Multiple downlink data aggregator and system
BACKGROUND OF THE INVENTION 
     The invention relates to satellite communications and, more particularly, to a method and apparatus for processing multiple TDMA frames from multiple transponder downlinks. 
     A typical Satellite Communications Controller (SCC) has sufficient capacity to receive data from a single transponder only. However, as the use of satellite communications increases and satellites become increasingly loaded, it becomes necessary for communications networks to receive data from more than one transponder. Therefore, in order to utilize existing TDMA terminals in a multi-transponder mode, it is an object of the invention to provide an interface between the multiple transponders and the single SCC which will enable the SCC to process data as if it were receiving transmissions from just a single transponder. Preferably, the interface would appear to the SCC as if it were a modem and the use thereof would not require substantial modification to the terminal. 
     One known technique for engaging multiple transponders in a network is known as &#34;transponder hopping.&#34; With transponder hopping, a plurality of stations acting in concert can transmit to or receive from more than a single transponder. The use of transponder hopping is discussed by Schmidt in his U.S. Pat. No. 4,115,661 which issued Sept. 19, 1978, and in his article &#34;The Application of TDMA to INTELSAT IV Satellite Series&#34; which appeared in the COMSAT Technical Review, vol. 3, no. 2, pages 257-75 (Fall 1973). Since one of the main characteristics of transponder hopping is that a plurality of terminals on one side of a downlink communicate with one another, with each of these terminals being able to receive only one downlink at a time, a high degree of burst scheduling must take place so that the communication can be orderly. It is therefore another object of the invention to provide a method of multiple transponder networking which does not require burst scheduling between earth terminals. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are met by providing a multiple downlink data aggregator between multiple demodulators operating on different transponders and a single SCC-type TDMA terminal. The data aggregator extracts traffic addressed to the local SCC from several downlinks and places it in a single TDMA frame, called a pseudo-frame. The pseudo-frame is then sent to the local SCC which processes it as if it were a single, standard TDMA frame. 
     The data aggregator includes a separate receive chain for each downlink, with each receive chain having a frame synchronizer. Each synchronizer performs standard TDMA receive-side functions for initial acquisition and steady state synchronization by generating apertures and controlling codeword tolerances. The device includes a frame format processor which receives and decodes FRB (Frame Reference Burst) messages for establishing apertures, and perhaps for forming a pseudo FRB for the pseudo-frame which is supplied to the SCC. This approach further allows each transponder reference station to schedule bursts independently. A result is that burst time planning is greatly simplified and a higher degree of satellite utilization is achieved. 
     In each of the receive chains, the received signals are provided from the frame synchronizer to an error correcting decoder which error corrects both the traffic channel information and the assignment messages. The corrected assignment messages can then be fed back to the data synchronizer to ensure proper code word aperture generation, etc. Since the SCC is typically designed to monitor the quality of the communications link, the assignment and space signalling messages are also provided to temporary storage buffers without error correction. The encrypted portions of the signal, usually the traffic channels, are then decrypted and the addresses of the traffic channels are examined to determine which of the channels are addressed to the local SCC. The selected traffic channels are stored in ping-pong receive buffers. The selected traffic channels are then multiplexed into a single pseudo-frame together with assignment and space signalling messages. Depending upon the particular desired format of the pseudo-frame, new assignment messages may or may not have to be generated. 
     The data aggregator according to the present invention is outboard to the SCC and allows the SCC to function in a multi-transponder network with only minor modification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following embodiments and the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating the position of a data aggregator according to the present invention within an otherwise conventional system; 
     FIG. 2 is an illustration of the data aggregation concept; 
     FIG. 3 is an illustration of a first data aggregation technique, called Channel Stuffing; 
     FIG. 4 is an illustration of a second data aggregation technique, called Triple Traffic Burst Pseudo-frame; 
     FIG. 5 is an illustration of a third data aggregation technique, called Single Traffic Burst Pseudo-frame; 
     FIG. 6 is an illustration of a fourth data aggregation technique, called Preassigned Burst Stuffing; 
     FIG. 7 is a block diagram of a data aggregator according to the present invention; 
     FIG. 8 is a block diagram of a portion of the data synchronizer of FIG. 7; 
     FIG. 9 is a more detailed block diagram of essential portions of the FEC and CRC decoders of FIG. 7; 
     FIG. 10 illustrates essential portions of the data encryption/decryption unit of FIG. 7; 
     FIG. 11 is a block diagram illustrating essential portions of the FEC and CRC encoders of FIG. 7; and 
     FIG. 12 is a timing chart illustrating XRB transmit timing delays. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is designed primarily for use with SCC-type TDMA terminals, and will be described in that context. However, it should be noted that the data aggregation technique provided by the present invention will be useful as well in other types of communications networks, and the invention is not limited only to use in a satellite communications system. 
