Patent Publication Number: US-6665292-B1

Title: Transmission and reception of TCP/IP data over a wireless communication channel

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part application of an application entitled TRANSMISSION OF TCP/IP DATA OVER A WIRELESS COMMUNICATION CHANNEL, Ser. No. 09/407,646, filed Sep. 28, 1999, now abandoned, which claims priority to a provisional application entitled SYSTEM AND METHOD FOR WIRELESS INTERNET SERVICE, Serial No. 60/151,282, filed Aug. 27, 1999, each of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to transmission of Internet data over a wireless communication channel. More particularly, this invention relates to transmission of Internet data using digital video broadcast format over a satellite communication channel. 
     BACKGROUND OF THE INVENTION 
     Explosive growth of the Internet, including Internet related commerce, has greatly increased the demand for Internet access. This demand crosses all geographic and demographic boundaries, from developed urban neighbors and cities to remote and developing nations. Presently access to the Internet is typically through copper wire. In developed nations such infrastructure exists in the form of public telephone networks. However, in developing countries and remote locations, there may not be a copper wire infrastructure in place, and development of such an infrastructure may be excessively costly, making its development unpractical. In addition, as Internet use increases, access through existing copper wires may become unacceptably slow due to the limited bandwidth of the copper wire and increased volume of Internet traffic. Because of these and other limitations on the existing infrastructure, a high-speed, reliable Internet communication service over a satellite link is desirable. A satellite link can provide Internet access to developing countries, as well as remote locations, where there is no copper wire infrastructure in place. Moreover, Internet access over a satellite link can provide an attractive alternative to copper wire, even if the infrastructure is in place, by providing high-speed, more reliable Internet access. 
     The Internet is based on an architecture developed by the Defense Advanced Research Projects Agency (DARPA). This architecture is generally referred to as the transmission control protocol/Internet protocol (TCP/IP) suite. The TCP/IP protocol suite is organized into four layers: network access layer; Internet layer; TCP layer; and process layer. 
     FIG. 1 is a block diagram illustrating an exemplifying system for discussion of the TCP/IP model. FIG. 1 shows two nodes  20  and  22  connected to the Internet  24 . As illustrated, the Internet comprises multiple subnetworks  26  and  28 . Data is transferred between subnetworks through a router  30 . In a node, network access layer  32  provides access to the subnetwork to which the node is connected. At the network access layer, a network header  34  is added to the data to be sent over the subnetwork. Various standards have been developed for transmission of data across subnetworks, also referred to as Local Area Networks (LAN). These standards have been collected into the Institute of Electrical and Electronic Engineers (IEEE) 802 specification. Within this specification, the IEEE 802.3 defines a LAN utilizing a carrier sense multiple access collision detection (CSMA/CD) protocol. In CSMA/CD protocol when a node on the network wants to transmit data, it senses the communication channel to determine if the channel is being used. If the channel is in use, the node does not transmit and waits for a time when no traffic is sensed to transmit. The transmitting node also monitors the channel as it transmits to determine if another node transmits at the same time. If two nodes transmit at the same time, there is a collision, which both nodes detect. Following detection of a collision, both transmitting nodes will discontinue transmission. The nodes will then wait independently random periods of time and then attempt to retransmit. A popular implementation of the CSMA/CD protocol is the Ethernet. 
     If data is to be transmitted between two nodes which are located on different, interconnected, subnetworks the Internet Protocol (IP) is used. At the IP layer  36 , functions for routing across multiple networks are provided. The IP layer provides an IP header  38  which comprises the destination address. To facilitate movement of data between two subnetworks, the router  30  is used. The router  30  receives a message from one subnetwork and examines the IP header  38  for the destination address. Knowing the destination address, the router transmits the message to the appropriate subnetwork where the destination node is located. 
     The Transmission Control Protocol (TCP) layer  40  provides for a reliable, error free, data exchange. The TCP layer  40  receives a data block  42  from the process layer  44 . If the data block  42  is too large, the TCP layer  40  divides the data block  44  into messages  46 . A TCP header  48  is added to each message  46 . The TCP header comprises a destination port, sequence number and checksum. The destination port identifies which port at the destination node is to receive the message  46  transmitted. The sequence number identifies the individual data packets so the messages  46  can be sequenced in the proper order at the destination node. The checksum provides a method of verifying the integrity of the data received at the destination node. 
     The process layer  44  contains the logic needed to support various user applications. Applications which operate on top of the TCP/IP include, for example, Simple Mail Transfer Protocol (SMTP) for basic electronic mail, File Transfer Protocol (FTP) for file transfer from one system to another, hyper text transfer protocol (“HTTP”) for transfer of Web pages and TELNET which provides remote login capability. 
     At the destination node, the process described above is performed in reverse. Traffic on the subnetwork is received by the network access layer  32 . The network header  34  is removed and the remaining data passed to the IP layer  36 . At the IP layer, the IP header  38  is removed and examined. If the message is addressed to the destination node, the remaining data is passed to the TCP layer  40 , otherwise the message is ignored. The TCP layer removes the TCP header  48 . The integrity of the message  46  is then verified using the checksum from the TCP header  48 . If the data block that was sent has been divided into multiple data packets, the TCP layer  40 , using the sequence number from the TCP header  48 , places the messages  46  in proper order. After the TCP layer  40  has rebuilt the data block  42  that was transmitted, it passes the data block  42  to the appropriate port at the process layer  44 . 
