EP2087750A1

EP2087750A1 - Communications systems

Info

Publication number: EP2087750A1
Application number: EP07824287A
Authority: EP
Inventors: Mark Wentworth Rayne; Andrew Iain Nicol
Original assignee: Sepura Ltd
Current assignee: Sepura Ltd
Priority date: 2006-10-23
Filing date: 2007-10-23
Publication date: 2009-08-12
Also published as: GB2443320A; GB0720747D0; GB2443320B; WO2008050110A1; GB0621047D0

Abstract

A TETRA system (1) has TETRA V+D radio coverage as far as a boundary (3). A TETRA terminal (7) operating in direct mode can set up a circuit mode data call with a packet mode (gateway 6). The TETRA DMO terminal (7) can then send data packets to the packet mode gateway (6) through the DMO circuit mode data channel. The packet mode gateway (6) sends the data packets on to, e.g., a database (1), via the TETRA V+D infrastructure using TETRA packet data protocol. Alternatively terminal (7) can send the data packets to gateway (6) inside specially labelled SDS messages. This provides a method of extending packet data coverage to black spots in the TETRA V+D coverage.

Description

Communications Systems

The present invention relates to mobile communications systems and in particular to the transmission of packet data in mobile communications systems .

Many mobile communications support the transmission of packet data and the use of packet-data using applications. For example, the TETRA (TErrestrial Trunked RAdio) systems support the transmission of packet data in its V+D (voice and data) trunked networks . It is also known to have mobile communications systems in which mobile terminals can operate independently of the fixed (trunked) network infrastructure. An example of this is TETRA direct mode operation (DMO) . The Applicants have realised that it would be desirable to be able to transmit packet data in a system where communications terminals can operate in a so-called direct mode. However, direct mode operation, such as TETRA DMO, does not normally support the transmission of packet data.

According to a first aspect of the present invention, there is provided a method of operating a mobile communications system in which mobile communications terminals can transmit to each other independently of the fixed network infrastructure, the method comprising: transmitting packet data from or to a mobile terminal that is operating independently of the fixed network infrastructure by sending that data through a non-packet data communications channel that is Supported for transmissions independent of the fixed network infrastructure. According to a second aspect of the present invention, there is provided a mobile communications terminal of a communications system, the mobile terminal comprising: means for transmitting to another mobile terminal independently of the fixed network infrastructure of the communications system; and means for, when the mobile terminal is operating independently of the fixed network infrastructure, transmitting packet data or receiving packet data by sending that data through a non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure . In the present invention, packet data is transmitted by or to direct mode operating terminals (units) using existing communications protocols supported in direct mode operation, by sending the packet data over the existing direct mode communications channels. In other words, the packet data is transmitted (or received) by tunnelling the packet data over an existing (already supported) direct mode communications channel or channels (using existing direct mode communications protocols and already specified and supported communications messages or data bursts) .

By "tunnelling" is intended, as is known in the art, the concept of embedding or placing the packet data inside another (the non-packet data) communications protocol (communications message and/or data burst) . Thus, for example, the packet data is embedded in the user or information part of the communications protocol (such as an SDS message) and/or sent via a (non-packet data) circuit mode connection (data call) . This can be used to allow, for example, packet data transmission to be extended to direct mode operating terminals, but without the need to specify or support a specific packet data protocol for direct mode operation. The packet data can be any suitable such data, such as, e.g., IP data. The terminal can preferably both send and receive packet data in this way. Preferably the terminal can send and receive data consecutively or concurrently, as desired.

The communications channel (protocol) that the packet data is sent (tunnelled) over can be any suitable such channel (protocol) that is supported for a direct mode operating communications terminal . Thus , in the case of a TETRA system, for example, it could comprise, and preferably does comprise, using a circuit mode connection (e.g. a circuit mode data call) and/or one or more SDS messages (both of which are already supported for TETRA direct mode operation) to transmit the packet data.

Thus, in one particularly preferred embodiment, the packet data is transmitted by using a circuit mode call which is set up to transmit data.

Thus, according to a third aspect of the present invention, there is provided a method of operating a mobile communications system in which mobile communications terminals can transmit to each other independently of the fixed network infrastructure, the method comprising: transmitting packet data to or from a mobile terminal that is operating independently of the fixed network infrastructure using a circuit mode call. According to a fourth aspect of the present invention, there is provided a mobile communications terminal of a communications system, the mobile terminal comprising: means for, when the mobile terminal is operating independently of the fixed network infrastructure, transmitting or receiving packet data using a circuit mode call . Where the packet data is sent (tunnelled) using a circuit mode call, then any suitable and desired data exchange protocol can be used. Preferably the protocol includes acknowledgement and repeat mechanisms, numbering of data blocks and windowing, and/or flow-control mechanisms . Suitable such protocols would include PPP, X-modem, Z-modem, etc..

The protocol is preferably transparent (as the higher stack layers can ensure order and content are preserved) .

In a particularly preferred embodiment where packet data is sent using circuit mode data (a circuit mode call), forward data protection is used (i.e. extra data is embedded to help recovery at the receiving end) . This may help the reliability of the data transmission.

In another particularly preferred embodiment, the packet data is transmitted using a short data message (by embedding it in a short data message) , such as a text message, such as, and preferably, in a TETRA system, using the TETRA SDS (short data service) protocol .

Thus, according to a fifth aspect of the present invention, there is provided a method of operating a mobile communications system in which mobile communications terminals can transmit to each other independently of the fixed network infrastructure, the method comprising: transmitting packet data to or from a mobile terminal that is operating independently of the fixed network infrastructure using a short data message.

According to a sixth aspect of the present invention, there is provided a mobile communications terminal of a communications system, the mobile terminal comprising: means for, when the mobile terminal is operating independently of the fixed network infrastructure, transmitting or receiving packet data using a short data message.

Where transmission of packet data using a short data service, e.g., SDS messages in TETRA, is used, the short data messages preferably carry an appropriate indication, such as an appropriate protocol identifier (PID) in TETRA, indicating that they carry packet data. The SDS messages may also and preferably do carry sequence numbers to allow the packet data to be reassembled in the correct order.