     A basic principle behind the approach taken in the present invention is the realization that an SCC can pass on no more than 13 Mbits/sec of user information. On the other hand, TDMA frames from any one transponder are usually transmitted at a rate of 48 Mbits/sec. Since the SCC is designed to extract 13 Mbits/s from the 48 Mbit/s TDMA frame, the SCC will function satisfactorily if its input is in the form a 48 Mbit/s TDMA frame. It is, therefore, a function of the data aggregator to independently synchronize to each transponder network, to extract traffic addressed to the local SCC from the plurality of transponder frames, to place the selected traffic in a single TDMA frame, hereinafter referred to as a pseudo-frame, and to send the pseudo-frame to the local SCC. The SCC will then treat the pseudo-frame as if it were receiving a single 48 Mbit/s TDMA frame from one transponder. 
     In one embodiment of the present invention, shown in FIG. 1, the data aggregator 2 accepts three independent, however loosely synchronous, TDMA frames from three transponders, extracts only desired channels, and aggregates them into one TDMA pseudo-frame for the local SCC 4. More particularly, the satellite communications controller (SCC) 4 first emits a burst through MODEM 6. This signal is up-converted in an up-converter 12, amplified in a high power amplifier 20 and transmitted from earth station 24 to satellite transponder A (XPONDA, not shown). The downside in turn receives bursts from the same transponder and, roughly simultaneously, from transponder B (XPONDB, not shown) and transponder C (XPONDC, not shown). These bursts are down-converted in down-converters 14, 16, and 18, respectively, and passed to the data aggregator 2. The data aggregator 2 performs the above-mentioned function of forming a pseudo-frame by assembling the FRB (frame reference burst), the XRBs (transmit reference bursts), and the traffic channels addressed to the local SCC ports from all the bursts in the three TDMA frames. The aggregation of data into pseudo-frames will be discussed in greater detail with reference to FIGS. 2 through 6. 
     Assume that a satellite communications controller, arbitrarily designated SCC #A2, has already transmitted a burst to transponder A (XPONDA), and that shortly thereafter transponders A, B and C (XPONDA-XPONDC) have transmitted Frames A-C, respectively, which signals were received by the system of FIG. 1 and demodulated at MODEM A, DEMOD B, and DEMOD C, respectively. FIG. 2 generally illustrates the frame formats received from the various transponders as well as the format of the generated pseudo-frame. The labels A1, A2, A3, etc. are transmit node identification codes (NIDs) indicating the source of each of the respective bursts in frame A received from transponder A, and the NIDs B1-B4 and C1-C4 similarly indicate the sources of the respective bursts in frames B and C received from transponders B and C, respectively. The symbol &#34;X&#34; indicates a channel which is addressed to the SCC number A2. The data aggregator 2 in FIG. 1 examines the frames received from the various transponders and assembles in a single pseudo-frame all of the traffic channels addressed to the SCC #A2. The various techniques for generating the pseudo-frame will be explained with reference to FIGS. 3-6. 
     In each of FIGS. 3-6, the lines of data labelled FRAME A-FRAME C are illustrated as they might appear following demodulation from their respective modems/demodulators 6, 8, 10 in FIG. 1. These TDMA frames originate from transponders XPONDA-XPONDC, respectively, and each includes a frame reference burst (FRB) of a type well known in the art which contains information indicating where each burst begins, and transmit reference bursts (XRBs) of a type also well known in the art for indicating how many off-hooks there are at each SCC, the reference SCC using this information in the demand assignment algorithm. In each of the different schemes of FIGS. 3-6, the pseudo-frame maintains essentially the same TDMA frame structure as the frame (e.g. FRAME A) which would normally have been received by the SCC #A2, and the pseudo-frame is synchronized to the local SCC receive frame. FRAME A-FRAME C are further comprised of transmit node identification codes (NIDs) A1-A4, B1-B4, and C1-C4, respectively indicating the sources of the various bursts. Traffic channels in each burst addressed to SCC #A2 are designated by an X appearing in the appropriate data stream. 