     While the TCP/IP and Ethernet protocols are used for transmission of data on networks such as the Internet, Digital Video Broadcast (DVB) is used to send Motion Picture Expert Group (MPEG) data over a satellite link. In the TCP/IP model, DVB would be located at the network access layer providing an interface to the physical transmission medium comprising the satellite channel. 
     The DVB protocol is the result of a market led consortium of public and private sector organizations in the television industry. This consortium led to the development of the DVB standard published by the European Telecommunications Standard Institute (ETSI). The DVB standard describes the modulation and channel coding system for satellite digital television broadcast. This standard is compatible with the MPEG-2 coded TV services. The MPEG-2 protocol generates digital information separated into “data pockets.” DVB simply receives the data pockets and places them into “data containers” for transmission over a wireless media. Within the DVB protocol, no restriction exists as to the kind of information which can be stored in these data containers. 
     As digital TV continues to gain acceptance and becomes common place, the demand for DVB receivers will greatly increase. Once DVB receivers come to the market in large numbers, it is expected that the commonality of design for a large market will enable costs to be kept down. 
     Thus, DVB can provide a cost effective means of video data transmission via satellite. However, no analogous standard, or cost effective way, has been developed to transmit TCP/IP data via satellite. 
     Therefore, there is a need in the technology for a means for and method of transmitting TCP/IP data over a satellite link. 
     SUMMARY OF THE INVENTION 
     Demand for high-speed, reliable Internet access is increasing. This increased demand is occurring where there is no “copper wire” infrastructure in place, such as developing nations and remote areas. In addition, where the “copper wire” infrastructure is in place, its capacity is often exceeded by the demand placed upon the system. The invention provides a communication system for transmission of Internet or other TCP/IP data over a satellite link. 
     One embodiment provides Internet service to a plurality of remote units in communication with a satellite. The satellite is also in communication with a hub station. The hub station is further in communication with a plurality of content servers. Data from the content servers is communicated to the hub station via the Internet. The hub station then transmits the content server data received from the Internet to the plurality of remote units over the satellite link. The hub station formats the data, to be transmitted over the satellite link, in a format compatible with the DVB standard. 
     Further, a hub station receiving a block of Internet data for transmission to a plurality of remote units over a satellite communication channel formats the data within DVB frames such that fragment headers occur at variable locations within the DVB frame. A value, stored within the DVB frame, indicates the location of the fragment headers within the DVB frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an block diagram illustrating an exemplifying system for discussion of the TCP/IP model 
     FIG. 2 is a block diagram illustrating an exemplifying system in which the invention may be embodied. 
     FIG. 3 is a diagram describing the structure of a TCP transport packet of a transport layer. 
     FIG. 4 is a diagram describing the structure of an IP packet of a network layer. 
     FIG. 5 is a diagram describing the structure of an exemplifying Media Access Control (MAC) data frame at the network access layer. 
     FIG. 6 is a block diagram describing the structure of a Digital Video Broadcast (DVB) header of a DVB data frame. 
     FIG. 7 is a block diagram illustrating conversion of a MAC data frame into a DVB data frame. 
     FIG. 8 is a block diagram of a transmitter system of a hub station. 
     FIG. 9 is a flowchart describing the steps executed in the conversion of a MAC data frame into a DVB data frame. 
     FIG. 10 is a block diagram of a receiver system of a remote unit. 
     FIG. 11 is a flowchart describing the steps executed in the conversion of a DVB data frame into a MAC data frame. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a block diagram illustrating an exemplifying system in which the invention may be embodied. The system in FIG. 2 provides high-speed, reliable Internet communication service over a satellite link. 
     In particular, in FIG. 2, content servers  100  are coupled to the Internet  24  which is in turn coupled to a hub station  104  such that the hub station  104  can request and receive digital data from the content servers  100 . The hub station  104  also communicates via satellite  106  with a plurality of remote units  108 A- 108 N. For example, the hub station  104  transmits signals over a forward uplink  110  to the satellite  106 . The satellite  106  receives the signals from the forward uplink  110  and re-transmits them on a forward downlink  112 . Together, the forward uplink  110  and the forward downlink  112  are referred to as the forward link. The remote units  108 A- 108 N monitor one or more channels which comprise the forward link in order to receive remote-unit-specific and broadcast messages from the hub station  104 . 
     In a similar manner, the remote units  108 A- 108 N transmit signals over a reverse uplink  114  to the satellite  106 . The satellite  106  receives the signals from the reverse uplink  114  and re-transmits them on a reverse downlink  116 . Together, the reverse uplink  114  and the reverse downlink  116  are referred to as the reverse link. The hub station  104  monitors one or more channels which comprise the reverse link in order to extract messages from the remote units  108 A- 108 N. For example, in one embodiment of the exemplifying system, the reverse link carries multiple access channels in accordance with assignee&#39;s co-pending application entitled METHOD AND APPARATUS FOR MULTIPLE ACCESS IN A COMMUNICATION SYSTEM, application Ser. No. 09/407,639, filed Sep. 28, 1999, the entirety of which is hereby incorporated by reference. 
