a  provides a unique identifier that is used to indicate the particular Adapter that has been allocated the bandwidth A value of 0 for the field  607   a  indicates Aloha (and Nonallocated Ranging) bursts; the value of 0xFFFF may be used to indicate unassigned bandwidth. Other values are dynamically assigned. The NCC  411 A may impose other reserved values, or structure on these values, but the Adapter may only know what is explicitly assigned to it and 0. The Ranging field  607   b  specifies whether the burst is allocated for normal or ranging bursts. Even though an adapter may be designated as ranging, that adapter may be able to send Encapsulated Datagrams over the Inroute; and an active user may have Ranging turned on/off to test or fine tune it&#39;s values, with minimal impact on performance. The Reserved field  607   c  should have a value of 0 upon transmission and ignored on reception. The Burst Size field  607   d  is in terms of slots and includes the aperture and burst overhead. 
     For each Frame, the IRU  109   a  may receive another Bandwidth Allocation Packet from the Inroute Group it is currently expecting to receive bandwidth allocation on. The IRU  109   a  may need to scan the entire table to obtain the necessary information to transmit data, and process acknowledgements. In an exemplary embodiment, the Burst Allocation field  605   c  may contain the following fields: Inroute Group, Inroute Index, Frame Number, BurstID, Burst Offset, Burst Size, and Acknowledgement Offset. Since the IRU  109   a  can be monitoring two Inroute Groups, the IRU  109   a  may need to confirm the Inroute Group based on the MAC Address of the packet  605 , and only process the Bandwidth Allocation Packet  605  for which IRU  109   a  expects to use bandwidth. The Inroute Index is the Cumulative Burst Offset DIV Slot Size of a frame, and is used as an index into the Frequency Table field  603   g  of the Inroute Group Definition Packet  603 . Frame Number within the Bandwidth Allocation field  605   c  can come from the Frame Number field  605   b  of the packet  603 . A BurstID field may be the 4 least significant bits of the Index into the Burst Allocation field  605   c . The Cumulative Burst Offset starts at 0, and increases with the each Burst Size. The Burst Offset is effectively the Cumulative Burst Offset MOD Slot Size of a Frame. The Burst Size may come from the Burst Allocation packet ( FIG. 6D ). An Acknowledgement Offset field is an Index into the Burst Allocation Table of the entry. 
     This uses 1 packet per Inroute Group per Frame, or 535 Kbps of bandwidth for 25 active users per inroute, 75 Inroutes per Group, and 300 inroutes. Since it is transmitted on the Inroute Group&#39;s Multicast address, each IRU may only have to process 134 Kbps. 
     To ensure that active users do not experience degraded performance or data lost by any load balancing at the NCC  411   a , at least ten frames prior to moving an IRU  109   a  to a different Inroute Group (but on the same NCC  411   a ), the IRU  109   a  may be notified, so that it can begin to monitor both Inroute Group streams. This feature permits the system  100  to scale. The IRU  109   a  may need to continue monitoring both streams, until all outstanding Inroute Acknowledgement packets are received, or have been identified as lost. There may be at least 1 frame time with no bandwidth allocated between bursts that are allocated on different Inroutes; this ensures that the IRU  109   a  may be able to fill all its assigned slots, and have at least 1 frame time for tuning. The above requirement may apply to bursts that are defined across consecutive Bandwidth Allocation Packets, and when moving between Inroute Groups on the same NCC  411   a . However, if this requirement is not met, to avoid transmission across multiple frequencies, then transmission may be disabled during one of the assigned frames, rather than permitting tuning during a transmission. There may be at least 1 complete frame with no bandwidth allocated between normal and Ranging bursts, thereby ensuring that the IRU  109   a  may be able to fill all it&#39;s assigned slots, and yet have at least 1 frame time for tuning and adjusting transmission parameters. After the Bandwidth Allocation packet (which moves an IRU  109   a  to a different Inroute Group) is sent, the NCC  411   a  may continue to receive bursts under the old Inroute Group for a time in excess of the Round Trip Delay. The NCC  411   a  should be prepared to accept these frames, and to acknowledge them, and the IRU should continue to monitor the Acknowledgements from the old Inroute Group. An IRU  109   a  may not have its bandwidth moved to a different Inroute Group, while the IRU  109   a  is still monitoring a previous Inroute Group the IRU  109   a  has just been moved from—i.e., the IRU  109   a  need only monitor up to 2 Inroute Groups. 
     An adapter may only be assigned multiple bursts during a single frame time under three conditions. First, if these bursts are all on the same Inroute. Second, the bursts are adjacent to each other (i.e., back to back) in the frame. The adapter may transmit one packet for each allocated burst, but without the Burst Overhead of turning the Radio on and off for each packet. In the third case, all of the bursts, except the last, may be large enough for the maximum sized packet (largest multiple of the slot size ≦256), but only the first burst may have the Burst Overhead/Aperture included in its size. Accordingly, the system  100  is constrained to no more than 6 bursts per frame to support 256 Kbps Inroutes. 
     Once an AssignID is assigned to an adapter on an Inroute Group, the assignment may not change while the adapter remains active—except as part of a move between Inroute Groups. Once an AssignID is assigned to an adapter on an Inroute Group, it may be left unused for five super frame periods after it is no longer in use. 
     It is important to note that if an Inroute Group advertises that it has Aloha or Nonallocated Ranging bursts, than it may have some number of those bursts defined every frame time—e.g., for the next ten frame times. Furthermore, the number of bursts should be evenly spread across all frames in the Super Frame. Failure to meet this requirement may result in higher collision rates, and increased user latency. 
     The IAP packet is used to acknowledge each Inroute packet for assigned bandwidth with a good CRC, regardless of the presence of any encapsulation data. Besides allowing for faster recovery to inroute packet errors, this may also allow measurement of the inroute PER at the IRU. Aloha and Nonallocated Ranging packets are acknowledged explicitly. 
       FIG. 6   e  shows the structure of an inroute acknowledgement packet, according to an embodiment of the present invention. An inroute acknowledgement packet contains the following fields: a Frame Type field  609   a , a Frame Number field  609   b , and an ACK field  609   c . For this type of packet, the Frame Type field  609   a  is given a value of 4. The Frame Number field  609   b  specifies the Frame that the acknowledgement applies, which may be less than the current Frame Number. The ACK field  609   c  is a bitmap, that matches the entries for this Frame in the Burst Allocation field  605   c  of the Bandwidth Allocation Packet  605 . To determine what was acknowledged, the IRU  109   a  may determine which bursts were assigned to it by the Bandwidth Allocation Packet  605 , recalling the data that was transmitted during those bursts. The value of N is derived from the length of the IP Datagram, and may match the value of N from the associated Bandwidth Allocation Packet  605 . 
     This uses 1 packet per Inroute Group per Frame, or 57 Kbps of bandwidth for 25 Active Users per Inroute, 75 Inroutes per Group, and 300 inroutes. Since it is transmitted on the Inroute Group&#39;s Multicast address, each IRU may only have to process 15 Kbps. 