     FIG. 3 illustrates a technique referred to as &#34;Channel Stuffing&#34;. In this technique, the FRB and XRBs of FRAME A are passed to the SCC #A2 without modification and the data channels addressed to SCC #A2 ports in FRAME B and FRAME C are packed into the channel slots of FRAME A where the original channels were addressed to ports other than SCC #A2. The pseudo-frame consists of exactly the same number and size bursts as in FRAME A, so that the bursts will all start at the same location as in FRAME A, and there is no need to modify the FRB assignment field. Burst #A2, SCC #A2&#39;s own burst, may contain traffic channels from other bursts or may be left unchanged depending on the routing procedure of the SCC port traffic. Since each burst may contain traffic channels from other frames, the SCC may not be able to identify the originating node of the channels. If the SCC requires the burst NID in processing received channels, it may interface with the data aggregator where all the necessary information would be available. 
     FIG. 4 illustrates a further aggregation technique which may be called &#34;Triple Traffic Burst Pseudo-frame&#34;. In addition to the FRB and XRBs of Frame A, the pseudo-frame in FIG. 4 consists of three traffic bursts, one of which is from SCC #A2 located in the same position as in Frame A. All the data channels addressed to SCC #A2 ports are merged into two bursts A1 and A3, and the SCC #A2 burst remains unchanged. This configuration requires different transmit and receive frame compositions, and the data aggregator must create a new FRB having new burst assignments for SCC #A2 according to the new burst positions and lengths. As in the case discussed above, the NIDs may be obtained from the aggregator if the SCC needs them for routing purposes. 
     In the previous two aggregation methods, the burst position of the SCC #A2 burst is preassigned for every pseudo-frame so that the timing information for this burst is directly derived from the assignment table. However, the technique of FIG. 5, which may be called &#34;Single Traffic Burst Pseudo-Frame&#34; makes it possible for the aggregator to generate a single traffic burst if some modifications of the SCC software/hardware are allowed. Here, the pseudo-frame consists of the FRB and XRBs from FRAME A, but all valid SCC port channels are assembled in one traffic burst under the node A1. 
     The burst A1 in the pseudo-frame will begin at the same location as the burst A1 in the frame A. The FRB from FRAME A will therefor correctly indicate the starting position of the burst and, since the SCC can ignore all subsequent assignments in the FRB, the control fields of FRAME A can be passed to SCC #A2 without modification. The assignment table obtained by the SCC can then be used only for transmit timing purposes. With this method, simpler pseudo-frame generation and higher data concentration can be achieved. 
     Another data aggregation technique, with less impact on current SCC designs, although less efficient, is the allocation of a traffic burst slot in the pseudo-frame for each desired burst from three transponders. This may be called &#34;Preassigned Burst Stuffing&#34;. As shown in FIG. 6, each traffic burst destined to the SCC #A2 ports is packed by deleting unwanted channels and is placed in the preassigned pseudo-frame burst slot. The NIDs remain unchanged in the pseudo-bursts. The SCC #A2 burst position and length are also unchanged, thus requiring that the length and position of the SCC #A1 burst also remain unchanged. 
     The drawbacks of this method are that the data aggregator has to have advance knowledge of the maximum traffic from each SCC destined to SCC #A2, and that the maximum allocated traffic may exceed the capacity of one frame. However, an advantage is that the SCC can identify the transmit nodes of traffic channels without having to retrieve this information from the aggregator. 
     Although the invention should not be construed as limited to this embodiment, a specific embodiment of the data aggregator will now be presented. 
     A block diagram of one example of the data aggregator is shown in FIG. 7. The basic building blocks are triplicated SCC receive chains 5, 15, 25, address decoders 48, 52, 56 and data registers 50, 54 and 58 to extract desired traffic channels, alternate receive buffers 66-76 for channel/burst multiplexing, and a single transmit chain 35 to generate a pseudo-frame. In the figure, it is assumed that each SCC includes data encryption and decryption devices. The function of each unit will now be described in greater detail. 
     The data synchronizers 30-34, which are known in the art, perform the receive clock synchronization with the FRBs and provide codeword apertures to the demodulator. Since the aggregator does not have transmit functions, it is not necessary to generate transmit timings as in a standard synchronizer. The data synchronizers operate in a manner well known in the art, and the XRB aperture signals for receiving each of the XRBs are generated in a well-known manner such as illustrated in FIG. 8. A frame reference burst code word is detected and the phase of the FRB detection signal is compared with a locally generated FRB aperture signal in a phase comparator 102. In accordance with the phase comparison signal, the phase of the oscillator signal from local oscillator 103 is corrected in a phase correction circuit 104 and provided to a counting means 106 which is used to generate the FRB aperture signal and XRB aperture signals synchronized to the FRB aperture signal. The design and operation of data synchronizers are well known in the art, and the data synchronizers will obviously include many additional components. The diagram of FIG. 8 is provided merely to illustrate the basic synchronization technique which may be employed. 
