GB2283391A - Nethod of operation of a trunked radio system for data transmission - Google Patents

Abstract

A trunked radio system is described of the type in which a trunking controller (30) controls the operation of the system by transmitting channel grant commands and a talkgroup indications for assigning channels to talkgroups of radios. Each radio (14, 24) is by default a member of a paging group and receives messages transmitted by the host radio (40) on a paging group channel. For transmission of data, each radio (14, 24) is arranged to temporarily assign itself to a session data group, such that on transmission of a session data group channel grant by the trunking controller, the radio responds to that channel grant by tuning to a session data group channel. The radio performs a multiple access contention process to access the session data group channel and commences data exchange with the host computer on that channel. A trunking radio data modem (12, 22, 42) is described which enables this operation on a normal trunking system. <IMAGE>

Description

Method Of Operation Of A Trunked Radio Svstem For Data Transmission Field of the Invention This invention relates to a trunked radio system and a method of operating such a system for data transmission. The invention also, separately, relates to a trunking radio data modem.

Background to the Invention A trunked system, basically, groups a number of radio channels. One of the channels is designated as the control channel. Whenever a radio unit wants to initiate a call, the radio automatically requests a channel by sending some information on the control channel.

There are two ways of assigning a channel in a trunked system. One way is grouping the users so that each time one unit requests a channel (through the control channel), the channel is assigned to all members of the group (a "talkgroup"). The alternative is performing private call addressing so that the channel will be assigned to the initiating and designated units only.

Trunked system are mainly used to transport voice information.

Shared voice and data transmission over the same system, using the same radio unit, is becoming more and more demanded in the marketplace.

Usually, remote mobile data units communicate with a central or host computer.

One possible way to perform this mixed data and voice service could be to designate two different talkgroups, one for data and a second one for voice.

As the radios are shared for voice and data users, grouping the data units to a common data group does not provide satisfactory performance. Assigning a channel to all members of a data group while only two members actually transfer data, prevent the voice users from using the radio and is inefficient and even disruptive to the primary voice service. This method is used in some existent shared voice and data systems.

On the other hand, using the private call method is also inefficient. In this method the data units would have to request a channel each time they want to initiate a data transaction, without the ability of using the channel that was already assigned for other data units. The loading of the control channel is increased, decreasing the voice (and data) service performance. Moreover, every data transaction incurs the overhead of channel set-up time (the time it takes to set-up a channel from a request on the control channel) and the teardown time (the time it takes the system to deassign a channel). Usually, data communications is performed with short data burst with a duration significantly inferior to the set-up and tear-down times.In addition, the private call method requires the external modem unit to maintain a list of radio IDs and, at the start of a data transaction, communicate to the radio the ID of the addressed unit, thereby complicating the operation.

There is a need for an improved manner of communication data on a trunking radio system.

Summarv of the Invention According to the present invention, a trunked radio system is provided comprising a host computer and an associated radio; a number of remote units with associated radios; and a trunking controller having means for transmitting a channel grant command and a talkgroup indication for assigning a channel to a talkgroup of radios. Each remote radio has means for responding to a channel grant command transmitted to a talkgroup to which that radio belongs.Each radio is by default a member of a paging group and receives messages transmitted by the host radio on a paging group channel and for transmission of data, each radio is arranged to temporarily assign itself to a session data group, such that on transmission of a session data group channel grant by the trunking controller, the radio is arranged to respond to that channel grant by tuning to a session data group channel, by performing a multiple access contention process to access that channel and by commencing data exchange with the host computer on that channel.

The invention also relates to a method of operating a trunked radio system. At the trunking controller a channel grant command is transmitted with a talk group indication for assigning a channel to a talkgroup of radios.

Each remote radio unit locally considers itself a member of a paging group and receives messages on a paging group channel. The remote unit receives an instruction (DGS) to commence data transmission and temporarily assigns itself to a session data group. On receiving a session data group channel grant from the trunking controller, the remote unit responds to that channel grant by tuning to a session data group channel, by performing a multiple access contention process to access that channel and by commencing data exchange with the host computer on that channel.

The multiple access contention process preferably includes monitoring the channel for transmissions by other units and accessing the channel when monitored as being free, but may employ aloha contention resolution or slotted aloha.

