US 20030012152 A1
In a radio frequency communications system, a data carrier activation method is implemented such that the carrier is switched off when no data is available for transmission. If repeated signalling information is required to be transmitted, only a predetermined number of repeats is transmitted before the carrier is switched off. The data input for transmission are compared with an idle sequence with different bit alignments to detect the presence of idle signalling, and the carrier is switched off if a match is found. When more user data is received, the carrier is switched on and frames (SM) are transmitted in synchronization with the timing of frames (SM) transmitted before the carrier deactivation. After carrier reactivation, a constant power preamble (P) may be transmitted to assist in level control in the transmitter (27; 40).
1. Communications interface apparatus for connection between a source of data, including both user data and signalling information, and a transmitter, such that said user data is transmitted by said transmitter on a modulated radio frequency carrier,
the apparatus being arranged to receive said data, to detect the presence of repeated signalling information and the absence of user data in said data, and to deactivate said carrier if the number of repetitions of said signalling information is equal to or exceeds a predetermined value, such that excess repetitions are not transmitted.
2. Apparatus as claimed in
3. Apparatus as claimed in
4. A method of carrier deactivation, comprising:
receiving data, including both user data and signalling information, and transmitting said user data on a modulated radio frequency carrier, the method including:
detecting the presence of repeated signalling information and the absence of user data in said data, and
deactivating said carrier if the number of repetitions of said signalling information is equal to or exceeds a predetermined value, such that said excess repetitions are not transmitted.
5. A method as claimed in
6. A method as claimed in
7. Satellite communications interface apparatus for connection between a source of data and an earth station transmitter,
the apparatus being arranged to format said data as a series of constant length frames and to selectively output said frames to said transmitter such that said output frames are transmitted on a modulated radio frequency carrier in an SCPC format,
the apparatus being further arranged to detect whether at least an initial portion of each of said frames contains no information or only redundant information, to control the transmitter to deactivate the carrier in response to a positive said detection, to reactivate the carrier in response to a subsequent negative said detection, and to transmit frames subsequent to said reactivation with a timing synchronised with that of frames prior to said deactivation.
8. Apparatus as claimed in
9. A method of satellite carrier activation, comprising:
receiving data, formatting said data as a series of constant length frames, and selectively transmitting said frames on a modulated radio frequency carrier in an SCPC format,
the step of selective transmission comprising detecting whether at least an initial portion of each frame contains no information or only redundant information, and deactivating the carrier such that said portion of the frame is not transmitted,
wherein, after the carrier is deactivated, subsequent frames are transmitted with a timing synchronised with that of frames transmitted prior to said deactivation.
10. A method as claimed in
11. A method of transmitting a data burst via satellite to a receiving terminal, comprising:
transmitting the data burst in a format comprising one or more frames having a variable power level modulation, preceded by a preamble having a constant power level.
12. A method as claimed in
13. A data burst signal comprising a frequency carrier modulated by a preamble having a constant power level, followed by one or more data frames having a variable power level.
14. Radio frequency communications apparatus for connection between a source of data and a radio frequency transmitter, the apparatus being arranged to divide said data in sequence into blocks and to compare a series of bits of a predetermined length at the end of a first block with multiple sequential series of bits of said predetermined length comprising a second block, and, if all of said series are equal, inhibiting transmission of said second block.
15. Apparatus as claimed in
16. Apparatus as claimed in
17. Apparatus as claimed in
18. A method of radio frequency communication, comprising:
dividing data for transmission into blocks in sequence;
comparing a series of bits of a predetermined length at the end of a first block with multiple sequential series of bits of said predetermined length comprising a second block, and
if all of said series are equal, inhibiting transmission of said second block.
