Abstract:
A method of enabling a call from a first DTE available via a public switched telephone network (PSTN or ISDN) to a second DTE remotely available over a satellite packet data network includes mapping a destination number to a service group, remote gateway associated with the service group and a connection port of the remote gateway for the second DTE, storing the mapped service group, remote gateway equipment associated with the service group and the port of the remote gateway associated with the second DTE and retrieving the service group, remote gateway equipment associated with the service group and the connection port for the second DTE in response to the entered destination number.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11/117,437 filed on Apr. 29, 2005, and claims priority from U.S. Provisional Application No. 60/566,370 filed on Apr. 30, 2004, which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to satellite communication, and in particular, to communication over mobile telecommunications services such as Inmarsat&#39;s “Broadband Global Area Network” (BGAN).  
       BACKGROUND  
       [0003]     The Inmarsat mobile telecommunications service, Broadband Global Area Network (BGAN) offers packet data, ISDN, and a low-rate voice circuit switched services. Within the packet data services, a BGAN user terminal (UT) may transmit and receive data up to 472 kbps on a shared channel or may choose streaming service on reserved bandwidth at the rate of 32, 64, 128 and/or 256 kbps. For secure voice calls or 9.6 kbps fax calls, BGAN offers a 64 kbps ISDN line. The bandwidth of the aforementioned low-rate voice service is insufficient for fax or secure calls.  
       SUMMARY  
       [0004]     In one general aspect, a method of initiating a secure call between a first secure terminal unit and a second secure terminal unit over a digital transmission link with a narrow bandwidth, the first secure terminal unit being connected to a first secure interface unit and the second secure terminal unit being connected to a second secure interface unit, includes transmitting a switching initiate request from the first secure interface unit to the second secure interface unit, returning a switching acknowledgement from the second secure interface unit to the first secure interface unit in response to the switching initiate request, and transmitting a first control message packet from the first secure interface unit to the second secure interface unit if the switching acknowledgement is received by the first secure interface unit.  
         [0005]     Embodiments may include one or more of the following features. For example, the method may include detecting a first tone signal sent to the first secure interface unit from the first secure terminal unit. Transmitting the control message packet includes transmitting the control message packet in response to the switching acknowledgement if the first secure interface unit detects the first tone signal.  
         [0006]     The method may further include detecting a first tone signal sent to the first secure interface unit from the first secure interface unit and processing the first tone signal by the secure interface unit to produce a secure transmission request bit. Transmitting the control message packet includes transmitting the control message packet in response to the switching acknowledgement if the secure transmission request bit is present.  
         [0007]     The method may also include switching the second secure interface unit from a normal mode to a secure mode after returning the switching acknowledgement from the second secure interface unit to the first secure interface unit. The first secure interface unit switches over from a normal mode to a secure mode in response to the switching acknowledgement.  
         [0008]     As another feature, the method may include sending a regenerated first tone signal from the second secure interface unit to the second secure terminal unit in response to the switching initiate request and detecting the regenerated first tone signal by the second secure terminal unit, receiving a second tone signal sent from the second secure terminal unit to the second secure interface unit in response to the regenerated first tone signal, and transmitting a second control message packet from the second secure interface unit to the first secure interface unit in response to the second tone signal. The method may also include sending a regenerated second tone signal from the first secure interface to the first secure terminal unit in response to the second control message packet.  
         [0009]     As another feature, the method may include detecting termination of the first tone signal by the first secure interface unit, changing the first control message packet to a third control message packet in response to the detected termination of the first tone signal, transmitting the third control message packet from the first secure interface unit to the second secure interface unit, and terminating the regenerated first tone signal from the second secure interface unit to the second secure terminal unit in response to the third control message packet.  
         [0010]     The method may further include receiving a first scramble signal sent from the second secure terminal unit to the second secure interface unit in response to the termination of the regenerated first tone signal, transmitting a fourth control message packet from the second secure interface unit to the first secure interface unit in response to the first scramble signal, regenerating the first scramble signal by the first secure interface unit in response to the fourth control message packet, and sending the regenerated first scramble signal from the first secure interface unit to the first secure terminal unit.  
         [0011]     The method may also include detecting a dropped carrier by the second secure terminal unit after the first scramble signal, changing the fourth control message packet to the third control message packet in response to the dropped carrier, transmitting the third control message packet from the second secure interface unit to the first secure interface unit, and dropping a carrier signal from the first secure interface unit to the first secure terminal unit in response to the third control message packet.  
         [0012]     In addition, the method may include detecting the first tone signal sent from the first secure terminal unit to the first secure interface unit in response to the dropped carrier signal from the first secure interface unit, encoding the first control signal as the first control message packet by the first secure interface unit, transmitting the first control message packet from the first secure interface unit to the second secure interface unit, regenerating the first tone signal by the second secure interface unit in response to the first control message packet, and sending the regenerated first tone signal from the second secure interface unit to the second secure terminal unit.  
         [0013]     The method may also include receiving a second scramble by the first secure interface unit sent from the first secure terminal unit upon completion of the first tone signal, transmitting a fifth control message packet from the first secure interface unit to the second secure interface unit in response to the second scramble signal, regenerating the second scramble signal by the second secure interface unit in response to the fifth control message packet, and sending the regenerated second scramble signal from the second secure interface unit to the second secure terminal unit.  
         [0014]     As a further feature, the method may include transmitting a single sixth control message packet from the first secure interface unit to the second secure interface unit, transmitting demodulated data from the first secure interface unit to the second secure interface unit using a first data frame after transmitting the single sixth control message packet, remodulating the data by the second secure interface unit in response to the single sixth control message packet, and sending the remodulated data from the second secure interface unit to the second secure terminal unit.  
         [0015]     The method may also include receiving the first scramble signal sent from the second secure terminal unit to the second secure interface unit in response to termination of the second scramble signal, sending the fourth control message packet from the second secure interface unit to the first secure interface unit in response to the first scramble signal, regenerating the first scramble signal by the first secure interface unit in response to the fourth control message packet, and sending the regenerated first scramble signal from the first secure interface unit to the first secure terminal unit. A single seventh control message packet is then transmitted from the second secure interface unit to the first secure interface unit to synchronize the transition to transmitting demodulated data in a second data frame. The method may be implemented by hardware, software, or a combination thereof.  
         [0016]     In another general aspect, a secure terminal unit providing secure communication between a local STU-III terminal and a remote STU-III terminal over a digital communication link includes a receiver configured to receive a switching acknowledgement in response to a switching initiate request, an interface configured to receive a tone signal from the local STU-III, a processor configured to produce a transmission request bit in response to the tone signal and to produce a control message packet in response to the switching acknowledgement if the transmission request bit is present, and a transmitter configured to transmit the switching initiate request and the control message packet over the digital communication link.  
         [0017]     In another general aspect, software utilized by a computer, such as, for example, an embedded microprocessor, includes a first code segment producing a secure communication initiate request, a second code segment identifying a secure communication switching acknowledgement in response to the secure communication switching request, a third code segment producing a secure transmission request bit in response to a secure transmission tone signal, and a fourth code segment producing a control message packet acknowledging identification of the secure communication switching acknowledgement if the transmission request bit is present.  
         [0018]     In another general aspect, a method of enabling a call from a first DTE available via a public switched telephone network (PSTN or ISDN) to a second DTE remotely available over a satellite packet data network includes mapping a destination number to a service group, remote gateway associated with the service group and a connection port of the remote gateway for the second DTE, storing the mapped service group, remote gateway equipment associated with the service group and the port of the remote gateway associated with the second DTE and retrieving the service group, remote gateway equipment associated with the service group and the connection port for the second DTE in response to the entered destination number.  
         [0019]     Embodiments may include one or more of the above or following features. For example, the destination number may have ten digits that identify the service group, remote gateway equipment associated with the service group and the port of the remote gateway for the second DTE. The ten digits may be in the format xxx-yyyy-zz such that xxx identifies the service group, yyyy identifies the remote gateway equipment associated with the service group and zz identifies the connection port of the remote gateway equipment.  
         [0020]     As another feature, the method may include authenticating a personal identifier from the first DTE. The authentication may include allocating a line card in response to the authenticated personal identifier and sending a dial-tone to prompt the first DTE for the destination number.  
         [0021]     As a further feature, the method may include checking for an active PDP context session, determining if the active PDP context session has sufficient bandwidth and sending a wakeup message to establish a new session if the active session has insufficient bandwidth. It may also include starting a new PDP context session with the remote gateway and sending a wakeup acknowledgement from to the server on the new PDP context session.  
         [0022]     Another feature may include sending period data to the second DTE to maintain a “keep alive” PDP context session over the satellite packet data network.  
         [0023]     The method may also include sending an invite message to the destination remote gateway, the invite message having the destination number with the service group, remote gateway and connection port, the type of call and the channel identification. A ring signal may be sent to the destination port in response to the invite message and a ring acknowledgement may be sent from the remote gateway to the server. An off hook state of the second DTE may be detected in response to answering the call by the second DTE. In addition, call establishment message may be sent from the remote gateway to the server.  
         [0024]     An on hook termination signal may be received from the first or second DTE to terminate the call and the termination signal may be sent to the other DTE. In additional, the channel may be terminated and the line card freed of resources.  
         [0025]     In another general aspect, a system to enable a call from a first analog communication device on a PSTN to a second analog communication device on a remote gateway with a link to a satellite packet data network includes a remote gateway having at least one port assignable to the first analog communication device, a remote terminal connected to the remote gateway that converts a packetized data stream from the satellite link to a circuit-switched analog voice band signal and converts a circuit-switched analog voice band signal from the remote gateway to a packetized data stream to be transmitted over the satellite link, and a server connected to the PSTN that stores a destination number that corresponds to a service group, remote gateway and connection port for the second analog communication device, converts a packetized data stream from the satellite link to a circuit-switched analog voice band signal and converts a circuit-switched analog voice band signal from the PSTN to a packetized data stream to be transmitted over the satellite link. The server may have a line card to perform data compression and data decompression. The server may also include a call server to perform authentication and call routing.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a general block diagram showing a network configuration according to the present invention.  
         [0027]      FIG. 2  is a block diagram of a secure interface unit (SIU) and a Secure Terminal Unit III (STU-III).  
         [0028]      FIG. 3  is a block diagram showing a switching function of the SIU.  
         [0029]      FIG. 4  is a flowchart illustrating initiation and establishment of a secure call.  
         [0030]      FIG. 5  is a state diagram illustrating detection and/or initiation of a secure call.  
         [0031]      FIG. 6  is a timing diagram of a full duplex call sequence.  
         [0032]      FIG. 7  is a timing diagram of a half duplex call sequence.  
         [0033]      FIG. 8  is a timing diagram of a full duplex to half duplex call sequence.  
         [0034]      FIG. 9  is a diagram of a network configuration according to the present invention.  
         [0035]      FIGS. 10 and 11  are message flow diagrams between a remote gateway and a PSTN. 
     
