Fraud detection and user validation system for mobile earth terminal communication device6035039Abstract In a mobile satellite system, a mobile communication system includes a fraud detection and user validation system and method for commissioning the mobile communication system in the mobile satellite system. The method includes the steps of generating a first input signal by the mobile communication system upon initialization, and transmitting the first input signal from the mobile communication system to the central controller. The method also includes the steps of generating a first security key in the central controller using the first input signal, and generating a second security key in the mobile communication system using the first input signal and transmitting the second security key to the central controller. The central controller verifies the mobile communication system that transmitted the first input signal to commission it in the mobile satellite system. The verification process is responsive to a comparison of the first security key with the second security key. When the mobile communication system has been verified, information is transmitted throughout the mobile satellite system to enable future access and use of the mobile satellite system by the mobile communication system. Claims What is claimed is: Description TECHNICAL FIELD
______________________________________
GC-S Outbound TDM signaling channel from the GC to the
METs
MET-ST Inbound TDMA signaling channel from the MET to the GC.
MET-SR Inbound random access signaling channel from the MET
to the GC.
FES-C Outbound communications and inband signaling channel
from a FES to a MET.
MET-C Inbound communications and inband signaling channel
from a MET to a FES.
GC-I Interstation signaling channel from the GC to an FES.
FES-I Interstation signaling channel from an FES to the
______________________________________
GC.
FIG. 9a illustrates the basic signalling architecture in the satellite communication system. The basic element of communication for signaling and control for the MSS signaling system is the Signaling Unit (SU). The SU consists of 96 bits organized in 12 octets of 8 bits each. The first 80 bits comprise the message, and the last 16 a parity check, computed using the CCITT CRC-16 algorithm. The SU itself may take a variety of forms, depending on its use. The format of a typical SU, in this case a MET request for access, is shown in FIG. 9. For transmission, the SU is convolutionally encoded at either rate 3/4 or 1/2, adding an additional 32 or 96 bits respectively. For the example given in FIG. 9, the meanings of the various fields are as follows: Message type: A 7 bit code which identifies the meaning of the SU; in this case a request for access to the MSS system for call placement. MET-GC Signaling Protocol (MGSP) Header: A 8 bit field comprised of several sub-fields giving particular information related to the protocol: message type (command, response, message), message reference identification, and the number of times the message has been retransmitted. RTIN: Reverse Terminal Identification Number--the MET's Electronic Serial Number, by which it identifies itself in transmissions on the MET-SR channel. Digits 1-10: The first 10 digits of the addressed telephone number in the PSTN or private network, in hexadecimal. If the 10th digit is set to "C", an address of greater than 10 digits is indicated. CRC: The 16-bit error detection code (Cyclic Redundancy Code). The frame formats used in the GC-S, MET-SR and MET-ST channels are closely related, and are based on a common 360 millisecond superframe established on the GC-S channel. The frame formats and relationships of the out of band signaling channels are shown in FIG. 10. In FIG. 10, all timing relationships in the MSS system signaling scheme are determined from the GC-S frame structure. The GC-S is operated in the QPSK mode at an aggregate rate of 6750 b/s. The stream is divided into superframes of 360 ms, comprising three 120 ms frames. Each frame is in turn comprised of a 24-bit unique word (UW), six SUs, eight flush bits and 10 unused bits, for a total of 810 bits and 120 ms. The first frame of a superframe is identified by inversion of the UW. Mobile terminals throughout the area covered by any beam receive GC-S channels with a total uncertainty of approximately 32 ms, primarily due to their geographical locations. The received superframe boundary establishes the four 90 ms "slots" in the MET-SR random access channels, which operate in the BPSK mode at 3375 b/s. The actual random access burst is comprised of a 24-bit preamble, a 32-bit UW, a 128-bit SU (96 bits rate 3/4 coded), and eight flush bits, for a total of 192 bits in 56.9 ms. This allows a 33.1 ms guard time between bursts. Mobile Terminals select a MET-SR channel and slot at random from among the permitted choices. The MET-ST TDMA channels, which also operate in the BPSK mode at 3375 b/s, are comprised of bursts which are equal in length to the GC-S frame, and which are also timed on the received frame boundary. The TDMA burst is made up of a 24-bit preamble, a 32-bit UW, a 192-bit SU (96 bits rate 1/2 coded), and eight flush bits. The total length of the TDMA burst is 256 bits in 75.9 ms, which allows a guard time of 44.1 ms. Mobile Terminals always respond to commands received on the GC-S on a MET-ST channel which corresponds in number to the position of the command SU in the TDM frame. For example, the MET will respond to a command in SU slot 2 on MET-ST channel 2, and so forth. The response is always transmitted in the second frame time after receipt of the command, so that there is a minimum of 120 ms in which the MET can prepare its response. The initial phase of establishing a call is handled by out-of-band signaling on the GC-S, MET-SR and MET-ST channels. This phase culminates in assignment of a pair of communication channels to the MET and FES. When these elements receive and tune to the communication channels, further signaling and control functions are accomplished using inband signaling. The communication channels, FES-C and MET-C, use a variety of related TDM formats which are determined by the intended use of the link, i.e., voice, data, or facsimile and one of three possible primary modes: call setup (entirely signaling), communication (no signaling), or in-band signaling (an occasional subframe of 128 bits is used for signaling/control). The same 96-bit SU described above is used to accomplish in-band signaling. A typical example of a communication channel format, in this case voice mode in-band signaling is shown in FIG. 11. The outbound TDM, inbound TDMA, and inbound random access channels provide signaling between the GC and each of the METS in the associated control group. All communications on these channels will be passed in the form of 96 bit (12 octet) messages known as signaling units. Each signaling unit will begin with a 1-octet messages type field and end with a two-octet cyclic redundancy check. The MET to GC Signaling Protocol (MGSP) serves as the layer two protocol for these channels. Communications from the group controller (GC) to the mobile terminals is provided by the Outbound TDM or GC-S channel. The primary function of this channel is to carry frequency assignments from the GC to individual METs. In addition, the Outbound TDM channel carries network status information which is received by all METs in a particular beam and control group. The outbound TDM channel operates at a rate of 6750 bits/s with rate 3/4 FEC. QPSK modulation and nominally 6.5 kHz channel spacing (other spacings are under investigation) is employed. These parameters are identical to those of the communications channel and were chosen to reduce MET complexity. Inbound TDMA (MET-ST) channels are used by the MET to respond to actions initiated by the GC, such as responding to the call announcement issued by the GC to check a MET's availability to receive a PSTN originated or MET to MET call. The Inbound Random Access (MET-SR) channels are used by METs to request frequency assignments and for other MET initiated actions. The inbound random access and TDMA channels each operate at