19. A method as claimed in
20. A method as claimed in
21. A method as claimed in
22. A satellite earth station including apparatus as claimed in
23. A satellite earth station including apparatus as claimed in
24. A satellite earth station including apparatus as claimed in claim 14.
 This application is a divisional application of U.S. application Ser. No. 09/262,084 filed Mar. 4, 1999, which in turn claims the benefit of priority from foreign applications United Kingdom 9804640.2 filed Mar. 4, 1998 and United Kingdom 9804639.4 filed Mar. 4, 1998.
 The present invention relates to a data communication method and apparatus, and in particular such an apparatus for carrier activation in a satellite communication system.
 In satellite voice communication systems, it is known to switch the carrier off in one direction over the satellite link when the party transmitting in that direction is not talking. This technique is known as ‘voice activation’ or more generally ‘carrier activation’ and is described for example on page 55, section 3.2 of ‘Satellite Communications—Principles and Applications’ by Calcutt and Tetley, First Edition 1994, published by Edward Arnold. The average English speaker only talks during about 40% of the time during a telephone conversation, and therefore a satellite power saving of up to 4 dB can be achieved by this technique.
 The document U.S. Pat. No. 5,481,561 mentions that carrier activation could be applied to voice, facsimile and data communications, but recognizes that this is difficult to realize in practice.
 Carrier activation in fax calls has been implemented in the Inmarsat-M™, Inmarsat-B™ and Inmarsat-mM™ satellite services. The deterministic nature of the ITU T.30 protocols, to which Group 3 fax terminals conform, is used to detect when one terminal is about to receive page data and will therefore not be transmitting; the carrier for transmission by that terminal is then switched off.
 However, duplex data calls are generally not considered suitable for carrier activation, because data may be sent continuously in both directions.
 According to one aspect of the present invention, there is provided a transmitter in a satellite communications system, which receives input data in a format which may include an idle signal indicating that there is no user data present, compares the input data with a bit pattern corresponding to said idle signal in more than one relative bit alignment, and ceases transmission if a match is found.
 An advantage of this aspect is that carrier activation may be implemented even when byte alignment is not preserved between transmitting and receiving applications.
 According to another aspect of the present invention, there is provided a transmitter in a satellite communications system which assembles data and signalling information for transmission over the satellite, determines which of said signalling information need be transmitted in order to maintain the communications link over the satellite and ceases transmission if there is no data and only unnecessary signalling information to be transmitted.
 According to another aspect of the present invention, there is provided a single channel per carrier satellite communications system in which signals are transmitted in a constant length frame structure and carrier activation is implemented such that frames transmitted after reactivation of the carrier are synchronised with the frame timing of frames transmitted before the deactivation of the carrier. The interval between transmission of frames may be an integral number of frame periods, or an integral number of fractions of a frame period, such as quarters of a frame period.
 An advantage of this aspect of the invention is that a receiver may receive and decode the frames transmitted after the reactivation of the carrier without having to reacquire the frame timing. Furthermore, carrier activation may be implemented in this way as an additional feature to an existing satellite SCPC system without modification of frame formatting protocols.
 According to another aspect of the present invention, there is provided a method and apparatus of inhibiting transmission of a block of repeated data by detecting whether the last byte of a previous block is the same as each byte of the current block and inhibiting transmission of the current block if this is the case. Preferably, the carrier on which the blocks are transmitted is deactivated or reduced in power during the period in which the current block would otherwise be transmitted.
 According to another aspect of the present invention, there is provided a method of transmitting a burst of information after a period of carrier deactivation, in which a constant power level preamble is transmitted before the information. Advantageously, this assists in automatic level control of the transmitter.
 Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a diagram of a communications link between data terminals through a PSTN and a satellite network;
FIG. 2 is a functional block diagram of a mobile earth station and its associated interface to a data terminal;
FIG. 3 is a functional block diagram of a fixed earth station and its associated interface to a PSTN;
FIG. 4 shows the channel format used over the satellite link in a first embodiment of the present invention;
FIG. 5 is a flowchart of a carrier activation algorithm in the first embodiment;
FIG. 6 shows the timing of SCPC frames in the first embodiment;
FIG. 7 is a diagram of the frame format used over the satellite link in a second embodiment;
FIG. 8 shows an HDLC transmission and reception process including zero insertion and removal;
FIG. 9 is a flowchart of a carrier activation algorithm in the second embodiment;
FIG. 10 shows the timing of SCPC frames in the second embodiment;
FIGS. 11a to 11 c shows the timing of SCPC frames and the contents of encoded blocks transmitted in those frames, in a third embodiment;
FIG. 12 is a flowchart of an algorithm performed by the transmitting MIU on each data block in the third embodiment; and
FIG. 13 is a flowchart of an algorithm performed by the receiving MIU in the third embodiment.
 The overall layout of a satellite communications system, when used for data communications, is shown in FIG. 1. One example of such a system is the INMARSAT-B™ or INMARSAT-M™ satellite communications system, as described for example in Chapters 12 and 14 of “Satellite Communications: Principles and Applications” by Calcutt and Tetley, 1st edition, published by Edward Arnold. The following system is also described in W096/31040, the contents of which are incorporated herein by reference.
 A mobile DTE 2 is connected via an RS232C interface to a modem interface unit (MIU) 4. The MIU 4 simulates a Hayes—compatible modem and is able to decode Hayes-type commands from the mobile DTE 2, so that off-the-shelf communications software may be used in the mobile DTE 2. The MIU 4 does not perform modulation or demodulation in this case, since it is not connected to an analog line. Instead, the MIU 4 provides an interface to a mobile earth station (MES) 6 which allows communication via a satellite 8 to a fixed or land earth station (LES) 10. The LES 10 is connected to an LES MIU 12 which interfaces the satellite link to a network 14, in this case a public switched telephone network (PSTN), and functions as a modem to convert analog signals on the PSTN 14 to digital signals on the satellite link, and vice versa. A fixed DTE 18 is connected to the PSTN 14 through a modem 16 of standard type.
FIG. 2 shows the MES MIU 4 and the MES 6 in greater detail. The MES MIU 4 comprises a DTE interface 20, which provides an RS232 physical interface and emulates an AT.PCCA type modem, i.e. it complies with the minimum functional specification for data transmission systems published by the Portable Computer and Communications Association (PCCA), including the use of the AT command set and responses.
 Data received by the DTE interface 20 is sent to a buffer 22, which is in turn connected to an MES interface 24. The MES interface 24 implements, in ARQ (automatic repeat request) mode, a variant of the HDLC (High Level Data Link Control) protocol, as defined in ISO recommendations ISO/IEC 3309, ISO/IEC 4335: 1993 and ISO/IEC 7809: 1993. The particular version employed is ISO HDLC BAC 3.2, 4, 8, 10, 12 as defined in ISO 7809: 1993 (synchronous, two-way simultaneous, duplex, non-switched). A controller 26 controls the operation of the interfaces 20 and 24 and the flow of data through the buffer 22.
 The MES includes an RF modulator/demodulator 27, connected to an antenna 28, for RF modulating the output of the MES interface 24 and transmitting the output through the antenna 28 to the satellite 8, and for RF demodulating RF signals received from the satellite 8 through the antenna 28 and sending the demodulated signals to the MES interface 24. The MES 6 also includes access control and signalling equipment (ACSE) 30, for setting up and clearing the satellite link, which exchanges data with the controller 26 of the mobile MIU 4.
 The MES ACSE 30 communicates with a network control station (NCS) which allocates communications channels, supervises communications traffic through the satellite 8 and communicates with further ACSE at the LES.
 The mobile MIU 4, MES 6 and ACSE 30 may be integrated in a mobile unit and the antenna 28 may be integrated or connected externally with the mobile unit.
FIG. 3 shows the LES 10 and the LES MIU 12 in greater detail. The LES MIU 12 includes a modem 31 for demodulating analog signals from the PSTN 14 and modulating digital signals for the PSTN 14, and a modem interface 32 which supports modem protocols such as V.42 error correction, for communication with the modem 16.
