GB1574714A

GB1574714A - Communication systems

Info

Publication number: GB1574714A
Application number: GB289976A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1976-01-26
Filing date: 1976-01-26
Publication date: 1980-09-10
Also published as: FI770203A; CA1077631A; AR222291A1; DE2702803A1; JPS52113104A; IT1086661B

Description

(54) COMMUNICATION SYSTEMS (71) We, the POST OFFICE, a British body corporate established by Statute, of 23 Howland Street, London, W1P 6HQ; do hereby declare that 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 communication systems and especially but not exclusively to radio-telephone (radiophone) systems.

A radiophone system is one which provides at least some of its subscribers with a vehicle mounted or otherwise portable telephone instrument which has a radio connection to a telephone system enabling telephone conversation to take place between subscribers whilst one or both of them is moving.

As at present proposed, radiophone systems retain a contiguous group of radio channels, typically spaced at 25 kHz intervals, for a mobile to a fixed base station transmission, and a further group of channels for transmission from the fixed base station to a mobile.

These transmit and receive channels and used in pairs and together form a communication circuit, the term "channel" being reserved for communication in a single direction. In this specification, in addition to the above terms, the term "call" is used to mean intercom munication between subscribers and the term "call attempt" covers the operations made to attempt to establish a communication circuit between a calling subscriber and a called subscriber.

In radiophone systems as at present proposed, when a call attempt is made from a mobile subscriber, monitoring equipment associated with the mobile telephone instrument is caused to search for an idle communication circuit and when one is found it is seized and held for the duration of the call or call attempt if it is unsuccessful. It has been found that operation of a radiophone system in this way results in up to 50% of the total seized circuit time being wasted on unsuccessful call attempts and the time to set up successful calls. It is clearly desirable to eliminate as far as possible waste of communication circuit time because of the expense involved in providing a large number of alternative communication circuits for radio phone use.

It is therefore on object of the present invention to provide an improved communication system in which efficient use is made of the speech channels available.

According to the present invention there is provided a communication system for providing communication between subscribers via a common control centre, the system including a plurality of subscriber's terminals, each terminal including (i) tunable radio transmitting and receiving means capable of providing communication of speech between the subscriber's terminal and the common control centre on any one of a plurality of radio channels; (ii) signal generating means effective to generate and transmit a call request signal to request the control centre to set up a call between the subscriber and another subscriber, a first channel request signal to indicate that the subscriber is ready to speak, and a second channel request signal to indicate that the subscriber is ready to listen; (iii) tuning means responsive to a control signal from the common control centre to tune the radio transmitting and receiving means to a particular channel for communication thereon; wherein the control centre includes (i) call set up means responsive to the call request signal from the subscriber selectively to set up a call between the subscriber and another subscriber; (ii) memory means for storing data relating to calls which are in progress between subscribers; (iii) allocation means responsive to the memory means and to the first and second channel request signals from a particular one of the subscribers participating in each call to initiate the allo cation of a radio channel for speech transmission of the particular subscriber on detection of the first channel request signal and to initiate the allocation of a channel for speech transmission to the particular subscriber on receipt of the second channel request signal in such a way that for each call not more than one channel is allocated for speech transmission between subscriber and the common control centre at any instant, and that no channel is allocated for speech transmission until a call has been set up;(iv) means responsive to the allocation for speech transmitting selectively the control signal to a particular subscriber participating in a call to indicate which channel is allocated to the call at the time. The subscriber's terminals may be mobile telephone sets.

It will be appreciated that an advantage of a communication system according to the invention is that because a channel is allocated only when a speech is expected, the waste of channel time arising from the seizure of a channel or two channels forming a circuit during the setting-up time of the call can be avoided, and that in addition the waste of channel time arising from the dedication of a channel to a particular call is also avoided. It is possible that the common control centre may be unable to find a free channel to allocate to a particular call at the time when the channel is required, but with appropriate choice of the number of channels in relation to the communication traffic it is anticipated that the delay at finding a free channel would not exceed a few seconds.

