GB1602840A - Telecommunication systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TELECOMMUNICATION SYSTEMS (71) We, SPERRY LIMITED (formerly SPERRY RAND LIMITED), a British Company, of Sperry House, 78 Portsmouth Road, Cobham, Surrey, KTll lJZ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to telecommunication systems and has for its object to provide a telecommunication which is relatively simple but reliable in operation.

According to the present invention a telecommunication system comprises a base station and at least one relay station having a telecommunication link with the base station by which, in use, messages are transmitted between the base station and the or each relay station, each message comprising a predetermined number of Words each having a predetermined number of data bits, one or more of the relay stations having a satellite station in telecommunication therewith for the transmission of said messages, the base station, the or each relay station and the or each satellite station each being provided with means for detecting an error in a received message and, in the event of an error being detected, for discarding the entire message containing the error.

This simple error check and message discard feature is effective in ensuring that a false message can never be acted upon and yet avoids the complexity of systems having a sophisticated error check and error correction facility. However, a system in accordance with the present invention may have provisions for notifying the sending station that the previously transmitted message contained an error so that the sending station may either retransmit the message or merely log the fact that an error occurred.

In a preferred embodiment each message is made up of five Words with each Word containing eleven Bits, namely one start Bit, seven data Bits, one Word parity Bit and two stop Bits. The Word parity Bit provides a check on any error occurring in that Word and the fifth Word is a Message Parity Word which checks for errors in the message as a whole, i.e. as between Words as opposed to between Bits of a given Word. Thus the Word and Message parity checks provide a comprehensive error check and in the event of an error being detected, the receiving station discards the entire message.

Each relay station having a satellite station associated therewith may be provided with means for storing messages intended for the satellite station and subsequently transmitting them to the latter when the telecommunication link is free.

The message storing means may also be used in a similar manner for messages to be transmitted from the satellite to the base station, via the relay station. This message storing facility enables a simplex telecommunication link to be used between the relay and satellite stations which minimises the number and cost of components.

A digital telecommunication system in accordance with the present invention will now be described with reference to the drawings accompanying the Provisional Specification in which: Figure 1 is a diagrammatic representation of the telecommunication system, Figure 2 is a block diagram of part of the base station shown in Figure 1, Figure 3 is a block diagram of a relay station of the system having a satellite station associated therewith, Figure 4 is a block diagram of part of the satellite station associated with the relay station of Figure 3, Figure 5 is a block diagram of a component common to Figures 2, 3 and 4, Figure 6 is a block diagram showing further details of components common to Figures 2, 3 and 4, and Figure 7 is a block diagram similar to that of Figure 2 but showing a modification.

Referring first to Figures 1, 3 and 4, the telecommunication system comprises overall a base station 1 in telecommunication via respective duplex links 2 with two relay stations 3, 4. The relay station 4 has associated with it a satellite station 5 via a simplex telecommunication link in the form of a cable 6 being provided between the two.

Each relay station 3, 4 comprises a transmitter/receiver 7 connected to a controller 8 operable to control the passing of incoming messages to output data peripherals 9 and the receipt of data from input data peripherals 11 for the construction of messages to be transmitted to the base station I. Relay station 4 has a store-andforward controller 12 connected to the controller 8, the controller 12 being connected to the cable 6 and operable to store messages intended for the satellite station 5 until such time as the cable 6 is free for transmisSion purposes. The controller 12 is also operable ta store messages from the satellite station 5 for the base station 1 until such time as the associated duplex link 2 is free for transmission purposes.

The satellite staion 5 has a controller 13, inputperiherals 14 and output peripherals 15 generally similar to thase af the relay stations 3, 4.

Figure 3 shows the controller 8 and store-and-fafward controller 12 of the relay 4 in greater detail. The controller 8 comprises a deserialiser 16 for messages received from the base station 1. Each message have a five word format as fol lows: - Header Word I - Synchronisation and message routing with different codes defining different message destinations and origins.

Header Word 2 -- Peripheral address for the next two data words.

Data Word 1 - A seven bit data word.

Data Word 2 - A seven bit data word.

Message or Longitudinal Parity Word A seven bit parity check for the pre vitus four Words and the Parity Word.