    
     DETAILED DESCRIPTION  
       [0036]      FIG. 1  shows a general network configuration that includes a Land Earth Station  102  connected to a Mobile Earth Station  104  via a satellite  106  providing a communication link  108 . As shown in  FIG. 1 , a Public Switched Telephone Network (PSTN)  112  connects POTs (Plain old Telephones)  114 , cellular telephones  116  and STU-III terminals  118  to the Land Earth Station  102 . Another STU-III terminal  118  is directly connected to the Mobile Earth Station  104  which may be a mobile unit, for example, on a ship, or an airplane.  
         [0037]     As shown in  FIG. 2 , the STU-III terminal  118  is connected to the SIU  200 . The STU-III terminal includes a handset  202  and a keypad  204 . A separate call button  206  can be depressed to initiate a secure call.  
         [0038]     The SIU  200  includes a digital signal processor  208  connected to the STU-III terminal  118  by a telephone interface  210 . The digital signal processor  208  utilizes a memory or storage device  212  and communicates with a transmit module  214  and a receive module  216  that attach to a terminal  218 .  
         [0039]     Typically, the STU-III terminal  118  is connected to the SIU  200  by a communication jack  220 . In another implementation, the components of the SIU  200  are integrated into a secure terminal unit and/or SIUs can be located at the Land Earth Station  102  or the Mobile Earth Station  104 .  
         [0040]      FIG. 3  shows a block diagram of the switching architecture of the SIU  200 . The telephone interface  210  is connected to a secure interface unit (SIU) protocol function  304 , a switch-in signal (SWIS) detector  306 , and an AMBE+2 encoder  308 . A transmit switch  310  toggles between connecting the output of the SIU protocol function  304  or the output of the ABME+2 encoder  308  to a transmission output terminal  312 .  
         [0041]     For normal voice transmission and other non-secure communication, the transmission switch  310  connects the AMBE+2 encoder  306  to the transmission output terminal  312 . The input to the AMBE+2 encoder is an analog voiceband signal and its output is a baseband data signal. In other implementations, the voice encoder may utilize another algorithm such as, for example, AMBE or AMBE+.  
         [0042]     The SWIS detector  306  monitors the signal from the telephone interface  210 , continuously looking for a 2100 Hz Echo Suppressor Disable (ESD) tone or a 2100 Hz Echo Suppressor/Canceller Disable (ESCD) tone, which tones signal a desire to initiate a secure call. Once either the ESD or ESCD signal are detected, the transmit switch connects the SIU protocol function  304  to the transmission output terminal  312 . The AMBE+2 encoder  308  is disconnected and may cease operation to conserve processing resources.  
         [0043]     A Voice Encoder Parameter Analyzer  314  (VPA) samples the output of the AMBE+2 encoder  308  is an additional input to the SWIS detector  306 . Processing of the ESD or ESCD signals by the AMBE+2 encoder  308  produces an output signal having a predetermined parameter pattern, which corresponds to the ESD/ESCD input at the voice encoder. For example, let the predetermined parameter pattern template be: P 1 , P 2 , . . . Pn, and the AMBE+2 encoder output for the i th  frame is p 1 (i), p 2 (i), . . . pn(i), the weighted error vector will be  
         E   ⁡     (   i   )       =       ∑     j   =   1     n     ⁢       w   j     *                p   j     ⁡     (   i   )       -     P   j            .             
 
