GB2227395A

GB2227395A - Improvements in or relating to communications systems

Info

Publication number: GB2227395A
Application number: GB8928483A
Authority: GB
Inventors: Raymond Thompson; Richard Andrew Bishop
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1986-02-28
Filing date: 1989-12-16
Publication date: 1990-07-25
Anticipated expiration: 2006-02-28
Also published as: GB8605083D0; GB2227395B; GB2188213A; GB2188213B; GB8928483D0

Abstract

A digital communications system comprises a plurality of single channel radio access terminals each including a transmitter and receiver. Central equipment is also provided which includes transmitters and receivers and arranged to interface the terminals with a switching arrangement. The system handles packet traffic, on channels having a finite bit rate capability. Each packet has an identity header for recognition by a particular terminal. The central equipment includes a coder arranged to code the packets so that the available channel capacity is occupied. <IMAGE>

Description

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 mobiie access to a digital communication system such as the Ptarmigan Communications System sold by The Plessey Company plc such a system is described in the following articles: SCRA. The User Requirement and Subsvstem Solution A.F. Evers. I.E.E. Conference Record "Communications 1976." The Design of VHF Radio Equipment for the Ptarmigan SCRA Subsystem. R. Thompson, I.E.E. Conference Record "Communications 1976." As currently designed and used.

such a system requires each mobile subscriber to have a transmitter/receiver and associated control and encryption equlNnent. 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 radio frequency required which is important because of availabilitv constraints and the systems capacity to survive iamming.

According to the present invention there is provided a digital communications system comprising at least one single channel radio access terminal for providing a digital pulse stream representative of input data and for communication with a central equipment provided with a digital communications channel having a finite bit rate capability, each radio access terminal is arranged to provide the digital pulse stream representative of the input data in a manner such that a plurality of the digital pulse streams may be accommodated by the digital communications channels. wherein the radio access terminal is provided with control circuitry, arranged to communicate with a multiplexer and coder arranged to establish a signal connection to a plurality of vocoders1 and the central equipment is provided with a plurlity of multiplexers. transmitters and receivers arranged for communication with the radio access terminal, said central equipment being provided with output means arranged to communicate with a switching arrangement.

According to the present invention there is provided a digital communications system comprising a plurality of single channel radio access terminals each including a transmitter and receiver means, and central equipment including transmitter and receiver means. and arranged to interface the terminals with a switching arrangement, wherein the system is arranged to handle packet traffic, each packet having an identity header for recognition by a particular terminal.

and, the central equipment is provided with a coder arranged to code the packets so that available channel capacity is occupied.

The opportunity to achieve this increased capability comes from two emerging technologies. narrow band voice terminals (vocoders) and packet switching systems. The following arrangements describe new ways of combining these technologies with mobile access systems such as SCRA systems, and should be read with reference to the drawings, wherein; Figure 1 shows an arrangement whereby several vocoders share on SCRA equipment, and, Figure 2 shows an arrangement handling packets of data coded to occupy six channels.

A wide variety of techniques 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 modulation coding requiring 16 kb/s. The system operates as follows.

Referring to figure 1, the standard SCRA terminal transmitterireceiver and control unit are supplemented with a multiplexericoder. This connects the 16 kb/s signal available from the SCRA terminal to the vocoders.

For instance if these narrow band vocoders operate at 2.4 kb/s up to six could be used simultaneously with an SCRA terminal, depending on the detailed system design. With the continuing work on narrow band vocoder technology this maximum number will increase as the bit rate of the output from the vocoders reduces. If a "Concentrator" function is provided in the multiplexer equipment a greater number of subscribers could be accommodated.

The standard SCRA Central equipment is supplemented with a pool' of multiplexers for use with multiple subscriber SCRA terminals affiliated through that SCRA Central equipment. In current designs.

the SCRA Central equipment can normally accept a 32 channel group connection ro the switch connecting the subscribers onto the main trunk network. When not operating in a Cluster mode, 18 of the channels are spare and can be used for the additional channels muliplexed together at Terminals.

As many subscribers on such a system will have 16 kb/s voice subsets, the SCRA Central equipment may provide code conversion, digital analogue-digital (DAD) could alternatively be provided in the switch.