a rate of 2400 bits/s with rate 3/4 FEC. DPS modulation and nominally 7.5 kHz channel spacing is employed. This modulation scheme has been selected because of its robust performance in the presence of frequency offset and timing errors. It also exhibits superior performance relative to conventional BPSK in the presence of band-limiting and hard-limiting. Each control group has associated with it a number of L-band beams over which it operates. In each of these L-band beams a control group has associated with it a distinct set of outbound TDM, inbound TDMA, and inbound random access channels. The number of signaling channels of each type in each set is determined based on the level of signaling traffic flowing between the GC and the METs in that-control group in that L-band beam. As signaling traffic levels change, new signaling channels of each type are allocated to or deallocated from a particular set of channels. The frequencies used for outbound TDM, inbound TDMA, and inbound random access channels are included in the status information carrier in the bulletin board signaling units transmitted on the outbound TDM channel. Each MET is assigned to one of the outbound TDM channels in the control group and beam to which it belongs. Each control group supports up to 16 outbound TDM channels in each beam. Each outbound TDM channel has associated with it up to 6 inbound TDMA channels. An inbound TDMA channel will only carry messages that are responses to messages received on the outbound TDM channel with which it is associated inbound random access channels will not associated with a particular outbound TDM channel. A MET chooses a inbound random access channel at random from among those associated with its control group and beam each time a message is to be transmitted. Each control group can support up to 64 inbound random access channels in each beam. Up to 64 of these channels may be used system wide to meet the signaling requirements of a fully loaded system supporting 5000 circuits. Inband signaling channels (FES-C and MET-C) are provided between the FES and the MET. These channels are used to provide signaling for call setup and call release, and also provide the capability to pass other signaling information while a call is in progress. The FES-C and MET-C channels are operated in two separate modes in "call setup mode" only signaling messages are carried by the channel. In voice mode voice frames are carried by the channel, but the capability to inject signaling messages by occasionally dropping voice subframes exists. Frames containing inband signaling messages employ a unique word different from that used for frames containing only voice subframes. Interstation signaling channels (GC-I and FES-I) are used to pass signaling information between the GC and each of the FESs. These channels operate at a rate of 9.6 to 64 kbit/s and are implemented using either the available 5 MHz Ku-band satellite capacity or terrestrial links. The LAP-F protocol will be employed on those links to ensure reliable transfer of variable length signaling and network management messages. When a MET is idle (powered on and ready to receive a call) it will continuously receive an Outbound TDM channel in order to receive call announcements associated with incoming calls and obtain status information from bulletin board signaling units. Each MET will be capable of transmitting signaling information to the GC on any of the inbound random access channels or on any of the inbound TDMA channels associated with the outbound TDM channel that it is receiving. During a call a MET will receive and transmit all signaling information via the In-Band signaling channels. No signaling information will be sent to a MET via the outbound TDM channel during a call. Any signaling messages from the GC to the MET will be sent to the MET via the FES through the GC-I and FES-C channels. Each group controller supports at least one outbound TDM channel in each of its associated L-band beams. Each outbound TDM signaling channel is continuously transmitted and carries frequency assignments and networks status information from the GC to the METs. The outbound TDM channels are also used to poll idle METs to see if they can accept incoming calls. As this channel is the only way to signal information to a MET not engaged in communications, it must be as robust as possible under harsh fading and shadowing conditions. Another key element in the MSS system is the need for the METs to be as inexpensive as possible. Towards this end, the outbound TDM channel will have the same rate and modulation as the communications channels. This will maximize the commonality of the receive chain of the MET for communications and signaling. Note that as the demodulation process is much more complex than the modulation process, the inbound random access and inbound TDMA channels do not really require this level of commonality with the communications channel. The number of outbound TDM channels assigned to each set of signaling channels is determined by the traffic supported by the group controller in that L-band beam. Assignment of METs to outbound TDM channels is made based on a special identifier assigned to each MET as commissioning. This identifier is called the GC-S Selector Identifier code (GSI). The MET selects the outbound TDM channel to be used by dividing the GSI by the total number of outbound TDM channels available in the given beam. The number of TDM channels available is given in the BB of each TDM channel. The remainder of the four bit binary division process will form the number of the channel to be used. Each MET will receive only the outbound TDM channel assigned to it. This method allows METs in the same logical grouping to be assigned to the same outbound TDM channel as is needed for the Net Radio Service provided by the MSS System. It also allows the load on the outbound TDM channels to be redistributed quickly if a channel fails or a new channel is added. The 120 ms frame length was chosen because it would support 6 messages per frame and correspond to the slot size requirement (>120 ms) of the inbound TDMA channel. This allows a direct correspondence between outbound TDM frames and inbound TDMA slots for the purposes of TDMA synchronization and scheduling responses to outbound messages. Eight flush bits are included at the end of each frame to allow the decoder to reset to a known state at the beginning of each frame. This allows more rapid reacquisition following channel fade events. The modulation scheme and transmission rate for this channel will be the same as for the transmission channel, namely QPSK modulation at a transmission rate of 6750 bps. Signaling units within each frame will be coded with a rate 3/4 constraint length K=7 convolutional code. The outbound TDM superframe has a duration of 360 ms is and is made up of three outbound TDM frames. The superframe duration is the basic time interval over which message repetitions are done. Repetitions are used to increase the reliability of outbound TDM signaling units. Messages can be repeated in consecutive superframes. Studies by AUSSAT have shown that L-band fade events typically have durations ranging between 10 ms and 100 ms (2). Because the 120 ms frame would not provide adequate separation between message repetitions, the 360 ms superframe is used to reduce the chance of losing two copies of a message during the same L-band fade event. This repetition method is similar to that used in the AUSSAT system. Different numbers of repetitions may be used for different message types to provide different levels of reliability. The number of repetitions used for a particular message type will be a part of the signaling protocols and can be varied by the system operator. In addition to message repetitions, interleaving will be used to protect against burst errors. The interleaving is provided over a TDM frame and provides improved performance in the presence of short burst errors. The bulletin board is a set of signaling unit (SUs) that are periodically transmitted by the MCC on all outbound TDM channels. The bulletin board contains global information such as current network