GB2217955A

GB2217955A - Communications systems

Info

Publication number: GB2217955A
Application number: GB8909015A
Authority: GB
Inventors: John Duncan Harvey; Philip Whitehead
Original assignee: DOWTY INFORMATION SYSTEMS
Current assignee: DOWTY INFORMATION SYSTEMS
Priority date: 1988-04-20
Filing date: 1989-04-20
Publication date: 1989-11-01
Anticipated expiration: 2009-04-20
Also published as: GB8909015D0; GB8809258D0; EP0374212A1; WO1989010668A1; GB2217955B

Abstract

A multiple access communications system for use between a base (A) and a plurality of substations (B1 to Bn) with a first channel (C1) for transmission of data from the base (A) to the substations (B1 to Bn) and a second channel (C2) for transmission of data from the substations to the base. Each channel possesses frames of predetermined time length which are synchronised with frames of the other channel. The frames of the second channel (C2) comprise a plurality of time slots (t), divided in reserved slots (1 to m) which are allocated for the transmission of data, control slots (1 to k) which are allocated to individual substations (B1 to Bn) or groups of substations for transmission of requests to use one or more of the reserved slots (1 to m), special slots (1 to L) which are present for use when appropriate to supplement the reserved slots (1 to m) or the control slots (1 to k).

Description

COMMUNICATIONS SYSTEMS Technical Field This invention relates to multiple access communication systems and a method of operating the same, especially systems which allow communication of data between one or more base stations and a plurality of substations or mobiles.

Multiple access communication systems and in particular those employing mobile radio channel assignment are well known and are often referred to as trunking systems. Problems associated with such systems include difficulties in allowing the substations randomly to access the base station especially at busy periods. Simultaneous attempts at access by two or more substations can lead to a collision and mutilation of the data in question and subsequent attempts at access can cause an increasing amount of collisions, further mutilation and ultimately instability of the whole system.

A polling system under which a substation communicates with a base station only on request by the base station can be secure and allow high capacity of use but suffers from the disadvantage of lengthy delays for any substations waiting to transmit.

Various attempts have been made to try and overcome the problems associated with wholly random access systems and with polling systems and these have centred on controlling the timing and/or manner of access of the base station by the substation through the use of prearranged protocols.

One of the first such protocols was termed "Pure Aloha" under which a substation transmIts a request to the base station to transmit data and waits for an acknowledgment of the request. If none is forthcoming, the substation waits a random time before resubmitting the request.

A modified protocol was termed "Slotted Aloha" under which users of the system were allowed to transmit requests within a discrete timeslot. A related protocol was termed "Framed Aloha" under which the base station transmitted a message on a signalling channel to indicate which times lotus (arranged within "frames" containing a predetermined number of time slots) would be available to substations for making requests.

However, all such systems generally were not particularly efficient under normal traffic conditions and became unstable under heavy traffic conditions.

A still further protocol termed "Dynamic Frame Length Aloha" attempted to take account of varying traffic conditions by varying the number of timeslots per frame on the basis of an evaluation of the level of use made of the previous frame.

However, there were again problems with this protocol because of the limitations dictated by the system on the degree of variation of the number of slots in a frame and on the complexity of the systems employing the protocol.

Disclosure of the Invention The present invention is based on a protocol which can deal effectively with multi-access communication without resort to the complexity of a variable number of slots per frame and generally provides an efficient communication system.

In accordance with the invention, there is provided a multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substations to the base, wherein each channel possesses frames of predetermined time length which are synchronised with frames of the other channel and wherein the frames of the second channel comprise a plurality of time slots of which, in use:: i) a predetermined number (herein termed the "reserved" slots) are allocated for the transmission of data, and ii) a further predetermined number (herein termed the "control" slots) are allocated to individual substations or groups of substations for transmission of requests to use one or more of the reserved slots, and iii) a further predetermined number (herein termed the "special" slots) are present for use when appropriate to supplement the reserve slots or the control slots.