An outline of system operation is given below: CALL SET UP FROM TERMINAL The first vocoder to come off-hook instructs the Terminal to seize a 16 kb/s channel to the Central equipment SCRA. The Switch identifies the terminal and instructs the Central equipment SCRA that it is a multiple subscriber terminal. A multiplexer is assigned to -hat channel. Dialled digits. originating from the vocoder are then sent from the Terminal to set up the required call. Calls from other vocoders can be set up through their separatelv multiplexed channels, with some minor changes to the normal exchanges between the Switch and the Central SCRA equipment.

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

CALL SET UP TO TERMINAL A called subscriber is recognised by the Switch as being at a multiple subscriber terminal. The Central SCRA equipment is instructed to set up the connection to the Terminal in the normal way. It then assigns a multiplexer to route the traffic on the appropriate channel. If the Terminal/Central SCRA connection has already been set up a call is routed directlv 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 equipment which can give improved range, resilience to jamming and voice qualitv. The Terminal easily ascertains how many vocoders are off-hook and the Central equipment can gain this information from the Switch or by monitoring signalling traffic. In this way use of the radio connection can be automatically optimised.

Many data sources such as computers and displays provide traffic formatted into packets. Where narrow band vocoders are used it is possible to treat this traffic in a packet mode. Packet traffic is often characterised by very low packet to space ratio and thus can be exploited within an SCRA system 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 equipment. Six operating Terminals will be demultiplexing their individual traffic out of the composite signal. In a system sending packet traffic the Terminals individuai packets can be coded up to occupy all six channels, as shown in Figure 2. This improves the error rate at the Terminals.

As packets carry identity headers the Terminals can select those intended for their 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 of the packet system the Terminals can be instructed when to transmit their packets. In this way several Terminals can share a single 16 kb/s radio channel. Instructions to transmit can originate at the Switch or Central equipment depending on the system design.

If the system is operating this way on the Terminal to Central SCRA link it is possible to alter the arrangement described in a) above. Packet traffic then shares a single channel from the Central SCRA equipment. In this way one of the Central channel pairs (transmit and receive) can serve "n" Terminals (or Terminal subscribers where submultiplexing as described in the article bv R.

Thompson referred to above is used).

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 of the invention.

Claims

1 A digital communications system comprising at least one single channel radio access terminal for providing a digital pulse stream representative of input data and for communication with a central equipment provided with a digital communications channel having a finite bit rate capability, each radio access terminal is arranged to provide the digital pulse stream representative of the input data in a manner such that a plurality of the digital data pulse streams are accommodated by the digital communications channel, wherein the radio access terminal is provided with control circuitry , arranged to communicate with a multiplexer and coder arranged to establish a signal connection to a plurality of vocoders, and the central equipment is provided with a plurlity of multiplexers, transmitters and receivers arranged for communication with the radio access terminal. said central equipment being provided with output means arranged to communicate with a switching arrangement.

2. A digital communications system as claimed in claim 1 wherein, to set up a call from a radio access terminal, a vocoder is arranged to instruct the radio access terminal to seize a channel for communication with the central equipment, the switching arrangement is arranged to identify the radio access terminal and to instruct the central equipment that the radio access terminal is a multiple subscriber terminal, and to assign a multiplexer to the channel to allow dialled digits originated by the vocoder to be sent from the radio access terminal to the central equipment.

3. A digital communications system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

3. A digital communications system as claimed in claim 1 wherein, to set up a call to a radio access terminal, the switching arrangement is arranged to recognise a called subscriber as a multiple subscriber terminal, and to instruct the central equipment to set up a connection to the radio access terminal, said central equipment being arranged to assign a multiplexer to route traffic through the appropriate channel to a desired vocoder.

4. A digital communications system comprising a plurality of single channel radio access terminals each including a transmitter and receiver means, and central equipment including transmitter and receiver means and arranged to interface the terminals with a switching arrangement. wherein the system is arranged to handle packet traffic, each packet having an identity header for recognition by a particular terminal. and, the central equipment is provided with a coder arranged to code the packets so that available channel capacity is occupied.

5 A digital communications system substantially as herein before described.

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

7. A digital communications system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A digital communications system comprising a plurality of single channel radio access terminals each including a transmitter and receiver means, and central equipment including transmitter and receiver means and arranged to interface the terminals with a switching arrangement, wherein the system is arranged to handle packet traffic on channels having a finite bit rate capability, each packet having an identity header for recognition by a particular terminal, and, the central equipment is provided with a coder arranged to code the packets so that available channel capacity is occupied.

2. A digital communications system substantially as hereinbefore described.