GB2295523A

GB2295523A - Data radio equipment and method of controlling a radio and a radio modem

Info

Publication number: GB2295523A
Application number: GB9423932A
Authority: GB
Inventors: Salomon Serfaty; Danny Zadok
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1994-11-26
Filing date: 1994-11-26
Publication date: 1996-05-29
Also published as: AU698135B2; ZA959942B; AU3795495A; CN1143873A; GB9423932D0

Abstract

Data radio equipment has a radio 14 operable on a radio system, a radio modem 12 connected to the radio 14, and data terminal equipment 10 connected to the radio modem 12 via a port (130, Fig.4). A translating device 122 in the radio modem 12 receives signals from the data terminal equipment 10 via the port (130) and translates at least a part of a received modem control command into a radio command. A control element 126 in modem 12 is coupled to the translating device 122 and to the radio 14, for controlling the radio in response to the radio command to communicate with the radio system. The prior art requirement for the data terminal equipment 10 to have both a first port (26, Fig. 1) coupled to the modem and a second port (18) coupled to control the radio 14 is thus avoided. Data may be communicated between data terminal equipment 10 and the radio 14, between the radio 14 and a selected one of second radio devices 34, 36 and between the second radio device and a gateway 42, where extension parts of the modem command define the identity of the second radio device and one or more outputs 44, 46, 48, 52, 54, 56 of the gateway 42. The gateway outputs may be coupled to local and remote host computers 50, 72. <IMAGE>

Description

Data Radio Equipment and Method of Controlling a Radio and a Radio Modem Field of the Invention This invention relates to data radio equipment, such as mobile data equipment and, separately and in addition, it relates to a method of controlling a radio and a radio modem, for example where the radio communicates over a radio system such as a trunked radio system.

Background of the Invention MPT1327 is a standard for a trunked radio system that defines a radio link. It does not specify the kind of information that is distributed over the radio link.

MAP27 is an open interface between a data terminal equipment (DTE) and a mobile radio transceiver. It allows control of the radio and transmission of short messages, status messages and prescribed data.

When non-prescribed data is used, MAP27 may be used to control the radio and establish, through the MAP27 infrastructure, an RF link between two users.

In a non-prescribed data mode a user requests on a control channel a traffic channel from the system and the system responds with a traffic channel assignment. Once the RF link has been established it can be used to transfer data using any protocol, as long as it meets the constraints of the system (bandwidth, deviation, etc).

In the prescribed data mode the protocol for transferring data is specified in the MPT1327 standard itself. It allows also the transmission of status messages and short messages over a control channel without having to request a traffic channel.

For convenience the MPT1327 or other trunking system infrastructure will further be referred as the system.

In order to be able to perform data communication between a DTE over a radio path, a user has to connect the DTE to an RF modem that presents a physical RS-232 interface for the DTE and to connect the RF modem further to a mobile radio transceiver. There exists also a standard set of modem commands referred to as AT commands that allows a user to control a modem. These commands are called AT commands because their syntax always begins with "AT".

An example for a DTE connected to an RF modem and a mobile radio transceiver is shown in FIG. 1.

It is known to establish the radio connection manually through the radio control panel or the RF modem control panel.

Further it is known to issue the corresponding AT commands through an extra port of the DTE as shown in FIG. 1.

The DTE may be a portable personal computer or a simple terminal with limited space for ports and having only a single port and thus preventing the user from controlling the mobile radio transceiver via an interface. Additional ports involve extra space, weight and cost.

Summarv of the Invention According to a first aspect of the present invention, a method is provided of controlling a radio and a radio modem by a data terminal equipment where the radio is operable on a radio system and the radio modem has a port for communicating commands with the data terminal equipment. The method comprising the steps of: sending a modem control command from the data terminal equipment to the radio modem and receiving the modem control command at the radio modem; translating at least a part of the modem control command to a radio command at the radio modem; passing the radio control command to the radio and in response thereto controlling the radio to communicate with the radio system; and communicating data between the data terminal equipment and the radio modem via the port and between the radio modem and the radio system via the radio.

In this manner, the same port can be used for radio control commands and data, thereby reducing space requirements and cost in the data terminal equipment.