status, signaling channel frequencies and inbound random access channel congestion control parameters. Every MET processes the information in the bulletin board METs, on startup, and acquires the entire bulletin board before attempting to use the MSS system. At least one bulletin board SU is transmitted in every outbound TDM frame. Bulletin board SUs are also sent as "filler" SUs, i.e., sent when there are no other SUs pending on the outbound TDM channels. Bulletin board SUs do not occupy any fixed position in the outbound TDM frame. Bulletin board SUs are grouped into pages of related SUs. Each Bulletin Board page has an update number associated with it, which will be sent with each SU of that page. This number will be incremented by the NCC whenever the information in that page is updated. METs are required to build a local data structure that contains the contents of the bulletin board. Whenever a change in update number is detected for any page, the MET will update the entire data structure for that page with the contents of the bulletin board SUs that follow. The inbound TDMA channel is used by the METs to transmit responses to call announcement messages and for responses to other messages received on the outboard TDM channel. Each of the inbound TDMA channels is assigned to a particular outbound TDM channel. The number of inbound TDMA channel assigned to a particular outbound TDM channel depends on the traffic supported by that outbound TDM channel and is selectable by the network operator. The TDMA channel is divided into slots of 120 ms duration. Inbound messages consist of 96 bits before coding and 128 bits after rate 3/4 convolutional coding. The resulting burst will occupy 80 ms of the slot, allowing 40 ms of guard time. This guard time arises due to the uncertainty in round trip transmission time between the satellite and a mobile terminal. Mobile terminals derive their inbound frame timing (for both the TDMA and random access channels) from the outbound TDM frames. Inbound TDMA slots have the same duration as an outbound TDM frame. At a MET each TDMA slot boundary occurs at an outbound TDM frame boundary. If MET A is nearer to the satellite than MET B, MET A will receive the outbound TDM channel .DELTA.t sooner than MET B, where .DELTA.t corresponds to the difference in propagation times to the satellite for the two terminals. As a result, if both METs synchronize their transmit timing to their reception of the outbound TDM channel, MET B's responses to messages will take 2.DELTA.t longer to reach the satellite than MET A's responses. As additional guard time of 1 symbol time also must be included to account for the .+-.1/2 symbol synchronization uncertainty in the MET. This results in a total guard time requirement of 2.DELTA.t+1 symbol time. TDMA scheduling is done using a fixed relationship between outbound TDM channel time slots and inbound TDMA channels and slots. The response to a message received in the nth slot of the outbound TDM frame is transmitted on the nth TDMA channel assigned to that outbound TDM channel. The frequencies of the assigned inbound TDMA channels are contained in one of the bulletin board signaling units periodically transmitted in the outbound TDM channel. The response to an outbound message is transmitted in the TDMA time slot that begins 120 ms after the end of the TDM frame in which the outbound message was received. This should provide adequate time for message processing in the MET. The inbound random access channel is used by the METs to transmit call requests to the GC. It is also used to carry other inbound messages for MET originated actions. The number of inbound random access channels assigned to a particular control group in a particular L-band beam depends on the traffic supported by that control group in that beam and is selectable by the network operator. To provide reasonable call setup times and call loss probabilities these channels are typically be operated at a throughput of approximately 25% or less. As the random access channel is operating at a relatively low efficiency, one of the prime goals in its design is that it be bandwidth efficient. The frequencies used for the random access channels are transmitted in the bulletin board signal units. For each transmission, METs choose at random among the inbound signaling channels assigned to their control group. After transmitting a message, the MET waits a given amount of time for a response. If no response is received within this amount of time, the MET retransmits in a slot selected at random over some given number of slots. This procedure is repeated until either a response is received or a maximum number of transmissions is reached. The bursts on the random access channel are identical to those on the TDMA channel (i.e., modulation, coding, preamble, etc.). The MET-GC Signaling Protocol (MGSP) procedures send signaling units between GCs and METs via the GC-S, MET-ST and MET-SR channels. This protocol encapsulates functions such as channel selection, channel access, slot timing, error recovery and congestion control. Higher layer functions, such as call processing, use the protocol for communicating among themselves between the METs and GCs. The relationship of MGSP to other signaling layers in the GC and the MET is shown in FIG. 12. A transaction consists of a command message that is sent from an originating application to a destination application, to which the destination application replies with a response message. Each command and response consists of a signaling unit. The MGSP performs functions such as channel selection, error recovery using retransmission, and repetition of SUs to improve channel reliability. The MGSP at a MET also implements congestion control procedures for the MET-SR channels. Only one outstanding transaction exists between a MET and a GC in a given direction. However, two simultaneous transactions, one in each direction, are supported between a GC and a MET. MGSP also provides a only-way message service, that does not require a response from the receiver. The improved call setup protocol used to establish a MET originated voice call is shown in FIG. 13. When a MET user initiates a call, the MET formats and transmits an access request message via a random access channel. This message includes the call type and the destination phone number. The group controller chooses an FES to handle the call and sends frequency assignments to the MET via the TDM channel and to the FES via the interstation signaling channel. The FES frequency assignment also includes the call type, the destination phone number to allow the FES to complete the call, and an access security check field used to verify the METs identity. The access security check field is generated by the group controller using the MET frequency assignment and the MET key which is known only to the MET and the group controller. After the MET receives the frequency assignment, it transmits a scrambling vector message to the FES. This message contains the initial vector to be preloaded into the FES scrambler at the beginning of each voice channel frame. Letting the MET randomly pick this vector provides some degree of privacy on the Ku to L-band link. The scrambling vector message also contains an access security check field generated by the MET using its frequency assignment and its key. The FES compares this field with that received from the group controller to verify the identity of the MET. After receiving the scrambling vector message, the FES and the MET switch from call setup mode to voice frame mode and the FES completes the call to the terrestrial network user. The improved protocol used for PSTN originated calls is shown in FIG. 14. When a call from a terrestrial network user arrives at an FES, the FES makes a channel request using interstation signaling. This request contains the phone number received from the terrestrial network user. The group controller determines the MET identity based on the phone number and transmits a call announcement via the TDM channel. The MET acknowledges this announcement via the TDMA channel. This exchange allows the group controller to verify that the MET is