       FIG. 6   f  shows the structure of an inroute command/acknowledgement packet, according to an embodiment of the present invention. An inroute command/acknowledgement packet  611  is used to explicitly acknowledge Aloha and Nonallocated Ranging bursts, and to send commands to an Adapter. Acknowledgment packets are sent on the Inroute Group&#39;s Multicast address, and commands are sent on the All IRU Multicast address. These packets are multicast to reduce Outroute bandwidth, and since there is no IRU unicast address. The inroute command/acknowledgement packet  611  includes the following fields: a Frame Type field  611   a , a Reserved field  611   b , Number of Entries field  611   c , Frame Number field  611   d , Offset Table field  611   e , Padding field  611   f , and a Command/Acknowledgment field  611   g . For this type of packet  611 , the 8-bit Frame Type field  611   a  is set to a value of 5. A 3-bit Reserved field  611   b  is unused and set to 0 for transmission; the field  611   b  is ignored on reception. The Number of Entries field  611   c , a 5-bit field, specifies the number of entries in the Offset Table field  611   e . For Acknowledgments, the 16-bit Frame Number field  611   d  indicates the frame that is being acknowledged; for Commands, the field  611   d  specifies the frame that the command is directed towards. The Offset Table field  611   e  (with N×10 bits) provides a table of offsets for where each of the variable sized Command/Acknowledgment fields  613  begin. The size of the field  611   e  is known based on the Command field  613 , but can also be derived from the Offset for the next Entry, or the size of the IP Datagram for the last entry. Each offset is a 10 bit value, and starts from the beginning of the Offset Table field  611   e . The value of N is the Number of Entries. Padding field  611   f  varies in length from 0 to 6 bits and provides byte alignment at the end of the Offset Table field  611   e . A Command/Acknowledgment field  613  has a length of N×8 bits and provides a list of Commands or Acknowledgments, sorted by serial number (SerNr); these commands and acknowledgements are defined according to  FIGS. 6G–6L . It should be noted that no more than one Command or Acknowledgment can be sent to an adapter per packet. The value of N is derived from the length of the IP Datagram 
       FIG. 6   g  shows an Exemplary Ranging Acknowledgement. The acknowledgement  613  includes a Serial Number (Serial No.) field  613   a  (26 bits), a Command field  613   b  (4 bits), a Reserved field  613   c  (3 bits), an Inroute Group ID field  613   d  (7 bits), an Assign ID field  613   e  (16 bits), a Power Adjustment field  613   f  (8 bits), and a Timing Adjustment field  613   g  (8 bits). The SerNr field  613   a  specifies the serial number of the IRU  109   a . A value of 0 for the Command field  613   b  indicates a Ranging (and Nonallocated Ranging) Acknowledgment. When an adapter is using allocated Ranging, it may not receive Ranging Acknowledgements for each Frame, but the Encapsulated Datagrams may be acknowledged with the Inroute Acknowledgement Packet  609 . The Reserved field  613   c  is similar to the reserved fields described above. The Inroute Group ID field  613   d  indicates the Inroute Group for which future Ranging Bursts may be allocated. The Assign ID field  613   e  is used for future Bandwidth Allocation Packets  637 , whereby future Ranging Bursts may be allocated. If the Assign ID field  613   e  has a value of 0, Ranging may be terminated, thereby leaving the adapter inactive. Ranging can also be terminated by the clearing of the Ranging bit in the Burst Allocation field  605   c , but this should only be done if the Ranging had passed. The Power Adjustment field  613   f  is a signed 8 bit field that specifies power adjustment in increments of 0.1 dB. The Timing Adjustment field  613   g  indicates timing adjustments in units of μs. 
       FIG. 6   h  shows the structure of an exemplary Aloha Acknowledgement. This acknowledgement  615  includes a Serial Number field  615   a , a Command field  615   b , a Reserved field  615   c , an Inroute Group ID field  615   d , and an Assign ID field  615   e . These fields  615 ,  615   a ,  615   b ,  615   c , and  615   e  are similar to the fields  613   a ,  613   b ,  613   c ,  613   d , and  613   e , respectively, the ranging acknowledgement  613 . With this particular acknowledgement, the Command field  615   b  is given a value of 1. The Inroute Group ID field  615   d  specifies the inroute group that is to receive future bandwidth allocations. The Assign ID field  615   e  is an Id used in future Bandwidth Allocation Packets  637 , whereby future Bursts may be allocated. A value of 0 for the Assign ID field  615   e  acknowledges the data without assigning any bandwidth. If any Backlog is advertised from the Aloha packet, the packets may need to be flushed, since the adapter remains inactive and no synchronization is possible. 
       FIG. 6   i  shows the structure of a Disable ITU command, according to an embodiment of the present invention. A Disable ITU command  617  a Serial Number field  617   a  (26 bits), a Command field  617   b  (4 bits), and a Reserved field  617   c  (3 bits). As with the acknowledgement packets  613  and  615 , the Serial No. field  617   a  stores the serial number of the IRU  109   a . For this type of command, the Command field  617   b  is assigned a value of 2. Under this command, the IRU  109   a  may not transmit until it receives another command indicating that the IRU  109   a  may transmit. This setting, for example, is stored in nonvolatile memory on the IRU  109   a.    
       FIG. 6   j  shows the structure of an Exemplary Start Ranging Command. This command  619  includes a Serial Number field  619   a  (26 bits), a Command field  619   b  (4 bits), an Invalidate field  619   c  (1 bit), a Reserved field  619   d  (3 bits), an Inroute Group ID field  619   e  (7 bits), and an Assign ID field  619   f  (16 bits). In this case, the Command field  619   b  has a value of 3. If the adapter is inactive, this command  619  may start sending an Nonallocated Ranging packet. An active adapter may be informed by having Ranging bursts allocated. The 1-bit Invalidate field  619   c , if set, indicates that the Adapter may invalidate it&#39;s prior Ranging Info, and revert to the defaults, before sending it&#39;s Nonallocated Ranging packet. The Reserved field  619   d , Inroute Group ID field  619   e , and Assign ID field  619   f  are similar to the fields  615   c ,  615   d , and  615   e , respectively of acknowledge packet  615 . 
       FIG. 6   k  shows the structure of a Go Active Command and a Change Inroute Group Command. These commands include the following fields: a Serial Number field  621   a  (26 bits), a Command field  621   b  (4 bits), a Reserved field  621   d  (3 bits), an Inroute Group ID field  621   e  (7 bits), and an Assign ID field  621   f  (16 bits). For the Go Active Command, the Command field  621   b  has a value of 4, while the field  621   b  is set to a value of 5 for the Change Inroute Group command. In both commands, the Assign ID field  621   e  is used in future Bandwidth Allocation Packets, whereby future Bursts may be allocated. With respect to the Go Active Command, if the Assign ID field  621   f  has a value of 0, the data is acknowledged without assigning any bandwidth. If there is any Backlog advertised from the Aloha packet, the backlog of packets may need to be flushed, since the adapter remains inactive and no synchronization is possible. In the case of a Change Inroute Group command, an Assign ID field  621   e  with a 0 value can be used to make an adapter inactive (alternatively, the bandwidth allocation of the adapter is removed). 
     The structure of a Send Test Pattern Command is shown in  FIG. 6   l . This command  623  includes a Serial Number field  623   a  (26 bits), a Command field  623   c  (4 bits), a Reserved field  623   d  (3 bits), a Pattern field  623   d  (3 bits), and a Frequency field  623   e  (24 bits). With this command, the Command field  623   c  has a value of 6. It is noted that this command may inactivate the adapter. The 3-bit Pattern field  623   d  specifies the test patterns that can be programmed from the ITU registers. If the Pattern field  623   d  has a value of 0, then the test is terminated. The test can also be terminated if the Send Test Pattern Command is not repeated within four frame times. 
     The return channel burst structure may be defined by the burst structure required by the Burst Channel Demodulators (BCDs)  411   b . The 64 kbps OQPSK BCD  411   b  utilizes the frame structure, shown below in Table 3. The frame overhead is sized as 2 slots (112 bits) minus the aperture size. The Aperture size (125 microseconds) is 8 bits. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Micro- 
                   