     In one embodiment of the exemplifying system, each remote unit  108 A- 108 N is coupled to a plurality of system users. For example, in FIG. 2, the remote unit  108 A is shown as coupled to a local area network  116  which in turn is coupled to a group of user terminals  118 A- 118 N. The user terminals  118 A- 118 N may be one of many types of local area network nodes such as a personal or network computer, a printer, digital meter reading equipment or the like. When a message is received over the forward link intended for one of the user terminals  118 A- 118 N, the remote unit  108 A forwards it to the appropriate user terminal  118  over the local area network  116 . Likewise, the user terminals  118 A- 118 N can transmit messages to the remote unit  108 A over the local area network  116 . 
     In one embodiment of the exemplifying system, the remote units  108 A- 108 N provide access to the Internet for a plurality of users. For example, assume that the user terminal  118 A is a personal computer which executes browser software in order to access the World Wide Web. When the browser receives a request for a web page or embedded object from the user, the user terminal  118 A creates a request message according to well-known techniques. The user terminal  118 A forwards the request message over the local area network  116  to the remote unit  108 A, also using well-known techniques. Based upon the request message, the remote unit  108 A creates and transmits a wireless link request over a channel within the reverse uplink  114  and the reverse downlink  116 . The hub station  104  receives the wireless link request over the reverse link. Based upon the wireless link request, the hub station  104  passes a request message to the appropriate content server  100  over the Internet  24 . 
     In response, the content server  100  forwards the requested page or object to the hub station  104  over the Internet  24 . The hub station  104  receives the requested page or object and creates a wireless link response. The hub station transmits the wireless link response over a channel within the forward uplink  110  and forward downlink  112 . For example, in one embodiment of the exemplifying system, the hub station  104  operates in accordance with assignee&#39;s co-pending application entitled METHOD AND SYSTEM FOR FREQUENCY SPECTRUM RESOURCE ALLOCATION, application Ser. No. 09/407,645, filed Sep. 28, 1999, the entirety of which is hereby incorporated by reference. 
     The remote unit  108 A receives the wireless link response and forwards a corresponding response message to the user terminal  118 A over the local area network  116 . In one embodiment of the exemplifying system, the process of retrieving a web page or object is executed in accordance with assignee&#39;s co-pending application entitled DISTRIBUTED SYSTEM AND METHOD FOR PREFETCHING OBJECTS, application Ser. No. 09/129,142, filed Aug. 5, 1998, the entirety of which is hereby incorporated by reference. In this way, a bi-directional link between the user terminal  118 A and the content servers  100  is established. 
     To better understand the operation and advantages of the invention, descriptions of example packet structures are provided. FIG. 3 is a diagram describing the structure of a TCP packet  300 . The term “packet” commonly refers to a unit of databits, including data and control signals. The term “message” commonly refers to the user information or data being communicated. A message may be of any length. If necessary, the TCP layer fragments or splits the message into multiple packets for transmission. 
     As shown in FIG. 3, the TCP header  48  comprises a source port field  304  followed by a destination port field  308 , each having  16  bits, to identify the end points of a network connection. Each “host” computer may determine for itself how to allocate its ports. In a network, the term “host” commonly refers to one of a group of computers intended for running user applications (i.e., programs). The TCP header  48  further includes a sequence number field  312 . TCP accepts arbitrarily long messages from user applications, breaks them up into TCP datagrams not exceeding 65,536 bytes, and sends each datagram as a separate packet. Hence, the sequence number  312  is a 32-bit word indicating the location of the datagram in the original message. 
     Next, a piggyback acknowledgment field  316  is used by a receiving computer to indicate receipt of a particular packet on a parallel reverse connection. A TCP header length field  320  of 4 bits follows the piggyback acknowledgment field  316  to indicate how many 32-bit words are contained in the TCP header  48 . This information is needed because the header  48  includes a variable-length options field  360  which communicates data as may be agreed upon by the source host and destination host. 
     After several unused bits  322 , the TCP header length  320  is followed by six 1-bit flags. The first 1-bit flag is URG  324  which is set to 1 if an urgent pointer  356  is used, and set to 0 otherwise. The urgent pointer  356  is used to indicate a byte offset from the current sequence number  312  at which urgent data are to be found. The second 1-bit flag is ACK  328  which is set to 1 when a packet bears an acknowledgment, and set to 0 otherwise. For instance, a connection reply bears an acknowledgment, so its ACK bit  328  is set to 1. The third 1-bit flag is EOM  332  indicating the end of message when set to 1. The last packet of a message has the EOM bit  332  set to 1. All other packets will have the EOM bit  332  set to 0. The fourth 1-bit flag is RST  336  which is used to reset a connection that has become confused due to a host delay or breakdown. A host delay may occur due to congestion of packets over the network. A host breakdown (commonly referred to as a “crash”) may be caused by a variety of events, such as a power failure, a host processor reset, or an error in the host application software. The fifth 1-bit flag is SYN  340  which is used to establish synchronization for a connection request. A connection request has the SYN bit  340  set to 1 and ACK bit  328  set to 0 to indicate that the piggyback acknowledgment  316  is not in use. As noted above, the connection reply does bear an acknowledgment with its SYN bit  340  and ACK bit  128  set to 1. The sixth 1-bit flag is FIN  344  which indicates release of a connection. The FIN bit  344  is set to 1 to indicate that the sender has no more data, and set to 0 otherwise. 