The subscriber's terminals may have a manually operable switch means. The signal generating means may generate the first and second channel request signals in response to the operation of the switch means. Thus during a call the switch means may be moved to a first position by a subscriber wishing to speak causing the subscriber's terminal to signal to the control centre a request for allocation of a channel for speech signals from the subscriber. The control signal transmitted by the control centre keeps the subscriber's terminal tuned to whichever channel is in use for the call. As soon as the subscriber wishes to listen he moves the switch means to a second position which causes the terminal to signal to the control centre a request for allocation of a channel for speech signals to the subscriber. Channels are thus relinquished if they are not in use or likely to be used, and must be so relinquished several times in the course of a conversation.

This enables channels to become available for allocation to other calls, resulting in more efficient use of the channels.

The call request, and the first and second channel request signals may be transmitted over a speech channel on which the control centre expects to receive the signals in the form of a combination of tones which would be unlikely to occur in speech. Mternatively they could be transmitted over a signalling channel dedicated to the transmission of such signals.

This transmission may be digital and may be time division multiplexed with signals from other terminals.

The control signal from the control centre may be transmitted in a channel reserved for signalling. It may have cyclically recurring parts, e.g. time slots, respectively allocated to different calls being set up by the control centre. Each time slot may be divided into address information and message information.

The address information identifies the call to which the slot is allocated, and the message information may include information giving a representation of the progress in predetermined steps of the setting up of the call by the control centre.

Each subscriber's terminal may have a control signal receiver including a data interpreter section having a first unit for interpreting that part of the control signal which contains the address information so as to enable it to derive the relevant message information. Part of the message information may be interpreted by a second unit of the data interpreter to operate an indicator in the subsçriber's terminal. This would produce a representation of the steps reached in the setting up of a call. The representation may consist of various distinctive audible tones corresponding to ringing, busy, and number unobtainable. In addition another distinctive tone may be used to indicate that no channel is available although a call has been set up.

In order to increase the probability of a communication channel being immediately available at the time it is required once a call has been set up, limiting means may be provided for restricting the number of call attempts which can be accepted by the control centre when no communication channel is free.

The allocation means may provide for allocating a channel to a calling subscriber for the duration of a period in which the setting up of a call is most likely to occur in the anticipation of that event, so that the channel is immediately available when the call is set up. Such a period is likely to be from 30 to 50 seconds after the initiation of the call attempt.

Alternatively, or in addition, priority determining means may be provided for assigning differential priorities for allocation of channels to particular subscribers to the system. Variation means may be provided for varying the assigned priority of channel allocation according to the time that a particular conversion has been in progress. Preferably a control centre provides an interface between a plurality of telephone lines connected to a telephone network and radio sets at a base station. Hybrid transformers or equivalent circuits each associated with a respective telephone line are provided to split each line into transmit and receive legs, said transmit legs being connected through a first switching unit to any of the transmitters at said base station and the receive legs being connected through a second switching unit to any of the receivers at said base station. Said switching units are preferably operated by a control section under the control of a signal from said common signalling circuit which is time shared among the subscribers.

A mobile user may become dissatisfied with the quality of the call, for example, because he has moved out of the area covered by the base station transmitter or group of transmitters during the call. In order to overcome this difficulty his tenninal may be provided with a button, for example, which causes the terminal to send a signal requesting the base station to change to another channel or group of channels, which may be determined on the basis of the signal strength of the transmission from the terminal at several receivers in different localities. If such a re-allocation is not possible the old channel or channels may be retained or a warning tone given until a changeover can be nlade;such a re-allocation nay be given priority over other requests for a channel.

Facilities may be provided for suspending a call if, for example, some infonnation or a particular person has to be searched for, as it is desirable that a channel should not be kept idle dung this time. A signal could be sent by the mobile user to the common control centre to cause it to suspend the allocation of channels during a call, and the suspension could be ended either by another signal from the mobile user or by the other subscriber participating in the call speaking at one of a particular series of times in response to a continuously repeated enquiry from the common control centre.