 The modem interface 32 is connected through a buffer 34 to an LES interface 36, which implements protocols compatible with the MES interface 24, so that data can be exchanged between the LES MIU 12 and the MES MIU 4. A controller 38 supervises the operation of the modem interface 32, buffer 34 and LES interface 36. The LES interface 36 is connected to an RF modulator/demodulator 40 which modulates signals for transmission to the satellite 8 through an antenna 42, and demodulates signals received from the satellite 8 though the antenna 42. Call set-up and clearing are controlled by an LES ACSE 44 within the LES 10 which exchanges signals with the LES MIU 12, the MES ACSE 30, and the network control station (NCS).
 Although the system described above allows full duplex data communications, many user applications such as file transfer, database and email protocols communicate in half-duplex mode for reasons of design simplicity, even if files are to be sent in both directions. However, switching off the carrier during a call may cause the receiver to lose synchronisation with the transmitter.
 Moreover, in existing satellite communications protocols, some redundant signalling takes place when there is no user data to be sent. The carrier could be switched off during this signalling, but it must be determined which signalling is redundant and which is necessary.
 In the first embodiment, the MIU connected to both the LES 10 and MES 6 detects whether there is no information or only redundant information to be transmitted, and if so, sends a signal to the LES 10 or MES 6, which disables the transmitter thereof until the MIU indicates that information is ready for transmission. In the case where the LES 10 is receiving the carrier which is deactivated, the LES 10 signals this to the LES MIU 12, which maintains the connection with the PSTN modem 16. For example, if the V.42 protocol is being used, the LES MIU 12 transmits flags.
 As described above, the MIU formats the data to be transmitted into HDLG frames.
 Multiple HDLC frames are formatted into one single channel per carrier (SCPC) frame, as shown in FIG. 4. The transmission begins with a header portion P, followed by a sequence of fixed-length SCPC frames SM1, SM2, . . . SMn. The end of the transmission is indicated by an end signal E.
 Each SCPC frame SM is subdivided into four sections, each containing a header H1, H2, H3, H4, a data field D1, D2, D3, D4, and dummy bits (shaded). The data fields D1 and D2 together form one or more HDLC frame, which is repeated in the data fields D3 and D4, to increase the energy per bit. The contents of each HDLC frame depend on whether data or control information is being sent.
 If data is being sent, the HDLC frame has an information (I) format formed from the concatenated data fields D1 and D2. The HDLC frame includes control bytes C containing acknowledgement and frame number information indicating the sequence number of the transmitted frame and the sequence number of the last frame received correctly.
 Line control messages are sent as unnumbered information (UI) HDLC frames, more than one of which may be contained within the data fields D1 and D2. Flow control messages are sent in a supervisory (S) HDLC frame format.
 The LES MIU 12 and the MES MIU 4 are programmed to generate either RR (Receive Ready) or RNR (Receive Not Ready) HDLC flow control frames when no user data is received and no other HDLC signalling is required. The flow control frames indicate whether the MIU is ready to receive more data over the satellite link. In order to maintain this function, while implementing carrier activation, the MIU follows the algorithm shown in FIG. 5. The algorithm is intended as a modification of an existing MIU functionality and is therefore applied after the framing of data into HDLC and SCPC frames, including the generation of RR and RNR frames. The algorithm determines the carrier state which is signalled to the earth station to which the MIU is connected, in order to switch off the carrier.
 At the first iteration of the algorithm, at the beginning of a call, initial values of variables are set as follows:
 Flow control flag, FC=cleared
 Number of redundant flow control frames to be sent, X=1 (or a higher integer)
 Number of ‘Establish LCM’ to be sent, NE=3 (or another positive integer)
 Variable for detecting change in N(R), Nrp=0.
 At step S10, it is detected whether a new SCPC frame has been composed. At step S20, it is detected whether the SCPC frame is empty. If so, the carrier state is set as ‘OFF’ (S30) and the algorithm restarts.