 A decision value can be derived based on taking average of E(i) over N frames together with a threshold THD.  
         D     ESD   /   ESCD       =     {           1   ,             if   ⁢           ⁢     1   /     N   ⁡     (       ∑     i   =   0     N     ⁢     E   ⁡     (   i   )         )           &gt;   THD               0   ,         Else               
 
 Where P j &#39;s, w j &#39;s , N and THD are empirical values depending on the specific family member of the AMBE voice coder. 
 
         [0044]     The decision D ESD/ESCD  is incorporated in the SWIS Detector module as a complimentary measure to the detection algorithm, which is based on the input VF signal from the telephone interface. For example, detection of the predetermined parameter pattern causes the VPA  314  to pass a secure transmission request bit to the SWIS detector  306 . Thus, if detection of the ESD or ESCD signals is ambiguous due to weak or poor transmission quality, the SWIS detector  308  can verify that a secure transmission is desired from the VPA&#39;s  314  output of the secure transmission request signal.  
         [0045]     In one implementation, the verification process requires that the SWIS detector  306  detect both the ESD/ESCD signal and the secure transmission request bit. In another implementation, the SWIS detector  306  initiates a secure call if either of the ESD/ESCD signal or the secure transmission request bit is present. In a further implementation, the SWIS detector  306  uses the determination as to whether the secure transmission request bit is present only if the ESD/ESCD signal detection is ambiguous.  
         [0046]     A receive terminal  316  is connected to an AMBE+2 decoder  318 , a switch-in message (SWIM) detector  320 , and the SIU protocol function  304 . A receive switch  322  toggles between connecting either the output of the AMBE+2 decoder  318  or an output of the SIU protocol function  304  to the telephone interface  210 .  
         [0047]     The AMBE+2 decoder  318  produces an analog voiceband signal from a data signal which is received from the satellite. The SWIM detector  320  monitors the data signal at the receive terminal  316 . If the remote STU-III terminal starts to go to a secure mode, the remote SIU will detect the transition and translate ESD/ESCD tones into an Alternate Service Switch-In Message. Detection of the Alternate Service Switch-In Message by the SWIM detector  320  causes the receive switch  322  to disconnect the AMBE+2 Decoder  318  and to connect the SIU protocol function  304 .  
         [0048]     Another VPA  324  monitors the receive terminal  316  for when the remote AMBE+2 is fed with the ESD/ESCD tone signal. The output of the VPA  324  is connected to the SWIS detector  306 . Thus, when the predetermined parameter pattern with the unique characteristic is detected at the receive terminal  316 , the VPA  324  raises a flag bit to the SWIS Detector. When this flag bit is received, the SWIS detector  306  defers its decision to switch to secure mode. This prevents the SIUs at both ends from simultaneously attempting to switch to secure mode thereby causing a double switching problem. Such verification process can be achieved by implementations similar to those described above with respect to the VPA  314 .  
         [0049]     The functions performed by the transmit switch  310  and the receive switch  322  are described in terms of hardware as physical switches, however, the switching operations may be implemented in software or in a combination of hardware and software.  
         [0050]     Operations involved in initiation and termination of the secure call are illustrated in  FIG. 4 . In operation  402 , a user initiates a telephone call in an unencrypted clear voice mode. In operation  404 , the user at either end takes an action to instruct the STU-III terminal to initiate a secure call. In operation  406 , the STU-III terminals enter an initial call/modem training phase in which the modems exchange data rate information and training sequences. Once the modems are trained, the STU-IIIs enter a variable exchange phase in operation  408 . This is followed by a crypto sync/resync phase, in which the two crypto units in the STU-III terminals acquire synchronization to each other in operation  410 . Once this is achieved, the secure communication path between the two STU-III terminals is complete and users may communicate in a secure mode in operation  412 . In operation  414 , the secure connection is terminated and a clear voice mode telephone conversation may continue.  
         [0051]      FIG. 5  shows a state diagram for establishing a secure call from an initial state of operation in a POTS mode. In state  502 , the SIU monitors outgoing voice transmission to detect ESD or ESCD signals. If either the ESD or the ESCD signal is detected in two consecutive error free frames, in state  504  the SIU transmits a secure service switch-in (SWI) message over the satellite link to the remote SIU. The SWI message transmission is then repeated n times by the SIU. For example, the SWI message may be repeated for 10 frames.  
         [0052]     Once the repeat count has expired, the SIU waits for a switch-in acknowledgement (SWA) message in state  506 . If the initiating SIU receives the SWA message prior to expiration of the time out period, the initiating SIU goes into alternative service operation at state  510 .  
         [0053]     If the SIU does not receive the SWA message within a time out period, the SIU sends an alternative service termination (abort) message in state  506 . For example, the abort message may be 114 frames of all ones repeated N3 times. Once the abort message is completed, the initiating SIU goes back to state  502  to monitor outgoing voice transmission for the ESD/ESCD signals.  
         [0054]     In state  502 , the SIU also monitors the received satellite signal for an SWI message from the remote SIU. If a valid SWI message from the remote SIU is detected in two consecutive error free frames, the SIU sends the SWA message N2 times in state  512 . Once the repeat count has expired, the SIU begins alternate service operation in state  510 . The SIU monitors the received signal for an abort message and maintains alternate service operation unless the abort message is received. If the abort message is received, the SIU goes to state  502  and continues monitoring outbound transmission of the ESD/ESCD signals or inbound receipt of the SWI message from the remote SIU.  
         [0055]      FIG. 6  is an illustration of a timing diagram for establishment of a secure call in full duplex interoperable mode. The initiating STU-III generates the 2100 Hz ESD/ESCD signal  601 , which indicates that the user desires to initiate a secure call.  
         [0056]     The SWIS detector at the initiating SIU detects the ESD/ESCD signal with the assistance of the VAP, which analyses and detects a unique output from the AMBE+2 vocoder due to the ESD/ESCD signal input. In response, the initiating SIU transmits an SWI message  602  to the responding SIU. The initiating SIU then remains in voice mode and waits for a response from the responding SIU. The initiating SIU transmits 10 frames of the SWI message and then the initiating SIU changes to a CMP(ESCD) message, which is transmitted in consecutive frames so long as an ESD/ESCD signal is detected.  
         [0057]     The initiating SIU starts a timer 1.3 seconds after transmitting the first SWI-2100 message. At the end of this time period, if the initiating SIU does not receive a response from the remote SIU, the initiating SIU abandons the attempted mode switch and sends an abort signal (not shown) to the initiating STU-III. It then stops the abort signal and returns to voice mode once the ESD/ESCD signal is terminated.  