Althougll the invention has been described in connection with radiophone systems, the principle of setting up a call by means of signals which may be digital in nature transmitted over a special signalling channel, probably multiplexed with other such signals, before a speech channel is made available to the call, it may equally well be applied to any system where a scarce or expensive transmission medium is used in conjunction with a cheaper or more plentiful medium. For example, transatlantic transmission via the submarine cable or satellite is scarce and costly but is extended at both ends by relatively cheap and more plentiful land telephone lines. The invention could therefore be used in the allocation of channels of the transatlantic communication system under the control of centres at one or both sides of the Atlantic. As the signals used on the signalling circuit have the form of high speed digital data signals they would be unsuitable for direct application to the telephone lines for transmission to the subscribers, and therefore the control centres would need to be provided with suitable forms of signal translator to reduce the conventional tones used to indicate the progress of setting up a telephone call.

A radiophone system using a signal channel for signalling purpose runs the risk that a faulty equipment or a malicious person could jam the channel by continuous transmission, thus preventing the use of the channel by any of the mobile subscribers. This may be guarded against either by fitting a special timer to each mobile equipment wich overrides other controls and disables the transmitter if it transmits for more than a predetermined interval of time when tuned to the control channel, and by inserting a special detector circuit in the common control centre which would detect the jamming and would designate a channel normally used for speech to become the control circuit instead. This information could be transmitted to the mobile equipments because the transmission from the base station would be on a different frequency from that used by the mobile equipments.

If the call traffic becomes too large for a single control circuit, the common control centre could be arranged to withdraw two of the channels from normal speech traffic and use it as an additional control circuit. The channel numbers of the additional control circuit could be broadcast on the original control circuit in place of idle slot signals.

Inevitably, the identification signals for the mobile subscribers will be relatively large, for example 17 bits, so as to enable the particular subscriber to be identified amongst all the others available to the telephone system.

However, during any particular set of calls a complete identification need not be used the whole time, and the common control centre could be arranged to issue a short identifying signal of, say, 8 or 9 bits for the duration of the particular call or call attempt. This short identification signal could be stored temporarily in the mobile equipment and additionally in the storage means of the common control centre. The use of a short identification signal in this way would release eight bits for each message used in setting up a call, and these bits could be used to provide additional checks on the data transmission and/or provide a wider range of messages without altering the length of the message. Alternatively the use of the short identification signal could be used to shorten the messages and enable more calls to be accommodated by the signalling circuit.

It may be desirable in a radiophone system to discourage subscribers from making their call of too long a duration, particularly if the call traffic density is high. Such discouragement could take the form of a spoken message to both subscribers superimposed on their conversation at the start or adding pips at increasing frequency as time progresses. Alternatively the conversation could be cut off by the common control centre at a predetermined time after a warning dignal to that effect. The duration of the call could be increased after a predetermined time, and this, could be combined with the pipped tones of increasing frequency referred to above. Where separate channel allocation is provided, at each request for a reversal of transmission direction the request for a new channel could be given low priority compared with other users after the call has progressed for a predetermined duration. the conservation will thus be punctuated by gaps waiting for a free channel which could be of longer duration than usual.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic view of a control centre controlling several base station radio sets; Figure 2 shows schematically a mobile radio set for use in the system of the invention; and Figure 3 shows a control centre of in which individual transmit and receive channels are indicated.

Before describing the embodiment of the invention it is helpful to refer in detail to Figure 1. In Figure 1 there is shown a common control centre 101 from which a group of land lines 102 extend to base station radio transmitter and receiver sets 1 03. Lines 104, interfacing with a public telephone network (not shown) are connected via a switch 105 to the radio sets 103, there being more lines 104 than radio sets 103. Each of the radio sets 103 is arranged to transmit on a first single frequency selected from a first group of frequencies and receive on a second single frequency selected from a second group of frequencies; the two single frequencies together define a communication with a mobile radio set can be established, the mobile being able to receive frequencies of the first group and transmit frequencies of the second group.

Attached o each line 104 is a detector 106 capable of detecting and differentiating between the various states of the line during call progress, these states being silent, ringing tone, engaged tone, number unobtainable tone, call answered etc. Outputs from these detectors 106 are monitored by a control unit 107 which generates messages corresponding to each change in the state of call progress on each telephone line 104. These messages are routed through an access switch 108 to the appropriate radio sets 103 and are thence transmitted multiplexed together to mobile radio sets (Figure 2) over a common time shared signalling channel using a particular one of the frequencies of the first group which is reserved for this purpose; one of the frequencies of the second group is reserved for signalling from the mobile set to the control centre 101.