A response to the radio control command may be passed back to the radio modem and at least a part of the response may be translated to a response to a modem control command and passed to the data terminal equipment.

The communication between the data terminal equipment and the radio system is preferably transmitted in a half-duplex mode.

In accordance with a second aspect of the invention, data radio equipment is provided comprising a radio operable on a radio system, a radio modem connected to the radio; and data terminal equipment connected to the radio modem by means of a port. The equipment further comprises a translating device in the radio modem coupled to the port for receiving signals from the data terminal equipment via the port and for translating at least a part of a received modem control command to a radio command and a control element in the radio modem, coupled to the translating device and to the radio, for controlling the radio in response to the radio command to communicate with the radio system.

In accordance with a third aspect of the invention, a method is provided of communicating via a radio system between (a) a data terminal equipment coupled to a first radio device and (b) a gateway having a second radio device connected thereto. The method comprises the steps of: sending a modem control command from the data terminal equipment to the first radio device where the modem control command comprises a modem command part and an extension part; translating at least the modem command part into a radio command at the first radio device and in response thereto controlling the first radio device to request from the radio system a radio channel to the second radio device; communicating data between the data terminal equipment and the first radio device, between the first radio device and the second radio device and between the second radio device and the gateway, where the extension part of the modem command defines one of (i) the identity of the second radio device and (ii) one or more outputs of the gateway.

Thus a concise and flexible modem extension command is created which the data terminal equipment can use to select gateway inputs and/or outputs. The modem extension command is clear and easy-to-use and is compatible with existing modem control software. A detailed syntax is set out below.

In this manner a command structure is provided which is a modification of a structure primarily devised for a modem layer of communication and is now also used for controlling a radio layer.

In the case where the extension part defines one or more outputs of the gateway, it is preferred that the gateway is controlled to select, in response to the extension part, a set of gateway outputs for onward communication and an available one of said set of outputs is selected for onward communication.

In accordance with another aspect of the invention, a radio system is provided comprising a first radio device, a data terminal equipment coupled to the first radio device, at least one second radio device; a trunked radio system providing communication between the first radio device and the second radio device; and a gateway connected to the second radio device, having at least one output for onward communication to at least one host computer. The data terminal equipment has an interface for sending a modem control command to the first radio device, where the modem control command comprises a modem command part and an extension part.The first radio device has translating means for translating at least the modem command part into a radio command and control means, coupled to the translating means for controlling the first radio device in response to the radio command to request from the trunked radio system a radio channel to the second radio device. In one aspect the identity of the second radio device is determined by the extension part of the modem control command. In another aspect, where there are a plurality of gateway outputs, the gateway has control means for identifying and interpreting the extension part of the modem command and, in response thereto, selecting one or more outputs of the gateway.

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

Brief Descriotion of the Drawings FIG. 1 shows a prior art arrangement with a DTE, an RF modem and a mobile radio transceiver, FIG. 2 illustrates an arrangement according to a preferred embodiment of the present invention, FIG. 3 shows in a time chart how to control, according to the preferred embodiment of the present invention, with extended AT commands the different modem gateway inputs and outputs, FIG. 4 describes detailed an RF modem according to the preferred embodiment of the invention.

Detailed Description of the Drawings FIG. 1 shows a typical prior art arrangement with a DTE 10, an RF modem 11, a mobile radio transceiver 14 and an antenna 16. A first port 18 of the DTE 10 is connected via a first connection cable 20 to a port 22 of the mobile radio transceiver 14 and a second port 26 of the DTE 10 is connected via a second connection cable 28 to a port 30 of the RF modem 11. The RF modem 11 is connected to the mobile radio transceiver 14 with a link 25.

The mobile radio transceiver 14 is connected to an antenna 16 to transmit and receive radio signals.

With the connection cable 20 the DTE 10 is able to control the mobile radio transceiver 14 according to the MAP27 standard to request a traffic channel, to send status messages or short data messages and to receive handshake information.

AT commands and data are transferred between the second port 26 of the DTE 10 to the port 30 of the RF modem 11 and via the link 25 to the mobile radio transceiver.