     Flow control in TCP is handled using a variable-size sliding window. A 16-bit window field  348  is used to indicate how many bytes a source host may send beyond the number of bytes acknowledged by a destination host on a parallel reverse connection. The window field  348  is followed by a checksum field  352  to provide reliable connections. An error in transmission can be detected by the destination host by computing the checksum in the same way as the source host and comparing it against the value in the checksum field  352 . The value in the checksum field  352  is calculated by the source host by adding up all the data, regarded as 16-bit words, and then converting the resulting sum to its 1&#39;s complement, a standard computer operation. 
     The checksum field  352  is followed by the above-described urgent pointer field  356 . An options field  360  follows the urgent pointer field  356  to communicate optional data, such as buffer sizes during the link setup procedure. The options field  360  is followed by a data field  46 . The data field  370  comprises the message being communicated over the computer network. For instance, the message could be a word processing document, a computer program, a digital image, digitized voice information for a phone call, an electronic mail message and so forth. 
     FIG. 4 is a diagram describing the structure of an IP packet or datagram. An IP datagram comprises an IP header  38  followed by a data field  460 . The IP header  38  includes a 20-byte fixed part and a variable length optional part. The 20-byte fixed part of the IP header  38  includes a version field  404  which indicates to which version of the Internet protocol the datagram belongs. By including the version  404  in each datagram, it is possible to change protocols while the network is operating. Since the IP header  38  is not constant in length, an IP header length (IHL) field  408  is provided to indicate the length of the IP header  38  in 32-bit words. By definition of the IP, the minimum value of the IHL  408  is 5. The IHL field  408  is followed by a “type of service” field  412  which allows the host computer to inform the subnetwork the kind of service required. Various combinations of reliability and speed are possible by predefined service types. The “type of service” field  412  is followed by a “total length” field  416  which includes the total number of all bits in the datagram, i.e., both header and data bits. The maximum size of the “total length” field  416  is 65,536 bytes. 
     The IP layer may break up each TCP datagram into smaller fragments for transmission across the network. The elementary fragment unit is 8 bytes. Since the size of a datagram is a maximum of 65,536 bytes, there is a maximum of 8192 fragments per datagram. Hence, after the “total length” field  416 , an identification field  420  is used to allow the destination host computer to determine to which datagram a newly arriving fragment belongs. All fragments belonging to the same datagram contain the same value in the identification field  420 . 
     After an unused bit, two 1-bit fields follow the identification field  420 . The first 1-bit field is a “don&#39;t fragment” (“DF”) bit  424 . When the DF bit  424  is set to 1, network gateways are instructed not to fragment the datagram because the destination is incapable of reconstructing the fragments together into their original datagram. The second 1-bit field is a “more fragments” (“MF”) bit  428 . The MF bit  428  is used as a double check against the total length field  416  to ensure that no fragments are missing from the reconstructed datagram. Except for the last fragment, all message fragments have the MF bit  428  set to 1. 
     The two 1-bit fields are followed by a “fragment offset” field  432  which indicates the location or order of the current fragment in the datagram. As shown in FIG. 4, the “fragment offset” field  432  consists of 13 bits and, hence, there is a maximum of 8192 possible message fragments for each datagram. The “fragment offset” field  232  is followed by a “time to live” field  436  which is a counter used to limit packet lifetimes. Typically, a network gateway destroys packets having a lifetime exceeding 255 seconds. 
     After the IP layer at the destination host constructs a complete datagram, the IP network layer utilizes a protocol field  440  to indicate a typical transport protocol. TCP is one transport protocol, but other protocols such as transport protocols specified by the Open Systems Interconnection (OSI) standard (e.g., ISO  8073 ) may be used. A header checksum field  444  follows the protocol field  440  to verify the validity of the IP header  400 . The header checksum  444  is useful because the IP header  38  may change at a gateway, e.g., due to fragmentation into multiple fragments. A “source address” field  448  follows the header checksum  444  to indicate the source network number and host number of the data portion of the datagram. A destination address field  452  follows the source address field  444  to indicate the destination network number and host number of the data portion. Finally, an options field  456  follows the destination address field  452 . 
     FIG. 5 is a diagram describing the structure of a network header at the network access layer. The network header and data comprise a media access control (MAC) data frame  500 , typical of an Ethernet data frame. The IP datagram from the IP layer is encapsulated into the MAC data frame. The MAC data frame  500  comprises a header  502  and data  504 , and checksum  512 . 
     The header  502  comprises a 6-byte destination address  506  identifying the desired designation of the datagram. A 6-byte source address  508  identifying the originator of the datagram follows the destination address  506 . The source address  508  is followed by a 2-byte type field  510  identifying the type of data frame, such as for example Ethernet. The “type” field  510  is followed by a variable length data field  504 . The data field  504  can be from 46 to 1500 bytes in length depending on the size of the IP datagram. If the IP datagram requires more than 1500 bytes of data field  504 , the datagram is packetized into multiple MAC frames. The data field  504  is followed by a 4-byte checksum  512 . The value in the checksum field  512  is calculated by the source host by adding all the data, regarded as 16 bit words, and then converting the resulting sum to 1&#39;s complement, a standard computer operation. 
     FIG. 6 is a block diagram describing the structure of a DVB frame  600 . A standard DVB frame comprises a DVB header  602  and a data container  604 . The DVB header  602  begins with an 8 bit sync pattern  610 . Following the sync pattern field  610  are three single bit fields, the transport error indicator  612 , the payload unit start indicator bit  614  and the transport priority bit  616 . 