When a call is answered, the control unit 107 selects a free speech communication circuit if one is available and allocates it to the call.

If no free circuit is available, an announcer 109 is switched to the appropriate telephone line 104 through a switch 110 and emits a distinctive audible signal until a circuit becomes available. At the same time the mobile radio set is informed of this state via the common signalling channel.

Figure 2 shows a mobile radio set which includes a radio section 201 having a transmitter 201T and a receiver 201R. The transmitter 201T is modulated via a lead 203 by a transmitter control section 205T while a call is being set up or via lead 202 by a microphone 204 when a call is in progress, according to the position of a selection switch 209 which is controlled by a general control unit 205G. The control section 205T interprets information from a number generator 208 for specifying a party to be called. The general control unit 205G can also activate either the transmitter 201T and the receiver 201 (R via leads 211 and 216.

An output 206 from the receiver 201 R is monitored by a data interpreter unit 2051 of the control section 205. A part 205C of the data interpreter unit interprets signals received via the common signalling channel concerning the state of call progress. Each of these signals is stored in a store 213 until displaced by a further signal or cancelled. The stored signal activates a local announcement generator 214 which gives its indication by producing dis tinctive tones at a reproducer 207.

When the call is answered and a communication circuit is allocated, the control section 205G switches tuning circuits of the transmiiter 201T and receiver 201R to the correct circuit via leads 212 and operates switches 209 and 210. The control section 205cm may include a decoder which responds to a code number identifying the communication circuit to energize one of the leads 212 which may be connected to operate switches bringing into circuit appropriately pre-set tuning capacitors in the transmitter 201T and the receiver 201 R Thus during the call set-up time, the transmitter 201T may be kept switched off by the control section 205G except when necessary briefly to reply on the common return signalling channel to control queries from the base station. The user is made aware of the progress of the call attempt by distinct audible signals from the announcment generator 214.

Typically, for a system having 8,000 mobile radiophone subscribers, it has been found that during a busy hour there are about 1,550 requests from mobile subscribers attempting to make calls, resulting in 1,050 effective calls; in addition, there are typically 780 calls for mobile subscribers, of which about 400 are acknowledged and lead to 260 effective calls.

The control unit 107 generates a digital common control signal at 1,200 bits per second using for each message a 40 mS time slot, i.e. 90,000 slots per hour. This control signal is time shared between six radio sets 103, one at each of six base stations like that shown in Figure 1 with a total cycle period of 3 seconds, associated losses taking up 14,400 slots out of the 90,000 slots available. Time sharing enables the radio set of a mobile subscriber joining the system to moniter the common signalling channel for signal strength and to select that base station which is the strongest or, if that one is full, the next strongest and so on.

To make a call to a mobile subscriber, the mobile radio set is signalled from each base station in turin, and ideally the mobile set returns an acknowledgement to the most suitable base station. In the event of no ackknowledgement being received, the procedure is repeated once or twice. If an acknowledgement is received, the mobile set is repeatedly signalled every 3 seconds from the selected station either until the handset in the radiophone is lifted whereupon the control unit 107 operates to allocate a speech communi- cation circuit, or until the call from the land side is abandoned. These initial and subsequent signals to mobile set account typically for 13,300 of the 90,000 slots per hour available in the common control signal.

To make a call from a mobile radiophone, the station selected as described above on the basis of the signal strength of the signalling channel is monitored by tlie data interpreter 2051 for a time slot which is empty. On an empty slot being found, the mobile transmitter 2011 transmits its identity and call requirement (obtained by the subscriber keying the pulse generator 208) on the common mobile signalling channel in a time slot which is two slots later than that one containing the empty slot indication. The mobile set then moniters the selected base station signalling channel and responds only to signals containing its identity.

