GB2260881A

GB2260881A - Trunked system fallback operation

Info

Publication number: GB2260881A
Application number: GB9122761A
Authority: GB
Inventors: Timothy S Klein
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1991-10-26
Filing date: 1991-10-26
Publication date: 1993-04-28
Anticipated expiration: 2011-10-26
Also published as: HK1000623A1; GB9122761D0; GB2260881B

Abstract

A trunked radio system comprises a control station (12) and at least one subscriber unit (21) arranged to communicate over a plurality of channels (15-20). The control station (12) has means (14) for instructing the subscriber unit (21) to enter a fallback mode. The subscriber unit (21) has means for entering a fallback mode upon receipt of that instruction, whereby the subscriber unit (21) switches to a fallback channel. The control station (12) has means (14) during normal non-fallback operation for transmitting a message designating the fallback channel to be used in the event of entering fallback mode. The subscriber unit (21) has means for storing this fallback channel designation in memory (28) and for referring to it when instructed to switch to fallback mode, wherein the subscriber unit (21) tunes to the fallback channel stored in memory (28). <IMAGE>

Description

TRUNKED SYSTEM FALLBACK OPERATION FIELD OF THE INVENT10N This invention relates to a trunked radio system.

BACKGROUND OF THE INVENTION Trunked radio systems, specifically those adhering to MPT1327 protocol and MPT1343 performance specification, have a facility for providing a reduced level of operation for subscriber radio units. This reduced service is provided directly from a controlling base station when the system is subject to a partial failure. This mode of operation is termed "Fallback mode" and is invoked by the transmission of an 'Enter fallback' message from a control channel of the base station. Upon receipt of this message all subscriber radio units monitoring the control channel tune to a predefined "Fallback channel". The frequency of this channel is base station and control channel specific, such that each control channel of each base station may have a different fallback channel.

Each subscriber radio has a pre-set fallback channel assignment corresponding to the fallback channel of the area with which it is likely to be used.

A problem with this arrangement is that when a radio is located in an area wherein it monitors a control channel which has a fallback channel different from the one set in the radio, if an 'Enter fallback' message is received, the radio will not tune to the correct fallback channel, but to an erroneous channel, not providing a service.

SUMMARY OF THE INVENTION According to the present invention there is provided a trunked radio system comprising a control station and at least one subscriber unit arranged to communicate over a plurality of channels, the control station having means for instructing the subscriber unit to enter a fallback mode, the subscriber unit having means for entering a fallback mode upon receipt of that instruction, whereby the subscriber unit switches to a fallback channel, the control station has means during normal non-fallback operation for transmitting a message designating the fallback channel to be used in the event of entering fallback mode, the subscriber unit has means for storing this fallback channel designation in memory and for referring to it when instructed to switch to fallback mode, wherein the subscriber unit tunes to the fallback channel stored in memory.

According to the present invention there is provided a subscriber radio unit comprising a transceiver for transmitting and receiving messages over a plurality of channels, a memory for storing the channel value received from a periodic fallback channel designation message and means for switching to a fallback mode upon receipt of a fallback command, wherein the fallback channel used is the channel value stored in memory.

The means for transmitting and receiving control and fallback messages is preferably a control channel allocated to the control station.

Preferably the means for transmitting the fallback channel designation includes a facility for assignment of a plurality of fallback channels to a plurality of pre-defined groups of subscriber units as a plurality of fallback groups.

Preferably the message designating the fallback channel is transmitted periodically during normal non-fallback operation or immediately prior to the system entering fallback mode.

The message designating the fallback channel to be used is preferably a Broadcast telegram in accordance with MPT1327 protocol, wherein the parameters field contains the fallback channel designation and the fallback group information.

BRIEF DESCRIPTION OF THE DRAWING An exemplary embodiment of the drawing will now be described with reference to the drawings in which; FIG.1 shows a block diagram of a preferred trunked radio system in accordance with the present invention, and; FIG.2 is a flow diagram of the preferred method of operation of a subscriber unit in accordance with the present invention, and; FIG.3 is a block diagram of a preferred Broadcast telegram message in accordance with the present invention.