     Following the transport priority bit  616  is the 13-bit PID/Offset (PID)  618 . In a standard DVB header the PID is used to indicate the video channel the date corresponds to. Following the PID field  618  are a 2-bit adaptation field control  620 , a 2-bit scrambling control  622  and a 4-bit sequence number  624 . 
     Next, the DVB frame comprises a data container  604 . The data container provides 184 bytes of storage for data to be transmitted. 
     In one embodiment, the system comprises a hub station in communication with the Internet. The hub station receives data from the Internet and formats the data for transmission over a satellite communication link to a plurality of remote units. At the remote units the data is converted back to its original Internet data format and transmitted by the remote unit to end users. In other embodiments the remote unit may be, for example, hand held and use the internet data locally at the remote unit. 
     TCP/IP data, if formatted appropriately, may be able to be transmitted over a satellite link using DVB. Because DVB provides a standardized method of transmitting video data over a satellite link, costs of implementing a DVB satellite link should decrease as digital TV evolves. Thus, DVB can be an attractive data transfer platform. 
     In one embodiment, the TCP/IP frames are formatted into DVB frames so as to align the beginning of each TCP/IP data frame with the beginning of a DVB data frame and filling the DVB frame, or multiple DVB frames, until all the TCP/IP data is transmitted. This embodiment is attractive because synchronization of TCP/IP and DVB frames is simple, because the beginning of a TCP/IP frame always occurs at the beginning of a DVB frame. The size of a TCP/IP data frame is, on average, a different size than a DVB frame. If the TCP/IP data spans multiple DVB frames, not all of the DVB data frames will be filled. This results in non-optimal utilization of the DVB data frames, usually resulting in a portion of a DVB frame being empty. In the typical case, when the TCP/IP data does not fit exactly into a DVB frame, the unused portion of the DVB frame is wasted, decreasing the overall bandwidth of the communication system. 
     According to another, embodiment of the invention, the DVB header  602  fields and data container  604  are assigned values as described below. The DVB header  602  fields, which are included at the start of every DVB frame  600 , remain the same as in a standard DVB header except for the PID field  618 . In this embodiment of the invention, the PID field  618  is assigned a value indicating the location in the data container  604  where a new MAC frame begins. The values assigned to the PID are described in detail below. Use of the PID field to indicate the location where a new MAC frame begins within a DVB frame allows TCP/IP data to fill the DVB frames rather than to regularly transmit partially filled frames. In other embodiments, DVB header  602  fields other than the PID field  618  may be assigned a value indicating the location in the data container  604  where a new MAC frame begins One advantage of using the PID field  618  to carry a value indicating the location in the data container  604  of the MAC frame is that standard off-the-shelf DVB receivers can be used to receive the DVB data stream. A standard DVB receiver is capable of providing data output when seemingly random, or disrupted, values are detected in the PID field  618 . In addition, the PID field  618  contains enough data bits to carry information indicating the location of the MAC frame anywhere within the DVB frame. Other DVB header fields may not contain a sufficient number of bits to carry the information. 
     If one attempts to place information indicating location of the MAC frame into DVB header fields other than the PID fields  618 , off-the-shelf DVB receivers may detect errors. The detected errors can be catastrophic, causing the DVB receiver to provide no output except perhaps an indication that an error has occurred. In such a case, the DVB data is not output by the receiver and the system becomes inoperable. It may be possible to use DVB header  602  fields other than the PID filed  618  to contain the location value with this same advantage as long as the field does not interfere with the operation of a standard DVB receiver. 
     The DVB data container  604  comprises two portions according to the invention: a first fragment header  606  and a data packet  608 . The first fragment header  606  is transmitted at the beginning of the first fragment of a multiple fragment MAC data frame transmission. As will be discussed below, one MAC data frame may be packetized into multiple DVB data frames to improve the efficiency of transmitting the MAC data frame in a DVB data frame format. Because the header information only needs to be transmitted once, it is transmitted with the first fragment only. On subsequent data frames, the header information is not repeated. 
     The first fragment header  606  comprises a 16-bit packet length field  626 . The value placed in the packet length field  626  corresponds to the length of the data field  504  of the MAC data frame. Following the packet length field  626 , a 16-bit “type” field  628  contains the value from the 2-byte type field  510  of the MAC data frame. A destination address field  630  follows after the “type” field  628  and contains 4 bytes of the 6 byte destination address  502  of the MAC data frame. The 4 bytes used may be either the 4 most significant bytes, the 4 least significant bytes or any other combination of 4 bytes from the 6 byte MAC destination address  502  field. In the invention it is not anticipated that there will be more than 4 bytes of address space needed to identify all the remote units  108 A- 108 N of the system. If growth in the number of remote units  108 A- 108 N exhausts the 4 byte address space, other fields in the DVB header  602 , not required to support operation of a standard DVB receiver, may be allocated to remote unit  108 A- 108 N addressing. 
     The remainder of the DVB data container is the data packet  608 . The total length of a DVB data container is 188 bytes. Because there is a 4 byte DVB header  602  transmitted in every DVB data frame  600 , there is a maximum of 184 bytes available to carry other information. All of the 184 bytes may comprise the data packet  608  if there is no first fragment header  606  required in the DVB data frame  600 . If the DVB data frame  600  contains a first fragment of a MAC data frame, 8 bytes are allocated for the first fragment header  606  and 176 bytes are available for the data packet  608 . 