Each Word has an eleven Bit format as follesvs: Bit AStart Bits B to H - Data Bit Word Parity Bits J & IC. - Stop If the deserialiser 16 recognises the code of the Header Word 1, then it will proceed in one of two ways according to the routing indicated in the Header Word 1. If the routing indicates that the message is intended for the relay station itself, then Header Worder 2 is stored in a store 17 and the twO Data Words 1 and 2 stored in a buffer 18 under the control of a trans mjtter/receiver controller 19.If the Message Parity Word shows there to have been no error in the message, then the Data Words 1 and 2 are outputed to the peripherals 9, again under the control of controller 19, in accordance with the address contained in Header Word 2. If the routing data in Header Word 1 indicates that the message is intended for the satellite station 5, then the whole message is routed to the store-and-forward controller 12 and more specifically ta a buffer 21 and thence to a serialiser/deserialiser 22 which is controlled by a further transmitter/receiver controller 23, which components, together with a further controller 24, make up the controller 12. Since the system is designed to handle messages asyncronously (i.e.

messages are transmitted at random and the receiving station does not know in detail when a message might be received), some timing mechanism is required ta ensure that no errors occur as a result of attempted transmission during an existing transmission, for example. This timing function is effected by the controller 24 which monitors all receptions and transmissions and attends ta the necesary timing, being connected to the controllers 19 and 23 When a message is transmitted to the satellite station 5, it is serialised at 22 and sent down the cable 6 via a modem 25.

The message is received at a deserialiser 26 (Figure 4) via a modem 27 which form part of the controller 13 af the satellite station 5. The controller 13 is generally similar ta the controller 8 of the relay stations 3, 4 and further comprises a store 25 for the Header Word 2 of a received message, a buffer 29 for the Data Words 1 and 2 and a transmitter/receiver controller 31 which has a timing device 32 associated therewith, the latter also being connected ta the store 28. At the appropriate time, the data in the buffer 29 is outputed to the peripherals 15 in accordance with the address data in the Header Word 2.

Considering now outgoing messages from the relay 4, these can be generated either in the relay itself or in the satellite station 5. Taking the latter first, data for a message is taken from the input data peripherals and fed to a serialiser 33, each message having the same format of five Words as described above in relation to received messages. The Message Parity Word is pra- vided by a parity generator 34 and the serialised message is sent through the cable 6 via the modem 27 and is received in the serialiser/deserialiser 22 of the relay 4 via the modem 25. The message is transferred to a buffer 35 and subsequently transmitted to a serialiser 36 under the control of the controllers 23 and 24. The message is finally transmitted from the relay transmitter receiver 7 under the control of the controllers 19 and 24.

The message transmission portion of the relay controller 8 is generally similar to that of the sub-sation 5 and comprises the serialiser 36 and a parity generator 37. The data input for a given message is obtained from the input peripherals 11 and in the case of both the substations 5 and the relay 4, the Header Word 2 of the previously received message is used to dictate the channels from which data is to be obtaned for the next transmitted message. Thus if, for example, channels 1 and 2 of the output data peripherals 9 or 15 are involved in a received message, then the next transmitted message will take data from channels 1 and 2 of the input data peripherals 11 or 14 but the channels 1 and 2 of the output data peripherals may concern informaion different from those with which channels 1 and 2 of the respective input data peripherals are concerned.

It will be appreciated that the relay station number 3 operates as regards message reception and transmission in a manner similar to that described for relay station 4 given that it has no satellite station associated therewith. Messages transmitted from the transmitters/receiver 7 are received by a transmitter receiver 38 provided at the base station 1. This station also having a controller 39, input data peripherals 41 and output data peripherals 42.

Thus, the base station 1 receives and transmits messages in a manner similar to that described in connection with the relay stations 3 and 4.

Considering now the error check and message discard facility of the system in greater detail, Figure 2 shows the components of the controller 39 of Figure 1.

