Patent Publication Number: US-6993009-B2

Title: Method and apparatus for deriving uplink timing from asynchronous traffic across multiple transport streams

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application of Kelly et al. entitled “Precise TDMA Timing Based off of DVB Transport Stream Asynchronous Traffic, Possibly Shared Across Multiple Transport Streams”, Ser. No. 60/188,368, filed on Mar. 10, 2000, and of U.S. Provisional Application of Kelly et al. entitled “Two-way Communications System and Method”, Ser. No. 60/197,246, filed on Apr. 14, 2000, the entire contents of each being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to data timing sharing and recovery in a communication system, and even more particularly to derivation of precise TDMA uplink timing across multiple satellite asynchronous Digital Video Broadcast (DVB) transport streams. 
     2. Description of the Related Art 
     Using satellites for Internet and Intranet traffic, in particular multicasting of digital video through use of DVB and two-way broadband communication has recently received a great deal of attention. There are a number of applications using satellites in one or two-way data communications, and each presents unique timing and transmission problems which must be considered. Satellites can help relieve Internet congestion and bring the Internet and interactive applications to countries that do not have an existing network structure, as well as provide broadband interactive application support. 
     In a typical broadcast mode, geosynchronous satellites relay a signal from a single uplink station to a number of receivers within the “footprint” of the satellite. The satellite system covers a footprint, which could, for example, represent all or a portion of the continental U.S. When the signal carries packetized digital data, a geosynchronous satellite is an excellent mechanism for carrying multicast data, as a multicast packet need only be transmitted or “broadcast” once to be received by any number of remote receivers. Such a signal, by carrying both unicast and multicast packets, can support both normal point-to-point and multicast applications. 
     As one means of using satellite technology in this growing field, very small aperture terminals (VSATs) provide rapid and reliable satellite-based telecommunications between an essentially unlimited number of geographically dispersed sites. VSAT technology has established effective tools for LAN internetworking, multimedia image transfer, batch and interactive data transmission, interactive voice, broadcast data, multicast data, and video communications. The emergence of VSAT technology has provided a practical solution for broadband delivery. Using a system of deployed satellites in conjunction with the necessary ground-based infrastructure and VSAT terminals, users can potentially transmit and receive video, audio, multimedia, and other digital data hundreds of times faster than over conventional phone or terrestrial data lines. 
     The Internet Protocol (IP) is the most commonly used mechanism for carrying multicast data. Examples of satellite networks capable of carrying IP Multicast data include Hughes Network System&#39;s Personal Earth Station (PES) VSAT system and Hughes Network System&#39;s DirecPC® system. Combining VSAT delivery with standards-based IP multicast ensures users a less expensive and more flexible approach to achieving high-quality, real-time broadcasting. 
     As for digital TV transmission, MPEG-2 emerged as the digital entertainment TV compression standard (ISO 13818) for transmission media such as satellite, cassette tape, over-the-air, CATV, and new broadband multimedia data and interactive services wherein MPEG-2 packets are used as “data containers”. The MPEG-2 system standard simply defines a packet structure for multiplexing coded audio and video data into one stream and keeping it synchronized. Although the MPEG-2 standard does not prescribe which encoding methods to use, the encoding process, or encoder details, the standard does specify a format for representing data input to the decoder, and a set of rules for interpreting these data. Video can thus be encoded using inexpensive MPEG standards-based encoders that encapsulate the MPEG packets in IP multicast frames. 
     MPEG-2 defines two types of streams—the Program Stream which includes the packet structure above, and the Transport Stream, which offers robustness necessary for noisy channels, as well as the ability to include multiple, asynchronously multiplexed programs with independent time bases in a single stream. The Transport Stream is well-suited for delivering compressed video and audio over error-prone channels such as a satellite transponder. However, the MPEG-2 specification does not provide all the information necessary to ensure interoperability, data broadcasting, and delivery scheduling in a TV system. 
     In response to this need, DVB standards have been developed and published by the European Telecommunications Standards (ETSI), and have been globally adopted. DVB is fundamentally an MPEG-2 based system, which provides the basis of DVB video, audio, and transport across a variety of media such as satellite, cable TV, broadcast, etc. For this reason, DVB has defined a set of implementation guidelines for MPEG-2 in DVB which cover the minimum requirements for interoperability for baseline standard definition television (SDTV), high definition television (HDTV), and DVB Integrated Receiver Decoders (IRD). Data broadcasting is a key application of digital TV, and DVB has taken elements of MPEG-2 Digital Storage Media—Command and Control (DSM-CC) and produced specifications and guidelines which now provide the basis for most data broadcast applications around the world. 