Using, e.g., SDS messages to carry the packet data may be particularly advantageous, because it is a supported and reliable communications protocol and, e.g., would allow multiple terminals to transmit packet data to, e.g., a gateway, over a shared transmission channel .

It should also be noted that, as will be appreciated by those skilled in the art, a given packet data transmission could use a combination of communications protocols, such as a mixture of circuit mode calls and SDS messages . The choice of medium could, e.g., be made by configuration, or dynamically (in use) . For example, some data of a transmission , could be sent using one medium and then the transmission continued using a different medium.

It would also be possible to transmit data using two transmission media simultaneously (using mixed media) , such as by using both a circuit mode call and short (e.g. SDS) messages. Indeed, in a particularly preferred embodiment, the (packet) data transmission uses two different communications channels (protocols) , such as and preferably, a circuit mode call and short data (e.g. SDS) messages. In this case, a given terminal could, e.g., send packet data using two different communications channels (protocols) or could receive packet data using two different communications channels (protocols) or both. Most preferably in such an arrangement, one protocol (channel) is used to transmit data in one direction, and the other protocol (channel) is used for transmissions (e.g. signalling) in the other direction. Thus, in a preferred embodiment, the terminal will send (transmit) packet data using one communications channel (protocol) and receive (packet) data via (using) another (different) communications channel (protocol) .

For example, and preferably, a circuit mode connection may be used to send data in one direction, with short messages being used in the other direction, e.g., to carry data at a lower rate, and/or to send back (packet) acknowledgements of the data being transmitted by the circuit mode call. Thus, a preferred embodiment of the present invention comprises setting up a circuit mode connection (call) to perform a data, e.g., a file, transfer to or from the communications terminal, and then using short messages to send back packet acknowledgements, and/or to send data (in the other direction) at a lower rate. For example, in a TETRA system, as frame 18 is reserved for signalling from the recipient, it could be used to carry receipt acknowledgments by short (SDS) message to data being transmitted in circuit mode. This may help to enhance reliable reception of the data.

Indeed, it is preferred in general in the present invention for the recipient to return acknowledgements, preferably via a short data message or messages, of packet data that it receives. Thus, in a preferred embodiment, an acknowledgement or acknowledgements of packet data that is received are returned _.to the sender.

In one preferred embodiment, that is particularly applicable to a TETRA system, packet data block acknowledgements are sent using a stealing channel, preferably using the U-plane stealing channel (in a

TETRA system) . This would allow a fast switching method for packet data acknowledgements . Preferably, in a TETRA system this is done by inserting (i.e. not replacing user data) a MAC-U-SIGNAL PDU (Protocol Data Unit) , which is used, for example, and preferably, to allocate a timeslot (or multiple timeslots) for the receiving party to transmit a response, and in which the transmitting party pauses transmission. There are spare MAC-U-SIGNAL types which are reserved for proprietary use in TETRA and which could be used for this purpose. It will be appreciated that the packet data that is transmitted or received by the mobile terminal will have a destination or source, respectively. This could be, for example, another direct mode operating terminal such that, for example, and preferably, two DMO terminals can exchange packet data with each other by sending

(tunnelling) their packet data to each other either via a circuit mode connection or SDS signalling.

Thus, in one preferred embodiment, the packet data is transmitted (via the non-packet data communications channel) to or from another mobile unit (terminal) of the communications system (and preferably to or from another mobile terminal that is operating independently of the fixed network infrastructure (a direct mode terminal) ) . In one particularly preferred embodiment, the communication is between the mobile terminal and a device, such as, and preferably, a gateway, that has the ability to convert the packet data transmission between the protocol used to transmit it to or from the mobile terminal and another communications protocol, such as, and preferably, a packet data communications protocol.

Thus, in one preferred embodiment, the packet data is transmitted (via the non-packet data communications channel) to (or from) a device, preferably a gateway device, which device preferably can and preferably does, convert the packet data transmission between the protocol used to transmit it to or from the mobile terminal and another communications protocol, such as, and preferably, a packet data communications protocol. In one preferred arrangement of these embodiments the (gateway) device connects to a computer network, such as, and preferably, the Internet and/or an

Intranet. It preferably does so directly. This could then allow, e.g., direct mode operating terminals to connect (e.g. directly) to the Internet without going through the trunked voice and data network, and, e.g., accordingly provide a mechanism for extending direct mode operation packet data transmission to the interior of buildings .

Thus, in a preferred embodiment, the data transmission is to or from a device (a gateway device) that can connect, and that preferably ⁽ is connected, to a computer network independently of (and without the need to connect via) the communications system fixed infrastructure. The computer network is preferably the Internet and/or an Intranet. It is believed that such protocol conversion and gateway arrangements may be new and advantageous in their own right .

Thus, according to a seventh aspect of the present invention, there is provided an apparatus for use in a communications system, the apparatus comprising: means for converting a packet data transmission sent using a non-packet data signalling protocol to a transmission to be sent using a packet data signalling protocol and/or vice-versa. According to an eighth aspect of the present invention, there is provided a method of operating a communications system, the method comprising: converting a packet data transmission sent using a non-packet data signalling protocol to a transmission to be sent using a packet data signalling protocol and/or vice-versa. As discussed above, in a preferred embodiment of these aspects of the invention, the apparatus or method comprises means for or a step of converting a packet data transmission sent using a non-packet data signalling protocol independently of any fixed communications system infrastructure to a packet data communication to be sent via a computer network, such as, and preferably, the Internet and/or an Intranet (and vice-versa) . In these aspects and embodiments of the invention, the communications terminal is preferably a mobile communications system terminal or unit, preferably a TETRA terminal operating in direct mode, and the computer network is preferably the Internet and/or an Intranet.

In the above aspects and embodiments of the invention, the device with which the direct mode terminal is in communication could comprise, e.g., a computer or device coupled to a computer, which computer may then, for example, relay the packet data on to the

Internet or any other communications connection (such as WiFi or Bluetooth), to a wireless LAN, etc., that the computer may have, and vice-versa. Such an arrangement could and preferably does comprise, e.g., a PC connected to the Internet and/or to a private network (e.g. an

Intranet) by a LAN or wireless LAN. Such an arrangement could, e.g., effectively act as a direct mode to wireless LAN packet data gateway that could then be used indoors, and, e.g., provide a means of extending (e.g. TETRA) packet data coverage to the interior of buildings .