Typically 13 such replies are made if a 6 second contact is maintoinetl, this accounting for the use of about 20l 50 of (lie available time slots.

There would be about 42,000 of the original 90,000 time slots per hour to be shared between 1 5 5() randomly occurring initial requests for a call to or front a mobile subscriber, the chance of two mobile sets attempting to use the same empty slot being about I in 27, which is acceptably small.

A suitable message length to occupy each 40 mS time slot is 32 bits, which at 1200 bit/s would take about 27 mS, the remaining 13 mS being used for power run-down of one transmutter, power run-up of a succeeding transmitter and the transmission of synchronising signals.

Given that the 32 bit messages generated at the base station allow 21 information bits and 11 check bits, 17 of the information bits are used for mobile identity, this allowing at least 131,000 identities. The remaining 4 infonnation bits give adequate scope for instructions and response. A typical list using decimal code number (instead of binary) is shown below: Base Station O Synchronizing Signal 1 Mobile X - please acknowledge this call and ring your bell.

2 Mobile X - we know you have handset lifted, but have no available circuit.

3 Mobile X - NEXT slot will contain your allocated circuit number.

4 Section Full 5 Empty Slot 6 Mobile Y - your request for a call is receiving attention.

7 Mobile Y -- send first half of your wanted number.

8 Mobile Y - send second half of your wanted number.

9 Mobile Y - no tone yet received frown land. Are you hanging on? 10 Mobile Y - engaged tone received from land. Are you hanging on? 11 Mobile Y - ringing tone received from land. Are you hanging on? (2) Mobile Y - caller has answered but no circuit available yet.

(3) Mobile Y - next slot will contain your allocated circuit.

12 Mobile Y - number tinobtainable tone received from land. Re place handset.

13 Mobile Z - we have registered you in this area.

14 Mobile Z - do you still need regist ration in this area? Mobile Station 0 Mobile X - acknowledges your call and is ringing.

1 Mobile X - acknowledges your call and has lifted handset in answer.

2 Mobile X - has received circuit allo cation correctly.

3 Mobile Y - wishes to make a call 4(orl)Mobile Y - understands signal, hand set still lifted.

(2) Mobile Y - has recieved circuit allo cation 5 Mobile Z - wishes to register/remain registered in your area.

In addition to the instructions listed above both base station and mobile must be able to send numbers. These are in binary coded decimal form (4 bits per decade) so that up to 5 digits are contained in a time slot. The check bits would follow these.

In the embodiment of the invention, referring back to Figure 2, a push-to speak button 215 provides a positive control enabling a subscriber to switch manually between the receiver 201 R and the transmitter 201T the receiver and transmitter being controlled by the control section 205G to be exclusively operable i.e. a simplex system. On pushing or releasing the push-to-speak button 215, the mobile radio transmitter transmits on the common mobile signalling channel that it no longer requires the particular speech channel allocated to it and the common control centre at these station reacts by returning this channel to a pool of free speech channels for use by another mobile.

In response to this information a pool of free speech channels appropriate to the desired conversation direction as indicated by the position of the button 215 is monitored at the base station and a free channel is allocated to the conversation, the number of the channel allocated being transmitted to the mobile set via the common mobile signalling circuit. The channel number signals or, if a channel in either direction is not available, a channel absent signal is transmitted to the mobiles on the common signalling circuit from a transmitter 31 1T and confirmation of receipt is returned to the base station via the receiver 311 R The construction of the common control centre shown in Figure 1 is not suitable for the invention because the transmitter and receiver are switched together. Figure 3 shows an example of a common control centre of the invention and shows telephone lines 304 at a control centre 301, the lines 304 being split into transmit and receive channels by hybrids 312 in order that they may be switched separately to transmit channels 302T and receive channels 302R by switches 305T and 305R respectively. An announcer 309 generates a warning via switches 310 to the telephone lines 304 whenever a channel in either direction is not available when required; similar warning signals are sent to the mobile concerned via the common control signal channel by means not shown.

The system is advantageous where there are more simultaneous telephone conversations than available radio circuits; there may also be some difference in the number of transmit and receive channels if the conversations are predominantly one way, i.e. either to or from the mobile radio sets.