available before assigning bandwidth to the call. Frequency assignments are then made and the scrambling vector is transmitted by the MET. The call is then completed to the MET user. MET to MET calls are set up using a double hop connection through an FES. These calls are set up by the group controller and the FES as a MET to PSTN call setup concatenated with a PSTN to MET call setup. As a result the METs require no additional call processing for MET to MET calls. Advantageously, the MET combines three different features for the delivery and transmission of voice and data. These three features include: the ability to initiate and transmit a data call, the ability to initiate and transmit a facsimile digital call, and the ability to roam between satellite and terrestrial based wireless communication systems. The following documents, representing applicable transmission protocols, are hereby incorporated by reference: EIA/IS-41B Cellular Radio Telecommunications Inter-System Operations; EIA/TIA-553-1989 "Cellular System Mobile Station - Land Station Compatibility Standard"; EIA/TIA-557; EIA/IS-54B. Met Roaming The Mobile Telephone Cellular Roaming Service (MTCR) supplements cellular service, providing access where there is no cellular coverage. The "home" Mobile Switching Center (MSC) of the multimode MET, as defined in EIA/IS-41B, is either the terrestrial cellular mobile carrier (CMC) system or the satellite network system. The MET registers as a "visitor" in either the satellite MSC or a terrestrial cellular system MSC per the requirements of EIA/IS-41B. The visitor registration sequence is provided in FIG. 15. The gateway provides automatic roaming for METs outside the range of terrestrial cellular coverage in accordance with EIA/IS-41B. METs are identified with the same 10-digit telephone number in the terrestrial cellular and satellite networks. In the idle state, a mobile unit monitors the cellular and satellite signaling channels. The normal cellular procedure is used for terrestrial calls as defined in EIA/TIA 557. Each MET uses the cellular terminal ESN (electronic serial number) and the telephone number for the purposes of identification and registration on the CMC. Upon power up, the MET registers per the requirements of FIG. 16. If unsuccessful, it registers in accordance with the secondary selection, if applicable. If the mobile is in the coverage area of selected preferential service, the MET will not attempt to register as a roamer in another system until the MET detects the absence of preferential coverage. At that time, the MET attempts to register on the secondary coverage system as a roamer. If the MET is registered in a secondary coverage system as a roamer, and detects the availability of preferential coverage, it attempts to register (reregister) with the preferential system. However, once a call is established on the satellite system, it remains on the satellite system until completion. Reregistration only occurs after a suitable time delay at the MET to avoid constantly switching between networks. For all reregistrations, a suitable time delay is defined as follows. The MET, upon the loss of a primary service (satellite or cellular) waits, for example, a nominal 6 seconds before attempting to register in the alternate service (cellular or satellite). If the primary service is recovered prior to the expiration of the 6 second delay, the mobile will not attempt reregistration. When registered on the alternate service, the MET will wait a nominal 6 seconds, and will then continuously monitor the availability of the primary service. When the primary service becomes satisfactorily available, the MET will attempt to return the registration to the primary service. If the primary service is subsequently lost again, the MET will wait a nominal 15 seconds before attempting to register in the alternate service. If the primary service is recovered prior to the expiration of the 15 seconds delay, the mobile will not attempt reregistration. When registered on the alternate service, the MET will wait a nominal 15 seconds, and will continuously monitor the availability of the primary service. When the primary service becomes satisfactorily available, the MET will attempt to return the registration to the primary service. If the primary service is subsequently lost again (a third or more times), the MET will wait a nominal 30 seconds before attempting to register in the alternate service. If the primary service is recovered prior to the expiration of the 30 second delay, the mobile will not attempt reregistration. When registered on the alternate service, the MET will wait a nominal 30 seconds and will then continuously monitor the availability of the primary service. When the primary service becomes satisfactorily available, the MET will attempt to return the registration to the primary service. Once the MET has invoked any reregistration, a 5 minute timer will be started. The timer will be reset to 5 minutes for each reregistration. If the 5 minute timer expires, the reregistration delay will be set to a nominal 6 seconds and the cycle will start over again. If both services are lost, the MET will continuously monitor both services, and will attempt to register (reregister) on whichever service becomes satisfactorily available. Met Data Call FIG. 17a is a basic block diagram of the functions of the mobile earth terminal including the digital terminal equipment functions. The calling procedures for MET initiated data calls permit standard data terminal equipments (DTEs) connected to METs to place 2400 bps and 4800 bps data calls to appropriately equipped subscribers of the PSTN or members of private networks, similar to a data call by a standard modem. As discussed below, however, additional functions are required to effectuate the data call in the MET environment. The 2400 bps mode has a fall back rate of 1200 bps. The frame and data field formats for 1200 bps is identical to that for 2400 bps. The AT command set is a set of commands commonly used for exchange of control information between DTEs and dial up modems. DTEs connected to the MET use a subset of the AT command set to send commands to the MET and receive responses from the MET. The message sequence shown in FIG. 17b used to establish MET originated data calls. The protocol employed is specified in the event tree given in FIGS. 18-19. A data call is initiated by the transmission of an ATD command from the DTE to the MET. The ATD command contains the telephone number of the destination modem/DTE. The ATD command also contains the desired transmissions speed and the character format to be used. The message type field of the access request SU indicates that the call is a data call. Upon reception of the MET channel assignment SU, the MET transmits a scrambling vector SU to the FES via the MET-C channel. This message is continuously repeated until data frames are received from the FES. If no response is received after 5 seconds the MET ceases transmission and signals call failure to the user. The scrambling vector SU contains the initial scrambling vector to be used by the FES on the FES-C channel, the access security check field generated by the MET, and the desired character format and line speed for the connection. Upon successful reception of the scrambling vector SUs, the FES will compare the access security check fields received from the GC and the MET and initiate call release if the fields are not identical. If the access security check fields are identical, the FES will seize a circuit into the PSTN and initiate establishment of the terrestrial portion of the connection. The FES will also switch to data mode and begin transmitting data frames to the MET. Upon reception of the data frames from the FES the MET switches from the call setup frame mode to the data frame mode and continuously transmits data frames to the FES with NULL SUs in the in-band signaling frames until a "connect" SU is received from the FES. For 1200/2400 bps data calls, the MET and FES transmit null SUs in the SU field. Upon receiving data frames from the MET, the FES will begin transmitting ringing SUs to the MET in the in-band signaling