         [0058]     In response to the SWI-2100 message that the responding SIU receives from the initiating SIU, the responding SIU transmits an SWA message  603 A to the initiating SIU to acknowledge receipt of the SWI-2100 message and regenerates the ESCD signal  603 B which is sent to the responding STU-III. After sending 13 frames of the SWA message the responding SIU takes over control of the AMBE+2 vocoder and operates in secure mode and starts transmitting CMP(Idle) packets to the initiating SIU.  
         [0059]     When the initiating SIU receives the SWA message, it takes over control from the AMBE+2 vocoder and outputs silence  604  to the telephone interface. The initiating SIU changes operating mode from clear communication to secure communication. After completion of the mode switch, the signal sequence proceeds to establish a full duplex secure call.  
         [0060]     The responding STU-III detects the ESCD signal and after 1 second sends a Pseudo 1800 (P1800) signal  605  to the responding SIU. The STU-III can select either an interoperable mode (2.4 kbit/s) or alternate modes (4.8 or 9.6 kbit/s). The STU-III uses the P1800 signal  605  to indicate its attempt to diverge from the interoperable mode. This is done by inserting 3 phase reversals to at 32, 64, and 96 dibit positions in the beginning of the P1800 signal  605 . The responding SIU ignores the STU-III&#39;s indication of its capability of operation modes other than the interoperable mode, because the SIU supports only the interoperable mode.  
         [0061]     The responding SIU demodulates the P1800 signal  605  and detects the dibit data within 80 ms from the start of the P1800 signal  605 . The responding SIU changes the CMP(Idle) packet that it has been transmitting to the initiating SIU to CMP(1800) packets. After receiving the CMP(P1800) packets, the initiating SIU sends a regenerated P1800 signal  607  to the initiating STU-III by modulating the dibit pattern.  
         [0062]     The initiating STU-III terminates the ESC/ESCD tone within 90 ms after receiving the regenerated P1800 signal  607 . The initiating SIU detects the absence  608  of the ESC/ESCD tone and changes the message that it is transmitting to the satellite channel from the CMP(ESCD) to CMP(Idle) message  609 . The responding SIU detects the transition from the CMP(ESCD) to the CMP(Idle) message and terminates the ESCD tone to the responding STU-III  610 .  
         [0063]     Within 150 milliseconds of detecting the end of the ESCD tone, the responding STU-III terminal transmits a SCR1(GPA) signal sequence  611  to the telephone interface. The SCR1(GPA) signal is generated by a 23-bit GPA scrambler that is seeded by a seed vector. After completing the SRC1(GPA) signal, the responding STU-III drops the carrier  612 .  
         [0064]     The responding SIU detects the end of the P1800 dibit pattern and the start of the SCR1(GPA) signal. The responding SIU trains its demodulator equalizer using the SCR1(GPA) signal. In response to the SCR1(GPA) signal, the responding SIU terminates the CMP(1800) packet and starts transmitting a CMP(GPA) packet to the satellite channel  613 .  
         [0065]     The initiating SIU regenerates the SCR1(GPA) signal and sends it to the initiating STU-III terminal  614 . The regenerated SCR1(GPA) is used to train the initiating SIU&#39;s echo canceller and the initiating STU-III terminal&#39;s equalizer.  
         [0066]     When the responding SIU detects the dropped carrier  612  from the responding STU-III, the responding SIU changes the message to the satellite channel from the CMP(GMP) message to the CMP(Idle)  615 . Consequently, the initiating SIU drops the carrier  616  at the completion of the regenerated SCR1-GPA.  
         [0067]     The initiating STU-III detects the loss of carrier at the end of the SCR1-GPA signal and returns an ESD/ESCD tone that is sent to the initiating SIU  617 . The initiating SIU receives the ESD/ESCD tone and transmits a CMP(ESCD) message to the responding SIU  618 . The responding SIU regenerates the 2100 Hz ESCD tone and sends it to the telephone interface  619 .  
         [0068]     Following the ESD/ESCD tone, the initiating STU-III transmits the SCR1-GPA signal to the telephone interface  620 . The initiating SIU demodulates the SCR1-GPA signal and changes the outgoing message to the satellite channel from CMP(ESCD) to CMP(GPC)  621 . The initiating SIU demodulator uses the signal to train its demodulator equalizer.  
         [0069]     The responding SIU regenerates the SCR1-GPC signal and sends it to the responding STU-III  622 . The responding SIU uses this signal to train its echo canceller while the responding STU-III uses the SCR1-GPC signal to train its equalizer. Following the completion of the SCR-GPC sequence, the initiating STU-III enters the Variable Exchange and proceeds to Crypto Sync/Resync and subsequent data phases  623 . The initiating SIU transmits demodulated 2.4 kbit/s data to the satellite channel using the Transparent — 2400 data frame  624 . The initiating SIU transmits inserts one frame of CMP(Sync) message in between the CMP(GPC) and the first Transparent — 2400 frame and adjusts the pointers of its transmit slippage control buffer.  
         [0070]     The responding SIU receives the single CMP(Sync) frame and adjusts the pointers of its receive slippage control buffer. When it receives the Transparent — 2400 data frames, the responding SIU remodulates the received data and sends it to the telephone interface  625 .  
         [0071]     Within 300 ms after the responding STU-III detects the end of the SCR1-GPC signal, the responding STU-III begins transmission of a shorter (704 bits) SCRI-GPA sequence  626 . The responding SIU demodulates the SCR1-GPA signal and transmits it to the initiating SIU as a CMP(GPA) message  627 . The initiating SIU regenerates the SCR1-GPA signal and sends it to the telephone interface  628 . The initiating STU-III uses the short SCR1-GPA to train its demodulator equalizer.  
         [0072]     When the responding SIU detects the end of the SCR1-GPA signal  629 , the responding SIU sends one frame of CMP(Sync), adjusts its transmit slippage control buffer pointers, and then switches to transmission of Transparent-2400 data frames  630 . When the initiating SIU receives the CMP(Sync) frame, it adjusts its receive slippage control buffer. When the initiating SIU receives the subsequent Transparent — 2400 data frames, the initiating SIU remodulates the data and sends it to the telephone interface  631 .  
         [0073]     At this point, establishment of the secure call is completed between the two STU-III terminals. The SIUs continue to demodulate the modem signal from the local STU-III, transmit data through the satellite channel by the Transparent — 2400 data frames, and send the remodulated data to the local STU-III. The SIUs maintain the transparent channel for the STU-IIIs to proceed to subsequent phases of the secure call.  
         [0074]     A training failure may result due to a “hand-up” condition caused by waiting for certain signals and/or messages. To avoid the hand-up condition, a set of time-out timers is utilized to branch the program execution to a predetermined abort state as follows:  
                                           Init/Resp           Failure   SIU   Detection Criteria                   No response to SWI   Init SIU   1.3 sec after sending SWI       Init. ESD/ESCD   Resp SIU   Detect P1800 and CMP(ESCD)       tone lost       before P1800 received       No CMP(P1800) received   Init SIU   3 sec after SWA received       No CMP(GPA) or   Init SIU   2 sec after P1800 received       CMP(Idle) received       after CMP(P1800)       No CMP(Sync) received   Init SIU   6 sec after ending of               CMP(P1800)       No 1st CMP(Idle)   Resp SIU   3 sec after SWI received       received       No CMP(Sync) received   Resp SIU   4 sec after 1st CMP(Idle)       No 2nd SCR1 sequence   Resp SIU   2 sec after CMP(GPC)       received from local       received       STU-III       Lost message sync   Both   10 sec invalid message frame               sync (after sync established)                  
 