     As is typical, an FEC (Forward Error Correction) and CRC (Cyclic Redundancy Code) detector are used for decoding the received signals. Since the SCC may be designed to monitor the occurrence of errors in order to determine the quality of the communications link, the assignment and space signalling messages must be provided to the SCC without error correction, and these are stored in temporary buffers 82 and 86. However, since the aggregator requires the assignment messages to construct an assignment table used for the code word aperture generation for traffic bursts, for channel assembly and for creating a new assignment table for the SCC in some situations, the assignment messages and preferably the space signalling messages as well are error corrected together with the data fields, and error corrected information necessary for synchronization functions can be fed back to the synchronizers. The essential components of a typical FEC and CRC decoder are illustrated in FIG. 9. 
     The channel assembly and new assignment table are implemented in a control section 45. The original assignment messages without error correction are provided from each of the received chains to a channel assignment buffer 80, and the space signalling messages are provided from each of the received chains to a space signalling buffer 86. The five successive XRBs are stored in sequence in the buffer 86 as they arrive, and the processor 82 monitors the contents of the assignment buffer 80 and generates new channel assignment information in the new assignment buffer 84, some or all of which information may merely be passed from the original assignment buffer 80. 
     The data aggregator contains three independent data decryption devices 42-46, since each network may use a unique key which is updated once in every superframe. FIG. 10 illustrates the basic and well-known operation of a standard decryption unit. The triplicated decryption is necessary for generating an unencrypted pseudo-frame from independently encrypted frames. Decryption is typically performed on the data field only, with the address field being passed around the decryption device to the address decoder. 
     Every channel address is fed into an address decoder 48, 52 or 56 and the following data or signalling channel information is read into a data register 50, 54 or 58. If the channel address matches one of the local SCC port numbers, the contents of the address and data registers are transferred to one of receive buffers 66-76; otherwise, they are discarded. 
     Each receive chain has two receive buffers, e.g. receive chain A has buffers 66 and 68, each with 22.5 Kbytes of memory. The alternate buffer structure makes it possible to write data on one buffer while the other buffer is being read by the channel multiplexer 88. 
     All the buffer contents are now multiplexed by multiplexer 88 under control of controller 83 to form a pseudo-frame, and the data channels are reencrypted in data encrypter 90. Some portion of the bursts may need to be encoded in encoder 92, for instance when a new assignment table is created by the data aggregator. FIG. 11 portrays a standard FEC and CRC encoding configuration. 
     Some current SCC design plans assume that the data encryption is to be performed within the SCCs. However, this fact obligates the data aggregator to include both encryption and decryption devices--resulting in six encryption/decryption units for each SCC with aggregation. This number may, however, be reduced to four if the decryption function of the SCC is transferred to the data aggregator as shown. For system modularity purposes, the encryption unit 90 in the transmit chain may also be transferred to the aggregator so that the aggregator performs all the encryption and decryption functions. 
     The pseudo-frame delay timing considerations will now be discussed. 
     Each TDMA frame received by the data aggregator experiences one frame of delay to reach the SCC due to the channel assembly in the aggregator. The SCC derives its burst transmit timing from the delayed superframe reference burst. Since SCCs without data aggregators will not undergo this delay, there will be one frame of timing difference between the SCCs with and the SCCs without the data aggregator. The situation is illustrated in FIG. 12, where Δt is the nominal XRB transmit timing delay after the reception of the superframe reference burst SFRB absent aggregation. If the SCC with aggregation transmits its burst based on delay Δt, it reaches the satellite one frame later than its nominal position, which results in a collision with another XRB. However, this collision can be avoided by transmitting the XRB one frame earlier than its nominal delay. Therefore, it is necessary to change the SCC transmit timing procedure by one frame time. It should be noted that if Δt is less than 15 msec, the new transmit timing delay Δt&#39; becomes 300 msec-15 msec+Δt. 
     In addition to normal operations, the data aggregator must support the interface and IF Loopback Configuration as selected via the Monitor/Control system. In the interface loopback configuration, the reference modem will perform an interface wrap and the data aggregator will connect the modem signals directly to the corresponding lines at the SCC interface. In the IF Loopback mode, the data aggregator will be configured as in the Interface Wrap, therefore all control logic will be passed and signal lines will be connected directly between the modem and the SCC. 
     In the Interface and IF Loopback Modem, non-reference transponders will not transfer traffic to the SCC, however, the data aggregator will maintain synchronization with the transponder network. 
     The unit should also contain an apparatus for testing non-reference modems. 
     The extraordinary nature of the present invention is magnified by the fact that it requires so little modification to existing equipment. This benefit was achieved as a direct result of a design effort aimed at enhancing the capability of existing equipment with an innovative conglomeration of data structuring and hardware/software design.