Typically the system of the embodiment utilises at least one master signalling circuit per base station, each signalling circuit having a different pair of frequencies assigned to it.

The signalling procedures are much the same as described above with reference to Figure 1 except that to join the system a mobile monitors each base station master signalling channel in turn to find the strongest station. The base station master signalling channel and any subsidiary signalling channels are divided into numbered slots, there being each of them in turn as appropriate. Possibly up to 10 times slots would be taken while the mobile is waiting for its allocated slot to appear and the channel allocation process normally requires a further 6 time slots.

The mobile equipment may be provided with a button or switch which when the mobile user is dissatisfied with the quality of a call, e.g.

he is moving out of the area well covered by the base station transmission, he could press or operate, and the mobile would then either (a) return to the control channel and apply for a changeover as if it were a new call but using a slightly different code or (b) emit a special signal on the raffic channel, e.g. a combination of simultaneous tones not found in speech or music and hence detectable by suitable means at the base station or control. The central control terminal would reply with a new channel allocation.

If no new channel is immediately available either (a) the old one can be renominated or (b) the recorded announcement can be put on the land line and the call treated as any other awaiting a traffic channel. If the latter, priority can be given by placing the call at the head of the queue of waiters.

During a call a mobile user may find that a wanted correspondent of information has to be searched for at the land end. The channels could therefore be relinquished during this time. Thus the mobile could be provided with a button. etc., to be pressed when the user wished to suspend a call. Pressing the button would cause the mobile to emit a special signal either on the traffic channel or the control channel, and the central control would withdraw the channel or circuit for reuse by another mobile. Cessation of the call suspension will nearly always be due to the wanted land correspondent or information having been found. This can be detected either by placing a speech detector on the land connection at the control centre and allocating a new traffic channel in the event of any significant noise, or by playing a continuous announcement and rest cycle e.g. "Speak only if you are ready now" followed by a short silent period during which the speech detector is connected to line.

In the less likely case of the mobile user wishing to cease call suspension, this can be arranged via the common central channel.

To either case, if no radio channel is available and the end of the suspension period, it is dealt with by the "awaiting channel" procedure described above.

The techniques described herein can be used whenever it is desired to connect a scarce or expensive transmission medium in tanem with a cheaper or more plentiful medium, e.g. transatlantic transmission is scarce and costly but is extended at both ends by cheap land telephone links. On circuit is used as the control and all the others are traffic circuits used only when both correspondents are ready to converse.

Since the signals on the control circuit are in the form of high speed data unsuitable for the ear, they would be translated to the normal PSTN ear signals at each end of the link. The correspondents would thus hear tones with which they were familiar.

If the control channel receiver(s) at each base station also contain field strength measuring apparatus it is possible to reduce the complexity of the mobiles which is highly desirable because there are many more mobiles than base stations. The base station receivers could report by telemetry to the control centre, the field strength of every initial calling signal or call acknowledgement which they received, and the control centre could decide on the basis of this information the appropriate base station for the call instead of the mobile.

Sequential operation of the control channel has been described above. However, by careful equalisation of the line delays between the control centre and the various base stations it should be possible to signal from all stations simultaneously at rates approaching 1200 bit/s. This has two advantages over sequential signalling (a) extra control channel base stations (i.e. no traffic channels) can be added to the system for greater security and (b) air time is saved by omission of the base station switching periods and the sequential calling of mobiles, thus permitting more mobiles to be accommodated on the channel for a given bit rate.

A radiophone system using a single control channel for transmission by all mobiles runs the risk that a faulty mobile (or malicious person) may transmit continuously on the channel and thus prevent use of the channel by other mobiles. This may be guarded against in either or both of the two ways (a) by fitting a special timer to each mobile which turns it off if the mobile transmits for more than the permitted time when tuned to the control channel or (b) when the central control detects jamming it may designate a traffic circuit to become the control circuit instead. From the base station transmitters of the jammed circuit would be broadcast the number of the designated new control circuit instead of idle slot signals.

If the traffic becomes undesirably large for a single control circuit, the control centre could withdraw a circuit from those provided for speech traffic and nominate it as a control circuit. The circuit number of the new control circuit could be broadcast on the original control channel in place of idle slot signals.