               
               
                 Field 
                 Bits 
                 sec. 
                 Comments 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Radio Turn-on 
                 2 
                  31 
                   
               
               
                 Continuous Wave 
                 55 
                 859 
                 Divided into subparts 
               
               
                 CW Detect 
                 17 
                   
                 Needed for BCD to determine start 
               
               
                   
                   
                   
                 of burst 
               
               
                 Freq Est 
                 24 
                   
                 Needed for BCD frequency offset 
               
               
                   
                   
                   
                 estimation algorithm 
               
               
                 Freq Proc 
                 3.5 
                   
                 Time for BCD to process 
               
               
                   
                   
                   
                 frequency estimation 
               
               
                 HW Update 
                 10.5 
                   
                 Time to prepare DSPs and other 
               
               
                   
                   
                   
                 components for data 
               
               
                 Unique Word 
                 24 
                 375 
                 Unique word needed for burst 
               
               
                   
                   
                   
                 acquisition 
               
               
                 Dead Time 
                 3 
                  47 
                 All 1&#39;s 
               
               
                 PAYLOAD 
                 56*N 
                 875*N 
                 Each slot is 7 bytes of user traffic 
               
               
                 Postamble 
                 18 
                 281 
                 Parity bits at the end of the burst 
               
               
                 Radio Turnoff 
                 2 
                  31 
               
               
                 TOTAL Overhead 
                 104 
                   
                 2 slots - Aperture (8 bits) 
               
               
                   
               
            
           
         
       
     