     MPEG-2 was chosen as the basis for DVB source coding of audio and video, and for the creation of Program Elementary Streams and Transport Streams at the systems level. However, MPEG-2 standards are generic and are considered by the industry to be too wide in scope to be directly applied to DVB. Accordingly, industry guidelines have been established to restrict MPEG-2 syntax and parameter values, as well as suggesting preferred values for use in DVB applications to ensure interoperability across different media, a requirement which is frequently needed in the complex signal distribution environment. The core of DVB is its series of transmission specifications, including the DVB-S satellite transmission standard, based on QPSK or Offset QPSK (OQPSK), which is now the defacto world satellite transmission standard for digital TV applications. 
     Satellite DVB technology and the Internet Protocol (IP) have thus necessarily converged (“IP/DVB”) to allow users transparent access to a variety of broadband content, including live video, large software applications, and media-rich web sites. The borders between digital video broadcasting and computers have necessarily blurred —TV broadcasters transmit data, businesses broadcast multimedia applications, and even the most remote user can use interactive communications. From the outset of DVB, interactive applications have been perceived as being the cornerstones of the new generation of digital television. One of the strengths of DVB technology lies in the fact that it enables the point-to-multipoint transmission of very large amounts of data at high data rates while securely protecting against transmission errors. Such data may include audio and video but, in many applications, the data will be large files or other forms of generic information. 
     In support of these developments, VSAT systems, such as the Personal Earth Station mentioned above, allow commercial users to access one of a generally limited number of satellite return channels to support two-way communication. The choice of return or inbound channel is usually restricted to only a few of the possible channels preconfigured by a combination of hardware and/or software limitations. Other consumer-oriented hybrid systems, such as DirecPC® Turbo Internet, may use a dialup modem or other terrestrial link (as well as other non-satellite media) to send HTTP requests to the Internet, and may receive responses either via the outbound satellite channel, or a dialup modem connection. Some commercial systems may use a VSAT system terminal for Internet access to receive HTTP responses via the outbound satellite broadcast channel, and to send HTTP requests to the Internet through a VSAT inbound channel. Unfortunately, as these systems are mass-marketed to consumers and the number of users increases, the generally limited number of inbound channels can experience congestion and reduced user throughput as a result of an increasing number of users competing for a finite number of inbound satellite channels. The potential benefits that VSAT technology bring to consumers in the area of broadband delivery are necessarily diminished. 
       FIG. 1  partially depicts one-way satellite broadcast system  100  wherein One-Way NOC  110  transmits DVB transport stream  120  to through satellite  130  to multiple remote users  150  (1 to n). Each remote user  150  has a receiver (RCVR)  140  which receives and demodulates the data contained in DVB transport stream  120 . One-Way NOC  110  may also provide and receive information to/from the internet or an intranet through gateway  160 . The return link from remote users  150  to One-Way NOC  110 , e.g. a terrestrial line, is not shown. 
     As the use of two-way satellite networks has expanded into the consumer market, industry has further pursued internetworking of multiple satellite-broadcast networks and their associated independent inroute (“inbound”) or uplink channels. As the market expands, the number of possible uplink users further increases, and the previous approaches to allocation of return channels to users in fixed, predetermined groups necessarily requires additional hardware and system complexity in order to accommodate the increased uplink demand. Further, this approach becomes increasingly inefficient both in terms of hardware allocation, cost, and uplink channel utilization, since many of the available groups of uplink channels may be either heavily or lightly loaded or subject to load imbalance relative to other inroute groups because of each user being hard-configured for access to a specific inroute channel, or to only a limited number of channels. 
     Slotted-time uplink channels are commonly used and may be based on a Time-Division Multiple Access (TDMA) approach, wherein precise system timing is necessary to allow multiple users access to the necessary bandwidth and ability to transmit information in a multiplexed fashion on the return channel. TDMA allows a number of users to access a single radio frequency (RF) channel without interference by allocating unique time slots to each user within each channel. In TDMA, access is controlled using a frame-based approach. Transmissions are grouped into frames, with a frame synchronization (“sync”) signal usually being provided at the beginning of each frame. Following the frame sync, there are a number of time “slices” within the frame used for a burst transmission. In the simplest case, one time slice is allocated to each of the users having the need to transmit information. In more complicated systems, multiple time slices are made available to users based on transmission need or a prioritization scheme. After all time slices have elapsed, another frame synchronization signal is transmitted to restart the cycle. Thus, the frame sync serves as a system time reference that provides a common transmit timing source to each uplink user who transmits in a burst during a pre-assigned time slot. 