The invention, in a further aspect, relates to a trunking radio data modem for connection to a trunking radio, comprising: a first port for receiving commands and data from a data terminal and a second port for issuing commands and supplying data to a trunking radio. The modem has first means (e.g. a controller) for receiving a command from the first port to commence transmission of data, second means responsive to the first means for supplying to the second port a command for the trunking radio to assign itself to a session data group, third means for receiving from the second port an indication that a channel has been granted to the session data group and transmission means responsive to the third means for transmitting data from the first port to the second port.

it is preferred that the modem is arranged to receiving an end-of-session command from the second port and responds to receipt of the end-of-session command by supplying to the second port a command for the trunking radio to cease to assign itself to the session data group and to assign itself to a paging data group.

The channel access method described here offers an efficient way to access the channel for the data users without disrupting the primary voice service.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief description of the drawings Fig. 1 shows an example of an overall system in accordance with the preferred embodiment of the invention.

Fig. 2 is a state diagram showing different states of operation of a remote unit of Fig. 1 Fig. 3 illustrates elements of a remote unit of Fig. 1 in greater detail. Figs. 4 and 5 are timing diagrams illustrating operation of the unit of Fig. 3 and Fig. 6 is a timing diagram illustrating operation of the host computer of Fig. 1.

Detailed description of the Preferred Embodiment This invention relates to an interface between a mobile modem connected to a mobile terminal or host computer and a trunking radio, as depicted in Fig. 1, and a procedure implemented in the radio to access a communication channel.

Fig. 1 shows an example of two mobile terminals 10, 20 operating on a trunked system managed by a trunking controller 30. Mobile terminal 10 is connected over a standard (e.g. RS232) interface 11 to a data modem 12, which is connected over a novel interface 13 to a trunking radio 14 such as a Maxtrac Spectra(trade mark) radio manufactured by Motorola Inc. Also connected to trunking radio 14 is a microphone 15 for voice communication. Terminal 20 is likewise connected to a data modem 22 and, via interface 23, to a trunking radio 24 (and may have a microphone not shown).

Radios 14 and 24 communicate via a trunking base station 31 with a third trunking radio 40 which is connected via an interface 41 to a data modem 42 which is connected via a standard interface 43 to a host computer 44. Modem 42 is identical to modems 12 and 22. Interface 41 is identical to interfaces 13 and 23.

For simplicity, reference will be made to the combination of terminal 10 or 20 and modem 12 or 22 as a "remote unit" and the host 44 and modem 42 as a "host computer".

It will first be described how remote unit 10 which is willing to transmit data to host 44 opens a data session, following which it will be described how host computer 44 initiates a session with a remote terminal, this being the less common scenario.

Two channel types are defined and used for data communications.

These will be referred to as a "session channel" and a "paging message channel". These channels, as well as voice channels, are physically the same and are allocated by the trunking controller 30.

The trunked radio 14 has two data talkgroups programmed in it, in addition to any normal voice talkgroups. These will be referred to as the "session data group" (SDG) for communication on the session channel and the "paging data group" (PDG) for the paging message channel. By the expression "session channel" is meant a channel assigned to the SDG, while "paging message channel" means a channel assigned to the PDG.

The session data group is used for initiating (or resuming) sessions between the remote units 10, 20 and the host computer 44. The paging data group is used for sending short paging messages, initiated by the host computer 44, and for corresponding acknowledgements, if any, from the remote units.

A particularly useful feature is that in the paging message from the host, an indication is given as to whether an acknowledgement is required.

This is preferable to an automatic acknowledgement, since each remote unit has to contend for the channel in order to send its acknowledgement and the channel could become overloaded with acknowledgement messages.

For an explanation of the operation of the system, the case of a data session is first explained.

Whenever there is an open session between the host computer 44 and one or more remote units, the data communication between the host computer and those units is done on the session channel. It is noted that the host computer 44 can have a multitude of sessions open with several remote units at the same time.

If remote unit 10 wants to open a session with the host computer, it instructs radio 14 to monitor the control channel for an active session channel, that is, a channel assigned to the session data group. If there is an active channel being used by other remote units engaged in a session with the host computer, the radio is addressed directly to that channel. Observe that this monitoring does not entail a channel request from the remote unit. Once in the active session channel, the remote unit monitors the channel activity before transmission to avoid collisions. Otherwise the unit starts to transmit immediately. In other words, whenever a session channel is assigned for data communications any remote unit can "join" this channel without loading the control channel.Once the session is open, the remote unit and the host computer 44 will conduct their dialog on this assigned session channel.