 If the SCPC frame is not empty, the MIU detects (S40) whether the new SCPC frame is an ‘Establish LCM’ (line control message) which is transmitted during call set-up to establish the parameters of the call. If so (S50), the MIU sets the carrier state as ‘OFF’ (S55) if the counter NE (number of Establish LCM) is zero; if NE is not zero, it is decremented (S60) and the carrier state is set ‘ON’ (S65). In either case, the algorithm restarts. As a result, sufficient ‘Establish LCM’ frames are transmitted to ensure that one is received, before the carrier is deactivated.
 If the SCPC frame is not an ‘Establish LCM’, the MIU next detects (S70) whether Nrp=N(R), where N(R) is a variable defined in the HDLC protocol and represents the serial number of the next expected I (information) frame. If the current SCPC frames contains more than one HDLC frame each having an N(R) value, the most advanced N(R) value is taken. If Nrp#N(R), Nr, is set to N(R) (S80), the carrier state is set as ‘ON’ (S95) and the algorithm proceeds to step SI00.
 If Nrp=N(R), the MIU detects (S90) whether the SCPC frame contains only RNR or RR HDLC frames. If not, the carrier state is set as ‘ON’ (S95) and the algorithm proceeds to step S100. At step S100, the MIU detects whether the SCPC frame includes an RR frame. If so, the flow control flag FC is cleared (step S110) and the algorithm restarts. If not, the MIU detects (S120) whether the SCPC frame includes an RNR frame and sets the FC flag (S130) if it does. In either case, the algorithm then restarts.
 If the MIU detects at step 90 that the SCPC frame does contain only RR or RNR frames, this means that no user data is present, but the MIU must still determine whether the RR or RNR frames need to be sent to ensure flow control. At step 140, the MIU determines whether the last frame inside the HDLC frame is an RR or an RNR frame. If the frame is RNR, the MIU detects (S150) whether FC is set and if not, sets it (S160) and proceeds to step 190. If the frame is RR, the MIU detects whether FC is set, and if so, clears it (S180) and proceeds to step 190.
 At step 190, the carrier state is set as ‘ON’. The variable NFC, which is used as a counter of the number of redundant flow control indications remaining to be sent, is set (S200) to X−1, and the algorithm restarts.
 If FC is detected as set at step S150 or as clear at step 170, the MIU then detects (S210) whether NFC is zero, i.e. whether no more flow control indications need to be sent. If so, the carrier state is set to ‘OFF’ (S220) and the algorithm restarts. If not, the carrier state is set to ‘ON’ (S230), NFC is decremented (S240) and the algorithm restarts.
 The state of the carrier is redetermined for each SCPC frame and a decision is made as to whether to switch the carrier off for that SCPC frame. The SCPC frame length is constant. Thus, when the carrier is switched off and then on, the next SCPC frame timing is aligned with that of the previous transmitted frame, as shown in FIG. 6. In other words, the period for which the carrier is switched off is an integral number of SCPC frames.
 A second embodiment of the present invention will now be described, in which a 64 kbit/s channel is provided by the satellite link and is used by an ISDN application. In this embodiment, the network 14 is an ISDN and the satellite 8 has a multibeam user antenna for communication with the MES 6, in order to increase the gain of the user link and support a higher data rate. In this embodiment the LES MIU 12 provides an ISDN interface to the network 14, while the MES MIU 4 simulates an ISDN terminal adapter for the mobile DTE 2. Since the MES MIU 4 does not simulate a modem in this embodiment, it does not decode the Hayes™ AT command set and is preferably integrated with the MES 6. In the second embodiment, a 16 QAM modulation scheme is used for transmission, such that transmitted data has a variable power envelope. Further details of the modulation and coding schemes are described in co-pending application GB 9804639.4, the contents of which are incorporated by reference in so far as they relate to a 64 kbit/s satellite channel.