     All the fields in the Inroute packets, and Inroute related packets, may be encoded using a Big Endian (Network Byte Order) format. To be more specific, the bits in any structure defined for these packets may start with bit  7  of byte  0 , and after reaching bit  0  in each byte, they may wrap into bit  7  of the next byte. When a field has bits crossing over the byte boundary, the lower numbered bytes may have the higher place value. For example if an 13 bit field started on bit  2  of byte  7 , then the 3 most significant bits ( 12 : 10 ) would come from byte  7  bits  2 : 0 , the 8 next most significant bits ( 9 : 2 ) would come from byte  8 , and the 2 least significant bits ( 1 : 0 ) would come from byte  9  bits  7 : 6 . 
     As shown in  FIG. 6   m , the inroute packet format includes of a variable size header and 0 or more bytes of encapsulated datagrams. The encapsulated datagrams are sent as a continuous byte stream of concatenated datagrams, with no relationship to inroute packetization. Proper interpretation may require a reliable, ordered processing of all data bytes exactly once. To resolve problems due to data loss on the inroute, a selective acknowledgement, sliding window protocol may be used. As is the case for such sliding window protocols, the sequence number space may be at least twice the window size, and data outside the window may be dropped by the receiver. 
     Since the burst allocations may be of different sizes, and can vary over time, the windowing may be of a byte level granularity. For the same reasons, retransmissions may be less efficient, as the retransmission burst may not match the original transmission burst size. 
     For allocated streams, Inroute burst data may be retransmitted if not acknowledged in the Inroute Acknowledgement Packet for that Frame Number, or if that Acknowledgement is lost. After, for example, 3 retries, the adapter should classify the ITU as non functional and wait for user intervention. 
     If synchronization problems are discovered, the NCC  411   a  can force the adapter inactive by removing its bandwidth allocation. This may cause the adapter to reset its sequence number and datagram counter to 0, and start at the beginning of a new datagram. This may also cause the flushing of all Backlogged datagrams in the IRU. Since the sequence number is reset every time the adapter goes active, any data sent in Aloha or Nonallocated Ranging bursts may be duplicated due to retransmissions, if the acknowledgement is lost. 
     One of the “features” of the BCDs  411   b  is that multiple packets can be concatenated in a Burst, but if Bits  7 : 3  of Byte  0  are all 0&#39;s, and Bits  7 : 0  of Byte  1  are all 0&#39;s, then the BCD  411   b  may ignore the rest of the burst. To take advantage of this, when back to back bursts are allocated to the same adapter, it may not turn off the Radio, and may use the saved Burst Overhead for extra Payload. This may keep the required 1 to 1 mapping of allocated bursts to packets. Also, if the requirement of avoid 0&#39;s at the beginning of the packet is not met, the Backlog Indicator can be. 
     Active adapters that have no data ready to send may send Inroute packets of the full allocated burst size without any encapsulated datagrams to maintain channel utilization, and allow measurement of inroute PER from the NCC  411 A. This may be replaced to include periodic Network Management packets containing system profiling information. 
     A burst data frame (i.e., inroute packet) for Aloha (and ranging) bursts has the structure shown in  FIG. 6   m . The NCC  411 A can detect the type of burst from the frame numbering information in the packet header. The structure for the inroute packet include the following fields: a Serial Number Low field  625   a , a Backlog Indicator field  625   b , Padding Indicator field  625   c , Frame Number field  625   d , Burst Number field  625   e , a Length FEC field  625   f , a Length field  625   g , a Serial Number High field  625   h , a Destination ID field  625   i , a Backlog field  625   j , a Padding field  625   k , an Encapsulated Datagrams field  625   l , and a CRC field  625   m  The Serial Number Low field  625   a  stores the 8 least significant bits of the serial number. The serial number is split because of the BCD requirements with respect to the location of the Length field  625   g  and because of the need to have the first 13 bits non-zero. The 1-bit Backlog Indicator field  625   b  indicates the presence of the Backlog field. This should always be present for Aloha and Nonallocated Ranging bursts. The 1-bit Padding Indicator field  625   c  indicates the absence of the Padding field. This field should be encoded as a 0 to indicate padding is present. The reason that this is encoded this way, is so that the BCD requirement for having 1 of 13 specific bits set can be met. If they are not set, then the packet is already padded, and one byte of padding can be traded for enabling the Backlog. 
     The Frame Number field  625   d  stores the 2 least significant bits of the frame number, and may help the NCC  411 A to determine which burst was received. The 4-bit Burst Number field  625   e  indicates the burst slot that the Frame was transmitted in, assisting with identifying that burst as an Aloha type burst. The 8-bit Length FEC field  625   f  is the FEC value for the length, produced via table lookup in software. The 8-bit Length field  625   g  is the length of the burst and includes all the bytes starting with the Backlog Indicator field  625   b  through the CRC field  625   m . The 8-bit Serial Number High field  625   h  stores the 8 most significant bits of the of the Source adapter&#39;s serial number. The Destination ID field  625 I specifies the destination hybrid gateway. The Backlog field  625   j  indicate the number of bytes of Backlog that are present. It&#39;s encoded as a floating point number with a 2 bit exponent field and a 6 bit mantissa, and may be rounded up by the IRU. The end of the Backlog is indicated by 8 Backlog[7:6] ×Backlog[ 5 : 0 ]×2+SeqNr+size of the Encapsulated Datagram field. As such, it may include out of order, acknowledged data. It is only included to indicate increases in the size of the backlog, as measured from the IRU. The size of this field is sufficient for just under 2 seconds at 256 Kbps. The Padding field  625   k , if present, has its first byte indicating the total number of Padding bytes (N); all the other bytes are “Don&#39;t Care”. This field  625   k  is used to allow for stuffing packets to maintain link utilization when no data needs to be transferred, and to allow the padding of packets to the minimum burst size for Turbo codes. The N×8-bit Encapsulated Datagrams field  625   l  contains 0 or more bytes of encapsulated datagrams. There is no relationship between IP Datagram boundaries and the contents of this field; i.e., this field  625   l  can contain a section of an IP Datagrams, or multiple IP Datagrams. The value of N can be derived by subtracting the size of the other fields in the packet from the Length The CRC field  625   m  stores a 16-bit CRC; a burst with an invalid CRC is dropped and statistics retained. 
     As shown in  FIG. 6   n , the structure of another inroute packet include the following fields: a Sequence Number Low field  627   a , a Backlog Indicator field  627   b , Padding Indicator field  627   c , Frame Number field  627   d , Burst Number field  627   e , a Length FEC field  627   f , a Length field  627   g , a Sequence Number High field  627   h , a Backlog field  627   i , a Padding field  627   j , an Encapsulated Datagrams field  627   k , and a CRC field  627   l . The Sequence Number Low field  627   a  stores the 8 least significant bits of the Sequence, and thus, is 8 bits in length. The sequence number is split off because of a BCD requirement for the placement of the Length fields  627   f  and  627   g  as well as the need to avoid all 0&#39;s in certain bit positions. The 1-bit Backlog Indicator field  627   b  indicates the presence of the Backlog field. This should always be present for Aloha and Nonallocated Ranging bursts. The 1-bit Padding Indicator field  627   c  indicates the absence of the Padding field  627   j . This field  627   j  should be encoded as a 0 to indicate padding is present. The reason that this is encoded this way, is so that the BCD requirement for having 1 of 13 specific bits set can be met. If they are not set, then the packet is already padded, and one byte of padding can be traded for enabling the Backlog. 
     The Frame Number field  627   d  stores the 2 least significant bits of the frame number, and may help the NCC  411 A to determine which burst was received. The 4-bit Burst Number field  627   e  indicates the burst slot that the Frame was transmitted in. With the addition of the Inroute and Frame number it was received on, the NCC  411 A may be able to uniquely identify the source (SerNr) and destination (DestId). The 8-bit Length FEC field  627   f  is the FEC value for the length, produced via table lookup in software. The 8-bit Length field  627   g  is the length of the burst and includes all the bytes starting with the Backlog Indicator field  627   b  through the CRC field  627   i  m The 8-bit Sequence Number High field  627   h  stores the 8 most significant bits of the sequence number field that is used for the retransmission protocol. This is the Selective Acknowledgement, sliding window, byte address of the first byte of the Encapsulated Datagrams field. With a 32 Kbyte window size, this is large enough for 1 second at 256 Kbps. The Backlog field  627   j , Padding field  627   j , Encapsulated Datagrams field  627   k , and CRC field  627   m  are similar to the fields  625   j ,  625   k ,  625   l , and  625   m  of packet  625 . 