The controller comprises a sequencer 43 which is the heart of the system as this is the device which contains the routing and data address information for each Word to be transmitted. The sequencer 43 advances to a new set of routing and data address data as each complete message is transmitted. A transmitter controller 44 selects the data to be transmitted either from the fixed data in the sequencer 43 or from the input peripherals 41 using the peripheral addresses stored in the sequencer. A message or longitudinal parity generator 45 monitors the input to a serialiser 46 and updates its parity word as each Word is transmitted. When required, the Message Parity Word is placed onto the serialiser input and transmitted as all previous data. The sequencer 43 is now advenced to the next set of routing and address data.Messages sent to the base receiver 38 and sent to a deserialiser 47 which operates in a manner similar to the deserialisers 16 and 26 of the relay and satellite stations 4 and 5, respectively, to convert each complete eleven-bit serial Word of a message into a seven-bit parallel data word stripped of start, parity and stop Bits. The received data is loaded into a message buffer 48, and upon receipt of a complete message with no errors, the two data Words 1 and 2 are outputted to the required peripherals 42 as indicated by the peripheral address word (Header Word 2).

If any error is detected in a message, the message output sequence is bypassed and the data rejected. The transmitter/receiver controller 44 informs a time-out logic device 49 of the status of received messages. The time-out logic 49 enables the start of each message transmission sequence and is used to lock the transmitted data timing to the received data.

The sequencers 43 ensures that the messages are made up in correct order and contains the Header Word 1 and Header Word 2 generators (not shown). As the basic system only transmits two messages in a sequence, the Header Word 2 data is selectable from two preset codes. One of these codes addresses channels 1 and 3, the other addressing channels 2 and 4. A counter (not shown) counts the messages transmitted and selects first one Header Word 2 code, and then the other. The sequencer 43 will at any instant indicate the current message and word in that message which is to be transmitted next.

This information is decoded to enable Tristate drivers (not shown) which place peripheral addresses and data onto the buses at the appropriate time. The address of the Message Parity Generator 45 is placed onto the peripheral address highway during the fifth word of each message. The sequencer 43 also contains circuitry necessary to enable the buffer 48 to output data onto the output bus. This circuitry monitors the error flag and 'output information ready' status from the transmitter/ receiver controller 44. If no errors are present in the message then the output sequence will take place, providing the system is not in the process of an input cyclej in order to load the serialiser 46. If an input cycle is in progress, then the output sequence is delayed until the highways are free.The Most Significant Bit (M.S.B.) of the Header Word 2 data is used to transmit error status in each message. Received data errors will cause this error bit to be set.

Each of the Message Parity Generators 34, 37 and 45 of the satellite, relay and base stations 5, 4 and 1, respectively, monitors the data transmitted in Header Words 1 and 2 and Data Words 1 and 2. It produces a fifth seven bit data word which maintains odd longitudinal parity in similar bits of each word. The generator is considered to be a standard peripheral attached to the data and control bus. Whereas most peripherals in the system either are input or output devices, the parity generator does both. Also located on each parity generator 34, 37 and 45 are FAIL SAFE and REPLY generators (not shown). Neither of these functions are essential but they may be desired in certain systems.The FAIL SAFE flag gives an output if not output sequence has been initiated within one second and this output was designed in a described embodiment to turn off a system in the relay station 4 in the event of a fault leading to loss of communication.

The REPLY output is useful if the peripherals are used with a computer as this signal can be used to shorten an input or output cycle in the processor.

The buffers 18, 29 and 48 of the relay, satellite and base stations 4, 5 and 1, respectively, are identical and Figure 5 shows the components thereof in greater detail.

Each buffer comprises a serial store register 51 which can hold three, seven-bit parallel words. The output data of the associated deserialiser is clocked along this register until it holds Data Word 1, Data Word 2, and Header Word 2. The outputs of the three register stages 52, 53, 54 feed tristate bus drivers 55; 56; 57, 58, respectively.

Output data is placed onto the output bus when ordered by the associated transmitter/receiver controller. Data Word 1 is placed upon the output bus with the Bits A and B of Header Word 2 as the peripheral address (Address Bit G is not asserted during Data Word 1 output). Data Word 2 is placed upon the output bus with the bits C, D. E and F of Header Word 2 as the peripheral address (Address Bit G is always asserted during Data Word 2 output).

Each buffer also comprises two L.E.D.

indicators 59, 61 connected to respective error latches 62, 63. Indicator 59 displays errors received during the previous data word from the other end of the corresponding telecommunication link. The second indicator 61 displays errors generated at the end of the system in which the buffer is situated. Normally all error indications will be transmitted to the base station 1 for logging and any necessary action such as retransmission of the message concerned. It will be appreciated that an error may occur either at source or be due to a faulty transmission path.