In one particularly preferred embodiment, the device with which the direct mode terminal is in communication comprises a gateway that can, for example, and preferably, establish a packet data connection with or via the communications system fixed infrastructure (such as, in the case of TETRA, a TETRA V+D network) . (As is known in TETRA, for example, so-called "gateways" may be used to link communications terminals operating in direct mode outside the fixed network coverage to the fixed network) . This would facilitate, for example, effectively extending packet data coverage of the fixed network beyond the direct coverage of the fixed (V+D) network .

In such an arrangement, the gateway could, in effect, and preferably does act as a converter between DMO circuit mode data and/or DMO SDS messages and the V+D packet data protocol, thereby allowing packet data to be tunnelled through a DMO circuit mode data traffic channel or SDS messages to the V+D network via the "packet data"^' gateway. Thus, according to a ninth aspect of the present invention, there is provided a gateway apparatus for use in a TETRA communications system, the apparatus comprising: means for converting packet data into SDS messages and/or circuit mode data for transmission in direct mode operation and/or vice-versa.

According to a tenth aspect of the present invention, there is provided a method of operating a TETRA communications system, the method comprising: converting packet data into SDS messages and/or circuit mode data for transmission in direct mode operation and/or vice-versa.

Thus, in a preferred embodiment, the communication is with and the system or method further comprises a gateway device that can convert the received non-packet data protocol communication (e.g. circuit mode data or short data service) to packet data for onward transmission and vice-versa. The onward transmission could be and preferably is, e.g., directly onto a communications network (e.g. a TETRA V+D network) , and/or via a computer interface (e.g. RS732 or USB or Bluetooth) to an application connected to the computer or any network supported by the computer, e.g., by Ethernet cable, dial-up MODEM, broadband, Wi-FI, GSM-GPRS, 3G or Bluetooth, etc..

In these aspects and arrangements of the invention, as will be appreciated by those js-kilied in the art, there could be direct communication between the direct mode operating terminal, and the, e.g., gateway, or such communication could be via a repeater or repeaters, etc.. Where a repeater is used, the repeater preferably has appropriate addressing and/or store and forward functions (e.g. for SDS messages) to enable it to pass on the packet data appropriately, and/or to, e.g., allow its buffer, etc., the data if a gateway, etc., is busy or otherwise unavailable. Where a "packet data" gateway device is used, then it could, for example, comprise two separate radio transceiver units, one for communicating with the direct mode operating terminals, and one for communicating on the fixed network (V+D) side, or a single transceiver that can switch between the two could be used.

In the case of using a single transceiver for both the direct mode and fixed network transmissions, as will be appreciated by those skilled in the art, the, e.g., gateway is preferably configured to align the timing of these transmissions appropriately. For example, in the case of a TETRA system, the V+D and DMO TDMA (time division multiple access) structures are preferably aligned so that timeslot 1 in the DMO transmission is delayed by 1 timeslot relative to the V+D uplink slot allocated as the packet data channel. Such timing alignment can be achieved, e.g., by adjusting the timing of the DMO channel as soon as the packet mode gateway is assigned to a packet data channel, for example using the timing adjustment signalling that is specified for voice calls through TETRA gateways. Where SDS messages are used, the direct mode terminal can discover the "packet-data" gateway's timing from the gateway's presence signals.

In the case of using SDS messages, the gateway,- etc., preferably can identify them as carrying packet data and then unpack the data, so as to allow it to be "converted" to the packet data protocol.

The gateway, etc., may, and preferably does, include, for example, a data buffer to allow it to • buffer the packet data, to facilitate the conversion process .

It is preferred for the, e.g., gateway to broadcast ^" its availability to users /terminals . Such "presence" broadcasts preferably include, for example, information or indications relating to one or more or all of:

• gateway present, gateway supports packet data in SOS messages, gateway supports packet data in circuit mode calls, gateway has spare PDP contexts, gateway has spare traffic capacity, etc.. Where, in the case of a TETPvA V+D system, a so-called packet data protocol context needs to be established with the fixed infrastructure, the, e.g., gateway, can preferably establish this context, e.g., on a requesting terminal's behalf. The call set-up (e.g. circuit mode data call set-up) could include signalling ■to indicate this request, and/or, e.g., to indicate that a circuit mode data call is required.

Where such a PDP context is established, the gateway, etc., can preferably maintain an association between the packet data protocol (PDP) context and the terminal in question, so that, for example, incoming packet data can be linked to the correct terminal. In .a particularly preferred such embodiment, the PDP context link is retained after the data transmission is finished, so that it can be used again in the future for the terminal in question. A set of such associations could be maintained, e.g., on a first-in, first out, priority-based or some other basis .

Where a PDP context is established, then in one preferred embodiment, the context may be deactivated as soon as the circuit mode data call is terminated, although it is preferred to allow the context to time out or to be able to be explicitly closed (which may, e.g., involve flushing out of any queued data and reporting back) . In the case of using. SDS messages, the context is preferably deactivated a predetermined, short, time period (e.g. 5 minutes) after the direct mode terminal stops communicating.

Alternatively or additionally, the contexts may be de-activated on the basis of the longest idle period when all the contexts are used up, or when there is only a small number of contexts remaining free (so that the gateway can attempt to advertise free context capacity rather than reserving capacity for idle contexts) .

Whether circuit mode data or SDS messages, etc., are being used to transmit the packet data it would not , be necessary to align the packet data boundaries with the circuit data blocks or SDS messages, etc., although this can be done and may help with the data handling, as will be appreciated by those skilled in the art. In a preferred embodiment, the terminal operating in- direct mode can and preferably does present a packet data interface to the (client) packet data applications running on the terminal (even though, as discussed above, it may not actually transmit that data using a true packet data protocol) .