FIG. 2 illustrates an embodiment according to the preferred embodiment of the present invention. A DTE 10 is now connected with a single connection cable 32 to a RF modem 12. The RF modem 12 is connected to a mobile radio transceiver 14 with two links 24 and 25. The mobile radio transceiver 14 is connected to an antenna 16 to transmit and receive radio signals.

The mobile radio transceiver 14 is unchanged between FIG. 1 and FIG. 2.

Inside the RF modem 12 the connection cable 32 is connected to an AT packet assembler dissassembler and interpreter (AT PAD and interpreter) 122. That AT PAD and interpreter 122 is connected via a MAP27 application layer 126 to a first link 24 leading to the mobile radio transceiver 14 and is connected via a modem 124 to a second link 25 leading also to the mobile radio transceiver 14.

The AT PAD and interpreter 122 parses signals transferred via the single connection cable 32 from the DTE 10 and determines whether they form data or modem commands. The AT PAD and interpreter 122 is programmed to identify commands beginning with "AT" as referred to above.

In this manner, the AT PAD and interpreter 122 acts as dividing means for dividing modem commands and data. It also acts in reverse direction as combining means for combining responses and data where necessary.

Data is converted by the modem 124 into an audio signal and transferred via the second link 25 to the mobile radio transceiver 14.

Commands requesting an RF pipe (not shown) or its release are translated by the AT PAD and interpreter 122, relayed to the MAP27 application layer 126 and routed to the mobile radio transceiver 14 via the first link 24. Any system responses are routed back from the mobile radio transceiver 14 via the first link 24 to the MAP27 application layer 126 and further relayed back to the AT PAD and interpreter 122. Here the response is interpreted and an appropriate message (e.g. "connected" or "busy") is sent via the connection cable 32 to the DTE 10.

The applicant has found that with the novel RF modem 12 as described, a second port and a second connection cable are no longer required and the user can control the mobile radio transceiver 14 via a single interface to the RF modem.

In addition to the full control of the RF modem over the single link the applicant has implemented the usage of MAP27 status messages and MAP27 short messages by using extensions to the AT command set.

On the infrastructure side of the system of FIG. 2, there is a trunking system 33 (which has a trunking controller and may have a number of different repeater transceivers) and there is a number of fixed transceivers 34, 36 connected to different modem gateway input ports 38, 40 of a modem gateway 42. The modem gateway 42 is connected via a first set of output ports 44, 46, 48 to a local host computer 50 and via a second set of output ports 52, 54, 56 to a first set of line modems 58, 60, 62 and further to a public switched telephone network (PSTN) 64.

The modem gateway 42 has multi-way switching circuitry 41 connected to the input ports 38, 40 and the output ports 44, 46, 48 and it has command interpreting and control circuitry 43 connected to the switching circuitry 41.

From the PSTN 64 there are connections to a second set of line modems 66, 68, 70. These line modems 66, 68, 70 are further connected to a remote host computer 72.

For clarity reasons only a limited number of base stations 34, 36, modem gateway inputs 38, 40, modem gateway outputs 44, 46, 48, 52, 54, 56, one local host computer 50 and one remote host computer 72 are shown.

The first set of line modems 58, 60, 62 and the second set of line modems 66, 68, 70 are required for data transmission over the PSTN 64 and are merely shown for completeness.

It will now be described, with reference to FIG. 3, how extended AT commands, proposed by the applicant, can be used to control the different modem gateway inputs and outputs.

As a nonlimiting example, the case may be considered where it is intended to connect the DTE 10 via fixed transceiver 1 (34) to a modem gateway input 1 (38) and via modem gateway output 2 (54) to the telephone number 5658776.

In a step A the DTE 10 sends an extended AT command ATDT/IVO2/5658776 to the RF modem 12. This command is constructed with the following syntax: ATDT/Ii/Ox/phone number where ATDT is a standard modem command to initiate an automated telephone dial operation which signifies AT (command) Dial using Tones (rather than P=Pulses).

/Ii represents a requested input to the modem gateway, i.e. the fixed transceiver 34 or 36.

This is a first extension to the ATDT command.

/Ox represents the requested output and is a second extension to the ATDT command.

(NB "0" for "Output", not the numeral zero) /phone~ number indicates the telephone number to be dialled through the PSTN.

(Part of the standard modem command).