     TDMA requires a method for precise timing of the epochs of burst transmission to reduce burst overlap and consequent “collisions” of different users&#39; transmissions. Providing a common time reference for a limited number of remote network receivers receiving a single downlink or broadcast beam and sharing a limited number of uplink channels is relatively easy to accomplish, particularly when transmission and reception delays between the network control and the various users are well-characterized. For example, if synchronous operation is used, i.e., where the symbol rate of the digital transmission signal is precisely a multiple of the TDMA frame frequency, the TDMA frame rate can be locked to the system symbol clock at the network hub or earth station, and remote users can derive the frame rate from the recovered symbol timing. 
     However, frame timing sharing is more difficult with the evolution of multiple-beam satellites, and when sharing a larger number of different inroute or uplink channels among a large number of users. These users may be receiving different asynchronous broadcasts transmitted either through the same or different transponders on the same satellite or even on different satellites. Asynchronous digital transmissions have a symbol rate which is not a multiple of the TDMA frame rate. Establishing a common uplink transmission time reference for each of the users is more difficult due to the variety of delays and transmission paths in use, as well as the asynchronous nature of the broadcasts. 
     Several potential solutions for symbol timing recovery are available when asynchronous broadcast transmissions are used. For example, Global Positioning System (GPS) based timing, packetized elementary stream timing for Program Streams, or MPEG-2 Program Clock Reference (PCR) timing for Transport Streams may be used to synchronize a system. However, each of these solutions has relatively high-cost because of the additional processing and hardware requirements, including additional equipment at each of what could be a large number of remote user sites. 
     Currently, single, low-cost timing sources for sharing both frame and symbol timing throughout a communication system, particularly across multiple asynchronous transport streams is not available. 
     What is needed, therefore, is a relatively low-cost, accurate, and reliable system and method for sharing synchronized uplink data frame and symbol timing across a large network of geographically dispersed users receiving information across multiple transport streams, carriers, or satellites, without the necessity of involving multiplexing and modulation equipment. What is further needed is a system and method which solves the timing and uplink access problems associated with an increase in the number of users in the system, and which eliminates the need for major modifications or additions to network components required to transmit and receive the data. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforementioned problems of providing a low-cost and accurate system symbol and frame timing reference to dispersed uplink transmissions to reduce collisions of user transmissions, and to ensure all transmissions are synchronized in accordance with time slot allocations. 
     In one aspect of the invention, a communication system for sharing uplink timing information includes a common symbol timing reference and one or more control stations which each transmit different broadcast data streams in accordance with the common symbol timing reference. The first control station includes a delay tracker to determine the transmission delay associated with the first control station, and the second control station includes a delay tracker to determine the transmission delay associated with the second control station. Within each of the broadcast data streams, a common non real-time reference frame marker and a different delay message corresponding to the associated transmission delay are included. A remote (“local”) receiver receives one of the broadcast data streams. Each of the local receivers at their respective remote locations recovers their appropriate delay message, depending on the broadcast being received, and timestamps the non real-time reference frame marker with the associated local time of receipt. Timing recovery or correction circuits at each of the sites determine the system return channel uplink frame start time by correcting the associated local time of receipt with a local timing offset. The local timing offset is determined by the respective transmission delays, so that remote users can uplink messages in the proper time-slot(s) after the system uplink frame start time. This approach works even if different remote users receive the non real-time reference frame marker from different asynchronous broadcasts. 
     In a second aspect of the invention, a method for transmitting a frame synchronized message in a slotted-time communication system having a plurality of distributed user nodes and one or more control nodes includes receiving a reference marker in a local receiver of one of the plurality of distributed user nodes; timestamping the received reference marker with a local reception time; subsequently receiving a control node timing differential in the local receiver; correcting the local reception time by applying the control node timing differential and a local offset time; determining a start time for a system-wide return channel transmit frame using the corrected local reception time; and transmitting a remote user message during a preassigned period within the system-wide return channel transmit frame. 
     The present invention has a number of features that distinguish it over conventional digital timing recovery and sharing schemes. For example, the timing recovery method of the present invention uses an independent non real-time message structure to provide realtime TDMA timing to receivers for use in deriving uplink frame and symbol timing. The accuracy of this novel approach is determined at least in part by the jitter, or pulse-to-pulse variation, in each of the local receiver clocks, and the resulting system accuracy is comparable to more costly GPS-based solutions. 
