GB2111351A - Communication systems - Google Patents

Abstract

In communication systems in which a plurality of stations share a common transmission medium in time division multiplex each station is provided with means to identify an available time slot in the TDM frame and an unsynchronised clock pulse generator to time recurrences of that time slot.

Description

SPECIFICATION Communication systems The present invention relates to communication systems.

In particular the invention is concerned with communication systems of the kind in which a plurality of stations or ports of the system are arranged to utilise a common transmissin medium in time division multiplex.

According to one aspect of the present invention in a communication system in which any one station or port of the system is arranged to transmit to and/or receive from another station of the system information signals by way of a transmission medium, and in which a plurality of said stations may utilise said transmission medium in common in time division multiplex with each of said plurality of stations utilising said common transmission medium during an available time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, at least some of said stations include means responsive to information signals transmitted by others of said stations to identify an available time period, and unsynchronised means to generate timing signals of substantially a predetermined repetition rate by means of which the recurrence of said available time period in succeeding frames is identified and the transmission of information signals during said available time period initiated.

The transmission medium may be a common conductive path or set of conductive paths, such as a data highway or bus, and the information signals may be modulated on a carrier signal for transmission along said conductive path or paths.

Alternatively the transmission medium may be a radio transmission channel, the information signals being modulated for transmission between stations or ports of the system on a radio frequency carrier signal of a frequency common to at least a plurality of stations or ports of the system.

According to another aspect of the present invention in a cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio freqency carrier signal, and in which a plurality of such subscriber's sets may utilise the same carrier signal frequency in time-division multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency during a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, at least one unit of each subscriber's set includes means responsive to radio frequency carrier signals transmitted by units of other subscribers' sets to identify an available time period, unsynchronised means to generate timing signals of substantially a predetermined repetition rate, means responsive to said timing signals to identify the recurrence of said available time period in succeeding frames and means to initiate the transmission of modulated carrier signals during said available time period.

Preferably said means responsive to said timing signals includes counter means to count a predetermined number of said timing signals to define a time interval substantially of the duration of said sequence or frame.

A cordless telephone system in accordance with the present invention will now be described by way of example.

The system comprises a plurality of subscribers' sets having a base unit, which is linked to conventional exchange equipment by way of a line-pair, and a mobile unit which is linked by radio to the respective base unit. The subscribers' sets may all use the same carrier frequency or their respective radio links or may use one of a group of frequencies within an allotted frequency band.

The radio link between a base unit and the respective mobile unit has an effective range of say two hundred metres, and in general radio signals transmitted by the units of one subscribers set will be detectable by other sets within that range or in some cases well beyond that range. The sets are therefore arranged to operate in burst mode in time division multiplex, the speech signals originating at a mobile unit, for example, being sampled and pulse coded, and the coded signals generated during each of a succession of frame periods of approximately 20 milliseconds being stored for subsequent transmission as a group or burst at a high bit rate.

Each frame period is divided nominally into fifty time periods or slots, and the exchange of bursts of coded speech signals between the base unit and the mobile unit of any one subscribers' set may be arranged to take place in successive slots, so that the frame period effectively provides for twenty five two-way channels. Thus up to twenty five subscribers' sets may be used simultaneously within range of one another on any one carrier frequency.

The speech signals may be sampled at a rate of 8KHz and the magnitude of each sample coded to a four-bit code, so that during a frame period 640 coded speech bits are generated and these bits, together with bits for timing, alignment, identification and signalling, are assembled to form a burst of information signals totalling, say, 740 bits. At a bit rate of 2MHz this burst occupies approximately 370 microseconds of a 400 microseconds time slot, leaving a margin of some 30 microseconds for "guard band" purposes. If two adjacent slots are used for one subscriber's two-way or duplex channel these guard bands may be largely disposed at either end of the respective pair of bursts.

Each unit of a subscribers' set is provided with means to transmit and receive bursts of information signals, modulated onto a radio frequency carrier signal, and at least the base unit of each set is provided with means to monitor the radio frequency carrier signals transmitted by the units of any other subscribers' sets within range that are in use.If a subscriber wishes to make a call, or if there is an incoming call to a subscribers set, the monitoring means is used to locate any period within the frame period which is of at least the duration of two time slots during which there are no detectable transmissions to identify a period which is available for the subscriber to use An internal clock, which may be free-running but crystal-controlled to fairly close tolerance, is then used to time a period substantially equal to the frame period of 20 milliseconds from the start of the available period referred to above to mark the recurrence of that period in the next frame sequence of the subscribers' sets that are in use.If the monitoring means shows that the "available" period is then still available the subscribers' set can come into use, with say the base unit transmitting a burst to the mobile unit followed immediately by the mobile unit transmitting a burst to the base unit.