         [0075]     The SIUs monitor the modem carrier on the telephone interface for call interruptions. If a loss of modem carrier for 100 ms or more is detected, the SIU immediately drops the modulated carrier to the local STU-III and transmits an Abort message to the remote SIU. After completing the Abort message, the SIU starts the process of returning to clear communication in both directions.  
         [0076]     The SIUs continuously monitors the bidirectional data transferred by the Transparent — 2400 message. If the total number of 1 bits in 48 bits of the Transparent — 2400 message contains less than 1176 (98%) for 25 consecutive messages (1200 bits), then the SIU considers this to be an Abort message and terminates the secure operation as described above.  
         [0077]      FIG. 7  is a timing diagram showing establishment of a half duplex call. The initiating STU-III transmits a Pseudo-1800 Hz (P1800) signal ( 701 ) instead of the ESD/ESCD tone as described by timing signal  601  for the full duplex call establishment. The initiating SIU detects the P1800 signal and transmits out an SWI-1800 message ( 702 ) to the responding SIU.  
         [0078]     Upon receiving the SWI-1800 signal, the responding SIU transmits an SWA message over the satellite channel to the initiating SIU ( 703 A) and sends a P1800 VF signal to the responding STU-III ( 703 B) through the telephone interface.  
         [0079]     After receiving the SWA message ( 703 A), the initiating SIU switches to half duplex secure communication mode and outputs silence to the telephone interface ( 704 ). The initiating SIU continues to monitor the P1800 signal from the STU-III and after transmitting 12 frames of the SWI-1800 message ( 702 ), the initiating SIU transmits CMP(P1800) packets over the satellite channel to the responding SIU ( 705 ).  
         [0080]     After sending 1024 bits of the P1800 signal, the initiating STU-III drops the carrier ( 706 ). When the initiating SIU detects the dropped carrier, it changes the message to the satellite channel from CMP(P1800) to CMP(idle) ( 707 ). The responding SIU detects the CMP(idle) and stops sending the P1800 VF signal to the telephone interface ( 708 ).  
         [0081]     After a period of carrier off for 256 bits or 107 ms, the initiating STU-III restarts the P1800 signal ( 709 ). Upon receiving the P1800 signal ( 709 ), the initiating SIU changes its output message from CMP(Idle) to CMP(P1800) ( 710 ). When the responding SIU detects the change in message from CMP(Idle) to CMP(P1800) ( 710 ), the responding SIU resumes regeneration of the P1800 VF signal ( 711 ).  
         [0082]     After 784 bits of the P1800 signal, the initiating STU-III starts transmitting the SCR-GPC signal ( 712 ), which is used to train the initiating SIU&#39;s equalizer. When the initiating SIU detects the end of the P1800 signal and the start of the SCR1-GPC signal, the initiating SIU changes the message to the satellite channel from the CMP(P1800) signal to the CMP(GPC) signal ( 713 ). When the responding SIU detects the change from CMP(P1800) to CMP(GPC), the responding SIU changes its output to the telephone interface to begin sending a regenerated SCR-GPC signal ( 714 ).  
         [0083]     After sending 1024 bits of the SCR1-GPC signal, the initiating STU-III starts the data section of the half duplex transmission ( 715 ). The data section is encapsulated by SOM and EOM control data patterns at each end of the data section, respectively. When the SOM data pattern is detected, the initiating SIU stops the CMP(GPC) message and transmits a single CMP(sync) message frame ( 716 ) and then transmits Transparent — 2400 data frames to the satellite channel ( 717 ).  
         [0084]     When the responding SIU detects the transition to CMP(sync) message and then to the Transparent — 2400 data frames, the responding SIU stops sending the SCR1-GPC sequence, realigns the pointers of the slippage control buffer, and begins sending remodulated data from the Transparent — 2400 frames to the responding STU-III ( 718 ).  
         [0085]     When the initiating SIU detects the EOM data pattern and loss of carrier from the initiating STU-III ( 719 ), the initiating SIU begins transmitting a CMP(Idle) packet within 20 ms of the dropped carrier ( 720 ). The responding SIU detects the transition from the Transparent — 2400 signal to the CMP(Idle) packet and drops the carrier within 20 ms after completing the EOM data pattern ( 721 ).  
         [0086]     After detecting the loss of carrier following the EOM, the responding STU-III waits for at least 35 ms and then begins to transmit its first half duplex call initiation message with a P1800 signal of 784 bits ( 722 ). The responding SIU detects and demodulates the P1800 signal and changes its outgoing message to the satellite channel from CMP(Idle) to CMP(P1800) ( 723 ). When the initiating SIU detects the CMP(P1800) message from the satellite channel, the initiating SIU sends a regenerated P1800 VF signal to the initiating STU-III ( 724 ).  
         [0087]     Following the P1800 signal, the responding STU-III begins transmission of the SCR1-GPA signal ( 725 ), the SOM data pattern ( 726 ), the data traffic ( 727 ), and the EOM data pattern ( 728 ) to the responding SIU. The responding SIU detects the signal sequence from the telephone interface and the responding SIU begins transmitting the CMP(GPA) ( 729 ), CMP(Sync) ( 730 ), and the Transparent — 2400 data frames ( 731 ) to the initiating SIU. In turn, the initiating SIU regenerates the VF signal ( 732 ) and remodulates the signal with the data traffic ( 733 ) carried by the Transparent — 2400 data frames.  
         [0088]     The responding SIU detects the end of the responding STU-III&#39;s transmission ( 734 ) and changes its outgoing message to the satellite channel from the Transparent — 2400 frame to the CMP(Idle) message ( 735 ).  
         [0089]      FIG. 8  is a timing diagram showing establishment of a full duplex to half duplex call. The full to half duplex call occurs when the Initiating STU-III starts a full duplex call but the Responding STU-III is configured in half duplex mode. The signal sequence is identical to the full duplex call sequence up to  610  as described with respect to  FIG. 6 . Thus,  801  to  810  in  FIG. 8  correspond to  601  to  610  in  FIG. 6 , respectively.  
         [0090]     While being configured in half duplex mode, the Responding STU-III terminates the P1800 signal and drops the carrier after 1024 bits (426 ms) of the P1800 signal ( 811 ) has been transmitted regardless of the state of the ESCD being received by the Responding STU-III. The loss of carrier after the P1800 signal signifies the transition from full duplex to half duplex operation.  
         [0091]     The Responding SIU detects this event and changes the call sequence establishment from a full duplex to a half duplex call. The Responding SIU changes the message transmitted to the satellite channel from CMP(1800) to CMP(Idle) ( 812 ). When the Initiating SIU detects the switch of incoming message from CMP(P1800) to CMP(Idle), the Initiating SIU drops the carrier ( 813 ).  
         [0092]     The call sequence continues as described in the half duplex call sequence as described in  FIG. 7 , with sequence  722  to  735  of  FIG. 7  corresponding to  822  to  835  of  FIG. 8 , respectively.  
         [0093]     In another embodiment, transmission of voice communication can be accomplished over the Inmarsat mobile telecommunications service, “Broadband Global Area Network” (BGAN). BGAN offers packet data, ISDN, and a low-rate voice circuit switched services. Within the packet data services, a BGAN user terminal (UT) may transmit and receive data up to 472 kbps on a shared channel or may choose streaming service on reserved bandwidth at the rate of 32, 64, 128 and/or 256 kbps. For secure voice calls or 9.6 kbps fax calls, BGAN offers a 64 kbps ISDN line. The bandwidth of the aforementioned low-rate voice service is insufficient for fax or secure calls.  
         [0094]     For ISDN service, users are expected to pay around $5.00 USD per minute. For BGAN 32 kbps streaming, the service charge is around US$1.50 per minute. For BGAN packet data on shared channel, the service charge is about $5.00 per mega byte.  
         [0095]     Referring to  FIG. 9 , circuit-switched services can be transmitted over the BGAN packet-switched network. An Inmarsat BGAN terminal  910  provides an access point to the BGAN network. The BGAN terminal  910  provides a communication link  915  to an Inmarsat I-4 satellite  920 , which in turn has a communication link  925  with an Inmarsat satellite access station  930 . A packet data server  935  and a server  940  are connected to the satellite access station  930 . The packet data server  935  is connected to the Internet  945 . The server  940  is connected to the PSTN  950 , which can be used to send voice or fax signals by a telephone  955  or fax machine  960 .  
         [0096]     The BGAN terminal  965  is connected to a remote interface  970 . The remote interface  970  emulates a computer interface to the BGAN terminal  965  with the same signaling and data packet format as does a remote computer. A telephone  975 , fax machine  980  and personal computer  985  are connected to the remote interface  970 . The remote interface  970  provides an access port for a PC user.  
         [0097]     More specifically, the remote interface  970  functions as a bridge or converter between the circuit-switched data terminal equipment (DTE) or peripheral device and the packet-switched service provided by the Inmarsat BGAN network. The DTE is a generic term for communication devices such as the telephone  975 , fax machine  980  and STU-III secure phone, etc., which hook up to an analog telephone line, known as Plain Old Telephone Service (POTS). The remote interface converts the analog voice band signal of DTE to IP network standard packets and transports them over the Inmarsat BGAN network.  
         [0098]     The server  940  terminates the packet switched network, i.e., the BGAN network link and converts the data packets back to an analog signal and transports the analog signal to the circuit-switched public telephone network (PSTN)  950 . The server  940  includes the following components: 1. Call Server—support authentication, IP routing, call control, operation management, and billing functions; 2. IP PBX—provides the PSTN interface, with FXO ports interface via an external gateway device. There is a prepaid module to support the basic calling card functions; 3. Line Card—provides the data compression/decompression function to work with the remote gateway to support fax, secure phone and phone services.  
         [0099]     The system enables users to make secure voice (such as STU-III and FNBDT) and fax calls with the following advantages: a. Substantial cost saving (60% to 70% from the ISDN rate); b. simultaneous calls among voice and fax lines up to the aggregated bandwidth limit allowed. The higher the call density, the cheaper each call gets; c. allow terrestrial (e.g. office or home) calling BGAN UT at substantial savings.  
         [0100]     Each peripheral device  975 ,  980 ,  985  is programmed and associated with a particular remote interface  970 . Calls are routed to the associated remote interface  970  from the BGAN terminal  910  via the Inmarsat SAP (satellite access point)  930 . Upon receiving the call request, the server switches the call through a PBX or a digital switching facility to the public switched telephone network (PSTN)  950 . The server  940  is composed of channel cards that process all the secured voice call or fax calls interface unique to the mobile satellite communications environment. The server  940  can be located in the area where it has low-cost access to the PSTN for international long distance. For large group BGAN users, it may also be feasible to locate the server  940  at the group user&#39;s home headquarters.  
         [0101]     Referring to  FIGS. 10-11 , message flow from the remote gateway to the PSTN and from the PSTN to the remote gateway are described as follows.  
         [0000]     Initial Call—Originating  
         [0000]    
       