In a step B the AT PAD and interpreter 122 recognizes ATDT/I1/02/5658776 as an AT commands and routes ATDT/Il to the MAP27 application layer 126. This recognition is simply performed by parsing for the character "/" and separating out the four parts of the command. The RF modem 12 translates I1 into the MPT1327 ID of the fixed transceiver 1 (34) and issues the appropriate MAP27 commands to establish the connection to this MPT1327 ID by requesting a non-prescribed data channel from the system 33 via the first link 24 and the mobile radio transceiver 14.

The system 33 responds by assigning traffic channel 3 (as a nonlimiting example) for the communication between the mobile radio transceiver 14 and the fixed transceiver 1 (34). The mobile radio transceiver 14 and the fixed transceiver 1 (34) then switch over from a control channel to the traffic channel 3. An RF pipe (not shown) between the mobile radio transceiver 14 and the fixed transceiver 1 (34) thus has been established.

Although not shown in FIG. 3 in case the fixed transceiver 1 is already busy, a busy signal would have been issued from the system 33 to the mobile radio transceiver 14 and relayed back to the DTE 10.

Once the RF link is established, MAP27 is no longer needed until the connection needs to be released.

In a step C the AT PAD and interpreter 122 sends via modem 124, second link 25, mobile radio transceiver 14 and fixed transceiver 1 (34) the command 02/5658776 to a modem gateway 42 as non-prescribed data. The RF modem is at this point already talking to the modem gateway 42 via the RF infrastructure. The command interpreting and control circuitry 43 of the modem gateway 42 now interprets the string by parsing the incoming modem command extension and causes switching circuitry 41 to select output 2 (54). The command interpreting and control circuitry 43 then dials the remote host computer 72 via line modem 60, PSTN 64 and line modem 68.

After the host computer 72 has answered the call a connect indication is relayed back to the AT PAD and interpreter 122 of the RF modem 12. This connect indication is then interpreted by the AT PAD and interpreter 122 and further sent via the single connection cable 32 to the DTE 10.

In a step D data communication between the DTE 10 and the remote host computer 72 begins in a half-duplex way known in the art which need not be described further.

As described above this extended AT command allows the DTE 10 to control the modem gateway 42. The fixed transceivers 34 or 36 at the modem gateway inputs are identified by translating, in the radio modem 12, the identifier I1, I2, etc into an actual radio transceiver identification number (by a simple, preprogrammed look-up operation) and requesting from the radio system 33 a connection to that identified transceiver.

But the user does not always want to use this full control of the modem gateway 42. By the simple omission of the modem gateway input specification /Ii the modem gateway input will be chosen at random or availability.

As already described above in FIG. 2 there are two kinds of modem gateway outputs. The first group, 44, 46, and 48 is directly linked to a host computer 50 and the second group 58, 60, and 62 is connected via line modems 58, 60, 62, PSTN 64, line modems 66, 68 and 70 with a remote host computer 72.

It is also possible to select a group of outputs. For example, the line modem outputs 52, 54, and 56, (or a subset of those outputs) can be identified as group "A" and the host computer outputs 44, 46 and 48, (or a subset of those outputs) can be identified as group "B". Using the syntax: ATDTnl/OA/5658776 or ATDT/I1/OB one of these sets (or subsets) can be selected, leaving the gateway 42 to make the exact choice.

What is different between these two kinds of modem gateways is the type of the output, and another extended ATDT command is proposed with the the following syntax: ATDTtlilTs/phone~nllmber where ATDT is a standard modem command to initiate an automated telephone dial operation.

/Ii represents a requested input to the modem gateway.

/tIx represents a requested type of modem gateway output (extension to the standard AT commands).

/phone~number indicates the telephone number to be dialled through the PSTN.

For example, ATDT/I1/T2/5658776 specifies a request for gateway input number 1 (34), gateway output type 2 and requests a dialling to telephone number 5658776.

As already described above, by the simple omission of the modem gateway input specification /Ii the modem gateway input will be chosen at random or depending on availablility.

The qualifier /T can be used to select a particular type of output. If for example two types, i. e. T1 and T2 are defined, we can use them to indicate a request for a direct connection or a connection through a PSTN.