^■Although the present invention has been described above with particular reference to packet data transmissions from a direct mode operating terminal to a given destination, it would also or instead be possible for the invention to be applied, analogously, to the reverse process of packet data that is to be transmitted to the direct mode operating terminal. in this case, incoming packet data would, e.g., be converted by the, e.g., gateway from a packet data protocol to a circuit mode data call or SDS messages and then transmitted accordingly to the direct mode terminal . Thus, the present invention also extends to the reverse processes of transmitting packet data to a terminal using the techniques described herein. It will be appreciated that in a preferred embodiment, and according to a further aspect of the present invention, the present invention provides a method of transmitting packet data in a mobile communications system comprising:

a) a DM-MS (Direct mode mobile station) detecting the presence of a packet-mode gateway,- b) a DM-MS application deciding it wants to exchange packet data with a V+D (voice+data) connected data service; c) the DM-MS requesting a circuit-mode call set up for packet data; d) a packet-mode gateway negotiating PDP (Packet data protocol) context activation with the V+D network, and informing the DM-MS that the call setup is successful; e) the DM-MS commencing sending circuit-mode data to the packet-mode gateway; f) the packet-mode gateway buffering the circuit-mode data and locating the first complete data packet; g) the packet-mode gateway asking the V+D network for permission to start transmitting the data packets, and getting allocated to a packet data channel; h) the packet-mode gateway sending timing adjustment messages to the DMO (direct mode operation) channel to align the DMO timing to the allocated packet data channel ; i) the packet mode gateway starting transmitting the data packets to the V+D network, while continuing to receive circuit-mode data containing further data packets; and, when the DM-MS has finished exchanging packet data with the V+D network, j) the DM-MS clearing the circuit-mode data call, and the packet-mode gateway deactivating the PDP context . ^■

Similarly, in another preferred embodiment, and according to a further aspect of the present invention, the present invention provides a method of transmitting packet data in a mobile communications system comprising:

a) a DM-MS detecting presence of a packet-mode gateway; b) a DM-MS application deciding it wants to exchange packet data with a V+D connected data service; ■ c) the DM-MS commencing sending SDS messages to the packet-mode gateway with the "packet data" PID; d) the packet-mode gateway extracting the data packets from the SDS messages and buffering them; e) the packet-mode gateway negotiating PDP context activation with the V+D network; f ) the packet-mode gateway asking the V+D network for permission to start transmitting the data packets, and getting allocated to a packet data channel ; g) the packet-mode gateway sending timing adjustment messages to the DMO channel to align the DMO timing to the allocated packet data channel; h) the packet mode gateway transmitting the data packets to the V+D network, while continuing to ■ receive SDS messages containing further data packets ; and i) when the DM-MS has failed to exchange any data packets with the packet-mode gateway for a short period of time (e.g. five minutes) , the packet-mode gateway deactivating the PDP context.

The communications terminal in the present invention can take any suitable or desired form. It should be a mobile terminal (mobile station) of a mobile communications system. The mobile station may, e.g., be portable or, e.g., vehicle mounted, etc., as is known in the art.

The various processes, etc., of the present invention to be carried out in or by the system infrastructure can be performed in any suitable and desired components of the system infrastructure and/or communications terminal .

The communications system of the present invention can be any suitable such system. The present invention is particularly applicable to mobile communication systems, such as the TETRA system. Thus the present invention also extends to a communications terminal and to a method of operating a communications terminal of a mobile communications system, and to a mobile communications system and a method of operating a mobile communications system, that is in accordance with and/or that can be operated in accordance with, the present invention.

Thus, the present invention also extends to a mobile communications system comprising one or more mobile communications terminals that are in accordance with the present invention.

In these aspects and embodiments of the invention, the communications system preferably comprises, as discussed above, a gateway apparatus or device to which packet data may be transmitted by a (and preferably each) mobile terminal when the terminal is operating independently of the fixed network infrastructure of the communications system by sending that data through a non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure, and/or from which packet data may be transmitted to a (and preferably each) mobile terminal , when the terminal is operating independently of the fixed network infrastructure of the communications system by sending that data through a non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure, and which gateway apparatus or device can convert or translate the packet data transmission between the protocol used to transmit it to or from the mobile terminal and another communications protocol. In these arrangements, the gateway apparatus or device preferably comprises, as discussed above, means for connecting to a computer network independently of the communications system's fixed network infrastructure, and/or means for establishing a packet data connection with or via the communications system's fixed infrastructure.

In all the aspects and embodiments of the present invention, the mobile communications system is preferably a TETRA system. As will be appreciated by those skilled in the art, all of the aspects and embodiments of the present invention described herein can and preferably do include, as appropriate, any one or more or all of the preferred and optional features of the invention described herein.

The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the method or a method herein described when installed on data processing means, a computer program element comprising computer software code portions for performing the method or a method herein described when the program element is run on data processing means, and a computer program comprising code means adapted to perform all the steps of a^' method or of the methods herein described when the program is run on a data-processing system. The invention also extends to a computer software carrier comprising such software which when used to operate a communications system or terminal comprising data processing means causes in conjunction with said data processing means said system or terminal to carry out the steps of the method of. the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out herein. The present invention may accordingly suitably be embodied as "a computer program product for use with a computer system. Such an implementation may comprise a series of computer readable instructions either fixed on a tangible medium, such as a computer readable medium, for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques . The series of computer readable instructions embodies all or part of the functionality previously described herein.

Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink-wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or .World Wide Web . A number of preferred embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows schematically the use of a. packet mode gateway;

Figures 2 and 3 show alternative embodiments of a packet mode gateway; and

Figure 4 shows a timing diagram for the embodiment of Figure 3. The preferred embodiments of the invention will be described with particular reference to a TETRA. mobile communications system. However, as discussed above, while the present invention is particularly applicable to TETRA mobile communications systems, as will be appreciated by those skilled in the art, it is not exclusive to those systems and can be applied to other mobile communications systems and communications systems equally.

Figure 1 illustrates the use of a packet mode gateway. A fixed data service 2 (e.g. a police database of stolen vehicles) has a wired connection to a TETRA

V+D network 1 using packet data. TETRA terminals 4 and

5 can interrogate the database 2 over the TETRA V+D radio interface using packet data protocol. However the TETRA V+D radio coverage only extends as far as boundary 3. Therefore TETRA terminals 7 and 8 outside boundary 3 are unable to communicate directly with the TETRA V+D network. However they are able to communicate with other TETRA terminals using direct mode operation (DMO) , can set up connections with each other to exchange circuit mode data, and can exchange DMO SDS (short data service messages) messages. Radio equipment 6 acts as a "packet mode" gateway that is able to operate both in TETRA V+D mode and TETRA DMO, either simultaneously, or quasi-simultaneousIy . In this embodiment as will be discussed further below, a TETRA terminal 7 'may set up a circuit mode data call with the packet mode gateway 6, and the packet mode gateway establishes a packet data protocol (PDP) context with the TETRA V+D network. Then when the TETRA DMO terminal 7 sends data packets to the packet mode gateway

6 through the DMO circuit mode data channel, the packet mode gateway sends the data packet on to the database 1 via the TETRA V+D infrastructure using TETRA packet data protocol (and vice versa) . Alternatively terminal 7 can send the data packets to gateway 6 inside specially labelled SDS messages .