     This timing sharing approach also allows several DVB transport streams to easily share timing among a common set of TDMA uplink channels, and further allows a DVB transport stream to be used to derive the precise TDMA timing, even when the data must traverse across multiple LANs and DVB multiplexers before being transmitted as part of the transport stream. 
     Further, an asynchronous data source may be used to provide the system frame timing to many remote network sites, even across multiple transport streams, carriers, or satellites without the necessity of multiplexing and modulation equipment. In this approach, the modulated broadcast streams use a central clock timing to ensure symbol timing is shared evenly throughout the system, and the ability to share both symbol and frame timing is substantially independent of the asynchronous broadcast signal being received. 
     In addition, the method and system of the present invention simplifies adding a large number of new uplink users who can share a set of TDMA channels by allowing some receivers on each of several transport streams to synchronize to the same uplink timing, because each of the transport streams has specific system symbol and frame timing information associated therewith which is sourced from a centralized clock and non real-time reference timing pulse. 
     Finally, the method and system of the present invention allow expansion to an (essentially) unlimited number of users on the same return channels, and allows these users to all use the same symbol and frame timing derived from different transport streams. 
     These and other features and advantages of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating a preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention provided by this detailed description will become apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts a conventional one-way satellite broadcast system; 
         FIG. 2  provides a representation of the two-way satellite communication system of the present invention; 
         FIG. 3  portrays the preferred protocol IP/DVB layering of the broadcast signal associated with a superframe message used in the present invention; 
         FIG. 4  provides a block diagram of the Return Channel Transceiver of the present invention; 
         FIG. 5  depicts the NOC Return Channel Equipment Interface; and 
         FIG. 6  shows the communication timing delays associated with the NOC broadcast to the remote users. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the method and system of providing TDMA system timing of the present invention is described below. Although described generally in terms of Hughes Network Systems&#39; Two-Way DirecPC® for ease of discussion, the thrust of the communication timing sharing system and method of the present invention could be embodied in other forms with only slight variations as to the detailed implementation. It also will be obvious to skilled artisans in the relevant art that all features of the invention will not be described or shown in detail for the sake of brevity and clarity. 
     An exemplary one-way conventional satellite broadcast system  100  is depicted in  FIG. 1 . The present invention is designed to control the burst timing of a group of return channels that share the same frame timing, as previously mentioned. For simplicity, this system is characterized in  FIG. 2  as including one or more Network Operations Center (NOC)  210  (also commonly known as a “hub”, “outroute”, “control node”, “control station”, or “earth station”, etc.), at least one satellite  130  having uplink and downlink transponders, system time reference  240  which provides common symbol timing to each NOC  210  in the system, one or more (i.e.,  1  to n) remote users  150  at a user node, each having a satellite receive and transmit capability provided by an associated transceiver  230 . NOC  210  preferably provides access to the internet or an intranet through gateway  160 . NOC inroute receiver  620  (shown on  FIG. 6 ) may be collocated with NOC  210 , or may be separate from NOC  210 . 
       FIG. 2  also illustrates two NOCs  210 , i.e. NOC 1   210   a  and NOC 2   210   b , which each provide at least one DVB Transport Stream  220  (e.g.  220   a  and  220   b ) to satellite  130  for further retransmission. The DVB transport stream retransmitted from satellite  130  is shown merely as DVB transport stream  220  for clarity, which may differ from DVB transport stream  120  ( FIG. 1 ) only in the uplink frame timing information contained therein, as discussed below. Each NOC  210  in the system of the present invention may provide support for several receive or outroute channels. However, application of the method and system of the present invention is not intended to be limited to a system having a specific number of NOCs  210  or remote users  150 . Further, NOC  210  in  FIG. 2  is distinguished from NOC  110  in  FIG. 1  by NOC  210  having the ability to support receiving and processing return channel traffic from remote users  150 . 
       FIG. 2  illustrates a return channel transceiver (“transceiver”)  230  which provides an integrated uplink (or “return channel”) capability. The capability added by transceiver  230  provides two-way broadband communications via satellite  130 . The receive channel in transceiver  230  could, for example, operate at a rate of 48 Mbps, and the transmit channel in transceiver  230  is preferably a VSAT-like TDMA channel. Depending on consumer requirements, the channel rates for the transmit, “return”, or “inroute” channel could be, for example, 64 kbps, 128 kbps, 256 kbps, or possibly even higher, as consumer needs arise. A group of multiple transmit channels may also be shared among several independent DVB transport streams  220 , whether transmitted from the same or different NOC  210 . The return channel preferably contains a link-layer protocol, at the burst level, to provide for a substantially lossless channel. 