field of the data frames. Upon detection of the PSTN going off-hook, the FES will stop sending ringing SUs and will begin transmitting answer SUs to the MET in the in-band signaling field of the data frames. Upon completion of the handshake and bit rate selection procedures between the FES terrestrial modem and the PSTN user modem, the FES will stop transmitting answer SUs and will begin transmitting "Connect" SUs to the MET. Upon receiving a "Connect" SU from the FES, the MET continuously transmits "Connect Acknowledgment" SUs. Upon receiving a "Connect Acknowledgment" SU from the MET, the FES will cease transmitting connect SUs to the MET. For data calls the DTE must be involved in the call release procedure. In all other respects the call release procedures for both MET and PSTN initiated data call release is the same as those specified for MET initiated calls. MET originated call release occurs when the DTE issues an ATH command to the MET. When PSTN initiated call release occurs the MET issues a NO CARRIER indication to the DTE. The sequences for MET and PSTN initiated data call release are shown in FIGS. 20-21. The MET continuously transmits "On Hook" SUs until it receives an "On-Hook Acknowledgment" SU or until timers TM5 or TM7 expire. The data field portion of the data call is used to transport asynchronous data characters as shown in FIG. 22. Each data character byte shall consist of either a 7-bit data character representation with a parity bit or an 8-bit character representation. All data characters are transmitted least significant bit first. If a 7-bit data character representation with a parity bits used, the least significant bit shall contain the parity bit. If a 7-bit data character representation without a variety bits used, the least significant bit contains a 0 bit. Valid data characters are transported in the first L data character bytes of the frame, where L is the value of the length parameter contained in the length indicator bytes. All other data character bytes preferably contain a 10101010 pattern. The format of the length indicator bytes shall be as shown in FIG. 23. The length indicator bytes each contain a 6 bit length parameter that indicates the number of valid data character in the total frame. In addition the length indicator byte contains 2 parity bits used for error detection. The order of transmission of the bits in the length indicator byte is also as indicated in FIG. 24. For PSTN-originated data calls, channel assignment is as specified for PSTN originated voice calls. The GC will determine that the call is a data call and will indicate this in the call type field of the call announcement and MET channel assignment. Upon reception of the MET channel assignment SU the MET transmits a scrambling vector SU to the FES via the MET-C channel. This message is continuously repeated until a response is received from the FES. If no response is received after 5 seconds the MET ceases transmission and signal call failure to the user. Upon successful reception of the scrambling vector SUs, the FES will compare the access security check fields received from the GC and the MET and will initiate call release if the fields are not identical. If the access security check fields are identical, the FES will begin transmitting the ring command SU to the MET. Upon reception of the ring command SU from the FES, the MET signals the MET user either by generating an audible ringing sound or by sending a RING response to the DTE, sets Tm8 to 12 seconds and also transmits the ring command acknowledgment SU until the ATA command is issued by the MET DTE, or upon expiration of timer Tm8. Upon receiving the ring command acknowledgment from the MET, the FES will transmit a call setup complete SU to the GC to notify it that the channel has successfully been established, and will begin transmitting null signal units to the MET. If timer TM8 expires, the MET initiates a call release. When the MET DTE issues the ATA command the MET switches to the data frame mode, stop Tm8, and set timer Tm9 to 20 seconds. Upon detection of the MET switching to data frame mode, the FES will switch to data frame mode and will signal off-hook to the PSTN. The FES will then complete the modem bit rate selection and handshake procedures with the PSTN user modem based on CCITT Recommendation V.22bis. Upon completion of the modem bit rate selection and handshake procedures, the FES will begin continuously transmitting a connect SU to the MET. The FES continues sending the connect SU until a connect acknowledge SU is received from the MET. Upon receiving the connect acknowledge SU, the FES ceases transmitting connect SUs. Upon receiving the connect SU from the FES, the MET stops timer Tm9 and sends the connect acknowledge SU to the FES. The MET ceases transmitting connect acknowledgement SUs within 2 seconds of the time at which receipt of connect SUs ceases. If timer Tm9 expires, the MET initiates a call release. Met Facsimile Call FIG. 25 is a basic block diagram of the functions of the mobile earth terminal including the facsimile interface unit functions. Facsimile interface units (FIU) are installed in the Feeder Link Earth Stations (FES) and in the MET which communicate with each other by a digital communications channel established in the facsimile data mode. These units enable a terrestrial user's CCITT Group 3 facsimile terminal equipment (FTE) to be interconnected with a MET user's CCITT Group 3 FTE (or between two MET users) via the digital satellite system. The FIUs perform two basic functions. First, they demodulate the facsimile voiceband signals in the FTE-to-satellite direction (and remodulate the baseband digital signals in the satellite-to-FTE direction). Second, they perform protocol conversions so that the facsimile protocols become compatible with the transport channel constrains of the basic service configuration. To perform these functions, the FIUs consist of several standard elements including Telephone Tone Generators and Detectors; a CCITT V.21 Modulator and Demodulator; and Control Logic for baseband message formatting, facsimile process control, facsimile protocol conversation, call establishment, call control, and call clearing. The physical interface is preferably characterized using an RJ-11 connector, a 600 ohms signal impedance, a Line Supervision (Detection of Off-Hook and On-Hook), a Minus 24 volt, 30 ma nominal DC source for loop (supervisory) current. The interface preferably provides ringing voltage of 86 Vrms and support up to 5 ring loads, or provides a source over the linear range from 64 Vrms at 50 ms down to a minimum of 40 Vrms at 100 ma. The required generated signaling tones are:
______________________________________
[480 + 620 Hz (0.25 sec's on and 0.25 sec's off)]
Busy [480 + 620 Hz (0.5 sec's on and 0.5 sec's off)]
Ring-back
[440 + 480 Hz (1 sec on and 1 sec off)]
Dial [Reference CCITT E.180:350 + 440 Hz (continuous) -
10 Dbm0 nominally]
______________________________________
The FIUs transmit and receive digital signals to an from the satellite 2.4 kbit/ss Data channel in blocks of 288 bits as shown in FIG. 26, which illustrates the sub-field structure of the data channel. In addition, the FIU internally partitions each of the 288-bit data-field into 36, 8-bit data cells. The Digital Facsimile Protocol provides line-state indication by means of the following messages, carried as "line control packets", which are transmitted at full rate (i.e., 2.4 kbit/s) over the data channel. The coding is described below:
______________________________________
Line state
______________________________________
0001 Idle
0010 CED Connection
0100 Spare
0111 Binary Coded Signal Connection
1000 FIU Capabilities Control Packet
1011 Synchronizing Signal Connection
1101 Preamble Connection
1110 Message Connection
______________________________________
The coding is associated with the voiceband signal states as shown below:
______________________________________
Indication Line State
______________________________________
Idle No signal on the telephone circuit
CED Connection
2100 Hz Called Station Identification
(CED) signal on the telephone
circuit
Binary Coded Sig.