The use of all 17 mobile identity bits in every signalling message to or from the mobile is unnecessary. If the mobile is provided with a memory, the central control could issue a short identity - say 8 or 9 bits - to last only for the duration of the call or call attempt. This can be done by addition of a further base station message. Use of this message as early as possible in the call attempt then saves about 8 bits per message for the rest of that call. These 8 bits may be used for: a. adding extra check bits to the messages.to improve security.

b. providing space for 8 extra bits of message to give a wider message range.

c. reducing the number of bits in all slots by about 3.

d. reducing the number of bits in certain slots by 8 and hence shorten the slot duration.

Long calls may be undesirable on a radiophone system if they prevent use of the system by others in heavy traffic situations. It may be advisable to provide means of long call discouragement such as a. before a radio circuit is allocated an announcement is made to the land correspondent requesting brevity.

b. cut off of the conversation after a prede terminal time, the cut off being preceded by a warning signal; c. superimposing pip tones on the conversation after a predetermined time. The pips may either be at regular intervals or at decreasing intervals.

d. increasing the tariff after a predetermined time. This could be conveniently combined with the pip tones of(c) above.

e. Where speech channels are allocated at each change of direction of communication, after a predetermined time, each request for a reversal of transmission direction could be given a low priority compared to other users. Thus the conversation would be punctuated with "waiting for channel" gaps to a much greater degree than usual.

WHAT WE CLAIM IS: 1. A communication system for providing communication between subscribers via a common control centre, the system including a plurality of subscriber's terminals, each terminal including i tunable radio transmitting and receiving means capable of providing communication of speech between the subscriber's terminal and the common control centre on any one of a purality of radio channels; ii signal generating means effective to generate and transmit a call request signal to request the control centre to set up a call between the subscriber and another subscriber, a first channel request signal to indicate that the subscriber is ready to speak, and a second channel request signal to indicate that the subscriber is ready to listen: iii tuning means responsive to a control signal from the common control centre to tune the radio transmitting and receiving means to a particular channel for communication thereon; wherein the control centre includes i call set up means responsive to the call request signal from the subscriber selectively to set up a call between the subscriber and another subscriber; ii memory means for storing data relating to calls which are in progress between subscribers; iii allocation means responsive to the memory means and to the first and second channel request signals from a particular one of the subscribers participating in each call to initiate the allocation of a radio channel for speech transmission from the particular subscriber on detection of the first channel request signal and to initiate the allocation of a channel for speech transmission to the particular subscriber on receipt of the second channel request signal, in such a way that for each call not more than one channel is allocated for speech transmission between a subscriber and the common control centre at any instant, and that no channel is allocated for speech transmission until a call has been set up; iv means responsive to the allocation means for transmitting selectively the control signal to a particular subscriber participating in a call to indicate which channel is allocated to the call at the time.

2. A system according to claim 1 wherein the subscriber's terminal has a manually operable switch means and the signal generating means generates the first and second channel request signals in response to the operation of the switch means.

3. A system according to claim 1 or 2 where in the call request and channel request signals from the subscriber's terminal are transmitted via a channel of the plurality of channels to the common control centre in the form of a combination of tones which would be unlikely to occur in speech.

4. A system sccrding to claim 1 or 2 wherein the call request and channel request signals from the subscriber's terminal are transmitted to the common control centre via a channel reserved for signalling on which the signals relating to a plurality of calls are transmitted in time division multiplexed form.

5. A system according to any preceding claim wherein the control signal further provides a representation of the progress in predetermined steps of the setting up of each call by the common control centre, the control signal having cyclically recurring parts respectively allocated to different calls being set up by the control centre; and each subscriber's terminal includes an indicator responsive to the control signal to produce a representation of the steps reached in setting up a call requested by the particular terminal.