          1. Secure Phone/Fax/Telephone Off Hook  
          2. RG collects the user dialed DTMF digits  
          3. After Dialing Timeout, RG build the INVITE message with the DTMF digits collected in the TO field; RTP Payload type is set to identify what type of call it is making; then send the INVITE to Call Server.  
          4. The Call Server receive the INVITE, it select the next available line card and circuit based on the call type (from payload type). Then it forwards the INVITE to the line card selected for processing.  
          5. After receiving the INVITE, the line card Ring the select circuit  
          6. Responding to the Ringing Signal from Line Card, the PBX makes its circuit go Off Hook.  
          7. The Line Card send the DTMF dialing based on the number from the “TO” field.  
          8. The Line Card send “100 TRYING” message response back to Call Server; and the PBX send the PRI “Setup” message to the CO.  
          9. The Call Server receives the “100 TRYING” message, and sends it to the Remote Gateway.  
          10. The Line Card send “180 RINGING” message response back to Call Server.  
          11. The Call Server receives the “180 RINGING” message, and sends it to the Remote Gateway. At this time the PRI connect message is received at the PBX.  
          12. The PBX will change its loop polarity (apply “Reverse Battery” to the analog loop).  
          13. Upon detecting the “Reverse Battery”, the Line Card should send the “200 OK” indicating that the call is connected by the remote PSTN user. At this time, the voice path is open on from the PSTN user to the Line Card; and the Call Server starts the Billing timer.  
          14. The Server receives “200 OK” from the Line Card, and sends “200 OK” to the Remote Gateway. The Remote Gateway should stop the “Ring Back tone”.  
          15. The Remote Gateway should generate the “Reverse Battery” to the phone.  
          16. The Remote Gateway sends an “ACK” message back indicating the call is connected end to end.  
          17. The Call Server receives the “ACK”, and sends the “ACK” to the Line Card.  
          18. The Line Card and Remote Gateway start to exchange the RTP package. The Call Server is acting as the IP package router, and does not process the RTP packets.  
       