     The receive channel in transceiver  230  receives a DVB transport stream  220  which preferably uses an IP packet format which may include packets arranged in accordance with the Multiprotocol Encapsulation (MPE) standard. A preferred superframe message  300  is depicted in  FIG. 3 , wherein the frame marker is not necessarily transmitted in every frame. The stream preferably has DVB compliant MPEG-2 formatting supporting multiple MPE messages in a single MPEG frame. The transport stream may include fixed-size 204 byte MPEG packets, which could contain 188 bytes of user traffic and 16 bytes of forward error correction (FEC) data, for example. An MPE header may also preferably include specific media access control (MAC) data fields to indicate the type of media or traffic contained in the data stream, e.g., unicast, multicast, conditional access, or return channel broadcast messages, and other data fields to indicate whether the packet is encrypted. FEC at various rates is also preferably supported, e.g. FEC rates of 1/2, 2/3, 3/4, 5/6, or 7/8. Further, the header of each frame may also contain a Packet Identifier (PID) to distinguish between elementary streams so that remote user  150  may filter the message by PID. For ease of discussion, DVB transport stream  220  will be referred to hereinafter as a “broadcast”. 
     Turning to  FIG. 4 , transceiver  230  preferably supports TCP/IP applications, e.g. web browsing, electronic mail and FTP, and also multimedia broadcast and multicast applications using IP Multicast, e.g. MPEG-1 and MPEG-2 digital video, digital audio and file broadcast. Transceiver  230  provides a high-speed, over-the-air return channel as an alternative to a low-speed terrestrial link. Transceiver  230  contains receiver (RCVR  140 ), processor  420 , RF transmitter (RF XMTR)  430 , timing recovery section  440 , and Transmit Unit (TU)  450 . RF XMTR  430  modulates and transmits, in burst mode, the in-bound carrier to satellite  130  and NOC  210  (shown on  FIG. 2 ). RF XMTR  430  may operate with, and be controlled by RCVR  140  via processor  420 , which also could master RCVR  140  by use, for example, of a Universal Serial Bus (USB) adapter (not shown). Configuration parameters and inbound data from processor  420  may be input to RF XMTR  430  through a serial port (not shown), and transmitter status information from RF XMTR  430  may also be provided through the serial port to processor  420 . TU  450  conditions the outgoing data signal by incorporating the appropriate signal protocols and modulation scheme, e.g. a IP/DVB protocol and TDMA using QPSK techrnques. 
     RCVR  140  receives the appropriate broadcast from satellite  130  through antenna section  460 , and provides appropriate timing-related signals to timing recovery section  440 . Timing recovery section  440  corrects or compensates the time of receipt of the received frame marker in accordance with timing information contained in the received broadcast signal. Timing recovery section  440  further enables RF XMTR  430  through processor  420  and TU  450  to transmit at the appropriate time in accordance with a TDMA time-slot allocation scheme. Significant cost savings can potentially be realized by having RF XMTR  430  mainly comprise firmware-controlled hardware without the necessity of having its own dedicated CPU and embedded software. Finally, antenna (ANT)  460  propagates and receives signals to/from satellite  130 . 
     A discussion of the nature and approach of the synchronized timing system and method of the present invention follows.  FIG. 5  shows return channel equipment (RCE)  510  at NOC  210  (shown on  FIG. 2 ) and its interface with NOC timing section  550 . RCE  510  reassembles packets received from remote users  150  (shown on  FIGS. 2 and 4 ) over the return channels into IP packets for further processing. Frame timing transmitted in the broadcast stream to remote users  150  (shown on  FIGS. 2 and 4 ) and ultimately used for uplink timing in the return channels is derived from a pulse from NOC frame pulse generator (NOC FPG)  520  in RCE  510 . NOC FPG  520  allocates bandwidth, coordinates the aperture configuration, and sends framing pulses to burst channel demodulator (BCD)  530 . The number of BCDs  530  supported by RCE  510  is preferably at least 32, to allow redundant equipment support for at least 28 return channels. Multiple sets of return channel equipment  510  may be provided in a networked cluster arrangement (not shown) within each NOC  210  to allow for processing of a large number of return channels, preferably up to 100,000 or more, for example. Return channel traffic from the remote users provided from the NOC RF section  610  (see  FIG. 6 ) and routed through system signal distribution section  540  is applied to BCD  530  to demodulate return channel data received from the remote users. 