The frame period timed by the internal clock of any one subscribers' set may differ from the frame periods timed by the respective clocks of the other subscribers' sets that are in use. The internal clock is therefore used to time a short delay, once the available period has commenced, before transmission is commenced, to act as the guard band referred to above. Once the available period has been taken into use the monitoring means of any nearby set which is waiting to come into use will register the activity during that period and for the duration of the call will have to look elsewhere in the frame for an available slot or pair of slots.

Since each subscribers' set which is active times its own frame period, guard band and burst transmission, without synchronisation from any outside source or from other subscribers' sets, the actual time interval before the frame cycle repeats is dependent upon the internal clocks involved, and where all the available time slots are in use the time interval, which is nominally equal to the frame period, will be effectively the sum of all the respective time slots and guard bands between the time slots as timed by the sets that are in use.

The 100 bits added to the 640 coded speech bits in each burst may be allotted as follows:- 32 bits to allow the clock oscillator of a mobile unit to lock onto the clock frequency of the respective base unit.

12 bits to provide a sequence identifying the start of a channel.

40 bits to provide a unique identification codeword for each base unit and its associated mobile unit, and 1 6 bits for signalling, dialling and status indication.

It will be appreciated that the self-timing arrangement described above may be utilised in any communication system in which a plurality of stations share a common transmission medium in time-division multiplex, whether the medium is a radio frequency or other form of carrier signal, radiated or conducted on a common transmission path, or simply a common conductive path over which the coded signals may be exchanged.

Where data from any one of a plurality of different sources is to be transmitted to any one of a plurality of destinations the transmission system comprises a plurality of ports interconnected by a common data highway. Each port includes a control arrangement to monitor the activity on the data highway, to identify which ports are in use and to determine which time slots in the frame period are free. Each port is arranged to receive from the source that it serves a generally continuous stream of data bits, these bits being stored at the port, or assembled, to be transmitted to the required destination port as a burst of pulses at a much greater bit rate than the incoming stream. As in the case of the cordless telephone system described above each port includes an asynchronous but accurate high frequency oscillator to time the frame period for that port and to generate the high bit rate clock signals to time each burst of pulses transmitted by that port. Because of the asynchronous operation the system does not require a master timing source or an absolute time reference.

The ports need not be uniquely associated with respective sources, but may be connected to a source or to a destination when required.

Claims (8)

1. A communication system in which any one station or port of the system is arranged to transmit to and/or receive from another station of the system information signals by way of a transmission medium, and in which a plurality of said stations may utilise said transmission medium in common in time division multiplex with each of said plurality of stations utilising said common transmission medium during an available time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, wherein at least some of said stations include means responsive to information signals transmitted by other of said stations to identify an available time period, and unsynchronised means to generate timing signals of substantially a predetermined repetition rate by means of which the recurrence of said available time period in succeeding frame is identified and the transmission of information signals during said available time period initiated.

2. A communication systemin accordance with Claim 1 wherein the transmission medium is a common conductive path or set of conductive paths.

3. A communication system in accordance with Claim 2 wherein the information signals are modulated on a carrier signal for transmission along said conductive path or paths.

4. A communication system in accordance with Claim 1 wherein said transmission medium is a ratio transmission channel, the information signals being modulated for transmission between stations or ports of the system on a radio frequency carrier signal of a frequency common to at least a plurality of stations or ports of the system.

5. A cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio frequency carrier signal, and in which a plurality of such subscriber's sets may utilise the same carrier signal frequency in timedivision multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency during a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, wherein at least one unit of each subscriber's set includes means responsive to radio frequency carrier signals transmitted by units of other subscribers' sets to identify an available time period, unsynchronised means to generate timing signals of substantially a predetermined repetition rate, means responsive to said timing signals to identify the recurrence of said available time period in succeeding frames and means to initiate the transmission of modulated carrier signals during said available time period.

6. A cordless telephone system in accordance with Claim 5 wherein said means responsive to said timing signals includes counter means to count a predetermined number of said timing signals to define a time interval substantially of the duration of said sequence or frame.

7. A cordless telephone system in accordance with Claim 6 wherein said counter means is arranged to count a further predetermined number of said timing signals after the commencement of said available time period, before the initiation of transmission of modulated carrier signals, to define a guard band or period.

8. A cordless telephone system substantially as hereinbefore described.