300 bit/s (non-permeable) binary
Connection coded procedural signal on the
telephone circuit
Synch. Signal
Modem synchronizing (or
Connection training signal on telephone circuit)
Preamble 300 bit/s binary coded preamble signal
Connection on the telephone circuit
Message Connection
Facsimile message on the telephone
circuit
______________________________________
The special line state "FIU Capabilities Control Packet" is not associated with an analog line state but with in-band signaling between FIUs. Line control packets are generated whenever a line state transition occurs, and generally always precede the transmission of information (associated with the new line state) over the digital channel. The indication in the line control packet applies to all associated 8-bit data cells of the satellite channel that immediately follow it until a new line control packet is generated. Hence, these line control packets are used a headers of new information. All non-preamble signals of the 300 bit/s binary coded procedural signalling, which are specified in CCITT Recommendation T.30, are transferred to the re-modulating (distant) FIU in the form of a demodulated baseband digital data stream. A re-modulating (or modulating) FIU is defined as the FIU which is receiving data from the satellite channel for modulation and transmission to the customer FTE. A demodulating FIU is defined as the FIU which is receiving data from the customer FTE for demodulation and transmission to the satellite channel. During a call, an FIU will perform both modulating and demodulating functions, as the signal direction between the end-customer FTEs will change several times. The facsimile message signal (including the TCF signal) is also transferred to the re-modulating FIU in the form of a demodulated digital data stream. Reception of the modem synchronizing signal on the telephone circuit is indicated by the demodulating FIU to the re-modulating FTU by transmitting the "Synchronizing Signal Connection" line control packet. The FIU at the modulating end generates a modem synchronizing signal according to this indication. Detection of the CED signal on the telephone circuit is indicated by the demodulating FIU to the re-modulating FIU by transmitting the "CED Connection" line control packet. (The actual CED signal cannot be transferred to the re-modulating FIU since it is not digital by nature.) The tonal signalling procedures defined in CCITT Recommendation T.30 are not generally accommodated by the Digital Facsimile Protocol. The Group 3 procedures recommended by CCITT are generally supported by the FIU. Therefore, the FIU is not required to detect the use of tonal signalling procedures. Eventually a Group 1 or Group 2 FTE should clear the call on the analog circuit when it does not receive proper responses from the FIU. Line control packets are generated whenever a line state transition occurs, and generally always precede the transmission of information (associated with the new line state) over the digital channel. Because these packets are transmitted in-band over the 2.4 kbit/s data channel, they are generated by the demodulating FIU in the FTE-to-satellite direction and removed by the remodulating FIU in the satellite-to-FTE direction. The first bit of the line control packet must be coincident with a data cell boundary. When generating line control packets these are constructed by utilization of 18 data calls as follows: The first 9 data calls (72 bits) are comprised of nine repetitions of the "11111111" binary octet and are used as a line control preamble to indicated that the following 9 data cells contain line state transition information. The next 8 data cells (64 bits) are comprised of 16 repetitions of the appropriate 4-bit code for the new line state. The final data cell (8 bits) comprises the unique binary word "11000011" which is used to precisely align the user data associated with the new line state. The new line state indicated by a received line control packet does not become effective until the last bit of the line control packet has been received on the satellite channel. Consequently, since line control packets are 18 data cells long, a line control packet inserted into the satellite data stream effectively adds 60 ms to the length of the previous line state. This is true for receipt of all line control packets except an "Idle" packet when it has been preceded by either a "Binary Coded Signal Connection" packet or a "Message Connection" packet. In either of those two cases, the "Idle" line control packet must be immediately processed (removed from the satellite data stream) when the first, rather than the last bit of the packet is received, thus avoiding a situation where 60 ms of invalid data would be modulated at either the V.21 or V.27ter mode. To accomplish this "early line control packet detection," the FIU must incorporate a look-ahead buffer between the satellite channel and the FIU software in which it can look ahead 60 ms into the received satellite data stream and identify a valid "Idle" line control packet while in the "Binary Coded Signal Connection" or "Message Connection" line state. Since, in the special situations, the "Idle" line control packet is removed all at once (instead of removing it over the course of 18 data cell times), the look-ahead buffer will shrink in size as "Idle" line control packets are detected and removed. When the FIU switches from its "modulating" role to its "demodulating" role, the look-ahead buffer may be empty due to the removal of "Idle" line control packets. The buffer must already be filled with satellite data by the time an FIU switches from the "demodulating" role to the "modulating" role. This filling can take place when the FIU is likely to be receiving idle fill over the satellite when it is in the "demodulating" role. Furthermore, the look-ahead buffer must accommodate two line control packets since, at most, there will be two transitions from "Binary Coded Signal Connection" or "Message Connection" line state to "Idle" line state during a single line turnaround of the half-duplex T.30 facsimile protocol. That is, the buffer must be about 288 bits in size. This will add, at most, 120 ms to the overall delay through the FIU. The facsimile interface unit (FIU) interworks with a user's CCITT Group 3 facsimile terminal equipment (FTE) through a telephone circuits in accordance with CCITT Recommendation T.30, and with the FIU at the distant end through the satellite channel in accordance with the Digital Facsimile Protocol. To accomplish this, the FIU shall comprise the following standard circuit elements: CCITT Recommend. V.27 ter [R-27] voice-band data modem; CCITT Recommendation V.21 [R-28] voice-band data modem; 2100 Hz tone generator and detractor; 1100 Hz interrupted tone generator and detector; processor/controller; and elastic buffer, multiplexer/demultiplexer, and interface with the satellite channel unit. After the FIUs have been switched-in by the FES or the MET in a remote-originated mode (FTE at the opposite FIU has originated the call), the FIU generates the CNG signal (a 1100 Hz interrupted tone) and transmits it to the called user. The CNG signal is terminated following the detection of either the CED signal or a 300 bit/s binary coded procedural signal on the incoming telephone circuit in accordance with the procedures defined in CCITT Recommendation V.25 [R-29]. The characteristics of the CNG signal comply with the relevant requirements specified in CCITT Recommendation T.30. The called FTE may transmit the CED signal following the connection of the telephone circuit at the called end. The called FIU shall detect the onset and termination of the CED signal on the telephone circuit and shall inform the calling FIU of this e vent by inserting the "CED Connection" line control packet in the satellite channel. The CED signal detector response threshold and times shall be in accordance to CCITT Recommendations T.4, T.30, and V.25 [R.29]. The insertion of the "CED Connection" line control packet in the data channel occurs in the first data-cell to be transmitted after the CED detector output has changed from OFF to ON. The "CED Connection" line state is changed to "Idle" by transmissions of the "Idle" line control packet in the data channel as soon as the ON-to-OFF transition of the CED signal is detected and a data-cell boundary becomes available, but subject to the restriction that a "Idle" signal gap of 75.+-.20 ms must be inserted (i.e. 22.+-.6 data cells) between the CED signal termination and the following signal (DIS) which is encoded according to the V.21 modulation scheme. That is, there must be at least one data cell between the "Idle" line control packet and the subsequent "Preamble Connection" line control packet (since transmission of the "Preamble Connection" packet contributes 60 ms to the length of the "Idle" line state). It is the responsibility of the FIU to be aware of the (possibly different) on-to-off and off-to-on signal detect times for CED and V.21 Preamble and adjust the generated satellite line control packets accordingly so as to insure a 75.