     To better understand the operation of the invention, a description of an example set of MAC data frames packetized into DVB data frames is provided. FIG. 7 illustrates three example MAC data frames  702 ,  704  and  706 . Packetizing three MAC data frames  702 ,  704  and  706  into five DVB data frames  708 ,  710 ,  712 ,  714  and  716  is described. 
     MAC data frame  702  comprises a header section  718  and a data section  720  corresponding to  502  and  504  of FIG. 5 respectively. DVB data frame  708  comprises a DVB header  722 , a first fragment header  724  and a data packet  726 , corresponding to  602 ,  606  and  608  of FIG. 6, respectively. 
     The first portion of the DVB data frame  708  is the DVB header  722 . The DVB header is at the beginning of every DVB data frame to conform to the DVB specification. Because this is the beginning of a MAC data frame, the first fragment header  724 , as discussed in relation to FIGS. 5 and 6, is built and located after the DVB header  722  in DVB data frame  708 . With the DVB header  722 , and the first fragment header, there are 176 bytes remaining in DVB data frame  708  for data. In this example, there are more than 176 bytes of data in the MAC data section  720 . Therefore the first 176 bytes of data from the MAC data section  720  are carried in DVB data packet  726 , filling DVB data frame  708 . The additional data in  720  is carried in the next DVB data frame  710 . 
     DVB data frame  710  begins with a DVB header  728 . Because this is a continuation of MAC data frame  702 , there is no first fragment header. With no first fragment header there are 184 bytes available in the DVB data packet  730  for MAC data. In this example, there are more than 184 bytes of data remaining in the MAC data portion  720  so the entire DVB data packet  730  is filled with data from MAC data frame  702 . The additional data remaining in MAC data frame  702  is carried in DVB data frame  712 . 
     DVB data frame  712  begins with a DVB header  732 . As described in relation to DVB data frame  710 , there is no first fragment header and thus there are 184 bytes of data available for MAC data portion  720 . In this example, there are less than 184 bytes of data remaining in MAC data portion  720 . Thus the last segment of data portion  720  is located in the first portion  736  of DVB data packet  734 . 
     If the remaining portion  738  of the DVB data packet  734  is at least 8 bytes in length, so as to accommodate an entire first fragment header, information corresponding to the next MAC data frame  704  is also carried in the  712 . In this example, the remaining portion  738  of the DVB data frame  712  contains more than 8 bytes. Thus the header  740  of MAC data frame  704  is converted into a DVB first fragment header  742 . If there are at least 4 bytes still available in the remaining portion  738  of DVB data packet  734  following the first fragment header  742 , MAC data  744  is carried in a section  746  of the remaining data portion  732 . In this example, there is enough data in MAC data frame  738  to fill section  746  of DVB data frame  712 . The data remaining in MAC data frame  704  is carried in the next DVB data frame  714 . 
     DVB data frame  714  begins with a DVB header  748 . Because frame  714  is not the first fragment of MAC data frame  704 , there is no first fragment header. In this example, the remaining data portion  744  of MAC data frame  702  is carried in the data packet  750  of DVB data frame  714 . The remaining portion  752  of DVB data frame  714  does not contain at least 8 bytes. Because there are less than 8 bytes available, the remaining portion  752  of DVB frame  714  cannot carry the entire first fragment header from MAC data frame  706 . In one embodiment the first fragment header is not split between two DVB data frames, therefore, the remaining portion  752  of DVB data frame  714  is unused. In other embodiments, the first fragment header may be divided between two DVB data frames. 
     In the embodiment where the first fragment header is not divided between two DVB data frames, the procedure described advances to the next DVB data frame  716 . DVB data frame  716  begins with DVB header  754 . The header  754  is followed by a first fragment header  756  which corresponds to the MAC header  758  of MAC data frame  706 . Data from MAC data frame  706  is carried in a data packet  762  in the same manner as described above. 
     In the example discussed in FIG. 7, first fragment headers do not always occur at the start of a DVB data frame. Allowing first fragment headers to occur at a variable location within a DVB data packet, provides for efficient utilization of the DVB data frame. As illustrated in FIG. 7, if first fragment headers could only occur at the beginning of a DVB data frame, large portions of DVB data packets could go unused following the completion of one MAC data frame to the beginning of the next DVB data frame. Placing first fragments at a variable location within the data packet of a DVB data frame provides efficient use of the DVB data frame, however, the receiving system must be able to locate the first fragment header in the data packet. 
     According to one embodiment of the invention, location of the first fragment header is provided by the PID field  618  in FIG. 6, of the DVB data frame. In other embodiments of the invention, location of the first fragment header may be provided in other DVB header  602  fields which do not adversely affect DVB transmission. In the embodiment utilizing the PID field  618  for indicating location of the first fragment header, as the DVB data frame is being built, if a first fragment header is located within the data packet  608  of the DVB data frame, the value of the PID is set to correspond to the number of bytes into the data packet the beginning of the first fragment header is located. If the first fragment header follows immediately after the DVB header, then the PID is set to 4, which corresponds to the beginning of the first fragment header following the 4 byte DVB header. If there is a first fragment header located somewhere else within the data packet of the DVB data frame, the PID is set to a value corresponding to the offset from the start of the DVB data frame to the start of the first fragment header. If the data packet contains only data, with no first fragment header then the PID is set to a value of 252. In one embodiment valid PID values are multiples of 4, thereby aligning data on 4 byte boundaries. 