Figure 6 shows the basic components of the error check and message discard facility which is identical for the base station 1 and each relay and satellite station 3, 4 and 5. The deserialiser 16, 26 or 47 of the station concerned sends message parity check data to a checker 64 having its output connected to an error latch 65 which is additional to the error latches 62 and 63 of Figure 5. The deserialiser 16, 26 or 47 also sends a framing error signal and a Word parity error signal to the error latch 65. The error latch 65 is connected to an output controller 66 which sends output signals, in normal circumstances, to the associated message buffer 18, 29 or 48, the output signals being in the form of ENABLE WORD 1 and ENABLE WORD 2 signals and an output strobe signal which actually places data on the various data highways.The three status inputs to the error latch 65 are stored therein at different times in a cycle controlled by a receiver message timing mechanism 66 which forms part of the associated transmitter/receiver controller 19, 31 or 44. If one or more error signals is received by the error latch 65, a Received Data Error output signal is sent from the later to the output controller 65 which inhibits the normal operation thereof so that no data contained in the received messages is outputed to the intended recipient peripherals i.e. the message is discarded. This is a most important aspect of the error check and message discard facility in that an incorrect message never reaches the intended recipient so that inappropriate action can never be taken.

Message transmission is unconditional in the sense that messages are transmitted without a transmission path integrity check.

If messages are not received by a station, then replies from that station are constructed using data from predetermined input peripherals, whereby a fail-safe mode of operation exists. When transmission path integrity returns, the associated transmitter/receiver controller will then allow message transmissions in the desired sequence.

The number of input and output data peripherals associated with the base, relay and satellite stations can be selected at will and the system lends itself well to a modular construction of peripherals, whereby they can be added or subtracted without disrupting the overall system. The described embodiment has been used in a marine environment in which the base station has been based either on land or on a vessel, with each relay station being a surface vessel and the satellite station being a submersible vessel.The system has been found reliable and utilitarian with the provision of error check and message discard facility and the store-and-forward controller 12 which allows the simplex cable 6 to be employed without having to suffer from the inherent limitation thereof by way of being able to transmit in one direc tion only at a given time.The messages transmitted from the base station 1 to the satellite station 5 will concern in the main throttle and rudder commands so as to position the submersible satellite as required for the work which it is undertaking and these commands will tend to be the subject of repetitive messages so that if an error is detected in d received message resulting in the entire message being discarded, then no great harm is done.

The system also lends itself well to being computer controlled from the base station 1 and Figure 7 shows the modification required to the controller 39 of the base station to this end. It will be seen that a computer, in the form of a central processor unit 67, with associated memory 68, and a buffer 69 replaces the sequencer 43 if Figures 2 and 7 are compared. The computer controls all data transactions with the input and output peripherals. As before, the deserialiser 47 converts the serial messages into seven-bit parallel Words under the control of the controller 44. In this embodiment, the base receive buffer 48 merely stores each seven-bit word as it is deserialised and upon receipt of a message with no errors, it informs the computer by way of an interrupt or busy bit in a status register that data is ready for collection.The serialiser 46 can only accept data at a fixed rate which is set by the output baud rate. Therefore, the send buffer 69 is provided which allows a complete message to be stored. The serialiser 46 takes data from the send buffer 69 as required. The second buffer data is inputed from the computer memory 68 as soon as possible after the first word of the present send buffer message has been despatched to the serialiser 46. The Message Parity Generator 45 is treated as a peripheral of the computer, as are all of the data input and output peripherals 41 and 42. The generator 45 uses the same address as the send buffer 69 so that it can update its parity word as a message is loaded into the send buffer. When the parity word is required, it is loaded into the computer memory 68 and then sent as a normal word to the send buffer 69.

The computer software controls the routing and peripheral addressing as the necessary data is placed in store and the program must cause the data to be used as necessary to address appropriate peripherals to input data prior to transmission.

The software also handles the ouput of data to peripherals using the data received from the base receive buffer 48.

Data transmission between the base station 1 and relay stations 3 and 4 can be over any medium which will permit the required data transmission rate which in the described embodiment is 1,200 bauds per second but can be up to 30,000 bauds per second. Thus the transmission medium may be a radio link, a telephone line, an optical link or ultrasonic link.

