GB2188213A - Improvements in or relating to communications systems - Google Patents

Abstract

A digital communications system comprising a radio access terminal for providing a digital pulse stream representative of input data for communication with a central terminal. A digital communications channel is provided having a finite bit rate. The radio access terminal provides a digital pulse stream representing the input data in a manner such that a plurality of digital pulse streams can be accommodated by the digital communications channel. The system may also handle packet data transmissions. <IMAGE>

Description

SPECIFICATION Improvements in or relating to communications systems The present invention relates to improvements in or relating to communications systems.

Single Channel Radio Access (SCRA) may be used to provide mobile access to s digital communication system such as the Ptarmigan Communications System sold by The Plessey Company plc (see References). As currently designed and used, such a system requires each mobile subscriberto have a transmitter/receiver and associated control and encryption equipment. When the subscriber has a call in progress, either voice or data, a duplex 16 kb/s radio circuit is occupied.

The present invention proposes an improvement in this type of system to enable the number of subscribers supported by SCRA equipments to be substantially increased. This is important in reducing the amount of expensive equipment required. It also reduces the amount of radio frequency required which is important because of availability constraints and the systems capacity to survive jamming.

Accordingly, there is provided a digital communications system comprising a radio access terminal for providing a digital pulse stream respresentative of input data and for communication with a central terminal provided with a digital communications channel having a finite bit rate capability, wherein the radio access terminal is arranged to providethedigital pulsestream representative ofthe input data in a manner such that a plurality ofthe digital pulse streams may be accomodated by the digital communications channel offinite bit rate capability.

The opportunity to achieve this increased capabilitycomesfrom two emerging technologies, narrow band voice terminals (vocoders) and packet switching systems. The following configurations are new ways of combining these technologies with mobile access systems such as SCRA.

Awide variety oftechniques are now available which allow voice traffic to be carried by digital signals at rates of substantially less than 16 kb/s. The current SCRA equipment uses a delta modulaion coding requiring 16 kb/s. Figure 1 showsan arrangement whereby several vocoders can share one SCRAterminal equipment. The system operates as follows.

The standard SCRA terminal transmitter/receiver and control unit are supplemented with a multiplexer/coder. This connects the 16 kb/s signal available from the SCRAterminal to the vocoders.

For instance if these narrow band vocoders operate at2.4 kb/s upto 6 could beused simultaneouslywith an SCRA terminal, depending on the detailed system design. With the continuing work on narrow band vocodertechnologythis maximum number will increase as the bit rate ofthe outputfrom the vocoders reduces. If a "concentrator" function is provided in the multiplexerequipmenta greater numberof subscribers could be acommodated.

The standard SCRA Central equipment is supplemented with a 'pool' of multiplexersfor use with multiple subscriber SCRAterminals affiliated through the SCRA Central. In current designs, the SCRA Central can normally accept a 32 channel group connection to the switch connecting the subscribers onto the main trunk network. When not operating in a Cluster mode, 18 ofthe channels are spare and can be used forthe additional channels multiplexed together at Terminals.

As manysubscribers on such a systemwill will have 16 kb/s voice subsets, the SCRA Central may provide code conversion, digital analogue-digital (DAD) if required.

The "pool" of multiplexers/DAD could alternatively be provided in the switch.

An outline of system operation is given below: Call set up from terminal Thefirstvocoderto come off-hookinstructsthe Terminal to seize a 16 kb/s channel to the Central SCRA. The Switch identifies the terminal and instructs the Central SCRA that it is a multiple subscriberterminal. A multiplexer is assigned tothat channel. Dialled digits, originating from thevocoder are then sent from the Terminal to set up the required call.

Calls from othervocoders can be set up through their separately multiplexed channels, with some minor chages to the normal exchanges between the Switch and the Central SCRA.

The last vocoderto go on-hook instructs the SCRA terminal to shut down the connection to the Central in the normal way.

Call set up to terminal Acalled subscriber is recognised by the Switch as being at a multiple subscriberterminal. The Central SCRA is instructed to set up the connection to the Terminal in the normal way. It then assigns a multiplexerto route the traffic on the appropriate channel. If the Terminal/Central SCRAconnection has already been set up a call is routed directly through to the appropriate vocoder.