In a typical system comprising a base station and 'n' substations, the substations can be divided into a number of groups so that this number does not exceed the number of reserved slots in a frame of the second channel. Each group is then allocated a particular control slot in each frame within which any member, i.e. substation, of that group is allowed randomly to attest to communicate with the base station.

If the communication from a substation includes a request subsequently to transmit data, an acknowledgment from the base station will generally include information regard ng which reserved slot or slots of a defines subsequent frame should be used for the tranwrission.

If traffic is moving freely between the base station and the substations, and ize-versa, and no individual user is suffering undue delay, the special slots of the second channel frames may remain unused. However, when some delay is detected by the system, then one or more special slots may be temporarily allocated either to the existing number of reserved slots (especially if the detected delay is mainly concerned with data transmission) or to the existing number of control slots (especially if the detected delay is mainly caused with access to the base station by the substations) or special slots may be allocated to both reserved and control slots as appropriate.

Thus the special slots make the system flexible in its response to the different traffic conditions that might otherwise cause delays in a system with a fixed number of slots per frame for submitting requests or transmission of data.

Description of the Drawings The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a multiple access communication system according to one embodiment of the invention, and Figure 2 is a schematic representation of the frame composition of the two channels in the system of Figure 1 for the transmission of data from the base to a plurality of substations and from each substation to the base.

The Best Mode of Carrying Out the Invention The illustrated system comprises a base station A and 'n' substations or mobiles B1 to Bn. The base station A has a radio transmitter Tx through which it communicates with all of the substations on a first radio channel C1, and the substations B1 to Bn all communicate with the base station A on a second radio channel C2 which is received by a radio receiver Rx at the base station. A control processor C at the base station controls operation of the system through channel Cl in response to communications received on channel C2 from the substations, as described hereafter.

Both radio channels C1 and C2 transmit data arranged within frames, as shown in Figure 2.

Generally, all the frames of each channel, and preferably the frames of both channels, are of fixed length or integral multiples thereof. The frames of the two channels are synchronised either "in phase", i.e. such that the frames of respective channels begin and end simultaneously, or alternatively "out of phase", i.e. such that the frames of the respective channels are offset by a predetermined amount of time.

Each frame of the second radio channel C2 comprises a plurality of time slots of which a predetermined number "m" of reserved slots 1 to m are allocated for the transmission of data, and a further predetermined number "k" of control slots 1 to k are allocated to individual substations B1 to Bn for the transmission of control, command and short data messages including requests to use one or more of the reserved slots. Generally, if there are more substations "n" than control slots "k" then the substations are divided into a number of groups 1 to k each associated with a respective control slot 1 to k in which the substations of that group can all make requests at random to use the reserved slots.The identity 1 to n of each substation is either implicit or it is allocated to it by the base station A at the time of registration when it communicates first or subsequently with the base station A. The group identity or control slot 1 to k is also normally allocated by the base station A.

Each frame of the first radio channel C1 therefore comprises data that prepares the substations for reception, timing data that signifies the start of a frame for synchronisation, substation identity data, and group data.

A communication from a substation B1 to Bn te the base station A can either be restricted to a request by the substation for permission subsequently to send data to the base station, or alternatively if the amount of data is sufficiently small to be included in a single slot, the substation may simply send that data.

Included in the latter would be simple signing on, and off the system, acknowledgments of information received from the base station and a variety of formal transmissions of that kind.

As with other random access systems, any communication from a substation to the base station which is not acknowledged because, for example, of mutilation caused by collision between two substations of the same group attempting simultaneously to use their common reserved slot must be re-transmitted.

If the communication from a substation includes a request subsequently to transmit data, an acknowledgment from the base station will generally include information regarding which reserved slot or slots 1 to m of a defined subsequent frame should be used for the transmission.