     In addition, NOC FPG  520  provides framing pulses to NOC timing section  550 . NOC timing section  550  includes NOC delay receiver  551  and echo timing receiver  552  which measure packet delays associated with internal NOC delays and NOC-satellite delays, respectively. These receivers can considered to function as “delay trackers” which help in ascertaining the aforementioned delays. These delays are determined from signals provided from system signal distribution section  540  through uplink module  560  and downlink module  570  to NOC delay receiver  551  and echo timing receiver  552 , respectively. Uplink module  560  translates the signal from NOC signal distribution section  540  into a form suitable for NOC delay RCVR  551 . For example, the signal from NOC signal distribution section  540  may be provided as an intermediate frequency (IF) from the outroute broadcast before transmission, and which may be converted by uplink module  560  to an L-band signal, for example. Similarly, downlink module  570  could, for instance, translate an IF signal from NOC signal distribution section  540  which represents the broadcast signal as “echoed” or received from satellite  130  into another L-band signal provided to echo timing receiver  552 . By using this arrangement, NOC delay RCVR  551  and echo timing RCVR  552  could replicate portions of RCVR  140  in order to achieve greater equipment commonality within the system. 
     NOC timing processor  553  processes the delay information from NOC delay receiver  551  and echo timing receiver  552 . NOC timing section  550  provides the appropriate frame timing information to NOC multiplexer section (NOC MUX)  580 . NOC MUX  580  combines broadcast data intended for the remote users  150  with the frame timing information from NOC timing section  550 , and provides a packetized data signal to system signal distribution section  540  for transmission to satellite  130  through the NOC RF section  610 , and ultimately to remote users  150 . 
     NOC FPG  520  periodically causes RCE  510  to send a superframe marker pulse to NOC delay receiver  551  and echo timing receiver  552  through NOC timing section  550  once every integral number of TDMA frames, e.g. 8 frames or 360 milliseconds (ins). At the same time, it sends a superframe header which is included in the broadcast stream transmitted from NOC  210  (shown on  FIG. 2 ) for reception by a RCVR  140  (shown on  FIG. 4 ) located at one or more remote users  150  (shown on  FIGS. 2 and 4 ), and which is also received in the broadcast by NOC echo timing receiver  552  from satellite  130  (shown on  FIGS. 2 and 4 ). 
     The equipment, signals, and subsystems of each of NOC  210  (shown on  FIG. 2 ) and transceiver  230  (shown on  FIG. 2 ) are preferably interconnected via one or more local area networks (LAN) (not shown) and, even more preferably, are interconnected in accordance with a so-called open system architecture which allows modifications and upgrades to be more easily accomplished as improvements in software and hardware become available. 
     The concept in the timing approach of the present invention is to provide information to RCVR  140  (shown on  FIG. 4 ) so that transceiver  230  (shown on  FIG. 2 ) may precisely time its burst transmission time as an offset of the received superframe header. The superframe header received in a superframe numbering packet (SFNP) transmitted in the broadcast is used by every remote user  150  (shown on  FIGS. 2 and 4 ) to synchronize their transmit start of frame marker to the superframe marker pulse generated by NOC FPG  520 . This packet is used to lock network timing for the return channels, and as a beacon to identify which satellite network is being connected to. Remote user  150  (shown on  FIGS. 2 and 4 ) may also be configured to receive several PID addresses, including the one to be used with its associated NOC FPG  520 . Further, each NOC FPG  520  may be allocated its own private PID to ensure that remote users  150  (shown on  FIGS. 2 and 4 ) receive traffic only from their assigned NOC FPG  520 . 
     However, receipt of the SFNP by itself is not sufficient because there are delays from the time that NOC FPG  520  generates the superframe header until the time the receiver actually receives the SFNP. 
     As shown in  FIG. 6 , the delay in receipt of the superframe header is equal to three separate delays—an internal NOC outroute delay, a NOC-satellite transmission time delay, and a transmission delay from the satellite to each of the specific remote users  150 . The latter two items, NOC-satellite delay and satellite-remote user delay, are known parameters determined during a standard satellite-user “ranging” process during system initialization. However, these values can change slightly due to satellite drift along a vertical axis with respect to the surface of the earth. 
     To be able to adjust for satellite drift, a known process called “echo timing” is implemented at NOC  210  to measure changes in position of satellite  130 . This measures the transmission time from NOC  210  to satellite  130  and, from this measurement, determines the satellite drift relative to NOC  210  which is used to approximate the drift of satellite  130  from the position of remote user  150 . These values are used to correct the ranging values determined during initialization. The NOC-to-satellite portion of the satellite delay is sent in the SFNP message and is determined as the difference between timing signals from NOC delay receiver  551  (shown on  FIG. 5 ) and echo timing receiver  552  (shown on  FIG. 5 ). Each remote user  150  preferably has a preconfigured value for the satellite-to-remote user delay that is determined during system installation. The NOC delay at ranging is stored, and the change in NOC delay is applied to the receiver-satellite delay to approximate the time delay associated with satellite drift. The NOC-satellite drift timing is preferably provided in a subsequent SFNP message to remote users  150  so that current drift timing, relative to the initial ranging NOC-satellite echo delay, can be determined for an upcoming transmit frame. 
     In addition to not knowing the satellite drift, remote user  150  does not know the delay within NOC  210 , i.e. NOC outroute delay, which can vary in real-time. The internal NOC delay measures the delay from the time the superframe marker pulse is provided by NOC FPG  520  (shown on  FIG. 5 ), until the time the frame pulse is actually transmitted in a message on the broadcast from NOC  210 . 