It will be appreciated that other contention methods can be used instead of merely monitoring channel activity. For example, "aloha" contention can be used as is well known or, in the case of a time-division-multiplex system, slotted aloha can be used. The present invention is equally applicable to such arrangements.

By default, the host computer monitors the session channel once it is granted to any of the remote units. It responds, in that channel, to transmissions initiated from the remote unit.

The trunking controller 30 monitors all assigned channels for activity.

If there is no activity in a given channel (any channel), a time-out-timer is started. If the time out timer expires and no transmission has been conducted for this period of time (called hang-time), the channel is deassigned.

A channel may have been deassigned to the session data group while a session is still active. As an example, one may consider a query from the remote unit 10 to the host computer 44 that takes some long processing time.

Since the host computer has knowledge that the session is still open, it will request a channel from the trunking system 30, 31 using the session data group. Only remote units having open sessions with the host computer 44 will respond to this call. In other words, the host computer will request a channel using the session data group only to resume a session that was not previously completed.

As a consequence of this scheme, only remote units having open sessions with the host computer 44 are involved in the data transmission. All other users are free to continue with their main voice usage.

Considering now data paging, the operation is as follows.

If the host computer 44 wants to send messages to one or more remote units 10, 20, it will do so on the paging message channel or, in other words, by requesting a channel using its paging channel group. Any remote unit that does not have an open session will monitor, on the control channel, for this paging channel group.

When messages from the host computer are received, each remote unit checks if the data is addressed to it. If, for example, the data message is not directed to remote unit 10, it indicates to its radio 14 to revert to the control channel.

Consequently remote units 10 will be on the assigned paging message channel for a relatively short time, freeing its radio 14 for voice transmission via microphone 15.

The above described operation is illustrated further in Fig. 2. That figure shows the various states that may be occupied by a remote unit and its associated radio.

The default state is state 100, in which the remote unit is monitoring the control channel, awaiting a "go to channel" command or a voice PTT ("push to talk") command.

On receipt of a voice PTT command from microphone 15, the remote unit goes to state 101 for voice communication and on release of PTT it returns to state 100.

On receipt of a paging channel grant command, the remote unit goes to state 102 and is tuned to the paging channel. In this state, the radio considers itself to be, in effect, a member of the paging data group. If, when in this state, the remote unit receives a paging message with the unit's own address, it will go to state 103 and will register that a session is now active. State 103 can be reached directly from state 100 by the unit's own initiation (by a special "DGS" command described below which precedes a data PTT command from terminal 10).

Considering the case where the terminal itself has initiated the session, terminal supplies a data PTT command to the modem 12 and if the channel is not busy, the remote unit goes to state 104 and transmits the data to the trunking base station 31 over an inbound channel for retransmission on an outbound channel to the host computer 44.

From state 103, receipt of a session channel grant command from the trunking base station 31, either through request from trunking radio 14 over the control channel or upon initiation by the host computer 44, again on the control channel, the remote unit goes to state 105 where it is able to receive data from the host computer via the base station 31.

The scheme described also enables a host computer 44 to initiate sessions with remote units (an infrequent requirement). Its short message can indicate the remote unit that it wishes to initiate a session. The remote unit, in this case, will do so using the SDG.

To support this mode of operation, a minimum set of signals is required to interface the mobile external modem 12 to the mobile trunked radio 14. This minimum set of signals is comprised of the following: Data PTT: this is a signal from the modem to the radio to indicate that it requests to transmit data.

Channel Grant (CG): this is a signal to the modem from the radio to indicate that the radio is currently on an assigned data channel.

Data Group Select (DGS): This is a signal from the modem to the radio.

The modem uses this signal to indicate to the radio to select the SDG, if it communicates during an open session, and to select the PDG otherwise.

These signals pass across the interface 13 in Fig. 1 and are illustrated in Fig. 3 which shows the modem 12 and trunking radio 14 in more detail. As is shown, the modem 12 comprises a controller 200 and buffer storage 201. The radio 14 comprises a controller 204, buffer storage 205 and an r.f. unit 206. The radio controller 204 tunes the r.f. unit 206 as illustrated by control line 205.