 As shown in FIG. 7, the format used for data transmission in this embodiment comprises SCPC frames SM1, SM2 . . . SMn each having as a header a unique word UW to assist synchronisation in the receiver. The end of a sequence of SCPC frames is indicated by an end of data signal E. Each SCPC frame contains two subframes SF1 and SF2. Each subframe SF is encoded from an input frame IF1, IF2 which contains a data field D of fixed length. (in this case 2560 bits) and a signalling field S. Each data field D contains HDLC frames transmitted by an ISDN application on the mobile DTE 2 or the fixed DTE 18.
 In ISDN applications, an idle state is indicated by transmitting a continuous sequence of HDLC flags (binary 01111110 or hex 7E). However, user data may coincidentally contain this bit sequence. Therefore, the applications follow a procedure as shown in FIG. 8. At P10, the user data is assembled for transmission. At P20, any sequence of 5 set bits together (11111) is detected and a zero (0) is inserted after them. The following bits are all shifted one bit position to allow the zero to be inserted. This technique is known as ‘zero insertion’. As a result, the user data cannot replicate the flag sequence. At P30, HDLC flags are generated if there is no user data to send and the HDLC frames are transmitted.
 At P40, the HDLC frames are received by the receiving application, flags are detected and the user data is separated from them. At P50, a zero is removed after every set of 5 sequential set bits, in a reverse operation to that of P20, to restore the user data to its original form for input to the application at P60.
 The user data is formatted 8-bit bytes and the data field D comprises an integral number of bytes (320 in this case). However, zero insertion destroys the original byte alignment of the user data, so that HDLC flags may no longer appear as binary 01111110. Instead, the HDLG flags may appear as any of the following bytes shown in Table 1:
 In this embodiment, the MIU performs the algorithm shown in FIG. 9 in order to detect an SCPC frame consisting entirely of flags, which therefore need not be transmitted. At step T10, the MIU assembles the data content of the input frames IF1 and IF2 of the current SCPC frame. At step T20, the MIU checks whether the value of the last data byte of the preceding SCPC frame had any of the hex values shown in Table 1 above. If so, the MIU then detects (T30) whether all of the data bytes in the current SCPC frame are equal to the last data byte of the preceding SCPC frame. If so, this indicates that the entire current SCPC frame consists of HDLC flags and an ‘idle’ state is set (T40). If either of the tests of T30 and T40 are not satisfied, the ‘idle’ state is not set (T50).
 If the ‘idle’ state is set, the MIU controls the MES 6 or LES 10 to which it is connected to switch off the carrier for the duration of the current SCPC frame. When a transition occurs to the ‘idle’ state, the MIU appends an end signal E to the end of the last transmitted SCPC frame, as shown in FIG. 10. Subsequently, when a transition out of the ‘idle’ state occurs, the new SCPC frames are transmitted with the same frame timing as the previously transmitted SCPC frames, so that the start of the new SCPC frame occurs an integral number of frame periods after the start of the previously transmitted SCPC frame.
 The receiving MIU, on detecting the end signal E without an indication from the ACSE that the call has been cleared, determines that the transmitting MIU has detected an idle state. Since ISDN is a synchronous protocol, the receiving MIU must continue to transmit signals to its associated DTE. The receiving MIU repeats the last byte of the SCPC frame received before the end signal. Since this has previously been detected by the transmitting MIU to be an HDLC flag or a bit-shifted version thereof, the repeated bytes will be detected as HDLC flags by the receiving user application.
 In an alternative to the second embodiment, the MIU continuously checks the input user data without waiting for sufficient user data to be received to form a complete SCPC frame, and an idle state is detected as soon as any 8 consecutive bits have the binary value ‘0111110’, for example by reading the input bits into an 8-bit shift register and continuously comparing the contents with hex 7E. However, the transmission of the current SCPC frame cannot be interrupted immediately when a flag is detected without violating the frame format, so this option does not confer any advantage in implementing carrier activation and requires a greater processing overhead than the second embodiment.