Moreover, if there are several locally connected host computers, each one with a port interface to the modem gateway, it is possible to define in the system types for each one of the host computers. The gateway will connect the incoming call to the particular host computer defined in the type.

For example ATDT/T7 would choose a modem gateway input at random and connect to a local host computer which is connected to a modem gateway output type 7.

In case we have only one type of modem gateway outputs and the user does not want to specify the modem gateway in- and outputs the command can be reduced to ADTD/5658776.

By issuing the extended AT command the user can either choose one of the output ports of the modem gateway to connect to a directly connected host computer or to dial a remotely connected host computer (through line modems and the PSTN).

In order to be able to perform this operation the RF modem has to translate the extended ATDT command into a series of MAP27 commands as described above.

Status messages The following table 1 details the proposed extensions to support status messaging from the user's perspective. This table 1 also explains the purpose of the command as well as the corresponding MAP27 protocol message that is issued or received by the RF modem.

Table 1 - Extensions of AT Commands for Status Messages

Syntax Purpose Direction MAP27 message M~SS < ID > , < st&num; > # Send status DTE - > SEND STATUS number st&num; RF modem to destination ID M ABORTJ Abort send DTE - > DISCONNECT status RF modem M~OKJ Status sent RF modem - > STATUS ACK (ACK) DTE with successful transaction return.

M~ERROR(n)J Negative RF modem - > STATUS ACK ACK DTE with unsuccessful cause.

M~RS < ID > , < st&num; > # Receive RF modem - > RECEIVE status st&num; DTE STATUS from source ID The symbol J represents a carriage return + line feed.

As an example, a) M SS176,3 indicates that status number 3 is to be sent to ID number 176. The user could specify either the entire ID of the destination where the message is to be sent to or a short "directory" number that is looked up in a table contained in the RF modem.

b) ABORT can be send if no M~OK has been received to indicate to the mobile radio transceiver to abort the transmission of the status message.

The M~ERROR is used to indicate to the user that something has gone wrong. A non-exhaustive list of errors may be: 0 Invalid status (out of 0-31 range) or incorrect syntax.

1 Transaction aborted through M~ABORT command.

2 Message rejected.

3 Called unit unreachable.

4 Called unit's call diverted.

5-8 Reserved.

9 Unspecified (an unknown error has occurred).

Short messages MAP27 specifies five different types or formats of user data to convey messages using this service: a) Binary or free format, b) BCD characters, c) Telex (CCITT alphabet No 2) Recommendation S1, d) 7-bit ASCII (CCITT alphabet No 5) Recommendation V3 and V4, e) 8-bit ASCII or PC character set.

As in the status messages case, the following table 2 specifies the short messages support from the user's perspective.

Table 2 - Extension of AT commands for short messages.

Syntax Purpose Direction MAP27 message M~SM < ID > ,[Fl, Send DTE - > SEND SST [SOMEscEOM], < message > to RF modem SOM < message > EOM@ destination ID using format F M ABORTJ Abort send DTE - > DISCONNECT message RF modem M~OK# Short RF modem - > SST ACK with message sent DTE successful transaction return.

MERROR(n)J Negative RF modem - > SST ACK with ACK DTE unsuccessful cause.

M~RM < Site~ID > ,[F], Receive RF modem - > RECEIVE SST tSOM Esc EOM1. < message > DTE SOM < message > EOMJ from source ID using format F The symbol represents J a carriage return + line feed. The codes for the different formats may be as follows: 0 Binary format (each bit is represented as ASCII character).

1 BCD format (each digit is represented as ASCII character).

2 Telex characters (characters represented as ASCII characters).

3 7-bit ASCII characters.

4 8-bit ASCII characters.

The SOM Esc EOM characters are needed to preserve transparency. SOM is an ASCII character that indicates Start of Message. Esc character is used as a stuffing ASCII character. EOM is a delimiter that indicates End of Message. If the three characters are not specified, their default values are used. The following defaults are preferred: SOM = ', Esc = \ and EOM = ".

The following examples illustrates the usage of the short messages: a) M~SM176,0"'00011011"J implies that the binary string composed of the 8 following bits 00011011 is to be transmitted to ID 176.

b) M~SM176,3"'hello world" implies that the message hello world, using 7-bit ASCII format, is to be transmitted to ID 176.