DETAILED DESCRIPIION OF A PREFERRED EMBODIMENT Referring to FIG.1 a trunked radio system operating in accordance with MPT1327 protocol is formed by a main switch controller (MSC) 10 connected to a plurality of peripheral stations formed by trunked system controllers (TSC's) 12 and 27. Each TSC is connected to the MSC 10 by a wireline connection, 11 and 26 respectively. The TSCs 12 and 27 have substantially similar resources coupled to them but for clarity only those pertaining to the TSC 12 are to be described.

The TSC 12 is coupled to a number of control channel resources formed by base stations 13 and 14, typically conventional transceivers. In this manner the TSC 12 has means for transmission and reception of control messages on a number of dedicated control channels.

The TSC 12 is also coupled to a plurality of traffic transmission and reception frequency resources 15 to 20, typically conventional transmitters and receivers. In this manner the TSC 12 has access to a number of frequencies which are arranged as separately selectable paired channels comprising transmission and reception frequencies.

A number of subscriber units 21 and 22 each have a memory resource 28 and 29 respectively, and transmitters and receivers capable of variable frequency selection matching the frequencies of the resources 13 to 20, such that they transmit and receive control messages on one of the control channels created by the base stations 13 and 14, and can transmit and receive traffic messages to and from the TSC 12 via any one of the traffic channels created by the resources 15 to 20.

Each subscriber unit monitors a control channel. In this description reference will be made to the control channel resource 14, providing the control channel for the subscriber units 21 and 22.

The assignment of channels is controlled by the TSC 12 which communicates with the subscriber units 21 and 22 by sending control messages on the control channel, indicating to them the traffic channel which they must tune to.

Fall back operation may be required in circumstances when it is not possible to successfully assign a plurality of channels and so achieve trunking because of a malfunction. In this situation a limited communication service is maintained with the use of fallback operation, whereby the system switches to fallback mode and every subscriber unit monitoring the control channel uses the fallback channel to provide a reduced service in accordance with fallback operation.

Referring to FIG.1 and FIG.2, a subscriber unit 21, is established on the control channel, arrow 30. It commences normal operation 31, facilitating transmission and reception of control messages to or from the TSC 12 via the control channel of the base station 14 (arrows 23 and 24 respectively) and transmission and reception of user traffic via the traffic resources of the TSC 12 (arrows 25 and 26 respectively).

A Broadcast telegram message is transmitted on the control channel, announcing the current fallback channel for the base station 14 to be used in the case of fallback operation.

The subscriber 21 receives this message (box 32), stores to value of the designated fallback channel in its memory 28 (box 33) whilst maintaining a normal service in accordance with the state described by box 31. It will be appreciated that the Broadcast telegram message announcing the fallback channel may either be transmitted periodically during normal operation, or immediately prior to the base station 14 entering fallback mode. In the case of the latter, the telegram is required to be transmitted only once. For this situation, the base station 14 includes means to store the fallback channel allocation, such that if the coupling between the base station 14 and the TSC 12 is inoperative, the fallback channel assignment remains accessible to the base station 14.

If the base station 14 is required to switch to fallback mode, it sends an 'Enter fallback mode' message (in MPT1327 protocol an 'ALHF' telegram) on the control channel (arrow 24). The subscriber 21 receives this message (box 34) and refers to its memory 28 to obtain the previously designated fallback channel (box 35). The subscriber unit 21 tunes its transceiver to this channel (box 36) and resumes operation in fallback mode, providing a service in accordance with standard fallback operation (box 37).

If the base station 14 switches back to normal operation, it sends a 'Resume normal mode' message (in MPT1327 protocol a 'Move' or 'Clear down' telegram) on the fallback channel (arrow 26).

The subscriber unit receives this message (box 38) and reverts to normal operation (box 31).