     Some of the packets sent to the NCC  411   a  do not require an IP header. Therefore, bandwidth savings are made by sending much smaller datagram headers, as shown in  FIG. 6O . The packet  629  includes a 4-bit Reserved field  629   a , which should have a value of 0 during transmission and may be used to specify Encryption, Compression, or Priority values. A Datagram Counter/CRC field  629   b  (12-bits) stores a 12 bit Datagram Counter value, from which a 12 bit CRC can be calculated by software on this Encapsulated Datagram appended with the SerNr and DestId; and the result is stored in this field  629   b  over the Datagram Counter value. The purpose of this field  629   b  is to detect loss of Synchronization between the IRU  109   a  and the NCC  411   a , thereby ensuring uncorrupted reassembly, correct source and destination addresses, and no loss of datagrams. Failures on this CRC should be considered as a synchronization failure, and the IRU  109   a  should be forced to the inactive state by the NCC  411   a , so as to initiate resynchronization. The polynomial to use in calculating this CRC is X 12 +X 11 +X 3 +X 2 +X+1 ( 0 ×F 01 ), and the preset (initial) value is  0 xFFF. The packet  629  also includes a 4-bit Protocol Version field  629   c ; this field  629   c  may/be encoded as 0 to indicate Network Management datagrams. Further, this value may be explicitly prohibited from being sent from the Host driver, for Network Security reasons. Further the packet  629  contains an 8-bit Message Type field  629   e  for specifying the message type, a 16-bit Length field  629   f  for indicating the length of the datagram (including the header), and a Payload field  629   g , which is a variable length field (N×8 bits). The value of N is the Length field that is present for all Payload formats. 
       FIG. 6   p  shows the inroute payload format for IP datagrams. The datagram  631  includes a Reserved field  631   a , a Datagram Counter/CRC field  631   b , and a Protocol Version field  631   c , which are similar to that of the datagram of  FIG. 6O . In addition, the datagram  631  contains a Header Length field  631   d  (4 bits) for storing the IP header length, a Type of Service field  631   e  (8 bits) for specifying the type of service, a Length field  631   f  (16 bits) for storing the length of the entire datagram including the header, and a Rest of Datagram field  63 l g  (N×8 bits). Details of the rest of the IP frame are described in IETF (Internet Engineering Task Force) RFC  791 , which is incorporated herein by reference. The value of N is derived from the Length field. It should be noted that the prior header includes the first four bytes of the IP header. 
     A number of scenarios exist in which the NCC  411   a  may force an adapter to the inactive state. For example, if the NCC  411   a  detects a synchronization error with the adapter, arising from errors in the encapsulation layer of the protocol, or by the Protocol Version field  629   c  and Length field  629   f  of the payload  629   g . In addition, if the NCC  411   a  receives no inroute packets with good CRC from the adapter for 24 frame times, then the adapter becomes inactive. Also, if the NCC  411   a  receives no Inroute packets with good CRC containing encapsulated datagrams for a number of frame times configured at the NCC  411   a . Prior to that, the adapter may have its bandwidth allocation reduced due to inactivity. Inactivity may forced upon the adapter if the NCC  411   a  receives Inroute packets with good CRC containing encapsulated datagrams that have already been acknowledged (out of window or completely overlapping prior data) after a configured number of frame times from when it last advancing the SeqNr. This can be due to excessive retransmissions, or synchronization errors. Lastly, the adapter can be made inactive through an operator command. 
     An IRU  109   a  may become inactive if the IRU  109   a  does not receive any Bandwidth Allocation packets from its current Inroute Group, which has assigned the IRU  109   a  bandwidth for 24 frame times. If the Bandwidth allocation packet is not received, the IRU  109   a  may not transmit during that Frame, but may consider itself as remaining active. Reception of explicit commands from the NOC  113  may also change the state of the IRU  109   a  from active to inactive. Further, a USB Reset or a USB Suspend may cause the adapter to go inactive, and flush the adapter&#39;s Backlog. The adapter may go active again, based on received messages from the NOC  113 . Further, the IRU  109   a  may become inactive if a the adapter&#39;s transmit path is disabled because of various conditions, for example, loss of FLL lock, loss of Super Frame synchronization, and etc. 
     Each of the gateways to be supported by the NCC  411   a  is configured into the NCC  411   a . For each gateway ID, the NCC  411   a  has the gateway address to gateway IP address mapping. This mapping may be periodically sent to all of the receivers. The receiver uses the mapping transmission to determine which gateway id is associated with its gateway IP address and informs the IRU  109   a  which gateway ID to use for inbound messages when it first becomes active using an ALOHA burst. This may support modes where the gateway IP address is dynamically set at connection setup time. 
     The source address may be the lower 28 bits of the 32 bit transceiver serial number. This is used for packet rebuilding. Messages may be sent by serial number to a receiver for polling, bandwidth allocation, and retransmission support. 
     The network timing is designed to control the burst timing of a group of return channels, which share the same frame timing. The frame timing is derived from a pulse from the NCC  411 A. The NCC  411 A allocates bandwidth, coordinates the aperture configuration, and sends framing pulses to both the BCDs that receive the traffic and to timing units which measure packet delay. 
     The NOC  113  may provide return channel frame format information once every 8 TDMA frames. The TDMA frame time is 45 milliseconds. Therefore, the return channel “super frame” may be defined as 360 milliseconds. To properly coordinate the return channel frame timing, additional information is provided to the receiver so that the receiver may precisely time its burst transmission time as an offset of the received “super frame”. 
     Accordingly, the NCC  411   a  sends a super frame marker pulse once every 360 ms to the timing units  409 , and concurrently transmits a super frame IP frame (super frame header) to all IRUs  109   a . A frame pulse is sent to the BCDs  411   b  every 45 milliseconds. The delay between the super frame marker pulse and the associated frame pulse is a fixed time, which is denoted as the “space timing offset”. The space timing offset is calculated as the maximum round-trip time from the farthest receiver plus two frame times. The two frame times are provided as a buffer to ensure that the transceiver has sufficient time to process return channel frame format data and to forward the return channel data to the transmit indoor unit one-half frame time ahead of the frame transmit time. The super frame header is used by every transceiver  109  to synchronize the start of frame marker to the NCC  411   a  super frame marker. However, this information is not sufficient because there is a delay from the time that the NCC  411   a  generates the super frame header until the header is received by the receiver. 
     The super frame header delay encompasses the NOC delay, the transmission time to the satellite (from the NOC  113 ), and the transmission time from the satellite to the specific receiver. The transmission time from the satellite to the specific receiver is a known parameter that is determined during ranging. This value can vary slightly due to satellite drift along the vertical axis. To adjust for this variation, Echo Timing is implemented at the NOC to measure changes in the satellite position. Echo Timing measures both the transmission time from the NOC  113  to the satellite  107  and the satellite drift from the NOC&#39;s position (which approximates the drift from the receiver&#39;s position). The transceiver  109  is unaware of the delay in the NOC  113 , which can vary in real-time. Thus, a second IRU  409   d  is implemented in the NOC  113  to measure the NOC delay. A pulse is sent to this IRU  409   d  when the frame is supposed to be sent, and the IRU  409   d  detects when the frame was actually sent. This delay is broadcast in the Frame Time message to all return channels to adjust for the NOC delay when calculating the actual time of the start of the super frame. 