M~ERROR is used to indicate to the user that something has gone wrong. The same non-exhaustive list of errors as used for status messages may be used for the short messages: 0 Invalid message (limit exceeded) or incorrect syntax.

1 Transaction aborted through M~ABORT command.

2 Message rejected.

3 Called unit unreachable.

4 Called unit's call diverted.

58 Reserved.

9 Unspecified (an unknown error has occurred).

With the use of the extended AT command set, the user can transparently use a DTE 10 with a single connection cable 32 in order to perform both his communications to a host computer as well as status messaging and short messaging using the MAP27 protocol. This functionality can be implemented by software means in the RF modem.

Using this procedure, all the MAP27 interaction needed to establish a call is done without the user having to deal with MAP27 commands or front panel dialling.

In combination with FIG. 4 a more detailed description of the RF modem 12 according to the preferred embodiment of the present invention will be given. The RF modem 12 is connected via three physical ports 130, 132 and 138 to the other apparatuses of the preferred embodiment. A single connection cable 32, coming from a DTE 10 (not shown) is now connected to a physical RS-232 port 130 of the RF modem. Two ports 132 and 138 of the RF modem 12 are connected to a mobile radio transceiver 14 (not shown) with two links 25 and 24.

Inside the RF modem 12 the physical RS-232 port 130 is connected to an AT PAD and interpreter 122. That AT PAD and interpreter 122 is connected via a MAP27 application layer 126, via a MAP27 network 134 and a MAP27 link 136 to a MAP27 physical port 138. Via the first link 24 the MAP27 commands are routed to the mobile radio transceiver 14 (not shown).

The AT PAD and interpreter 122 analyses signals received via the port 130 and determines whether they form data or modem commands.

The AT PAD and interpreter 122 is programmed to identify commands beginning with "AT" as referred to above. Data is converted by the modem 124 into an audio signal and transferred to the RF physical port 132.

Once the user is connected to the host computer, data information is transmitted back and forth. Data information received from the user is packetized and passed to the modem 124 that performs the task of the RF link layer. The task of this layer is to correctly pass the information to the pier side at the fixed end. Error detection is normally appended to the packet. If an error is detected during the transmission, the packet is retransmitted until a positive acknowledgement is received. The modem does the modulation and error correction, if needed (for example, in a mobile environment). The modulation is designed so as to satisfy the constraints of the system (bandwidth, deviation, etc).

AT commands are relayed by the AT PAD and interpreter 122 to the MAP27 application layer 126. If an AT command that requires MAP27 processing (as the above described first part of the extended ATDT command) is received, that part is passed through the MAP27 application layer 126 to the MAP27 network layer 134 to produce the corresponding MAP27 network command. This MAP27 network command is then translated in the MAP27 link 136 into the recommended radio control signal. This translation can easily be implemented with a translation table, similar to Tables 1 and 2 and applying the principals of the syntax described above.

Modifications of detail can be made by one skilled in the art without departing from the principals of the invention as defined in the claims.

Claims

1. A method of controlling a radio and a radio modem by a data terminal equipment where the radio is operable on a radio system and the radio modem has a port for communicating commands with the data terminal equipment, the method comprising the steps of: sending a modem control command from the data terminal equipment to the radio modem and receiving the modem control command at the radio modem; translating at least a part of the modem control command to a radio command at the radio modem; passing the radio control command to the radio and in response thereto controlling the radio to communicate with the radio system; and communicating data between the data terminal equipment and the radio modem via the port and between the radio modem and the radio system via the radio.

2. A method as described in claim 1 further comprising the steps of: receiving a response to a radio control command from the radio system at the radio; passing the response to the radio control command to the radio modem; translating at least a part of the response to the radio control command to a response to a modem control command; and passing the response to the modem control command to the data terminal equipment.

3. A method as described in claim 1 wherein the communication between the data terminal equipment and the radio system is transmitted in a half-duplex mode.

4. Data radio equipment comprising a radio operable on a radio system, a radio modem connected to the radio; and data terminal equipment connected to the radio modem by means of a port, the equipment further comprising: a translating device in the radio modem coupled to the port for receiving signals from the data terminal equipment via the port and for translating at least a part of a received modem control command to a radio command, a control element in the radio modem, coupled to the translating device and to the radio, for controlling the radio in response to the radio command to communicate with the radio system.