Variations on this arrangement are possible to optimise the use of the 16 kb/s radio connection. For instance if only one call is in progress the multiplexer can code the traffic in a way that occupies the full 16 kb/s. This will provide an improved error rate at the Central which can give improved range, resiliance to jamming and voice quality. The Terminal easily ascertains how many vocoders are off-hook and the Central can gain this information from the Switch or by monitoring signalling traffic. In this way use ofthe radio connection can be automatically optimised.

Many data sources such as computer and displays provide traffic formatted into packets. Where narrow band vocoders are used it is possible to treatthis traffic in a packet mode. Packet traffic is often characterised by very low packet to space ratio and thus can be exploited within an SCRAsystem in several ways.

a) Sharing of the central to terminal connections In normal SCRA operation six channels are time-multiplexed together and transmitted from the Central. Six operating Terminals will be demultiplexing their individual traffic out ofthe composite signal. In a system sending packettraffic to the Terminals individual packets can be coded up to occupy all six channels, (see Figure 2). This improves the error rate attheTerminals.

As packets carry identity headers the Terminals can select those intended fortheir own subscriber.

Terminals would transmit their traffic in the normal way, if required coding up packets to occupy available channel capacity.

b) Sharing of the terminal to central connection With suitable design ofthe packet system the Terminals can be instructed when to transmittheir packets. In this way several Terminals can share a single 16 kb/s radio channel. Instructionstotransmit can originate at the Switch or Central depending on the system design.

If the system is operating this way on theTerminal to Central SCRA link it is possible to alterthe arrangement described in a) above. Packettraffic then shares a single channel from the Central SCRA.

In this way one of the Central channel pairs (transmit and receive) can serve Un" Terminals (orTerminal subscribers where submultiplexing as described in reference 2 is used).

References 1. SCRA, the User Requirement and Subsystem Solution A.F. Evers, I.E.E. Conference Record "Communications 1976".

2. The Design of VHF Radio Equipmentforthe Ptarmigan SCRA Subsystem. R. Thompson, I.E.E.

Conference Record "Communications 1976".

Although the present invention has been described with reference to particular embodiments, it is to be understood that modifications may be effected within the scope ofthe invention.

Claims (10)

1. A digital communications system comprising a radio access terminal for providing a digital pulse stream representative of input data and for communication with a central terminal provided with a digital communications channel having a finite bit rate capability, wherein the radio access terminal is arranged to provide the digital stream representative ofthe input data in a manner such that a plurality of the digital data pulse streams may be accommodated bythe digital communications channel offinite bit rate capability.

2. A digital communications system as claimed in claim 1 ,wherein the radio access terminal includes a transmitter and receiver and associated control circuitry, the control circuitry being arranged to communicate with a multiplexer and coder which establishes a signal connection to a plurality of vocoders.

3. A digital communications system as claimed in claim 1 or 2, wherein the central terminal is provided with a plurality of multiplexers and transmitters and receivers arranged for communication with the radio access terminal, said central terminal being provided with output means for providing communication with a switching arrangement.

4. A digital communications system as claimed in claims 1 or 2, wherein the central terminal is provided a plurality of transmitters and receivers arranged for communications with the radio access terminal, said central terminal being provided with output means for providing communication with a switching arrangement including a plurality of multiplexers for routing digital signalsthrough the switching arrangement.

5. A digital communications system as claimed in claims 3 wherein, to set up a call from a radio access terminal, a vocoder is arranged to instructthe radio access terminal to seize a channel for communication with central terminal, the switching arrangement is arranged to identify the radio access terminal and to instruct the central terminal that the radio access terminal is a multiple subscriber terminal, and to assign a multiplexerto the channel to allow dialled digits originated by the vodcoderto be sent from the radio access terminal to the central terminal.

6. A digital communications system as claimed in claims3 or4, wherein, to setup a call to a radio access terminal, the switching arrangement is arranged to recognise a called subscriber as a multiple subscriberterminal, and to instructthe central terminal to set up a connection to the radio access terminal, said central terminal being arranged to assign a multiplexer to route traffic through the appropriate channel to the desired vocoder.

7. A digital communications system as claimed in claim 1, wherein the system is arranged to handle packet traffic, each packet having an identity header for recognition by the radio access terminals, the central terminal being provided with a coder arranged to code the packets so that available channel capacity is occupied.

8. Adigital communications system substantially as hereinbefore described.

9. A digital communications system substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

10. Adigital communications system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.