     As discussed previously, data formatted as described in FIG. 7 is transmitted by the hub station  104  over the forward link  110  to the satellite  106 . The satellite receives the data from the forward link  110  and re-transmits them on a forward downlink  112 . The remote units  108 A to  108 N monitor one or more channels which comprise the forward link in order to receive remote-unit specific data or broadcast data from the hub station  104 . 
     When data formatted as described in FIG. 7 is received by a remote unit, the PID field  618  in the DVB header is read. If the PID value is between 8 and 180, the receiver knows that a first fragment header begins at this location within the data segment of the DVB data frame, with the data located between the beginning of the data packet and the first fragment header containing data from the preceding MAC data frame. If the PID value is 4, the first fragment header is located at the start of the data packet of the DVB data frame. If the PID value is 252, there is no first fragment header and all the data in the data packet belongs to the preceding MAC data frame. Thus the only DVB data frames which have a PID value of 252 are intermediate DVB data frames, containing fragments of a MAC data frame which spans multiple DVB frames, or the last DVB data frame of a fragmented MAC frame that has less than 8 bytes available in the data packet section of the DVB data frame. If there are at least 8 bytes available in the data packet of a DVB data frame, and there is no more MAC data ready to transmit, a first fragment header is added to the DVB data packet with a length field set to zero, indicating to the receiver that there is no more data presently being transmitted. 
     If there is a first fragment header in the data packet portion of the DVB data frame, the receiver parses the first fragment header for the destination address  630 , and the packet length  626 . The destination address  630  identifies which remote unit the transmission is intended for. The packet length field  626  represents the number of bytes contained in the MAC data field  504 . Thus, using the length of the MAC data field the receiver is able to reassemble the original message. 
     FIG. 8 is a block diagram of a hub station transmitter system. The transmitter system comprises an Internet interface  780 , a MAC to DVB converter  782 , a controller  784 , memory  786 , a standard DVB transmitter  788  and a RF transmitter  790 . The Internet interface  780  receives MAC formatted data from the Internet. The Internet interface  780  provides any buffering or signal conditioning required by the hardware interface to the Internet. The output of the Internet interface  780  is passed to the MAC to DVB converter  782 . Using the procedure described above, the MAC to DVB converter  782  formats the MAC data frames into DVB data frames. The controller  784  is in communication with the MAC to DVB converter  782 . The controller  784  may be, for example, a microprocessor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), discrete logic, an analog circuit, or other control circuitry. Communication between the controller  784  and the MAC to DVB converter  782  allows the controller  784  to monitor the data transmission process such as, for example, detecting errors and detecting if a message is directed to the hub station rather than to a remote unit through the RF link. The controller  784  is also in communication with memory  786  so that if a message from the Internet was directed to the hub station, the controller  784  can direct that message to memory  786  or take appropriate action as required by the message. The output of the MAC to DVB converter  782  is passed to a standard DVB transmitter  788 . The standard DVB transmitter  788  prepares the DVB frame for transmission over the wireless link. The output of the standard DVB transmitter  788  is transferred to the RF transmitter  790 . The RF transmitter interfaces to an antenna  792  and transmits the DVB data frame over the wireless link to the remote units. 
     FIG. 9 is a flowchart describing the packetizing of a MAC data frame into a DVB data frame performed by the hub station. The process starts in block  800 . In block  802 , a MAC data frame is received from the network access layer. Flow continues to block  804 , where the amount of space available in the current DVB data packet is determined. If there are not at least 8 bytes available in the data packet, flow continues to block  806 . In block  806  the data packet is sent. In block  808  the process advances to the next DVB frame and the PID is set equal to a value of 4 indicating that the MAC frame is starting at the beginning of the DVB frame. Flow then continues back to block  804 . In block  804  if there are at least 8 bytes available in the data packet floe continues to block  810 . In block  810 , the header and data are copied from the MAC frame to the DVB frame. In block  812 , it is determined if all the MAC data, from the MAC frame being processed, has been copied. If all the MAC data has not been copied flow continues to block  814 . In block  814  the DVB frame is transmitted. Flow continues to block  816  where the process advances to the next DVB frame. 
     Flow then continues to block  818  where it is determined if after the remaining MAC data is copied to the DVB frame, will there be sufficient space remaining in the DVB frame for another header. If there will not be sufficient space remaining after the remaining MAC data is copied flow continues to block  820 . In block  820  the PID is set equal to a value of 252 indicating that the entire DVB frame contains data from the same MAC frame. If in block  818  it is determined that there will be sufficient space remaining in the DVB frame after all the MAC data is copied flow continues to block  822 . In block  822  the PID is set to a value equal to the number of bytes from the beginning of the DVB data frame to where a new MAC frame will begin. After the PID value is set in either block  820  or  822  flow continues to block  824 . In block  824  the MAC data is copied to the DVB frame. Flow then continues to block  812 . 
     In block  812  if it is determined all the MAC data, from the MAC frame being processed, has not been copied flow continues to block  826 . In block  826  it is determined if there are additional MAC frames available to be processed. If there are additional MAC frames available to be processed flow continues to block  802 . If there are no additional MAC frames available flow continues to block  828 . In block  828  the DVB frame being processed is transmitted. Flow then continues to block  830 . In block  30  a new DVB frame is started, and the PID is set to a value of 4. Flow then continues to block  802 . 