+-.20 ms idle time between different signals. The called FTE transmits the CED signal following the connection of the telephone circuit at the called end. The called FIU detects the onset and termination of the CED signal on the telephone circuit and informs the calling FIU of this event by inserting the "CED Connection" line control packet in the satellite channel. The CED signal detector response threshold and times are in accordance to CCITT Recommendations T.4, T.30, and V.25 [R-29]. The insertion of the "CED Connection" line control packet in the data channel occurs in the first data-cell to be transmitted after the CED detector output has changed from OFF to ON. The "CED Connection" line state shall be changed to "Idle" by transmission of the "Idle" line control packet in the data channel as soon as the ON-to-OFF transition of the CED signal is detected and a data-cell boundary becomes available, but subject to the restriction that a "Idle" signal gap of 75.+-.20 ms must be inserted (i.e. 22.+-.6 data cells) between the CED signal termination and the following signal (DIS) which is encoded according to the V.21 modulation scheme. That is, there must be at least one data cell between the "Idle" line control packet and the subsequent "Preamble Connection" line control packet (since transmission of the "Preamble Connection" packet contributes 60 ms to the length of the "Idle" line state). The FIU detects the (possibly different) on-to-off and off-to-on signal detect times for CED and V.21 Preamble and adjusts the generated satellite line control packets accordingly so as to insure a 75.+-.20 ms idle time between different signals. A CED signal is generated by the called FTE after the telephone circuit has been connected between the called FIU and the called FTE. The onset and termination of the CED signal on the telephone circuit are detected by the called (demodulating) FIU and signalled to the calling (re-modulating) FIU by means of line control packets. The calling FIU shall generate a CED signal (2100 Hz tone) and shall transmit it to the calling FTE when the line control packet indicates "CED Connection". The OFF-to-ON instruction to the tone generator shall occur only after the entire "CED Connection" line control packet has been received. The ON-to-OFF instruction to the tone generator occurs when a new line control packet has been detected by the called FIU. Ideally, the "Idle" line control packet will be received following the "CED Connection" packet, but errors on the satellite channel may corrupt the "Idle" packet so that the "Preamble Connection" packet may be the next one detected. The characteristics of the generated 2100 Hz tone shall comply with the relevant requirements for the CED signal specified in CCITT Recommendations T.30, T.4, and V.25. The FIU receives and demodulates the binary coded procedural signals which are defined in CCITT Recommendation T.30 and which appear on the incoming telephone circuit. The binary coded procedural signals (except the TCF signal) are modulated at 300 bit/s in accordance with the CCITT Recommendation V.21 modulation system and are preceded by the preamble (sequence of repeated flags) specified in CCITT Recommendation T.30. The TCF signal is modulated at 2.4 kbit/s in accordance with CCITT Recommendation V.27 ter and is preceded by the modem synchronizing signal. The data is transmitted over the satellite channel in the same order as received over the telephone circuit (i.e., the first bit received over the telephone circuit by the demodulating FIU is the first bit to be sent over the satellite channel to the re-modulating FIU). The non-preamble portion of the V.21 modulated 300 bit/s binary coded procedural signal is defined as the portion of this signal which begins with the first non-Flag octet and ends with the last Flag received after the last HDLC frame. The preamble portion of the 300 bit/s binary coded procedural signal is defined as the portion of the V.21 modulated signal which begins with the first Flag and ends with the last Flag preceding the non-preamble portion (i.e. ends with the Flag preceding the first non-Flag octet of the binary coded information field). This definition is illustrated in FIG. 27. The demodulated data stream of the non-preamble portion of the 300 bit/s binary coded procedural signal is transmitted to the re-modulating FIU regardless of the error state of the signal. The binary coded Address, Control, and HDLC information fields of the demodulated data (with the exception of the NEF, DIS, and DTC signals) are not generally manipulated by the demodulation process and the FCS field indicates an error condition, the regenerated FCS sequence must be such that it will result in an error condition being detected by the distant FTE, assuming subsequent error-free transmission. Transmission of the data shall be immediately preceded by the transmission of the "Binary Coded Signal Connection" line control packet. When the demodulating FIU manipulates the DIS, DTC and DCS signals in the manner described in the preceding paragraphs, a new Frame Checking Sequence (FCS) must be computed. However, if the original (prior to manipulation) signal indicates an FCS error, the sequence computed must be such that it will result in an error condition being detected by the distant FTE assuming subsequent error-free transmission. The demodulated, non-preamble data stream of the 300 bit/s signal is transmitted to the re-modulating FIU over the satellite channel, preceded by the "Binary Coded Sequence Connection" line control packet. The line control packet is delayed by the same amount as that introduced in the preamble in paragraph 10 above and is further aligned to make the first bit of the line control packet coincident with the first bit of a data cell. When reception of the 300 bit/s signal ceases, an "Idle" line control packet shall be inserted in the satellite channel starting at the next available data cell. The response time for the Synchronizing/TCF signal detection is in accordance to CCITT Recommendations T.4, and V.27ter. The onset of the synchronizing portion of the synchronizing/TCF signal is signalled to the distant FIU by transmitting the "Synchronizing Signal Connection" line control packet. The transmission of the "Synchronizing Signal Connection" line control packet, signifying the transition from the "Idle" to the "Synchronizing Signal Connection" line state, occurs in the first data cell of the satellite data channel after a period of 75.+-.20 ms of idle activity has been transmitted over the satellite channel following the end of the previous (DCS) signal which was originally encoded according to the V.21 modulation scheme. That is, there must be at least one data cell between the "Idle" line control packet and the subsequent "Synchronizing Signal Connection" line control packet (since transmission of the "Synchronizing Signal Connection" packet contributes 60 ms to the length of the "Idle" line state. The FIU detects the (possibly different) on-to-off and off-to-on signal detect times for V.27 ter and V.21 and adjusts the generated satellite line control packets accordingly so as to insure a 75.+-.20 ms idle time between different signals. When the line control packet indicate "Synchronizing Signal Connection", the data stream sent to the distant FIU includes a binary all "zero" sequence irrespective of the demodulated sequence. The "Synchronizing Signal Connection" line state continues until local V.27ter modem training is complete and the first bit of TCF data is demodulated by the local V.27ter modem. The signal activity detector does not detect Segment 2 of the V.27ter training sequence ("No transmitted energy") as the end of the modem Turn-On sequence. Thus, the Synchronizing Signal Connection line state will apply for at least 225 ms (regardless of the signal activity detector output state). The demodulated and unscrambled TCF signal (originally a series of binary "zeros") is transmitted to the re-modulating FIU over the satellite channel, preceded by transmitting the "Message Connection" line control packet. To compensate for the possibility that the demodulating V.27ter modem interprets the end of the modern Turn-On sequence as TCF data, the FIU insures that the first 6 data cells (48 bits) of the TCF data transmitted over the satellite consist of binary "zeros", no matter what was actually demodulated by the V.27ter modem. The "Message Connection" line control packet and subsequent TCF data is delayed by the same amount as that introduced to the "Synchronizing Signal Connection" line control packet defined in paragraph 17 above, and is further aligned to make the first bit of the line control packet coincident with the first bit of the data cell. The end of the TCF signal is determined by the FIU, and the tail of the signal is discarded if the number of bits is insufficient to fill the data cell. An "Idle" line control packet shall then be transmitted to the distant FIU in the following data cell. The re-modulating FIU receives the data stream for the 300 bit/s binary coded procedural signals or the TCF signals (preceded by the "Binary Coded Signal Connection" or "Message Connection" line control packet, respectively) which are transmitted over the satellite by the demodulating FIU. The re-modulating FIU generates the voice-band data signals for the non-preamble portions of the binary coded procedural signals based on the data stream and the relevant line control packet, and transmits them to the FTE on the outgoing telephone circuit. In generating the modulated signals, no manipulation of the data is performed by the re-modulating FIU. If a new Frame Checking Sequence (FCS) is regenerated, and the original signal indicates an FCS error, the sequence computed must be such that it will result in an error condition being detected by the FTE, assuming subsequent error-free transmission. The modulation applied to the data which have been preceded by the "Binary Coded Signal Connection" line control packet are at 300 bit/s and in accordance with the CCITT Recommendation V.21 modulation system (as specified in CCITT Recommendation T.30). The entire data, after removal of the line control packet, are transmitted to the FTE. The transmission of the 300 bit/s modulated binary coded signal preamble commences immediately after reception of a valid "Preamble Connection" line control packet and continues for as long as no new line state transition in the form of a valid line control packet is received. The Demodulating FIU makes sure that there is at least a 75.