It will be appreciated that a default fallback channel is pre-set in the subscriber unit. The value of this channel is determined by the area in which the subscriber unit is likely to be used. If a subscriber unit has a default channel setting for one area, such as that of base station 13 and is monitoring the control channel of base station 14, which has a different fallback channel from that of station 13, no service would be available in fallback mode using the default channel. However, by transmitting a message in advance of an 'Enter fallback mode' telegram, which designates the fallback channel for the subscriber units during normal operation in accordance with this invention, a service is available to the subscriber unit in the event of fallback operation, because the subscriber unit is aware of the fallback channel in use.

Referring to FIG.1 and FIG.3 a message is transmitted by the base station 14 in accordance with MPT1327 protocol. The SYS field of the transmission identifies the system to which the transmission pertains. The transmission is recognised as miscellaneous message by the value 000 in the CAT field, and the value 11 in the TYPE field. The message is defined as a Broadcast telegram by the FUNC field value 100. The SYSDEF field conventionally has no assignment at the value 10000, and would therefore be used in accordance with the present invention to define the telegram as a Fallback channel assignment message. The channel assignment field, CHAN, within the PARAMETERS field contains the value of the designated fallback channel. If the CHAN field is zero then the subscriber unit is to use its own pre-set fallback channel.

The system may be arranged to divide the subscriber units into groups, wherein each subscriber unit has a pre-set group number. The group to which the assignment is applicable is identified in the FALLBACK GROUP field consisting of three bits, where; 000 denotes all groups; 001 denotes group 1 only and so on according to binary notation up to the 7th group denoted by 111.

The facilities available on the fallback channel are indicated in the FACILITIES field consisting of three bits, where; 000 denotes a traffic channel; 001 denotes RQE's (emergency requests) only, and; 010 denotes RQE's and RQS's (emergency and simple requests) only, and;.

all other values are available for future customisation.

There is also a field reserved for future use, RSVD which holds the default value 00.

It will be appreciated that although MPT1327 is a protocol for trunked radio systems, this invention is not limited to said systems and is applicable to any trunked radio system which can provide a fallback mode of operation. Moreover, this invention is not restricted to any one format of fallback operation or any control telegram protocol.

Claims

1. A trunked radio system comprising a control station and at least one subscriber unit arranged to communicate over a plurality of channels, the control station having means for instructing the subscriber unit to enter a fallback mode, the subscriber unit having means for entering a fallback mode upon receipt of that instruction, whereby the subscriber unit switches to a fallback channel, the control station has means during normal non-fallback operation for transmitting a message designating the fallback channel to be used in the event of entering fallback mode, the subscriber unit has means for storing this fallback channel designation in memory and for referring to it when instructed to switch to fallback mode, wherein the subscriber unit tunes to the fallback channel stored in memory.

2. A subscriber radio unit comprising a transceiver for transmitting and receiving messages over a plurality of channels, a memory for storing the channel value received from a periodic fallback channel designation message and means for switching to a fallback mode upon receipt of a fallback command, wherein the fallback channel used is the channel value stored in memory.

3. The system or radio of claim 1 or claim 2 wherein the means for transmitting and receiving control and fallback messages is a control channel allocated to the control station.

4. The system or radio of claim 1 or claim 2 wherein the means for transmitting the fallback channel designation includes a facility for assignment of a plurality of fallback channels to a plurality of pre-defined groups of subscriber units as a plurality of fallback groups.

5. The system or radio of any preceding claim wherein the message designating the fallback channel is transmitted periodically during normal non-fallback operation.

6. The system or radio of any of the claims 1 to 4 above wherein the message designating the fallback channel is transmitted immediately prior to the system entering fallback mode.

7. The system or radio of any preceding claim wherein the message designating the fallback channel to be used is a broadcast telegram in accordance with MPT1327 protocol.

8. The system or radio of claim 7 wherein the parameters field of the MPT1327 Broadcast telegram contains the fallback channel designation.

9. The system or radio of claim 7 or claim 8 wherein the parameters field of the MPT1327 Broadcast telegram contains the fallback group information.

10. A trunked radio system herein described with reference to and in accordance with FIG.1 and FIG.2 of the drawing.

11. A subscriber radio unit herein described with reference to and in accordance with FIG.1 and FIG.2 of the drawing.