     When the transceiver  109  receives a super frame packet, the transceiver  109  time-stamps the packet. This time-stamp is created, for example, using an internal  32 -bit counter free-running at 32.768/4 MHz. For the transceivers  109  to determine exactly when the super frame marker occurred at the outroute hub, software of the user terminal  101  subtracts the site&#39;s satellite delay and the NOC delay. The NOC delay is broadcast in the Frame Numbering Packet. This delay is calculated at the HUB by the Local Timing IRU. The NOC  113  also provides the NOC  113  to satellite portion of the satellite delay in this message as the difference between the local timing and echo timing IRUs  409 . The Receiver has a configured value for the satellite to receiver satellite delay; other than ranging, this is a fixed value. In this situation, the NOC delay at ranging is stored and the change in the NOC delay is also applied to the receiver satellite delay to approximate satellite drift. When ranging, the PC approximates this value from the location of the satellite, location of the receiver, NOC timing, and the space timing offset configured in the NOC. The ranging process adjusts this value, and the site stores the final value. 
     Once the super frame timing has been generated, the site may determine its transmission time such that the frame is received at the proper time at the NOC  113 . The time at which the site may transmit is a satellite hop prior to the time that the NOC  113  expects the data to be received. The transmission time is measured by starting with the fixed space timing offset later than the regenerated super frame time. The NOC delay and the receiver satellite delay may be subtracted from this timebase. The final adjustment, for satellite drift, is made by determining the NOC delay difference between current and ranging and applying it. 
     The “ranging” process, whereby a site on a NCC  411   a  is configured is described as follows. When the IRU  109   a  is configured, the host PC  101  provides parameters including a “range timing offset” for the receiver. At this point in time, the IRU  109   a  may not enable transmission if the ranging timing is zero. The IRU  109   a , however, may enable the MAC for the NCC  411   a  master list and receive this message locally. Thereafter, when IRU  109   a  acquires transmit timing and is requested by the PC host  101  to range, IRU  109   a  may select a NCC  411   a  based on having an available ranging burst. IRU  109   a  requests a ranging transmission by sending a message over the ranging burst using some default amount of power after some random number of frame backoffs. If no response is received and the burst is still available, IRU  109   a  may increase power and try again. If the burst is now allocated to a different user, IRU  109   a  may revert to selecting a NCC  411   a  based on available ranging bursts. Once ranging response is received, IRU  109   a  may start sending ranging data every frame; this data may include the frame number. Next, IRU  109   a  adjusts ranging time and power based on NOC response and continues to adjust until IRU  109   a  is within a close tolerance. IRU  109   a  then stores the values when ranging is successful. IRU  109   a  then enables normal transmission mode 
     The NCC  411   a  may be capable of requesting a site to enter ranging mode. When the site does enter this mode, the site may use the ranging burst it has been assigned. It may transmit normal traffic (or a small fill-type packet) to the NCC  411   a . The NCC  411   a  may adjust the timing and power for the site. These adjustments may be stored if the NCC  411   a  indicates a successful re-range of the site. 
     According the one embodiment of the present invention, the Return Channel requirements are largely based on a traffic model, which defines the traffic pattern for a typical user. The capacity requirements, for example, may be as follows. It is assumed that the system  100  is based on a 2-to-1 ratio of outroute transponders to return channel transponders. An exemplary requirement is approximately 22,000 users per transponder, so 45,000 users (4500 active) per transponder are required for the return channel. Given a 2-to-1 ratio, 300 64 kbps return channels per transponder are supported by system  100 , with 15 active users per return channel. Each NCC  411   a  supports up to 30 return channels (32 BCDs, in which 2 are backups). Since each return channel supports 15 active users, the bandwidth sizing may assume 450 active users for a NCC  411   a . The return channels may be scaled in sets of 30 return channels. 
     In the alternative, the system  100  may support a 5-to-1 ratio of outroute transponders to return channel transponders. In this case, the system  100  provides up to 600 64 kbps return channels per transponder, with 25 active users per return channel 
     The return channels on an NCC  411   a , according to an embodiment of the present invention, may support frequency hopping to provide increased efficiency of system  100 . A subset of return channels may be configured to support a contention protocol, such as Aloha. It should be noted that any equivalent contention protocol may be utilized in system  100 . A receiver may randomly select a return channel with Aloha slots. In turn, the NOC  113  may assign the receiver a stream on the same or a different return channel. The NOC  113  may change the frequency for the assigned stream when the site requires additional bandwidth, when another site requires additional bandwidth on the same return channel, or when the site may be used for a poll response on another return channel to keep the BCD  411   b  locked for the return channel. NCC polling is used to keep BCDs  411   b  locked. The NCC polling algorithm also ensures that bandwidth is not wasted polling sites that are known to be either good or bad. The NCC polling algorithm may poll sites based on a LRU used list. Both the least recently used and “known bad” list may be rolled through to periodically verify site health of all sites. When the NCC  411   a  changes the frequency for a site, the NCC  411   a  may, at a minimum, provide a single frame for the site to retune to the new frequency. 
     A user on the system may have bandwidth allocated in one of the following three states. In the first state, if the user has not transmitted traffic for a period of time, then the user may be inactive. When inactive, the user may use Aloha to send initial traffic to the NOC  113 . The second state is when the user is active. In this state, a periodic stream is setup for the user. The periodic stream, at 1 kbps, is sufficient to handle TCP acknowledgements assuming ack reduction timer of 400 milliseconds. In the third state, the user&#39;s transmit backlog exceeds a predetermined value, in which additional bandwidth is provided. Additional bandwidth allocations are supplied until the maximum is attained or the backlog begins to decrease. 
     A pure-Aloha system assumes that a packet is randomly transmitted in a slot when data transmission is requested. The standard efficiency of a pure-Aloha system is 7%; this means that, when over 7% of the system is loaded, there may be a high number of retransmits necessary, making the response time delays too long. With a 7% efficiency rate, each active user would get (64 kbps/return channel)*(1 return channel/15 users)*(0.07)=300 bits/sec. This is obviously not enough bandwidth. In addition, aloha return channels may have more difficulty applying future efficiency techniques because of the collision nature of the channel. 
     A diversity aloha system is an adjustment to the pure-aloha system in that every packet to be sent is actually sent 3 times. This channel becomes 14% efficient. This doubles the throughput to 601 bits/sec. 
     An Aloha/Periodic stream technique is based upon the idea of being able to forecast the type of traffic an active user may be transmitting over the return channel. For the forecasted traffic (which occurs a majority of the time), the user may have non-collision bandwidth available. When the traffic requirements exceed the forecasted level, the user may be provided with additional allocated bandwidth 
     An Aloha/Periodic Stream-PLUS technique builds upon the above Aloha-based concepts. Some of the capabilities that are provided in addition to the periodic stream are as follows: load balancing and minimal delay. The traffic is balanced to ensure that non-busy user (those not requiring additional bandwidth) are equally loaded on all return channels that support the streams. Also, a minimal delay algorithm, which is more fully described below, is employed to ensure that user traffic can be transmitted to the NOC  113  expediently. 
     The minimal delay approach relies on equally dividing all bandwidth, other than that used for users requiring additional bandwidth, among all other active users. A minimum (4 kbps or so) may be ensured for each user so other users may be unable to request additional bandwidth if every site does not have the minimum amount of bandwidth. This approach provides optimal results when the return channels are lightly loaded. As users become active, they are assigned to the return channels with the fewest number of users which leads to automatic load balancing. 
     In addition, some minimal burst size is defined for the per-user burst. This size results in a maximum number (denoted as M) of bursts per frame (which may be 3 (120 byte)−5 (71 bytes)) depending of frame analysis. On a given return channel, it is assumed that there are 357 burst bytes per frame time, which may be at least two bursts of traffic. As users are assigned to the return channel, they are provided bandwidth according to Table 4, below. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 No. of Users 
                 Burst/frame 
                 Approximate Rate 
               