This provides a method of extending packet data coverage to black spots in the TETRA V+D coverage.

Figure 2 shows one embodiment of how packet mode gateway 6 may be constructed. In this embodiment, the gateway 6 has two separate radio transceiver units . In the arrangement .shown in Figure 2, radio transceiver 21 operates on a TETRA direct mode net, communicating with TETRA DMO terminals using DMO frequencies and protocols 23. Radio transceiver 22 operates on a TETRA V+D network, communicating with the TETRA V+D network (and thus with various TETRA services, including data services 2) using TETRA V+D frequencies and protocols.24.

Where appropriate, the gateway arrangement illustrated in figure 2 may further include, e.g., filters or use increased the physical separation between the two antennas 29 and 30 to help reduce the possibility of receiver blocking, as is known in the art. Radio transceiver 21 can exchange data with TETRA DMO terminals using circuit mode data on a channel dedicated to exchange of data between two terminals (i.e. a "circuit"). It can also exchange SDS messages with terminals . When exchanging data with a DMO terminal (or

DM-MS - direct mode mobile station) a data exchange protocol 25 is used that allows blocks of data to be delivered reliably. This protocol includes acknowledgement and repeat mechanisms and numbering of data blocks and windowing mechanisms. Suitable examples of such a protocol include PPP, X Modem, Z-Modem etc.

The protocol used is designed to cope efficiently with the type of reliability problems encountered in mobile radio links and operates over a half-duplex link, where the data transmitter has to^' stop transmitting and switch to receive mode from time to time to receive acknowledgement messages from the data receiver. The protocol also provides flow control, so that the flow of circuit mode data can be halted if required to avoid buffer overflow in the packet data gateway 6.

In use of this arrangement, the application generating data in the DM-MS (e.g. terminal 7 in figure 1) would generate data packets (e.g. IP packets) . The DM-MS terminal 7 would then collect the IP data packets and send them through the circuit mode channel to the gateway 6 using the circuit mode data exchange protocol. In an alternative method, the data packets can be sent in SDS messages .

The DM-MS terminal 7 data exchange does not need to take particular account of the data packet boundaries and can send fixed length blocks of data, irrespective of the packet data boundaries. (Alternatively the circuit-mode data blocks may be aligned with packet boundaries, and this may reduce the time delay required to exchange end-to-end packet data acknowledgements.)

The receiving packet-mode gateway 6 then assembles the incoming blocks of data in their correct sequence (using the circuit-mode data exchange protocol 25) and stor.es them in a data buffer 27 in the gateway 6 in the correct sequence. The circuit-mode protocol module 25 then informs the V+D transceiver's packet data protocol module 26 via signalling inter-connection 29, that data is now available for transmission.

Similarly, if received by SDS messages, the contained data would be assembled in data buffer 27 in the correct sequence. ■ Packet data protocol module 26 now examines the contents of data buffer 27, and locates the packet data protocol headers (e.g. IP, TCP or UDP headers) . Then it extracts successive data packets and transmits them to the TETRA V+D network using the TETRA packet data protocol on a shared, radio channel (or packet data channel) that may also be used by other packet data users, as is known in TETRA.

In the TETRA V+D packet data protocol (EN 300 392-2 clause 28 (SNDCP) ) , a terminal sending packet data must first establish a PDP (packet data protocol) context by negotiation with the TETRA V+D network. Therefore when the DM MS terminal 7 first decides that it wishes to exchange packet data through the gateway 6, it may request the gateway 6 to create a PDP context .on its behalf. The request includes relevant information such as is described in the primitives in EN 300 392-2 clause 28 for the requesting PDP context activation by an application above the SNDCP layer. The request also includes the DM-MS's terminal 7 TETRA address so that the packet-mode gateway 6 can link incoming packet data to the correct DM-MS terminal 7 (however this is not necessary if the lifetime of the PDP context is limited to the lifetime of a DMO circuit mode data call) .

Some new signalling could be defined between the DM-MS terminal and the packet-mode gateway to allow the DM-MS to establish a "proxy" PDP context in the gateway. Alternatively, the packet mode gateway 6 could activate a PDP context automatically then the ^'DM-MS requests a circuit mode call setup or starts sending data.

The packet-mode gateway 6 is able to signal to advertise its ability to act as a packet mode gateway to nearby DM-MSs .

Once the PDP context has been set up, the packet mode gateway retains the PDP context (remembering the TETRA address of the applicable DM-MS) for future use by the DM-MS. TETRA V+D supports the existence of up to fourteen co-existing PDP contexts, so the packet data gateway 6 maintains PDP contexts for up to fourteen different DM-MSs at any one time. If a fifteenth DM-MS requests a PDP context, the packet data gateway 6 can refuse the fifteenth request or can erase the oldest or the least recently used PDP context, etc., as desired. Some kind of priority mechanism could also be used if desired.

Alternatively, in the situation where a DM-MS sets up a circuit-mode data call to the packet-mode gateway 6 or sends to the packet mode gateway an SDS message containing packet data, and the packet-mode gateway β has an active PDP context for that DM-MS, the packet mode gateway 6 can immediately and automatically request a packet data channel from the V+D network, using standard TETRA V+D packet data protocol (as described in clause 28 of EN 300 392-2) .

The packet mode gateway 6 waits until it detects that it has one or more complete data packets in its ' buffer 27 and then requests a packet data channel (in line with the TETRA packet data protocol, where the SNDCP layer waits until it receives a first data packet before requesting permission to start transmitting data packets) .