 An optional feature of the frame format of FIG. 10 is shown in dotted outline. In this arrangement, a short preamble P is transmitted at the beginning of a burst of frames SM, as soon as the carrier has been reactivated.
 The preamble P comprises a repeated sequence of the same 16 QAM symbol, having a power level equal to the average power level of the 16 QAM constellation. The sequence comprises 16 symbols transmitted at a rate of 33.6 kSymbol/s, having a total duration of 476 μs.
 The transmission of the preamble assists in automatic level control using a feedback loop in a high-power amplifier (HPA) in the MES RF modulator 27 and the LES RF modulator 40, so that the transmit power can be ramped up to the required level in 500 μs or less.
 If the preamble P were not transmitted at the beginning of each burst, the transmission would begin with a unique word which does not have a constant power level, and would then not allow the HPA level to stabilise in the required time.
 In another alternative to the second embodiment, when the carrier is switched off and new user data is input to the MIU, the next SCPC frame is transmitted as soon as sufficient data has been received for one subframe SF and that subframe has been encoded. Thus, the previous frame timing is lost and the receiver must acquire the new timing by detecting the unique word UW.
 In a third embodiment illustrated with reference to FIGS. I1 to 13, the MIU divides the baseband data for transmission into blocks d1 to dn each equivalent to 20 ms duration, shown in FIG. 11a. The blocks containing no user data are shaded. Each frame SM is of duration 80 ms and so contains four blocks. The MIU performs a carrier activation algorithm as shown in FIG. 12 on each block, prior to scrambling and encoding the data for transmission. As described in GB9804639.4, the coding is performed by a Turbo encoder including an interleaver into which one 20 ms block is loaded at a time. The Turbo encoder is reset every 40 ms so that the Turbo encoding algorithm is performed on 40 ms blocks corresponding to two 20 ms blocks or one subframe SF. Because the interleaver has a constraint length of half the total interleaver size, the Turbo encoder incurs only a 20 ms delay as shown in FIG. 11a. This technique is described in more detail in PCT/GB97/03551. Hence, the 20 ms blocks are convenient subdivisions of a whole frame on which to perform carrier activation detection.
 At step U10, the MIU starts processing the next 20 ms block d. At step U20 the MIU detects whether the block is the first block in a frame SM. A position pointer X counts the position of the current block within the frame, so that at step U20, the MIU detects whether X=0. If X is not zero, this indicates that a previous block in the current frame has already been sent for transmission. Because the MIU cannot interrupt a frame SM once transmission has begun, the current block is then output for scrambling and encoding at step U30 and the counter X is incremented modulo 4 at step U40, to indicate the frame position of the next block to be checked.
 If X is zero, indicating that the block, if transmitted, will be first block of a frame, then the MIU detects at step U50 whether the last byte of the previous block was equal to hex 7E, 3F, 9F, CF, E7, F3, F9, or FC. If not, this indicates that idle flags may not be present in the current block and the current block is output for transmission, at step U60. At step U70, X is set to 1, indicating that the next block will be the second block in the frame.
 If, on the other hand, the result of the test at step U50 is positive, the MIU detects at step U80 whether each byte of the current block is identical to the last byte of the previous block, as detected at step U50. If not, this indicates that the current block probably contains at least some data other than flags, so the data is output for transmission at step U90, and X is set to 1 at step U100. Otherwise, if the result of the test at step U80 is positive, the current block is not output for transmission at step U110, the carrier is turned off, and X is set to zero at step U120. As shown in FIG. 11b, the 20 ms slot d5 which would have been output at the beginning of a new frame is not transmitted, and instead an end signal E is transmitted and the carrier is turned off for the rest of the 20 ms period. In this case, the block d6 contains user data so that the carrier is turned on and a new frame Smn+1 is transmitted, beginning with block d6. In this way, although frame synchronisation is not maintained on carrier reactivation, synchronisation is maintained with blocks which represent a fraction of the total frame length, so that the receiver does not need to resychronise to any great extent.