Between the two controllers are three control lines 208, 209 and 210 designated DGS, Data PTT and channel grant (CG) for carrying the signals listed above.

When a session is set up, data flows over a data bus 212, 213, 214 between the terminal 10 and the r.f. unit 206. Other control lines 216 are provided between the terminal 10 and the controller 200 of the modem, for indicating to the modem requests to transmit data and other control signals which need not be described.

As examples of operation, some timing diagrams are shown in Figs. 4 and 5 for two cases. The figures show initiation of a transmission using the session data group. It is assumed that the data group select signal is high when a session channel is requested and low otherwise.

The figures show transmission in the session channel.

Referring first to Fig. 4, when a terminal 10 wants to initiate a session with the host computer 44, its modem 12 activates the DGS signal to indicate to the radio 14 that a session channel is requested. The radio controller 204 scans the control channel information to see if a session data group is already active. If a SDG is active, as is illustrated in Fig.4, the radio activates the channel grant (CG) signal and automatically joins the session channel. If, after some time-out, no channel grant signal is received, the modem interprets that no channel is assigned and requests one for the SDG by activating PTT (Fig. 5).

Once a channel grant signal is received by the radio 14 and indicated to the modem 12, the remote unit transmits its message and the radio 14 reverts to receive. Observe in this example that the host computer 44 ends the session by explicitly providing the pertinent information (end-of-session indication) to the remote unit. This is received in the buffer storage 201 of the remote unit and identified as an end-of session message. Once the session is ended, the remote unit deactivates its DGS and reverts to listen to the PDG on the control channel.

A remote unit can also end a session. This is done by the modem prepending an end-of-session message to the data and then switching Data PTT and DGS low.

Although not explicit in the above example, the DGS signal should remain active for as long as the session is open, independent of whether or not the channel is deassigned by the control channel, as explained above.

The case will now be considered of initiating a transmission using the paging data group.

The host computer 44 can initiate a transmission using this paging data group. Fig. 6 shows a timing diagram for transmission in the paging message channel. The signals shown in Fig. 6 are between the host computer modem 42 and the host computer radio 40.

The host computer modem 42 deactivates its DGS signal, indicating to its radio 40 that the radio 40 should temporarily consider itself a member of the PDG. At the same time, the modem 42 activates Data PTT as shown. A channel request with the paging data group (PDG) is issued by the radio 40 to the trunking controller 30 on the control channel indicating that a group call is requested to the PDG. The trunking controller sends, through the control channel, a short message indicating that a particular channel, for example channel no. 4, has been assigned to the requested group. This is a channel grant command and is indicated to the modem 42 by the rising Channel Grant signal on line 208.

Simultaneously, any remote unit radio listening to the control channel receives this message and, if the group matches its own, it goes to that channel. By default, all remote units not in an active session are members of the PDG. Each such unit indicates this to its respective radio by holding DGS low. Each such radio previously tuned to the control channel now re-tunes to receive messages in the paging message channel.

The host computer now sends a paging message (as shown) to one or more remote units, which message includes the unique identification number(s) of the unit(s) being paged, and, if required, indicates that an acknowledgement is requested. The message may also contain further indications as to what, if any, further transactions are to take place.

All members of the PDG receive the paging message. All units not identified in the message revert to the control channel. This occurs by operation of a procedure normal in trunking systems in which the trunking controller 30 instructs the units to revert to the control channel after a period of inactivity on the traffic channel (in this case the channel designated as the paging channel). In a normal trunking system this period of time is called the "hang time". As an alternative, a remote unit can independently revert to the control channel as soon as the complete paging message has been received and decoded and is determined not to be directed to that unit. In each case, the unit remains a member of the PDG.

If an acknowledgement to the message is requested, this is sent by the addressed remote unit(s) 10, 20, using the channel allocated to the paging data group. Fig. 6 illustrates this acknowledgement on the channel. Each addressed unit then temporarily assigns itself to the SDG, returns to the control channel and awaits a channel assignment to the SDG.

Further transactions between the units are not illustrated in Fig. 6, but continue as described before. For example, the host computer modem 22 or the remote modem 12, 22 of the paged unit can now request a channel for the SDG using the same procedure as is described above for the PDG, depending on the nature of the instruction contained in the paging message.