If the packet data gateway 6 does not have an active PDP context for a DM-MS requesting a circuit mode data call or attempting to send an SDS' containing packet data, the packet-mode gateway 6 can reject the circuit mode call request or SDS message, or can attempt to set up the PDP context immediately and automatically.

It would also- be possible to indicate in circuit mode data call set-up signalling when the call is required for packet data operation. This would permit the packet-mode gateway 6 to operate in alternative modes (e.g. to act as a circuit-mode voice call gateway as well) . Once the packet mode gateway 6 has activated a PDP context linked to the DM-MS ' s address, it will also be possible for the data service 2 connected to the V+D network 1 to send packet data to the DM-MS terminal 7, either spontaneously, or in response to packet data received from the DM-MS terminal 7. These could be IP protocol acknowledgements, or application level data.

The data service 2 linked to the V+D network 1 does not need to know the DM-MS ' s 7 TETRA address or the packet-mode gateway's TETRA address because the V+D network's SNDCP entity stores the cross reference between the DM-MS_. application' s IP address and the packet mode gateway's address and PDP context number. The packet-mode gateway 6 thus determines the DM-MS ' s TETRA address from the data stored with the relevant PDP context and then stores the incoming data in a buffer dedicated to storing data for the correct destination DM-MS terminal 7.

If a circuit-mode data call already exists between the correct DM-MS and the packet data gateway 6, the packet data gateway 6 can commence transmitting the data packet (s) to the DM-MS on its next DMO transmit opportunity. The circuit-mode data transmission protocol preferably supports sending circuit-mode data in both directions in alternate half-duplex transmissions along with transmitting acknowledgments for the data travelling in the opposite direction to facilitate this .

If, alternatively, the packet-mode gateway uses SDS messages to send packet data to DM-MSs, the packet mode gateway can attempt to send SDS messages to the relevant DM-MS at the next available opportunity on the DMO channel .

If no circuit mode data call exists at the time that incoming packet data is received by the packet-mode gateway 6, and the packet-mode gateway 6 is using circuit mode data to send the packet data to the DM-MS terminal 7, the packet-mode gateway 6 may attempt to setup a circuit-mode data call with the appropriate DM-MS terminal 7. In this case the DM-MS terminal 7 can be configured to recognise that a circuit-mode data call setup received from the gateway 6 must be for the purposes of exchanging packet data, or, for example, the call setup could indicate that the call is for the purposes of exchanging packet data, so that the DM-MS terminal 7 can use the correct protocol to handle the incoming data and can route it to its packet data applications.

If the destination MS (terminal 7) is not presently contactable, the packet mode gateway 6 may continue to receive and buffer the incoming data. The packet data gateway 6 can stop the flow to the particular PDP context avoid buffer overflow when required, by deactivating the affected PDP context. The PDP context is allowed to time out once the circuit-mode data call is terminated. This is also the case with respect to migration from one packet-data gateway to another as it can avoid the complication of there being activated PDP contexts for a single IP address on more than one packet-mode gateway.

Where SDS messages are used, the PDP context is deactivated as soon as possible following a short delay (e.g. 5 minutes) after the DM-MS stops communicating with the packet mode gateway. The process of sending packet data as circuit mode data is essentially the reverse of the process, already described, of sending the circuit mode data as packet data.

When the DMO transceiver 21 receives an indication from the TETRA V+D transceiver 22 via signalling connection 29 that one or more data packets have been received for the current circuit-mode call, the DMO transceiver 21 takes a suitably sized block of data from the packet data receiving buffer 28, and attempts to transmit it as a block of circuit-mode data or SDS data to the DMO terminal 7.

When the first block has been delivered, the DMO transceiver 21 can attempt to transmit further blocks, etc.. These blocks may be chosen without regard to the boundaries of the contained IP packets, since the blocks of data are reassembled by the DM-MS terminal 7. (However there may be some efficiency benefits in aligning the circuit-mode or SDS data blocks with packet boundaries, because this could reduce the time delay required to exchange end-to-end packet data acknowledgements . ) Figure 3 illustrates a second embodiment of the packet-mode gateway 6 that is constructed using a single transceiver 41. This gateway uses a single transmitting and receiving antenna 42 connected to a single transmitter and receiver, so that it is connected alternately to the transmitter or receiver, as required. The gateway is able to switch rapidly between receive and- transmit and between the frequency of the DMO net and the frequency of the V+D network. The gateway 6 operates using the TETRA TDMA structure, where a multiframe is divided into eighteen frames, and a frame is subdivided into four slots, each slot having duration 85/6 ms .

When the packet mode gateway 6 is actively receiving circuit mode data from a DM-MS and transmitting the contents to a V+D network using packet data protocol, the V+D and DMO TDMA (time division multiple access) structures are aligned so that timeslot 1 in the DMO transmissions is delayed by 1 timeslot relative to the V+D uplink slot allocated as a packet data channel (like the alignment used by TETRA gateways for relaying speech calls between DMO and V+D) .

This is illustrated in figure 4, which shows how the transmission and reception timeslots might be distributed in this arrangement.

Figure 4 shows an example of this operation in terms of circuit-mode data. However, similar timings will apply to the use of SDS messages.

In figure 4, row 69 indicates transmissions by the packet data gateway 6 on the DMO frequency and row 70 indicates transmissions by a DM-MS to the packet mode gateway. Row 67 indicates the TDMA frame number on the DMO channel, and row 68 indicates the slot number on the DMO channel. Thus in frame 2 slot 1, the DM-MS transmits some circuit-mode data, labelled cd5, to the packet-mode gateway. In frame 3 slot 1, and in the succeeding slot Is, the DM-MS transmits further portions of circuit-mode data cdβ , cd7 , cdδ and cd9 , etc. (At some times the DM-MS may have to pause to receive acknowledgements sent from the gateway to the DM-MS in slot 1; this is not shown in figure 4. )

Row 66 indicates transmissions by the packet-mode gateway 6 on the uplink frequency of the V+D network. Row 64 indicates the V+D uplink frame number, and row 65 indicates the V+D uplink slot number . The packet mode .gateway has been allocated to use a packet data channel located on slot 3 of the V+D channel, and the DMO timing is aligned, as shown, so that DMO slot 1 occurs one timeslot after V+D uplink slot 3.