     
         [0120]     Disconnect Call—Originating 
    19. The remote user terminating the call by putting the phone On-Hook.     20. The Remote Gateway send the “BYE” message to the Call Server     21. The Call Server receives the “BYE” message, it stops the Billing timer, and generates the CDR for the call; then sends the “BYE” message to the Line Card.     22. The Line Card performs the “Power Deny” to remove the power from the loop indicating the “call disconnect” event has arrived.     23. The Line Card sends “200 OK” to the Call Server; and the PBX sends the “PRI Disconnect” message to the PSTN.     24. The Call Server sends “200 OK” to the Remote Gateway.    
 
         [0127]     Initial Call—Terminating 
    1. IP PBX Receives the PRI SETUP message indicating a PSTN incoming call.     2. IP PBX Response with PRI CONNECT message to answer the call.     3. The Voice Path open between PSTN user and IP PBX PRI Trunk.     4. The Prepaid Module IVR starts to collect the Account Number, PIN Code, and Dialing number.     5. IP PBX select next available Trunk (based on the predefined numbering rule), the FXO port Off-Hook     6. Line Card detects the Off-Hook, sending Dial tone     7. IP PBX start the DTMF Dialing for destination number     8. Line Card collects the Dialing number, constructs “INVITE” message with Dialing number in the “TO” field.     9. Call Server receives the INVITE, analysis the Dialing number, identify the Remote Gateway; it start the “Wake Up” process via background IP service.     10. Remote Gateway receives the “Wake Up” message, response it with “Wake Up Ack” via background IP; then it starts a Streaming-32K call.     11. The Call Server receives the Streaming request.     12. The Call Server establishes the IP route for the Streaming connection.     13. The Call Server sends the “INVITE” message to the Line Card via the Streaming connection just established.     14. The Remote Gateway receives the “INVITE” and Ring the proper port.     15. The Remote Gateway responses with “100 TRYING” messages     16. The Call Server sends “100 TRYING” to the Line Card.     17. The Remote Gateway sends the “180 RINGING” messages.     18. The Call Server sends “180 RINGING” message; the Phone get Off-Hook     19. The Remote Gateway detects the Off-Hook, and sends “200 OK”, indicates the call is connected.     20. The Server sends the “200 OK” message to the Line Card     21. The Line Card responses with an “ACK” message to Call Server     22. The Line Card applies “Reverse Battery” to the analog loop, indicates the line is connected.     23. The Call Server starts the Billing.     24. RTP messages start exchange between Line Card, and Remote Terminal.    
 
         [0152]     Disconnect Call—Terminating 
    25. Remote PSTN user disconnects the call, via PRI DISCONNECT message.     26. IP PBX sends On-Hook to the analog loop; the Call Server stops the Billing.     27. Line Card sends the “BYE” message indicating remote disconnect request is received.     28. The Call Server sends the “BYE” message.     29. The Remote Gateway receives the “BYE” message, and applies “Power Deny” to the analog loop.     30. The Remote Gateway receives On-Hook, and send the “200 OK” message to Call Server. 
 
 The Call Server sends “200 OK” message to Line Card, and mark the port at Remote Gateway to be available. 
   
 
       DEFINITIONS &amp; ACRONYMS  
       [0159]     Acronyms, which may be used throughout the Specification, represent the following terms:  
         [0160]     ACKmsg Acknowledgement message  
         [0161]     Alternate Mode STU-III 4800/9600 bits/s operation mode  
         [0162]     SWImsg Alternate Service Switch-in message  
         [0163]     AST Alternate Service Transport  
         [0164]     Dibit Two bit sequence in QPSK modulation  
         [0165]     EOM End of Message  
         [0166]     ESCD Echo Suppressor/Canceller Disable  
         [0167]     ESD Echo Suppressor Disable  
         [0168]     FSVS Future Secure Voice System (STU program)  
         [0169]     GPA Modem scrambler for Initiating STU-III  
         [0170]     GPC Modem scrambler for Responding STU-III  
         [0171]     Interoperable mode STU-III 2400 bit/s operation mode  
         [0172]     KMC Key Management Center  
         [0173]     KSps Kilo Symbol per second  
         [0174]     LES Land Earth Station  
         [0175]     MES Mobile Earth Station  
         [0176]     NACKmsg Negative Acknowledgement Message  
         [0177]     NSA National Security Agency  
         [0178]     POTS Plain Old Telephone Service  
         [0179]     PSTN Public Switched Telephone Network  
         [0180]     QoS Quality of Service  
         [0181]     SCR1 Scrambled ones  
         [0182]     SDM Inmarsat System Definition Manual  
         [0183]     SOM Start of Message  
         [0184]     STU-III Secure Terminal Unit III  
         [0185]     VF Voice frequency  
         [0186]     Definitions for various message types and terms used throughout the Specification are defined as follows:  
         [0000]     1. Voice/SCIB Switching Message Format  
         [0187]     This category includes three messages: 
        Full duplex secure service switch-in Indication (SWI-2100) message     Half duplex secure service switch-in Indication (SWI-1800) message     Acknowledgement (SWA) message 
 