FIG. 13 shows an algorithm used by an MIU receiving the transmissions represented by FIG. 11, every time a new frame SM is received. At step V10, a new frame is demodulated and decoded. At step V20, the MIU detects whether the frame is followed immediately by an EOD signal. If not, at step V30 the contents of the received frame are output to the DTE 2 or 18, but otherwise the MIU detects at step V40 whether the last byte of the current frame is equal to hex 7E or its bit-shifted versions. If it is equal to one of these, at step V50 this last byte is repeatedly output to the DTE 2, 18 until the next frame is received or the call is cleared; this has the effect of transmitting a continuous series of flags to the DTE. If the result of step V40 is negative, the MIU outputs hex 7E flags continuously to the DTE at step V60 until the next frame is received or the call is cleared.
 The algorithms of FIGS. 9, 12 and 13 are designed specifically to look for an HDLC hex 7E flag, but may be modified to look for any repeating byte entirely filling a frame or block, and to turn the carrier off if the repeating byte is also the last byte in the previous transmitted frame or block. The receiving MIU would then output the repeated byte a number of times corresponding to the period for which the carrier is switched off. Thus, power can be saved by not transmitting repeated user data, as well as repeated flags. The receiving MIU infers that the last byte of the previous frame should be repeated if the carrier is switched off, but must maintain timing synchronisation to calculate the correct number of repetitions. However, since the carrier is switched off for an integral number of blocks or frames, the receiving MIU need only be able to detect the carrier deactivation interval with a resolution of one block or frame, so that the local clock reference of the receiving MIU would be sufficient.
 The above embodiments have been described with reference to an 8bit HDLC protocol, but are applicable to other communications protocols with different idle sequences. For example, in a 16-bit variant of HDLC, the idle flag is hex 7FFE, so the carrier activation algorithm would look for bit-shifted versions of that flag instead. Alternatively, some protocols may use an all-zero or all-one byte (e.g. hex 00 or FF) as an idle flag. In that case, there would be no need to look for bit-shifted versions of the idle flag, but the carrier would be deactivated if a block or frame contained all zeros or all ones. Other protocols use a repeating sequence of different bytes to indicate an idle state; for example MPEG-4 uses a repeating sequence of a pseudo-random synchronisation sequence and a header. If transmitting data under those protocols, the MIU stores at least the quantity of data from a previous block or frame corresponding to one repeat period of an idle sequence and compares this to the contents of the current block or frame to see if the sequence is repeated throughout the block or frame. Optionally, the MIU's may be operable with more than one protocol, each having a different byte length or flag sequence, and the protocol type is then signalled from the transmitting FIU to the receiving FIU during call set-up so that the parameters of the carrier deactivation algorithms can be set appropriately at the the receiving MIU.
 In the embodiments described above, the carrier transmitted by either the LES 10 or the MES 6 can be deactivated; in the former case, satellite power efficiency is improved, while in the latter case, MES battery power is saved. However, it is not essential that carrier activation should be implemented in both directions. For example, carrier activation may be an optional feature of the MES, so long as the LES 10 is able to perform the necessary reception protocols if carrier activation is implemented at the MES. The present invention is not limited to present or proposed Inmarsat™ satellite services, but may be applied to other satellite data services employing HDLC or other protocols.
 In the above embodiments, a carrier is deactivated completely if there is only redundant data to be sent. Alternatively, however, the power level of the carrier could be reduced and optionally a synchronising sequence such as a unique word transmitted at reduced power during the deactivation period; this reduces the power requirements of an MES if implemented on an MES MIU and of a satellite if implemented on an LES MIU. Hence, references herein to ‘deactivating’ a carrier encompass the continued transmission on a carrier at reduced power while not transmitting any user data or level signalling information.
 In the specific description above, the apparatus is illustrated in terms of functional blocks, for ease of explanation. However, these blocks do not necessarily correspond to discrete physical units.