+-.20 ms gap between the "Preamble Connection" line state and the preceding line state. The re-modulating FIU must insure that the length of the "Idle" line state indicated on the satellite is correctly reproduced on the analog circuit. If, due to channel errors, the "Preamble Connection" line control packet was not preceded by an "Idle" packet, then the re-modulating FIU must insure that there is 75.+-.20 ms of idle analog line state before the flag sequence begins. The transmission of the non-preamble 300 bit/s modulated binary coded (non-preamble procedural signals is initiated following the recognition of a line state change from "Preamble Connection" to "Binary Coded Signal Connection" signalled by the reception of a valid "Binary Coded Signal Connection" line control packet from the satellite data channel. The transmission of the non-preamble portion of the 300 bit/s modulated binary coded procedural signals follow the termination of the preamble with no interruption of signal energy on the outgoing telephone circuit. Furthermore, in order to preserve the octet integrity of the preamble being transmitted the 300 bit/s modulated binary coded procedural signals may additionally be delayed by a period of up to 8 data cells. The transmission of the non-preamble 300 bit/s modulated signals continues until a valid "Idle" line control packet is received from the satellite channel. The FIU "looks ahead" in the received satellite data stream to identify the "Idle" line control packet as soon as its first bit has been received. The "Idle" line control packet, once so identified, will be immediately removed from the satellite data stream and will not contribute an additional 60 ms of time to the existing "Binary Coded Signal Connection" analog line state. In generating the TCF signal, the modulation shall be in accordance with CCITT Recommendation V.27ter. When the most recently received line control packet indicates "Synchronizing Signal Connection", a modem synchronizing signal (the long sequence with protection against talker echo as specified in CCITT Recommendation V.27 ter) shall be generated by the re-modulating FIU. Transmission of the modem synchronizing signal is initiated only after the line state change from "Idle" to "Synchronizing Signal Connection" has been detected by the reception of a valid "Synchronizing Signal Connection" line control packet. If the Synchronizing Signal has been preceded by a signal encoded according to the V.21 modulation scheme in the same direction of transmission(i.e. DCS), the demodulating FIU has made sure that the duration of the "Idle" line state between the last flag of the "DCS" message and the start of the "Synchronizing Signal Connection" line state is at least 75.+-.20 ms. The re-modulating FIU must insure that the duration of this "Idle" line state is accurately reproduced on the analog circuit. If, due to channel errors, the "Synchronizing Signal Connection" line control packet was not preceded by an "Idle" packet, then the re-modulating FIU insures that there is 75.+-.20 ms of idle analog line state before the flag sequence begins. The synchronizing signal is followed, without an interruption of signal energy, by a signal modulated by the received TCF data (after removal of the "Message Connection" line control packet which will take an additional 60 ms). The received data is delayed as appropriate, to allow the first effective bit of data preceded by the "Message Connection" indication to coincide with the first bit which follows the synchronizing signal in the modulated signal. Ideally, the FIU will insure that data consisting of binary "zeros" is modulated in the event that v.27ter modem training completes before the "Message Connection" line control packet is received over the satellite. This situation could occur because of the tolerance allowed in V.27ter for the modem Turn-On sequence. Once the "Message Connection" line control packet is received, TCF data is modulated without modification by the FIU. The TCF signal is almost immediately terminated upon reception of a valid "Idle" line control packet from the satellite channel. The FIU "looks ahead" in the received satellite data stream to identify the "Idle" line control packet as soon as its first bit has been received. The "Idle" line control packet, once so identified, will be almost immediately removed from the satellite data stream and will not contribute an additional 60 ms of time to the existing "Message Connection" analog line state. The demodulating FIU receives and demodulates facsimile message signals modulated at 2400 bit/s using the CCITT Recommendation V.27 ter modulation system, as defined in CCITT Recommendation T.4. Each transmission of a message is preceded by the modem synchronizing signal specified in the CCITT V.27 ter. The onset of the synchronizing signal portion of the synchronizing facsimile message signal on the incoming telephone circuit is detected by the demodulating FIU and signalled to the re-modulating FIU by transmitting the "Synchronizing Signal Connection" line control packet over the satellite data channel in the first available data cell. When CCITT V.27ter modem training has completed, the demodulating FIU inserts a "Message Connection" line control packet in the satellite data stream as soon as the first bit of valid demodulated data is available. Transmission of the demodulated data over the satellite is preferably in the same bit order as it was received on the analog circuit. The demodulated facsimile message is not generally manipulated in any way by the demodulating FIU. The tail of the facsimile message signal is detected by the demodulating FIU. The "Idle" line control packet is then transmitted to the re-modulating FIU. The "Idle" line control packet is transmitted to the re-modulating FIU in the data cell following the last bit of the last HDLC frame containing the last bit message data. If insufficient data is demodulated to fill a complete satellite data cell, it will be padded with binary "zeroes" in order reach a data cell boundary. The re-modulating FIU receives the facsimile message data stream which is transmitted over the satellite channel by the demodulating FIU, preceded by a "Message Connection" line control packet. When the most recently received line control packet indicates "Synchronizing Signal Connection" the long modem synchronizing sequence with protection against talker echo specified in CCITT Recommendation V.27ter is generated by the re-modulating FIU. The re-modulating FIU generates a voice-band data signal for the facsimile message signal, based on the received data stream, and transmits it to the FTE using 2.5 kbit/s modulation in accordance with CCITT Recommendation V.27ter. The synchronizing signal is followed, without an interruption of signal energy, by a signal modulated by the received data, which is introduced by the "Message Connection" line control packet. Modulation of this data may be delayed to allow the first bit of data to coincide with the first bit which follows the modem synchronizing signal. It may also happen that local modem V.27ter training takes less time than that experienced at the demodulating FIU, as indicated by the local V.27ter modem completing its synchronizing phase before the arrival of the "Message Connection" line control packet. In this case, the FIU insures that binary data consisting of all "ones" should be inserted in the modulated data stream as fill until the arrival of the "Message Connection" line control packet. Then, insertion of fill will cease and the relevant data cells received over the satellite will be modulated on the analog circuit starting with the next available bit position in the modulated data stream. The Message signal is almost immediately terminated on completion of the last bit of data as indicated by receipt of the "Idle" line control packet. The FIU "looks ahead" in the received satellite data stream to identify the "Idle" lin | ||||||