               
                   
               
             
            
               
                 1 user 
                 Two bursts/frame 
                 90% 
               
               
                 2 users 
                 One burst/frame 
                 45% 
               
               
                 3 − M users 
                 One burst/frame 
                 [0.90/(3 − M)] × 100% 
               
               
                 M + 1 − 2M users 
                 One burst/frame 
                 [0.90/(M + 1 − 2M)] × 100% 
               
               
                 2M + 1 − 4M users 
                 One burst/frame 
                 [0.90/(2M + 1 − 4M)] × 100% 
               
               
                   
               
            
           
         
       
     
     If M is defined as 5, then up to 20 users may be supported with each user getting 2.5 kbps. If M is defined as 4, then the number of users supported per return channel is 16 which is above the required value. 
     The bandwidth allocation is based on pre-defining the size of the “periodic” burst. According to one embodiment of the present invention, it is assumed that three equally-sized bursts may be used. Since the 64 kbps frame has 57 7-byte slots, each burst may have a size of 19×7=133 bytes. 
     The algorithm also assumes a small number of return channels which are full of slotted Aloha slots. These slots may be sized to handle the normal first transmission from a user (which is either a DNS lookup or an actual request). The Aloha burst sizes may be also 98 bytes (14 slots) to support 4/frame. Fine tuning may be required using an ERLANG analysis on the arrival rate of packets from receivers in an inactive state. 
     When an Aloha burst is received, the user is assigned periodic bandwidth The bandwidth is given an inactivity timeout value in seconds. In particular, if no data are yet received for the user, the algorithm uses the configured long timeout. If past data indicates periodic individual packets, the configured short timeout is used; otherwise, the long timeout is employed. 
     When a receive packet indicates that the backlog is greater than a configured amount, additional bandwidth may be provided to ensure that the data can be transmitted within a configured amount of time, if sufficient bandwidth exists. This may require switching the user to another return channel. 
     The bandwidth allocation algorithm ensures, when possible, that only the periodic bandwidth users are moved to another frequency. This allows the high-throughput users to transmit with no single frames of downtime (which are required if the site must switch frequencies). When possible, the bandwidth is allocated to ensure that user traffic backlog is reduced within a certain number of frames. The total backlog above the amount needed for additional bandwidth is determined. The algorithm determines if the requested bandwidth can be met within the number of frames. If so, the bandwidth is allocated as needed; if not, then the algorithm starts by limiting the bandwidth for those users with the largest backlog, as more fully described below. 
       FIG. 7  shows a flow chart of the return channel bandwidth limiting process utilized in the system of  FIG. 1 . Bandwidth limiters are used in system  100  to ensure that a user does not monopolize bandwidth, thereby maintaining fairness in manner bandwidth is allocated. The total bandwidth allocated to a specific user may be limited by a fixed amount of bandwidth every frame. In step  701 , the transceivers  109  provide the NOC  113  with information on the amount of backlog that the transceivers  109  possess. The NOC  113 , as in step  703 , assigns a predetermined minimum amount of bandwidth to each of the active users. This minimum value is configurable depending on the capacity of the system  100  and the number of user terminals  101 . Next, the NOC  113  determines whether additional return channel bandwidth is available, per step  705 . In the event that this minimal amount of bandwidth does not completely satisfy the requirements of one or more of the transceivers  109 , the backlog information of all the transceivers are examined to determine which transceivers  109  are in need of additional return channel bandwidth, per step  707 . If bandwidth is available, the NOC  113  checks whether the system can honor all of the bandwidth requirements (as indicated by the backlog information) using a default bandwidth level (step  709 ). The default bandwidth level is drawn from a set of predetermined bandwidth levels. In an exemplary embodiment, the set of bandwidth levels may be derived from traffic statistics that are gathered from the transceivers  109 . If the default level cannot meet the additional bandwidth requirements of the particular transceivers  109 , then a bandwidth level is selected from a set of predetermined bandwidth levels according to a predetermined criteria, as in step  711 . The predetermined criteria can be specified dynamically by the NOC  113 . For example, the NOC  113  may specify the criteria such that the bandwidth level that is nearest the next to the highest backlog is selected. In this manner, any attempts by a single transceiver  109  to excessively consume the available bandwidth are curbed. Steps  709  and  711  may be repeated until a bandwidth level is reached in which some (or all) of the users&#39; backlogs are satisfied across the entire span of users. At which time, the NOC  113  allocates bandwidth to the transceivers  109 , as in step  713 , based upon the bandwidth level that was set. This approach advantageously ensures that all users receive a minimum amount of bandwidth before high bandwidth users are further bandwidth allocations. 
     Alternatively, another approach to limit bandwidth is to limit protocols such as ICMP so a user cannot monopolize a channel with PINGs. 
       FIG. 8  is a flow chart of the auto-commissioning process utilized in the system of  FIG. 1 . The auto-commissioning process enables the user to be on-line with the system  100  through an automated process that obtain the necessary configuration parameters for the transceiver  109  and ODU  307 . The transmit path may be configured through a utility which saves transmission parameters to the PC  101 , allows frame timing fine-tuning (referred to as “ranging”), and provides troubleshooting tools for the transmission portion (i.e., ITU  109   b ) of the transceiver  109 . The system  100  provides auto-commissioning without requiring a phone line. The purpose of auto-commissioning is to prepare the system to be operational. 
     A user may commission the two-way site with no access to a phone line or to the Internet  105 . In step  801 , the user installs software in the PC  101 . The PC  101  executes the auto setup program, as in step  803 . For example, when the user starts the setup program from a CD (compact disc), the user may enter location information. To be as user-friendly as possible, the information may be in terms of country, state/province (optional), and city. From this information, the PC  101  may estimate the latitude and longitude of the site and select a two-way “beacon” for the site based upon the information on the CD. The program instructs, as in step  805 , the user to point the antenna to the beacon satellite using predefined pointing values. The system  100  provides a default satellite  107  and associated default transponder, whereby a user terminal  101  undergoing the commissioning process may establish communication with the NOC  113 . 
     Upon a successful antenna pointing (and ranging), a temporary channel is established, as in step  807 , from the transceiver  109  to the NOC  113  via satellite  107 . This temporary channel may support either a connection-oriented or connectionless (e.g., datagram) connection. According to one embodiment of the present invention, the temporary channel carries TCP/IP traffic, thereby permitting the use of a user-friendly web access and file transfer capabilities. The software may be capable of communicating over the system  100  to an “auto-commissioning server” in the NOC  113  to perform the two-way interaction required to sign the user up for two-way access. 
     In step  809 , the NOC  113  collects user information, such as billing and accounting information, user antenna location, and service plan selection. Next, the NOC  113  downloads the network configuration parameters, antenna pointing parameters, and transceiver tuning parameters to the PC  101 , per step  811 . According to one embodiment of the present invention, the antenna pointing parameters include the following: satellite longitude (East or West), satellite longitude, satellite polarization, satellite polarization offset, and satellite frequency. The transceiver parameters may include a symbol rate, modulation type, framing mode, Viterbi mode, and scramble mode. Next, the PC  101  is configured based upon the received network configuration parameters (step  813 ). In step  815 , the user performs the antenna pointing process, as instructed by the program; this process is more fully described below with respect to  FIG. 9 . Thereafter, the PC  101  sets various other parameters relating to PC system settings, per step  817  (e.g., default directories for loading packages) and desired applications (e.g., webcast, newscast, etc.). 
       FIG. 9  is a flow chart of the antenna pointing operation associated with the auto-commission process of  FIG. 8 . In step  901 , the user enters the location of the antenna by specifying the ZIP code, for example. Based upon the ZIP code, the setup program displays the antenna pointing details, per step  903 . The user then points the antenna, as in step  905 , according to the antenna pointing details. Pointing involves physically directing the antenna assembly according to the parameters that are supplied by the setup program For example, the bolts in the antenna assembly may be tightened enough so that the antenna does not move, except the azimuth (horizontally around the pole). The user may adjust the elevation by 0.5 degrees every 2 seconds until the elevation is maximized. Next, the azimuth is gradually moved (1 degree per second) until it is maximized. 
     The program indicates whether the antenna is pointed to the correct satellite (step  907 ). If the antenna is not pointed to the correct satellite  107 , then the user adjusts the antenna position, per step  909 . The user checks whether the antenna is properly position to exhibit an acceptable signal strength, as indicated by the setup program (step  911 ). This measurement provides digital signal strength for a demodulated carrier. If the signal strength is below an acceptable level, then the user must re-adjust the antenna (step  909 ). This approach requires another person to read the PC Antenna pointing screen while the antenna is adjusted; alternatively, the user may listen to an audible tone. Upon obtaining an acceptable signal strength, the antenna process ends. 