A telecommunications system in accordance with the present invention may be large or small: an example of a large system might relate to the remote control and data monitoring of a large number of remote data gathering or monitoring stations. Each station could monitor several sensors which have to be located remote from the base station. An example of a small system might be the simple remote control of a vehicle with or without data feedback, or a simple automatic data logging system.

Other aspects of the illustrated embodiment are described and claimed in copending Patent Application No. 29675/77 (Serial No. 1602839).

WHAT WE CLAIM IS: - 1. A telecommunication system comprising a base station and at least one relay station having a telecommunication link with the base station by which, in use, messages are transmitted between the base station and the or each relay station, each message comprising a predetermined number of Words each having a predetermined number of data bits, one or more of the relay stations having a satellite station in telecommunication therewith for the transmission of said messages, the base station, the or each relay station and the or each satellite station each being provided with means for detecting an error in a received message and, in the event of an error being detected, for discarding the entire message containing the error.

2. A system according to claim 1, wherein means are provided on each station for notifying a transmitting station that an erroneous message has been received from it, means also being provided on each station for lodging the occurrence of the transmission of an erroneous message or means for retransmitting the message in question.

3. A system according to claim 1 or 2, wherein each relay station having a satellite station associated therewith is provided with means for storing messages intended for the satellite station and subsequently transmitting them to the satellite station when the telecommunication link is free.

4. A system according to claim 3, wherein the telecommunication link between a relay and an associated satellite station is a simplex link.

5. A system according ot any of the preceding claims, wherein the error de tection means comprise latch means to which are applied message parity error,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (18)

**WARNING** start of CLMS field may overlap end of DESC **. tion only at a given time.The messages transmitted from the base station 1 to the satellite station 5 will concern in the main throttle and rudder commands so as to position the submersible satellite as required for the work which it is undertaking and these commands will tend to be the subject of repetitive messages so that if an error is detected in d received message resulting in the entire message being discarded, then no great harm is done. The system also lends itself well to being computer controlled from the base station 1 and Figure 7 shows the modification required to the controller 39 of the base station to this end. It will be seen that a computer, in the form of a central processor unit 67, with associated memory 68, and a buffer 69 replaces the sequencer 43 if Figures 2 and 7 are compared. The computer controls all data transactions with the input and output peripherals. As before, the deserialiser 47 converts the serial messages into seven-bit parallel Words under the control of the controller 44. In this embodiment, the base receive buffer 48 merely stores each seven-bit word as it is deserialised and upon receipt of a message with no errors, it informs the computer by way of an interrupt or busy bit in a status register that data is ready for collection.The serialiser 46 can only accept data at a fixed rate which is set by the output baud rate. Therefore, the send buffer 69 is provided which allows a complete message to be stored. The serialiser 46 takes data from the send buffer 69 as required. The second buffer data is inputed from the computer memory 68 as soon as possible after the first word of the present send buffer message has been despatched to the serialiser 46. The Message Parity Generator 45 is treated as a peripheral of the computer, as are all of the data input and output peripherals 41 and 42. The generator 45 uses the same address as the send buffer 69 so that it can update its parity word as a message is loaded into the send buffer. When the parity word is required, it is loaded into the computer memory 68 and then sent as a normal word to the send buffer 69. The computer software controls the routing and peripheral addressing as the necessary data is placed in store and the program must cause the data to be used as necessary to address appropriate peripherals to input data prior to transmission. The software also handles the ouput of data to peripherals using the data received from the base receive buffer 48. Data transmission between the base station 1 and relay stations 3 and 4 can be over any medium which will permit the required data transmission rate which in the described embodiment is 1,200 bauds per second but can be up to 30,000 bauds per second. Thus the transmission medium may be a radio link, a telephone line, an optical link or ultrasonic link. A telecommunications system in accordance with the present invention may be large or small: an example of a large system might relate to the remote control and data monitoring of a large number of remote data gathering or monitoring stations. Each station could monitor several sensors which have to be located remote from the base station. An example of a small system might be the simple remote control of a vehicle with or without data feedback, or a simple automatic data logging system. Other aspects of the illustrated embodiment are described and claimed in copending Patent Application No. 29675/77 (Serial No. 1602839). WHAT WE CLAIM IS: -

1. A telecommunication system comprising a base station and at least one relay station having a telecommunication link with the base station by which, in use, messages are transmitted between the base station and the or each relay station, each message comprising a predetermined number of Words each having a predetermined number of data bits, one or more of the relay stations having a satellite station in telecommunication therewith for the transmission of said messages, the base station, the or each relay station and the or each satellite station each being provided with means for detecting an error in a received message and, in the event of an error being detected, for discarding the entire message containing the error.