In V+D uplink frame 3, the packet mode gateway discovers it has received enough data from the DM-MS to start transmitting one or more complete data packets to the V+D network. In V+D uplink frame 3 slot 3, (i.e. the packet data uplink slot) the packet mode gateway transmits a first part or segment of packet data pdl to the V+D network. (This data need not comprise a complete packet, as the packet data protocol does not require packet boundaries to correspond to slot boundaries.) Then, in successive V+D uplink slot 3s, the gateway transmits further packet data segments pd2 , pd3, pd4, pd5. , etc.

Unlike in the case of a voice call, the contents of the packet data segments do not need to correspond exactly to the data received from the DM-MS, as they can be delayed by a greater or lesser degree, and the quantity of data per slot can (and almost certainly will) differ between the DMO slots and the V+D slots. Data rate mismatch can^' be accommodated by missing out packet data up-link slots, or by pausing the delivery of circuit mode data from the DM MS using the flow control features of the circuit mode data protocol.

Row 63 of figure 4 indicates transmissions by the V+D base station (BS)' on the downlink frequency of the V+D network. Row 62 indicates the V+D downlink frame ^■ number, and row 61 indicates the V+D downlink slot number. In V+D downlink frame 6 slot 3, the V+D BS sends the packet-mode gateway acknowledgement message ackl that acknowledges receipt of the packet data delivered in data segments pdl, pd2 and pd3. In other V+D downlink slot 3s, the V+D transmits null PDUs (Protocol Data Units) , but it might equally well send other signalling messages or packet data, perhaps to other MSs.

It can be seen that with this arrangement, the packet mode gateway achieves transmitting and receiving . on the DMO channel and on the V+D uplink and downlink frequencies always at different times, once per frame, allowing it to relay packet data between V+D and DMO. This is achieved using a single transceiver and single antenna that is shared between receive and transmit. Since the packet-mode gateway 6 never receives and transmits at the same time, the packet-mode gateway receiver is never blocked by the packet-mode gateway transmitter.

In the above method, the packet-mode gateway 6 is allocated to a single-slot packet data channel. A multi-slot packet data channel could be used, particularly, e.g., if the capacity of DMO circuit-mode data or SDS messages supports it.

The correct alignment between the DMO and V+D timing in these . arrangements can be attained by adjusting the timing of the DMO channel as soon as the packet mode gateway is assigned to a packet data channel. The timing must be adjusted such that DMO slot 1 occurs one timeslot after the V+D uplink packet data slot. This timing adjustment can be performed, e.g., using the timing adjustment signalling that is specified for voice calls through TETRA gateways.

Where SDS messages are used, DM-MSs would discover ^■ the packet-mode gateway's present timing from the packet mode gateway's presence signals (gps) in figure 4, and then adjust their timing accordingly.

The other details of the packet-mode gateway of figure 3 and its operation then are the same as described for the embodiment of figure 2 given above.

As already indicated, in the above arrangements as an alternative to exchanging packet data between the packet-mode gateway and the DM-MS using circuit mode data, it would also be possible to tunnel the packet data through SDS messages. There could be some significant advantages to this method,^' because the TETRA DMO protocol provides reliable delivery of SDS messages, so that there would be no need to use some additional protocol, as might be required with circuit mode data. In the present embodiment of such an arrangement, each SDS message carrying packet data is given a special protocol identifier (PID) labelled "packet data" so that the DM-MS 7 and the packet-mode gateway 6 know how to handle these particular SDS messages . The SDS messages also contain sequence numbers, to facilitate reassembly of the data in the correct order.

In this operation, rather than the gateway 6 passing the SDS messages from the DM-MS terminal 7 straight to the V+D infrastructure 1, it unpacks SDS messages with the "packet data" identifier (PID) and stores the packet data they contain in a buffer so that the individual data packets can be extracted and transmitted to the V+D network 1 using the packet data protocol . Use of SDS messages for this purpose also makes it possible for multiple terminals to use the packet mode gateway 6 at the same time. (DM-MSs wishing to send an SDS to a gateway can contend for access using random access.) The SDS method may also make it easier for the packet-mode gateway 6 to send data back to DM-MSs when a DM-MS is actively sending packet data to the packet-mode gateway. With the SDS method, it would be useful to keep a PDP context active in the packet mode gateway for a short period of time after a DM-MS stops using the packet mode gateway. Although the above embodiments have been described with reference transmitting packet data between a mobile terminal and a fixed network infrastructure, as discussed above, it would also be possible to use the techniques of the present invention to allow two direct mode operating mobile terminals to exchange packet data, again without the need to use a special packet data protocol. Thus, in another aspect and embodiment, the invention comprises and extends to transmitting packet data between mobile terminals operating independently of the fixed network infrastructure by tunnelling that data through a non-packet data communications protocol (such as, and preferably, using a circuit mode connection or SDS signalling) .

It can be seen from the above that the present invention, in its preferred embodiments at least, provides a system for supporting packet data in TETRA DMO.

More particularly, the present invention, in its preferred embodiments at least, provides a method of extending packet data services to applications using a TETRA DMO radio terminal by using a packet data gateway that connects to a TETRA V+D network. The proposed packet data gateway acts as a converter between DMO circuit-mode data or DMO SDS messages and V+D packet data protocol, allowing packet data to be tunnelled though a DMO circuit-mode data traffic channel or SDS messages to the packet data gateway.

The invention, in its preferred embodiments at least, also permits terminals to continue to exchange data with a data service connected to the TETRA V+D network whenever a packet data gateway is present and available. In preferred embodiments of this invention, a packet mode gateway is provided that can exchange packet data using a packet data protocol via radio signals with a fixed radio infrastructure on the one hand, and can exchange packet data using a circuit mode protocol or SDS messages via radio signals with a DMO terminal on the other hand, such that packet data is effectively exchanged between the DMO terminal and the fixed radio infrastructure . This is achieved in one embodiment using circuit mode data by a method comprising:

a) DM-MS detects presence of a packet mode gateway b) DM-MS application decides it wants to exchange packet data with a V+D connected data service. c) DM-MS requests a circuit-mode call set up for packet data. d) Packet-mode gateway successfully negotiates PDP context activation with the V+D network, and informs the DM-MS that the call setup is successful . e) DM-MS commences sending circuit mode data to the packet-mode gateway. f) The packet-mode gateway buffers the circuit mode data and locates the first complete data packet . g) The packet-mode gateway asks the V+D network for permission to start transmitting the data packets, and gets allocated to a packet data channel . h) The packet-mode gateway sends timing adjustment messages to the DMO channel to align the DMO timing to the allocated packet data channel. i) The packet mode gateway starts transmitting the data packets to the V+D network, while continuing to receive circuit mode data containing further data packets . j ) When the DM MS has finished exchanging packet data with the V+D network, it clears the circuit mode data call. k) The packet-mode gateway deactivates the PDP context .