     As part of this process, the user may be assigned to a service that may be supported on a different satellite or the same satellite. If the service is on a different satellite, the user may re-point to another satellite and then should automatically be ranged and obtain service. 
     The IRU  109   a  supports an AGC (automatic gain control) circuitry in addition to the signal quality factor measurement. The AGC circuitry provides a raw signal strength measurement that indicates that the receiver is receiving energy from a satellite  107 . This provides the additional advantage that the signal can be measured prior to the demodulator being locked. However, the circuitry may lead to pointing to the wrong satellite if a nearby satellite has a carrier at the same frequency to which the receiver is tuning to lock to a carrier. 
     The antenna pointing for the IRU  109   a  is supported in two different modes. Using voltage emitted from the ODU  307 . It requires installation of the transmission equipment, and requires that the user have a voltmeter that can be attached to the ODU  307 . The second mode is to use a PC Antenna pointing program, which may be separate from the auto-commissioning setup program This is the approach used when the user either does not have transmission equipment or does not have a voltmeter to attach to the transmit ODU. 
     The first approach allows a user to be physically present at the antenna, without interaction with the PC while pointing the antenna. This approach assumes that IRU  109   a , ITU  109   b , power supply  109   c , dual IFL  303 , and ODU  307  have been properly installed. A voltmeter that measures, for example, 0–10 volts may be used. 
     The user performing the antenna pointing process may start the pointing program from the host PC  101 . This software places the equipment in a mode where, instead of transmitting any user traffic, it places the transmission equipment in a mode where voltage is supplied to the ODU  307  to emit on an F-connector on the back of the ODU  307 . This program also supplies an approximation of the pointing parameters for the antenna. These values should be written and used to point the ODU. The voltage on the F-connector can interpreted as follows. The voltage range of 0–4V indicates an AGC level. The higher the voltage, the stronger the signal. When the voltage is in this range, the modulator is not locked. If the signal remains over 3V for over 10 seconds, then it is likely that the antenna is pointed to the wrong satellite. A voltage of 5V indicates a lock to an outroute that does not match the commissioned characteristics. The most probable cause is pointing to an incorrect, adjacent satellite, which can be corrected by minor azimuth changes. The voltage range of 6–10V specifies an SQF Value, in which the higher the voltage, the stronger the signal. A value of 8.0 may equate to an SQF of 100, which is the minimal acceptable level for an installation. 
       FIG. 10  is a diagram showing the scalability of the system of  FIG. 1 . The system  100 , according to an embodiment of the present invention, can scale to accommodate millions of customers. Conceptually, the resources of the system  100  is subdivided numerous times until a small number of users are sharing a small number of resources. The layers to scaling are as follow: (1) system, (2) transponder-sets, (3) Return Channel Equipment, and (4) Return Channel. At the system layer, an extremely large number of users can be supported. For the transponder-sets, two or more outroutes may be supported; therefore, more than two sets of Return Channel Equipment  411   a  re used at this layer. The transponder set also includes the necessary equipment to support a transponder&#39;s worth of return channels, supporting up to 100,000 users. At the Return Channel Equipment layer, which includes up to 31 return channels, a set of users are configured to each set of RCE  411  during ranging time. This configuration may also be dynamically switched. At the Return Channel layer, when a user becomes active, the user may be assigned bandwidth on a specific return channel. Up to 16 active users may be supported per 64 kbps return channel. 
     The above scalable configuration is described from a “bottom up” point of view, starting with the return channel to the system level. The Return Channel uplink is a standard NOC  113  with the additional timing unit equipment required to perform timing on each transponder. This may require the standard NOC infrastructure, including hybrid gateways, satellite gateways, and uplink redundancy. In addition, a Portion of one rack for additional equipment is required. Two Timing Units are used per uplink transponder (each with 2 IRUs). A System IF Distribution module  403  to distribute return channel signal to the RCE sets. A portmaster may also be needed to support the serial connections to do monitor and control of the 10 sets of BCDs. It should be noted that RS232 limitations may require the portmaster to be within 60 feet of all RCE equipment sets. 
     The return channel equipment  411  receives the data from the return channels and prepares the packets to be sent to the appropriate hybrid gateways  419 . The Return Channel Equipment  411  includes the following for 30 return channels: 3 BCD Racks; 8 BCD Chassis, each with 4 power supplies; cards required to properly connect 8 BCD chassis to the NC-Bus, Redundancy Bus, and M&amp;C Bus; Network IF Distribution; 32 sets of BCD equipment; and two NCCs  411   a  (e.g., PCs with TxRx). 
       FIG. 11  is a diagram of a computer system that can execute and support the system interfaces and protocols of system  100 , in accordance with an embodiment of the present invention. Computer system  1101  includes a bus  1103  or other communication mechanism for communicating information, and a processor  1105  coupled with bus  1103  for processing the information. Computer system  1101  also includes a main memory  1107 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  1103  for storing information and instructions to be executed by processor  1105 . In addition, main memory  1107  may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  1105 . Computer system  1101  further includes a read only memory (ROM)  1109  or other static storage device coupled to bus  1103  for storing static information and instructions for processor  1105 . A storage device  1111 , such as a magnetic disk or optical disk, is provided and coupled to bus  1103  for storing information and instructions. 
     Computer system  1101  may be coupled via bus  1103  to a display  1113 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  1115 , including alphanumeric and other keys, is coupled to bus  1103  for communicating information and command selections to processor  1105 . Another type of user input device is cursor control  1117 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  1105  and for controlling cursor movement on display  1113 . 
     According to one embodiment, interaction within system  100  is provided by computer system  1101  in response to processor  1105  executing one or more sequences of one or more instructions contained in main memory  1107 . Such instructions may be read into main memory  1107  from another computer-readable medium such as storage device  1111 . Execution of the sequences of instructions contained in main memory  1107  causes processor  1105  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  1107 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and software. 
     Further, the instructions to support the system interfaces and protocols of system  100  may reside on a computer-readable medium The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  1105  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  1111 . Volatile media includes dynamic memory, such as main memory  1107 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  1103 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communication. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  1105  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions relating to the system interfaces and protocols of system  100  remotely into its dynamic memory and send the instructions over a telephone line using a modem A modem local to computer system  1101  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus  1103  can receive the data carried in the infrared signal and place the data on bus  1103 . Bus  1103  carries the data to main memory  1107 , from which processor  1105  retrieves and executes the instructions. The instructions received by main memory  1107  may optionally be stored on storage device  1111  either before or after execution by processor  1105 . 
     Computer system  1101  also includes a communication interface  1119  coupled to bus  1103 . Communication interface  1119  provides a two-way data communication coupling to a network link  1121  that is connected to a local network  1123 . For example, communication interface  1119  may be a network interface card to attach to any packet switched local area network (LAN). As another example, communication interface  1119  may be an asymmetrical digital subscriber line (ADSL) card, an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. Wireless links may also be implemented. In any such implementation, communication interface  1119  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  1121  typically provides data communication through one or more networks to other data devices. For example, network link  1121  may provide a connection through local network  1123  to a host computer  1125  or to data equipment operated by a service provider, which provides data communication services through a communication network  1127  (e.g., the Internet). LAN  1123  and network  1127  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  1121  and through communication interface  1119 , which carry the digital data to and from computer system  1101 , are exemplary forms of carrier waves transporting the information. Computer system  1101  can transmit notifications and receive data, including program code, through the network(s), network link  1121  and communication interface  1119 . 
     The techniques described herein provide several advantages over prior approaches to providing access to the Internet. Multiple transceivers are employed with the capability to transmit backlog information over a return channel via a satellite, wherein the backlog information specify an amount of queued traffic for the respective transceivers. A hub receives the backlog information and allocates a minimal amount of return channel bandwidth to each of the transceivers. The hub sets a bandwidth level associated with the return channel bandwidth that is available based upon a bandwidth level from a set of predetermined bandwidth levels, according to a predetermined criteria. The hub selectively allocates additional return channel bandwidth based upon the bandwidth level to the transceivers that require additional return channel bandwidth in excess of the allocated minimal amounts. Under this approach, return channel bandwidth can be managed efficiently. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.