The second radio channel C2 also comprises a predetermined number "L" of special slots 1 to L which are used when appropriate to supplement the reserve slots m or the control slots k. For example, if there is a delay due to the volume of data being transmitted, then the base station A may allocate one or more special slots temporarily to the reserved slots to complete the transfer of data. Alternatively, if there is a delay caused by limited access to the base station by the substations, then the base station may allocate one or more special slots temporarily to the control slots. Additionally, the base station A may allocate special slots to both the reserved slots and control slots as appropriate.

The control processor C at the base station A analyses the communications traffic to identify delays and their causes and to initiate the appropriate allocation of special slots 1 to L.

Channel loading and corruption of data are both monitored as relevant causes of delays.

Algorithms within the control processor C determine the appropriate response in allocating the special slots. The control processor also performs all the other central control functions according to stored algorithms, for example, the acknowledgment of requests from the substations and the allocation of control slots and reserved slots to the substations. In carrying out these functions it will be appreciated that the control processor allocates information and/or identification codes to the slots of each channel.

Such information and/or identification codes commonly include: i) means to synchronise the slot either "in phase" or "out of phase", ii) means to identify the slot with respect to its frame and the system in general, iii) control information such as a destination address or addresses of individual substations or groups of substations, iv) source information if, for example, the origin of the data (or whatever) being sent in the slot would otherwise be unclear.

The use of such information/identification means associated with individual slots is known to those skilled in the art.

Claims

1. A multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substations to the base, wherein each channel possesses frames of predetermined time length which are synchronised with frames of the other channel and wherein the frames of the second channel comprise a plurality of time slots, characterised in that:: i) a predetermined number (m) of said time slots (t) are reserved slots (1 to m) which are allocated for the transmission of data, and ii) a further predetermined number (k) of said time slots (t) are control slots (1 to k) which are allocated tc individual substations (B1 to Bn) or groups of substations for transmission of requests to use one or more or the reserved slots (1 to m), and iii) a further predetermined number (L) of said time slots (t) are special slots (1 to L) which are present for use when appropriate to supplement the reserved slots (1 to m) or the control slots (1 to k).

2. A system as claimed in claim 1 characterised in that the base has a control processor (c) which allocates substations (B1 to Bn) to the reserved slots (1 to m).

3. A system as claimed in claim 2 characterised in that if the number of substations (B1 to Bn) using the system exceeds the number k of control slots (1 to k), the control processor (c) divides the substations into k groups each with a control slot (1 to k) allocated to it.

4. A system as claimed in claim 1 or 2 characterised in that the control processor (c) analyses the communications traffic on said other channel (C2) to identify delays in communications and their causes.

5. A system as claimed in claim 4 characterised in that the control processor (c), if it identifies a delay due te the volume of data being transmitted, allocates one or more special slots (1 to L) temporarily to the reserved slots (1 to m).

6. A system as claimed in claim 4 or 5 characterised in that the control processor (c) if it identifies a delay caused by limited access to the base (A) by the substations (B1 to Bn), allocates one or more special slots (1 to L) temporarily to the contrcl slots (1 to k).

7. A method of operating a multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substations to the base, wherein each channe possesses frames of predetermined time length which are synchronised with frames of the other channel and wherein the frames of the second channel comprise a plurality of time slots, characterised in that:: i) a predetermined number (m) of said time slots (t) are reserved slots (1 to m) which are allocated for the transmission of data, ii) a further predetermined number (k) of said time slots (t) are control slots (1 to k) which are allocated to individual substations (B1 to Bn) or groups of substations for transmission of requests to use one or more of the reserved slots (1 to m), and iii) a further predetermined number (1) of said time slots (t) are special slots (1 to L) which are present for use when appropriate to supplement the reserved slots (1 to m) or the control slots (1 to k).

8. A multiple access communications system substantially as herein described by way of example with reference to the accompanying drawings.

9. A method of operating a multiple access communications system substantially as herein described by way of example with reference to the accompanying drawings.