     FIG. 10 is a block diagram of a DVB receiver section of a remote unit. The DVB receiver section comprises an RF receiver  880 , a standard DVB receiver  882 , a DVB to MAC converter  884 , a controller  886 , memory  888  and a LAN interface/driver  890 . The RF receiver  880  receives the wireless communication transmitted by the hub station. The output of the RF receiver  880  is passed to the standard DVB receiver  882 . The standard DVB receiver  882  formats the data into DVB data frames. Data transmitted by the hub station is formatted to comply to the industry standard DVB format. As noted above, using the PID field to indicate the location of the start of a MAC from within the DVB data frame permits use of a standard DVB receiver. Use of the PID field does not interfere with normal operation of the DVB receiver. Use of fields other than the PID field may cause errors or otherwise make the DVB receiver inoperable. As noted above, use of standard DVB receivers has several advantages, such as, for example, the reduced cost of the high volume standard receiver as compared to custom designs, and the ability to easily incorporate new advances in DVB receiver technology as they become available. Standard DVB receiver chips may be used, such as, for example, MITEL VP305, VP306 or any other receiver chip which complies with the DVB standard. 
     The output of the standard DVB receiver  882  is passed to the DVB to MAC converter  884 . In the DVB to MAC converter  884 , the DVB data frame is defragmented into the original MAC data frame that was transmitted by the hub station as explained with reference to FIG.  11 . The controller  886  is in communication with the DVB to MAC converter  884 . Communication between the controller  886  and the DVB to MAC converter  884  allows the controller  886  to monitor data reception by the remote unit. The controller  886  monitors, such as, for example, errors in the received message and if the message was directed to the remote unit or to be transferred to the LAN. The controller is also in communication with memory  880 . If a transmission is intended for the remote unit, such as, for example, a software upgrade for the remote unit, the controller  886  detects that the message is intended for the remote unit and directs the DVB to MAC converter  884  to route the message to memory  888 . Messages that are intended for the LAN pass from the DVB to MAC converter  884  to the LAN interface/driver  890 . LAN interface/driver  890  provides signal conditioning and the required interface and driver circuitry to interface the remote unit to a LAN. The controller may be, for example, a microprocessor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), discrete logic, an analog circuit, or other control circuitry. 
     FIG. 11 is a flowchart describing the defragmentation of a MAC data frame formatted into a DVB data frame performed by the remote unit. The process starts at block  900 . In block  902  a DVB data frame is received. Flow continues to block  904 , where it is determined if the DVB frame received contains data which is a continuation from the MAC data contained in the previous DVB frame. If the DVB frame received does not contain data which is a continuation of the MAC data contained in the previous DVB frame flow continues to block  906 . In block  906  it is determined if a MAC frame defragmentation, or reassembly, is in process. If a MAC frame reassembly is in process flow continues to block  908 . In block  908  the MAC frame which is being reassembled is discarded. The frame is discarded because the reassembly process is not complete, however, the DVB frame received does not contain data which is a continuation from the MAC data from the previous frame. In the manner described, a remote unit is able to synchronize itself to an incoming data stream if the remote unit is coming online initially, following a remote unit reset, or if a DVB data frame is lost or corrupted. Flow then continues to block  910 . 
     If in block  906  it is determined that a MAC frame reassembly is not in process flow continues to block  910 . In block  910  it is determined if there are more MAC frames in the DVB frame being processed. If there are no more MAC frames in the DVB frame, as indicated by a length field of zero or the end of the DVB frame has been reached, then flow continues to block  902 . If it is determined that there are more MAC frames in the DVB frame being processed flow continues to block  912 . In block  912  it is determined if the entire MAC frame is contained in the DVB frame being processed. If the entire MAC frame is contained in the DVB frame being processed flow continues to block  914 . In block  914  the data from the DVB frame is copied to the MAC frame. In block  916  the completed MAC frame is sent to the IP layer. Flow then continues to block  910 . If in block  912  it is determined the entire MAC frame is not contained in the DVB frame being processed, but spans multiple DVB frames, flow continues to block  918 . In block  918  the first fragment of the MAC frame is copied to a MAC reassembly buffer. Flow then continues to block  902  to receive the next DVB frame. 
     Returning to block  904 , if the DVB frame received does contain data which is a continuation of the MAC data contained in the previous DVB frame flow continues to block  920 . In block  920  it is determined if a MAC frame reassembly is in process. If a MAC frame reassembly is in process flow continues to block  922 . In block  922  the MAC data from the DVB frame is copied to the MAC reassembly buffer. Flow continues to block  924  where it is determined if the MAC frame is complete as indicated by a PID value not equal to  252  or if there is data remaining in the DVB frame. If the MAC frame is complete flow continues to block  926 . In block  926  the MAC frame is sent to the IP layer. Flow then continues to block  928 . If, in block  924 , it is determined that the MAC frame is not complete flow continues to block  928 . Returning to block  920 , if it is determined a MAC frame reassembly is not in process flow continues to block  928 . 
     In block  928  it is determined if there are more MAC frames in the DVB frame being processed. If there is additional MAC data in the DVB frame being processed flow continues to block  906 . If there is no additional MAC data in the DVB frame being process flow continues to block  902 . 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiment is to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.