     Thus, once every superframe, the internal NOC delay between the time the previous superframe header was supposed to have been sent, and the time that it actually was sent is broadcast in a SFNP message to all remote users  150 . This value, along with the “space timing offset” (STO), discussed below, is used by each remote user  150  to calculate the actual start time of the superframe. Remote user  150  uses the calculated superframe start time as the TDMA uplink frame time reference point for determining an upcoming transmit frame start time. Preferably, the internal NOC delay is routinely updated by NOC Timing section  550 , and is thereafter broadcast in a subsequent SFNP message to remote users  150 . 
     NOC FPG  520  (shown on  FIG. 5 ) pulses NOC delay receiver  551  (shown on  FIG. 5 ) and echo timing receiver  552  (shown on  FIG. 5 ). After a time interval approximately equal to the STO elapses, NOC FPG  520  (shown on  FIG. 5 ) provides a frame pulse to BCD  530  (shown on  FIG. 5 ). This frame pulse could be provided, for example, once every 45 ms, the preferred frame duration. The STO represents a calculation of the maximum round-trip time from the farthest remote user  150 , plus two frame times. A two frame delay is provided as a buffer to ensure that transceiver  230  at remote user  150  has sufficient time to process return channel frame format data, and to provide return channel data for transmission at least one-half frame time ahead of the actual frame transmit time. 
     The operation of the communication timing system of the present invention will now be described. NOC outroute  600  takes formatted data packets and transmits them on the DVB transport stream  220  (shown on  FIG. 2 ) to satellite  130  for further retransmission to remote users  150 . The data stream or “payload” information is transmitted following an appropriately formatted MPE header and initialization vector, if the packets are encrypted. 
     Included in the DVB transport stream  220  (shown on  FIG. 2 ) is a SFNP which provides a superframe marker, as well as the internal NOC delay and satellite drift correction for a previous superframe marker transmitted in a prior SFNP. 
     When remote user  150  receives a SFNP at their respective RCVR  140  (shown on  FIG. 4 ), the received superframe packet is tagged with a local time-stamp. This local time-stamp may be created using an internal counter (not shown), which preferably is a 32-bit counter free-running at 32 MHz, for example. Each of the remote sites must determine when the most recently received superframe marker actually occurred at the NOC outroute  600 . To do so, each remote user  150  subtracts its known satellite delay, corrected for drift, and the internal NOC delay provided in a subsequently received SFNP Message from the local time of receipt of the previously received superframe packet. 
     Once the superframe timing has been determined, each remote user  150  determines its upcoming transmission time relative to the local time of receipt of the superframe marker which is adjusted by a local offset time to determine the transmit frame start time such that the transmitted or uplink frame is received at the proper time at NOC  210 . The time at which the site must transmit is a satellite hop before the time that NOC  210  expects the data to be received. The transmission time is measured by starting at a time later than the regenerated superframe time by the fixed STO. The NOC delay and the receiver-satellite delay must be subtracted from this timebase. As discussed above, the final adjustment to account for satellite drift is made by determining and applying the difference between the current NOC delay and the ranging delay. Then, knowing the fixed frame length, e.g. 45 ms, the frame start time of a subsequent user transmit frame can be determined. 
     Knowing when the superframe marker should occur allows the remote user  150  to align the start of a transmit (Tx) frame marker in TU  450  (shown on  FIG. 4 ) with the NOC superframe marker pulse. TU  450  (shown on  FIG. 4 ) preferably has a free-running counter (not shown) that runs synchronously with an internal counter (also not shown) in its associated RCVR  140  (shown on  FIG. 4 ). After a period of time equal to the duration of a return channel frame, e.g. 45 ms, this TU counter value is latched, and an interrupt to its RCVR  140  (shown on  FIG. 4 ) is generated to read the value of the counter in RCVR  140  (shown on  FIG. 4 ). The local time at which this interrupt occurs is compared to when the interrupt should have occurred. This time difference is stored in TU  450  (shown on  FIG. 4 ) to correct for the proper transmit time start. RCVR  140  (shown on  FIG. 4 ) also provides a nominal frame length counter to TU  450  (shown on  FIG. 4 ) to adjust its frame timing. Once the frame timing is adjusted, a nominal value, e.g. close to 45 ms, will preferably be used on a continuing basis with minor adjustments to account for drifts between the counter and the timing pulse. Once TU  450  (shown on  FIG. 4 ) is aligned, there are only small corrections necessary to keep TU  450  (shown on  FIG. 4 ) synchronized to NOC  210 . Transceiver  230  (shown on  FIGS. 2 and 4 ) then uplinks a message at the appropriate time which is received by NOC RF section  610  and processed in NOC inroute receiver  620 . 
     The following describes some of the calculations that are performed in both NOC  210  and RCVR  140  (shown on  FIG. 4 ) to regenerate the proper frame timing. The timing variable “OFFSET” represents the aforementioned local offset time. For these calculations, Table 1 provides a listing and description of timing equation variables. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Timing Equation Variables 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 NOC Echo at Ranging 
                 Difference in time between a frame exiting a 
               