Once the host-to-remote transaction on the paging channel is ended, the host computer modem 42 maintains the DGS signal to its radio 40 active to initiate a call in the SDG or attend to a possible call in the SDG.

Remote-to-remote communication is also possible. A remote unit wishing to initiate a session with another remote momentarily "disguises" itself as a host and sends a page. The trunking infrastructure cannot distinguish between a remote and a host and merely repeats on an outbound channel whatever it receives on an inbound channel.

In summary, a new trunking channel access procedure is provided for mixed data and voice applications. Among the advantages of the arrangement are the following: a single data group is used for data sessions between remote units and host computers; in this way, the host computer 44 does not have to have a radio per active session, in the case multiple simultaneous sessions are desired; neither the host computer 44 nor the remote terminal needs to pass to the radio any ID information of the party it wants to call; only remote units involved in sessions with the host computer 44 can be active in the session channel, leaving all other radios free to entail a voice conversation with any other user; and a paging message channel is provided for the host computer 44 to send short messages to the remote units with minimal influence in the primary voice traffic of the radios.

Claims (6)

Claims

1 A trunked radio system comprising: a host computer (44) and an associated radio (40); a number of remote units (10, 20) with associated radios (14, 24); a trunking controller (30) having means (31) for transmitting a channel grant command and a talkgroup indication for assigning a channel to a talkgroup of radios, and means (204) at each remote radio for responding to a channel grant command transmitted to a talkgroup to which that radio belongs, wherein each radio is by default a member of a paging group and receives messages transmitted by the host radio on a paging group channel and for transmission of data, each radio is arranged to temporarily assign itself to a session data group, such that on transmission of a session data group channel grant by the trunking controller, the radio is arranged to respond to that channel grant by tuning to a session data group channel, by performing a multiple access contention process to access that channel and by commencing data exchange with the host computer on that channel.

2. A method of operating a trunked radio system comprising a host computer (44) and an associated radio (40), a number of remote units (10, 20) with associated radios (14, 24) and a trunking controller (30), comprising the steps of: at the trunking controller (30): transmitting a channel grant command and a talkgroup indication for assigning a channel to a talkgroup of radios, and at a remote radio unit (10):: locally causing the remote radio (14) to assign itself by default to a paging group and receiving messages on a paging group channel, receiving an instruction (DGS) to commence data transmission and temporarily assigning itself to a session data group, receiving a session data group channel grant from the trunking controller, responding to that channel grant by tuning to a session data group channel, performing a multiple access contention process to access that channel and commencing data exchange with the host computer (44) on that channel.

3. A method according to claim 2, wherein the multiple access contention process includes monitoring the channel for transmissions by other units and accessing the channel when monitored as being free.

4. A trunking radio data modem (12, 14, 42) for connection to a trunking radio (14, 24, 40), comprising: a first port (11, 21, 43) for receiving commands and data from a data terminal (10, 20, 44) and a second port (13, 23, 41) for issuing commands and supplying data to a trunking radio (14, 24, 40), first means (200) for receiving a command from the first port to commence transmission of data, second means (210) responsive to the first means for supplying to the second port a command for the trunking radio to assign itself to a session data group, third means (208) for receiving from the second port an indication that a channel has been granted to the session data group and transmission means (201) responsive to the third means for transmitting data from the first port to the second port.

5. A modem according to claim 4, wherein the first means (200) are arranged to receiving an end-of-session command from the second port and wherein the second means (210) are responsive to receipt of an end-of-session command by said first means for supplying to the second port a command for the trunking radio to cease to assign itself to the session data group and to assign itself to a paging data group.

6. A modem according to claim 5, in combination with a trunking radio (14, 24, 40) connected to the second port, wherein the trunking radio comprises: transceiver means (206) for receiving channel grant commands and associated group indications, channel selection means (204) coupled to the transceiver means for selecting channels in response to channel grant commands received and tuning means (205) coupled to the transceiver means and the channel selection means for controlling the transceiver means to tune to a channel selected by the channel selection means, wherein the channel selection means are coupled to the second port of the modem for response to the second means to cause the trunking radio to respond to a channel grant command having an associated session group indication.