2. A system according to claim 1, wherein means are provided on each station for notifying a transmitting station that an erroneous message has been received from it, means also being provided on each station for lodging the occurrence of the transmission of an erroneous message or means for retransmitting the message in question.

3. A system according to claim 1 or 2, wherein each relay station having a satellite station associated therewith is provided with means for storing messages intended for the satellite station and subsequently transmitting them to the satellite station when the telecommunication link is free.

4. A system according to claim 3, wherein the telecommunication link between a relay and an associated satellite station is a simplex link.

5. A system according ot any of the preceding claims, wherein the error de tection means comprise latch means to which are applied message parity error,

framing error and Word parity error signals and in the presence of one or more of these signals, an output signal is derived from the latch means which inhibits the further processing of hte message being handled, whereby it is discarded.

6. A system according to claim 5, wherein the error detection means further comprises an output controller connected to the output of the latch means.

7. A system according to claim 5 or 6, wherein each message is made up of five Words with each Word containing eleven Bits, namely one start Bit, seven data Bits, one Word Parity Bit and two stop Bits, the first Word being a Header Word for synchronisation and message routing, the second Word being a Header Word giving the peripheral address for Data Words, the third and fourth Words being the Data Words, and the fifth Word being a Message Parity Word for checking for errors between Words of a message, the Word parity error signal being derived from the Word Parity Bit, and the message parity error signal being derived from the Message Parity Word.

8. A system according to any of claims 5 to 7, wherein each relay station comprises a transmitter/receiver connected to a controller operable to pass incoming messages to output data peripherals and to receive data from input peripherals for the construction of messages to be transmitted to the base station.

9. A system according to claim 8 when appended to claim 4, wherein the controller of a relay station having a satellite station associated therewith is connected to a store-and-forward controller operable to store messages to and from the satellite station until the appropriate telecommunication link is free.

10. A system according to claim 8 or 9, wherein each controller comprises a first deserialiser to which received messages are applied, an address store for storing the address content of a received message, a first buffer for storing the data content of a received message, and a first buffer controller, the first buffer controller allowing the data stored m the first buffer to be outputted to the output data peripherals only if the error detection means have not detected an error in the message being handled, the first buffer controller being connected to the output of the error latch means.

11. A system according to claim 10, when appended to claim 10, wherein each store-and-forward controller comprises a second buffer for storing messages intended for the satellite station, a second deserialiser connected to the second buffer and a second buffer controller which allows the second buffer to output a message to the satellite only if the error detection means have not detected an error in the message being handled, the second buffer controller being connected to the output of the error latch means.

12. A system according to claim 11, wherein messages are transmitted asynchronously and timing means are connected to the buffer controller and to the second buffer controller to ensure that no errors occur as a result of attempted message transmission during an existing transmission.

13. A system according to any of claims 10 to 12, wherein the buffer of each controller and store-and-forward controller comprises means for displaying visually the existence of an error in a received message.

14. A system according to any of the preceding claims, wherein the base station comprises a transmitter/receiver connected to a controller operable to pass incoming messages to output data peripherals and to receive data from input data peripherals for the construction of messages to be transmitted.

15. A system according to claim 14, wherein the controller of the base station comprises a third deserialiser to which received messages are applied, a third buffer for storing the data content of a received message, and a third buffer controller which allows the third buffer to output a message only it the error detection means have not detected an error in the message being handled.

16. A system according to claim 15, wherein the controller of the base station further comprises a sequencer which holds routing and data address information, a serialiser, and a message parity generator, the third buffer controller also being operable to select data either from the input peripherals or the sequencer to construct a message to be transmitted, the message parity generator monitoring the input to the serialiser so as to update the parity word as each Word is transmitted, and inputting the- Message Parity Word to the serialiser at the appropriate time.

17. A system according to claim 16, wherein the sequencer is replaced by a data processing unit.

18. A telecommunication system substantially as herein particularly described with reference to Figures 4 to 6, or as modified by Figure 7, of the drawings accompanying the Provisional Specification.