 2. SWI-2100 Message Format 
       
 
         [0191]     The SWI message format is based on AMBE+2 encoded speech data frame format. The SWI-2100 shall be used by the SWIS detector to initiate the switch-in to a full duplex secure call when the 2100 Hz ESD/ESCD tone is detected from the telephone interface. The Initiating SCIB shall transmit 10 frames of SWI-2100 message.  
         [0000]     3. SWI-1800 Message Format  
         [0192]     The SWI-1800 message format is based on AMBE+2 encoded speech data frame format. The SWI-1800 shall be used by SWIS detector to initiate the switch-in to a half duplex secure call when the 1800 Hz signal is detected from the telephone interface. The Initiating SCIB shall transmit 12 frames of SWI-1800 messages.  
         [0000]     4. SWA Message Format  
         [0193]     The SWA message format is based on AMBE+2 encoded speech data frame format. The SWA message is used to acknowledge both the SWI-2100 and SWI-1800 messages by the Responding SCIB. The SWA message shall be transmitted consecutively for 13 frames.  
         [0000]     5. Detection Criteria  
         [0194]     The detection criteria for SWI-2100, SWI-1800 and SWA are the same: The message is considered detected when two consecutive error free frames are received.  
         [0000]     6. ABORT Message Format  
         [0195]     The ABORT message is the Alternative Service Termination message. It consists of a 114 frames of all ones. The SCIB shall output silence on the VF port while transmitting and receiving the ABORT message. The detection criterion of the ABORT message is a minimum of 98% of continuous ones received in a 25 consecutive frames (500 ms). Following the detection of ABORT message, the SCIB shall wait until the end of the message and the drop of modem carrier before returning to clear communication mode.  
         [0000]     7. SCIB Protocol Control Message Format  
         [0196]     The SCIB protocol control messages are under the general structure of Control Message Packet (CMP). Each CMP occupies one voice frame (20 ms), and consists of 6 octets.  
         [0000]     8. CMP(Idle)  
         [0197]     This message does not indicates any events from the sender and requires no action of the receiver.  
         [0000]     9. CMP(ESCD)  
         [0198]     This message is used to indicate that an ESC/ESDC tone has been detected. The CMP(ESCD) shall be transmitted in consecutive frame as long as the ESD/ESCD tone is detected.  
         [0000]     10. CMP(P1800)  
         [0199]     This message is used to indicate to the remote SCIB the presence of P1800 signal at the local telephone interface. The CMP(P1800) shall be transmitted in consecutive frame as long as the P1800 signal is being demodulated from the local SCIB.  
         [0000]     11. CMP(GPA)  
         [0200]     This message is used to indicate to the remote SCIB the presence of SCR1-GPA signal at the local telephone interface. The CMP(GPA) shall be transmitted in consecutive frame as long as the SCR1-GPA signal is being demodulated from the local SCIB.  
         [0000]     12. CMP(GPC)  
         [0201]     This message is used to indicate to the remote SCIB the presence of SCR1-GPC signal at the local telephone interface. The CMP(GPC) shall be transmitted in consecutive frame as long as the SCR1-GPC signal is being demodulated from the local SCIB.  
         [0000]     13. CMP(Sync)  
         [0202]     The SCRsync message is sent for a single 20 ms frame only to synchronize the transition between SCIB message exchange and the transparent data frame (TRANSPARENT — 2400).  
         [0000]     14. CMP Message Detection Criteria  
         [0203]     All the CMP message detection criteria are the same, that is, when two consecutive error free messages are received; except for the CMP(Sync). The CMP(Sync) consists of only a single frame. To increase the robustness of the synchronization of the two peer SCIB, the detection of CMP(Sync) incorporates the message state information in the SCIB.  
         [0000]     15. Transparent Data Frame (TRANSPARENT — 2400)  
         [0204]     The Transparent — 2400 data frame contains 48 bits of secure data demodulated by the V.26bis demodulator. The MSB of the lowest order byte is the first bit be to sent or received and the LSB of the highest order byte is the last bit be to sent or received.  
         [0000]     16. VF Signal Format  
         [0205]     The Voice Frequency (VF) signals concern in this SCIB protocol include: 
        2100 Hz ESD/ESCD echo suppressor/canceller disable tone     Pseudo 1800 Hz signal P1800     Scrambled Ones SCR1 (GPA)     Scrambled Ones SCR1 (GPC)     Start of Message (SOM)     V.26bis Modulation scheme 
 
 17. 2100 Hz ESD/ESCD 
       
 
         [0212]     The ESD and ESCD signals are both 2100 Hz±0.1%. The difference is that the ESCD contain phase reversal every 450±25 ms. The STU-III terminal can be configured to use either of the signal. The SCIB is required to detect both of the signals but regenerate the ESCD signal only.  
         [0000]     18. Pseudo 1800 Hz (P1800) Signal  
         [0213]     The P1800 signal consists of an 1800 Hz tone generated by modulating the V.26bis modulator with a sequence of 02 02 . . . 02 bit pairs (dibits). This produces line spectrums at 1800, 600, 1200, 2400 and 3000 Hz. The P1800 signal is terminated by modulating dibits sequence  3202 .  
         [0214]     In the case of half duplex operation, the P1800 signal is the first signal sent by the Initiating STU-III in place of 2100 Hz ESD/ESCD tone in the full duplex mode. The half duplex P1800 consists of two segments, the first segment is 1024 bit long and second segment is 784 bit long with a carrier off period of 256 bits in between. The first 1024 bits (512 dibits) of P1800 signal is not terminated with 3202 dibits pattern, but all the subsequent P1800 signal preceding the half duplex transmission in both directions will contain the 3202 transition dibit pattern at its end.  
         [0215]     The STU-III terminal, if configured, will include three 180° phase reversals (modulating by dibit  01 ) near the beginning part of this signal (precisely at 32, 64 and 96 dibit positions) to indicate the enhanced service capability (alternate mode operation). Since the SCIB protocol in this specification supports does not support the alternate mode operation, the V.26bis demodulator shall ignore the phase reversals and the modulator shall not regenerate the phase reversals.  
         [0000]     19. Scramble ones SRC1 (GPA) Signal  
         [0216]     The SRC1 signal bit sequence is generated by a 23-bit GPA scrambler, seeded by a Seed Vector as follows:  
         [0217]     Scrambler Polynomial: X-23+X-5+1  
         [0218]     Seed Vector: 01000100010001011100010  
         [0000]     20. Scramble ones SRC1 (GPC) Signal  
         [0219]     The SRC1 signal bit sequence is generated by a 23-bit GPC scrambler, seeded by a Seed Vector as follows:  
         [0220]     Scrambler Polynomial: X-23+X-23+1  
         [0221]     Seed Vector: 01000100010001011100010  
         [0000]     21. Start of Message (SOM) Signal  
         [0222]     The SOM signal is used in half duplex operation and has two patterns. The initiator SOM consists of following pattern (in dibits sequence):  
         [0223]     1332 0020 1202 2132 2032 1023 1312 1222  
         [0224]     The responder SOM sequence is a inversion of the initiator&#39;s SOM as follows:  
         [0225]     2001 3313 2131 1201 1301 2310 2021 2111  
         [0000]     22. V.26bis Modulation Scheme  
         [0226]     The ITU-T V.26bis modem scheme is used by the STU-III for 2400 bit/s operation, except that the training sequence of the standard V.26bis modem is replaced by the SCR1 GPA and SCR1 GPC sequences as defined in the previous paragraphs.