               
                 “HEr” 
                 modulator at the NOC and the time when the 
               
               
                   
                 same frame is received from the RF XMTR after 
               
               
                   
                 being echoed to the satellite. This is stored 
               
               
                   
                 by a receiver when it successfully ranges. This 
               
               
                   
                 value can be provided in terms of timing unit 
               
               
                   
                 counter units. 
               
               
                 NOC Echo current 
                 Current difference between the frame exiting a 
               
               
                 “HEc” 
                 NOC modulator and when it was received at the 
               
               
                   
                 NOC RF SECN after being echoed to the satellite. 
               
               
                   
                 The NOC timing section may periodically provide 
               
               
                   
                 this to all receivers in terms of timing 
               
               
                   
                 unit counter units. 
               
               
                 NOC Delay 
                 Amount of time that elapses between a 
               
               
                 “HD” 
                 superframe pulse and the superframe message 
               
               
                   
                 transmission by the NOC RF SECN. This may be 
               
               
                   
                 provided in each superframe (for the prior 
               
               
                   
                 superframe) in terms of the timing unit counter 
               
               
                   
                 units. 
               
               
                 Superframe Length 
                 Amount of time from one superframe pulse to 
               
               
                 “SFLen” 
                 the next provided in terms of timing unit counter 
               
               
                   
                 units. This pulse can occur periodically, e.g. 
               
               
                   
                 once every 360 milliseconds, so this value 
               
               
                   
                 provides a timebase for a receiver to convert 
               
               
                   
                 between timing unit counters and either 
               
               
                   
                 milliseconds or frames. 
               
               
                 Space Timing Offset 
                 The number of milliseconds between the 
               
               
                 “STO” 
                 superframe pulse and the frame pulse to the 
               
               
                   
                 BCD for the first frame of the superframe. 
               
               
                   
                 To convert this to counter units, the 
               
               
                   
                 equation is STO*SFLen/360. 
               
               
                 Local Echo 
                 A value which may be used to determine transmit 
               
               
                 “LE” 
                 timing specific to the remote user location. 
               
               
                   
               
            
           
         
       
     
     The equation for the frame timing at RCVR  140  provides a frame pulse counter offset (“OFFSET”) from the superframe being received at the remote user  150 , and is calculated as follows. All units used in the equation are referred to a NOC reference counter (not shown). The conversion to a remote counter is based on determining a ratio of the increase of the counter in a superframe in SFNP, and the increase of the counter at RCVR  140  during a superframe.
 
OFFSET≈STO−HEc−(HEc−HEr)−LE
 
     The ranging process, as previously discussed, is used to derive LE. When the ranging process begins, NOC  210  provides an estimated LE based on the location of satellite  130  and location of remote user  150 . Remote user  150  will fine-tune and correct LE, storing the correct value when the ranging process successfully completes. 
     In the system and method of the present invention, and with a preferred remote unit and return channel addressing scheme, there is essentially no limitation on the number (“n”) of remote users  150  which may uplink data on a return channel. A minimum of 2 24  (˜16 million) transceivers are preferably supported by the addressing scheme embodied within the DVB stream and, even more preferably, up to 2 28  (˜256 million) transceivers are supported. 
     Further, because the return channel is preferably a substantially lossless channel, compression techniques may effectively be employed to reduce bandwidth requirements. IP header compression has the potential to give a tremendous improvement in bandwidth, since such compression eliminates 10–15 bytes for every IP packet. 
     While a preferred embodiment has been described above in terms of a TDMA timing approach, this preferred embodiment is in no way to be considered limiting, and is provided only by way of example. As a further example, the method and system of deriving precise timing can be accomplished across any type of communication system having multiple users sharing the same media, and may find particular application in any slotted-time system that requires bit timing, e.g. a frequency-time system using a phase-locked loop (PLL) or frequency-locked loop (FLL) based upon the same timing standard. 
     It will be obvious that the present invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. The breadth and scope of the present invention is therefore limited only by the scope of the appended claims and their equivalents.