In another embodiment that uses SDS messages to transmit the packet data, the method includes:

1) DM-MS detects presence of a packet mode gateway m) DM-MS application decides it wants to exchange " packet data with a V+D connected data service, n) DM-MS commences sending SDS messages to the packet mode gateway with the "packet data"

PID. o) The packet-mode gateway extracts the data packets from the SDS messages and buffers them, p) Packet-mode gateway successfully negotiates

PDP context activation with the V+D network, q) The packet-mode gateway asks the V+D network ^■ for permission to start transmitting the data packets, and gets allocated to a packet data channel . r) The packet-mode gateway sends timing adjustment messages to the DMO channel to align the DMO timing to the allocated packet data channel . s) The packet mode gateway starts transmitting the data packets to the V+D network, while continuing to receive SDS messages containing further data packets. t) When the DM-MS has failed to exchange any data packets with the packet-mode gateway for a short period of time (e.g. five minutes) the packet mode gateway deactivates the PDP context .

The proposed methods also allow DMO terminals to exchange packet data with each other, e.g., by- tunnelling their IP data to each other through a circuit-mode connection or SDS signalling.

Claims

1. A method of operating a mobile communications system in which mobile communications terminals can

5 transmit to each other independently of the fixed network^' infrastructure, the method comprising: transmitting packet data from or to a mobile terminal that is operating independently of the fixed network infrastructure by sending that data through a 10 non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure .

2. The method of claim 1, comprising transmitting the 15 packet data using a circuit mode call.

3. The method of claim 1 or 2 , comprising transmitting the packet data using a short data message..

20 4. The method of claim 1, 2 or 3 , comprising transmitting data using two different communications channels .

5. The method of any one of the preceding claims, 25 comprising: transmitting data to the terminal using one communications protocol and transmitting data from the terminal using another communications protocol.

30.

6. The method of any one of the preceding claims, wherein the packet data is transmitted to or from another mobile terminal of the communications system.

7. The method of any one of the preceding claims, 35 wherein the packet data is transmitted to or from a device that can convert the packet data transmission between the protocol used to transmit it to or from the mobile terminal and another communications protocol .

8. The method of claim 7, wherein the device can connect to a computer network independently of the communications system's fixed network infrastructure.

9. The method of any one of the preceding claims, wherein the packet data is transmitted to or from a gateway device that can establish a packet data connection with or via the communications system's fixed infrastructure .

10. A method of operating a communications system, the method comprising: converting a packet data transmission sent using a non-packet data signalling protocol to a transmission to be sent using a packet data signalling protocol and/or vice-versa.

11. A method of operating a TETRA communications system, the method comprising: converting packet data into SDS messages and/or circuit mode data for transmission in direct mode operation and/or vice-versa.

12. A mobile communications terminal of a communications system, the mobile terminal comprising: means for transmitting to another mobile terminal independently of the fixed network infrastructure of the communications system; and means for transmitting packet data or receiving packet data by sending that data through a non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure when the mobile terminal is operating independently of the- fixed network infrastructure.

13. The terminal of claim 12, comprising: means for transmitting or receiving packet data using a circuit mode call when the mobile terminal is

5 operating independently of the fixed network infrastructure .

14. The terminal of claim 12 or 13, comprising: means for transmitting or receiving packet data

10 using a short data message when the mobile terminal is operating independently of the fixed network infrastructure.

15. The terminal of claim 12, 13 or 14, comprising:

15 means for transmitting and/or receiving packet data using two different communications channels when the mobile terminal is operating independently of • the fixed network infrastructure.

20 16. The terminal of any one of claims 12 to 15, comprising: means for receiving data using one communications protocol; and means for transmitting data from the terminal using ^'25 another communications protocol; when the mobile terminal is operating independently of the fixed network infrastructure.

17. The terminal of any one of claims 12 to 16,

30 comprising means for presenting a packet data interface to a client packet data application running on the terminal .

18. A mobile communications system comprising:

35 one or more mobile communications terminals as claimed in any one of claims 12 to 17.

19. The Communications system of claim 18, comprising: a device to which packet data may be transmitted by a mobile terminal operating independently of the fixed network infrastructure of the communications system and/or from which packet data may be transmitted to a mobile terminal operating independently of the fixed network infrastructure of the communications system, by sending that data through a non-packet data communications channel that is supported for transmissions independent of the fixed network infrastructure; the device comprising means for converting the packet data transmission between the protocol used to transmit it to or from the mobile terminal and another communications protocol.

20. The communications system of claim 19, wherein the device comprises means for connecting to a computer network independently of the communications system's fixed network infrastructure.

21. The communications system of claim 19 or 20, wherein the device comprises means for establishing a packet data connection with or via the communications system's fixed infrastructure.

22. An apparatus for use in a communications system, the apparatus comprising: means for converting a packet data transmission sent using a non-packet data signalling protocol to a transmission to be sent using a packet data signalling protocol and/or vice-versa.

23. A gateway apparatus for use in a TETRA communications system, the apparatus comprising: means for converting packet data into SDS messages and/or circuit mode data for transmission in direct mode operation and/or vice-versa.

24. A computer program element comprising computer software code portions ^■ for performing the method of any one of claims 1 to 11 when the program element is run on data processing means.

25. A method of operating a communications system substantially as herein described with reference to any one of the accompanying drawings .

26. A mobile communications terminal substantially as herein described with reference to any one of the accompanying drawings .

27. A mobile communications system substantially as herein described with reference to any one of the accompanying drawings.

28. An apparatus for use in a communications system substantially as herein described with reference to any one of the accompanying drawings .