6. A system according to claim 5 wherein the parts of the control signal are times slots and, when used, each time slot contains address information identifying the call to which it is allocated and message information indicating the step reached in setting up the call and each subscriber's terminal includes a control signal receiver, said control signal receiver including data interpreter section having a first unit tor interpreting that part of the control signal which contains address information so as to enable the particular subscriber's terminal to

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. may be used for: a. adding extra check bits to the messages.to improve security. b. providing space for 8 extra bits of message to give a wider message range. c. reducing the number of bits in all slots by about 3. d. reducing the number of bits in certain slots by 8 and hence shorten the slot duration. Long calls may be undesirable on a radiophone system if they prevent use of the system by others in heavy traffic situations. It may be advisable to provide means of long call discouragement such as a. before a radio circuit is allocated an announcement is made to the land correspondent requesting brevity. b. cut off of the conversation after a prede terminal time, the cut off being preceded by a warning signal; c. superimposing pip tones on the conversation after a predetermined time. The pips may either be at regular intervals or at decreasing intervals. d. increasing the tariff after a predetermined time. This could be conveniently combined with the pip tones of(c) above. e. Where speech channels are allocated at each change of direction of communication, after a predetermined time, each request for a reversal of transmission direction could be given a low priority compared to other users. Thus the conversation would be punctuated with "waiting for channel" gaps to a much greater degree than usual. WHAT WE CLAIM IS:

1. A communication system for providing communication between subscribers via a common control centre, the system including a plurality of subscriber's terminals, each terminal including i tunable radio transmitting and receiving means capable of providing communication of speech between the subscriber's terminal and the common control centre on any one of a purality of radio channels; ii signal generating means effective to generate and transmit a call request signal to request the control centre to set up a call between the subscriber and another subscriber, a first channel request signal to indicate that the subscriber is ready to speak, and a second channel request signal to indicate that the subscriber is ready to listen: iii tuning means responsive to a control signal from the common control centre to tune the radio transmitting and receiving means to a particular channel for communication thereon; wherein the control centre includes i call set up means responsive to the call request signal from the subscriber selectively to set up a call between the subscriber and another subscriber; ii memory means for storing data relating to calls which are in progress between subscribers; iii allocation means responsive to the memory means and to the first and second channel request signals from a particular one of the subscribers participating in each call to initiate the allocation of a radio channel for speech transmission from the particular subscriber on detection of the first channel request signal and to initiate the allocation of a channel for speech transmission to the particular subscriber on receipt of the second channel request signal, in such a way that for each call not more than one channel is allocated for speech transmission between a subscriber and the common control centre at any instant, and that no channel is allocated for speech transmission until a call has been set up; iv means responsive to the allocation means for transmitting selectively the control signal to a particular subscriber participating in a call to indicate which channel is allocated to the call at the time.

derive from the control signal the message information being addressed to it.

7. A system according to claim 5 or 6 wherein each subscriber's terminal includes audible tone generating means responsive to the indicator to provide an audible indication of the steps reached in setting up a call.

8. A system according to any preceding claim wherein the common control centre includes means for limiting the number of call requests which can be accepted by the centre when no speech channels are available.

9. A system according to claim 1, 2 or 3 wherein the control signal is transmitted to the subscriber's terminal via a channel reserved for signalling.

10. A system according to any preceding claim wherein the common control centre further includes estimating means responsive to the common control signal to estimate the time by which the setting up of a call from a particular subscriber to another will be completed, and the allocation means is responsive to the estimating means to allocate a speech channel to that particular subscriber from that estimated time.

11. A system according to any preceding claim wherein the common control centre includes a plurality of radio sets arranged in groups covering respective areas and means is provided for assigning a particular group of radio sets to a call in which is participating a subscriber located in the area covered by the particular group.

12. A system according to claim 11 wherein the means for assigning a particular group of radio sets to a call is responsive to a signal strength received from a subscriber participating in the call.

13. A system according to claim 12 wherein there is further provided means for reassessing the assigment of the subscriber to the group of radio sets in response to a signal requesting the reassessment from the subscriber's terminal.

14. A system according to any preceding claim wherein there is provided signalling channel transfer means effective to transfer at least part of the signals from one channel to another and inform the subscriber's terminals of the transfer.

15. A communication system according to claim 1 and substantially as herein described with reference to the accompanying drawings.