5. Data radio equipment as described in claim 4, the equipment further comprising: a receiving means for receiving a response to the radio command from the radio system by the radio; a coupling means for passing the response to the radio command to the radio modem; a translating device in the radio modem for translating at least a part of the received response to the radio command to a modem response signal; a combining means for combining the modem response signal and communication data onto the modem interface.

6. Data radio equipment as described in claim 4, wherein the communication between the data terminal equipment and the radio system is transmitted in a half-duplex mode.

7. A method of communicating via a radio system between (a) a data terminal equipment coupled to a first radio device and (b) a gateway having a second radio device connected thereto, the method comprising the steps of: sending a modem control command from the data terminal equipment to the first radio device where the modem control command comprises a modem command part and an extension part; translating at least the modem command part into a radio command at the first radio device and in response thereto controlling the first radio device to request from the radio system a radio channel to the second radio device;; communicating data between the data terminal equipment and the first radio device, between the first radio device and the second radio device and between the second radio device and the gateway, where the extension part of the modem command defines one of (i) the identity of the second radio device and (ii) one or more outputs of the gateway.

8. A method of communicating via a radio system between (a) a data terminal equipment coupled to a first radio device and (b) a selected one of a plurality of second radio devices connected to a gateway, the method comprising the steps of: sending a modem control command from the data terminal equipment to the first radio device where the modem control command comprises a modem command part and an extension part;; translating at least the modem command part into a radio command at the first radio device and translating the extension part into an identification number for the selected second radio device and, in response thereto, controlling the first radio device to request from the radio system a radio channel to the selected second radio device, communicating data between the data terminal equipment and the first radio device, between the first radio device and the selected second radio device and between the selected second radio device and the gateway.

9. A method of communicating via a radio system between (a) a data terminal equipment coupled to a first radio device and (b) a second radio device connected to a gateway having a plurality of outputs for connection to further data devices, the method comprising the steps of: sending a modem control command from the data terminal equipment to the first radio device where the modem control command comprises a modem command part and an extension part; translating at least the modem command part into a radio command at the radio device and in response thereto controlling the first radio device to request from the radio system a radio channel to the second radio device;; communicating the extension part from the first radio device to the gateway via the second radio device and controlling the gateway to select one of the gateway outputs for onward communication, where the selection is made at least in part in response to the extension part; and communicating data between the data terminal equipment and the first radio device, between the first radio device and the second radio device and between the second radio device and a selected output of the gateway.

10. A method according to claim 9, wherein the step of controlling the gateway comprises selecting, in response to the extension part, a set of gateway outputs for onward communication and further comprises the step of selecting, at the gateway, an available one of said set of outputs for onward communication.

11. A radio system comprising: a first radio device, a data terminal equipment coupled to the first radio device, at least one second radio device; a trunked radio system providing communication between the first radio device and the second radio device; and a gateway connected to the second radio device, having at least one output for onward communication to at least one host computer; wherein the data terminal equipment has an interface for sending a modem control command to the first radio device, where the modem control command comprises a modem command part and an extension part; the first radio device has translating means for translating at least the modem command part into a radio command and control means, coupled to the translating means for controlling the first radio device in response to the radio command to request from the trunked radio system a radio channel to the second radio device, where the identity of the second radio device is determined by the extension part of the modem control command.

12. A radio system comprising: a first radio device, a data terminal equipment coupled to the first radio device, at least one second radio device; a trunked radio system providing communication between the first radio device and the second radio device; and a gateway connected to the second radio device, having a plurality of selectable outputs for onward communication to at least one host computer; wherein the data terminal equipment has an interface for sending a modem control command to the first radio device, where the modem control command comprises a modem command part and an extension part; the first radio device has translating means for translating at least the modem command part into a radio command and control means, coupled to the translating means for controlling the first radio device in response to the radio command to request from the trunked radio system a radio channel to the second radio device; and the gateway has control means for identifying and interpreting the extension part of the modem command and, in response thereto, selecting one or more outputs of the gateway.