BACKGROUND OF THE INVENTION 
     This invention relates to an interface for interfacing between two different types of networks, such as a GSM cellular radio network and an integrated services digital network (ISDN). 
     This is a continuation of international patent application Ser. No. PCT/EP91/00853 filed on May 6, 1991 in the European Patent Office and designating the United States, now abandoned. 
     An ISDN network provides communications on two traffic channels (usually labelled B1 and B2) and a control channel (usually referred to as the D channel). The bandwidth of the traffic channels makes them capable of carrying 64 kB/s per channel and the control channel is capable of carrying up to 16 kB/s. 
     The GSM (Groupe Speciale Mobile) cellular radio system provides communications on a traffic channel carrying 13 kB/s of speech or 12 or 9.6 kB/s of data. The GSM system provides a control channel of up to 1 kB/s. 
     It is desirable in the manufacture of GSM equipment to provide an interface (usually referred to as the S interface) for connection of ISDN compatible equipment. Thus, for example, an ISDN telephone, fax, answering machine or other such equipment can be plugged into a GSM mobile transceiver and operated from a vehicle or other remote GSM subscriber equipment. 
     Manufacturers of ISDN compatible equipment propose to provide the facility of transmitting packets of data on the D channel. For example, a number of ISDN telephones could be installed on a fixed network and a supervision device might transmit data on the D channel. The devices could initiate a connection to a supervisor computer without initiating a call. ISDN service operators might charge lower fees for transmitting a packet of data on the D channel, rather than establishing a point-to-point communication on the B2 channel. Other uses for data on the D channel would include credit card checking, supervision and alarm systems, broadcast message services etc. 
     It is a problem with connection of ISDN equipment to GSM equipment that the GSM control channel does not have the capacity for supporting data from the ISDN control channel. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an interface for communicating between first and second data channels and first and second control channels, said interface comprising: means for mapping control information between the first and second control channels; means for mapping data, in a first state of operation, between the first and second data channels under the control of control information on the control channels; and means for mapping data, in a second state of operation, between the first control channel and the second data channel under the control of control information on the control channel. 
     In this way, packets of data on the ISDN D channel are recognised as being data and these are mapped across onto the GSM traffic channel in packet form. In this way, the packets of data on the ISDN channel are not lost. Furthermore, the breadth of equipment types that can be connected to a GSM mobile unit is increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a GSM radio having an ISDN interface and an ISDN device connected thereto. 
     FIG. 2 shows the three layers of the different channels of the GSM/ISDN interface in accordance with the preferred embodiment of the invention. 
     FIG. 3 shows the mapping of data in accordance with the preferred embodiment of the invention. 
     FIG. 4 shows a system employing the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a GSM radio comprising a transceiver part 10, a handset 11, a system bus 12, and an ISDN module 13. The transceiver 10 comprises a speech coder 14, a call processor 15, an RF module 16 and a bus interface 17. The handset 11 comprises a codec 20, a keyboard display interface 21, a bus interface 22, a loudspeaker 23 and a microphone 24. The ISDN module comprises an ISDN controller 25, an ISDN S interface 26, a bus interface 27 and a further interface 28. Attached to the ISDN S 26 is shown an ISDN telephone 30. 
     Another optional bus device 32 may be connected to the bus 12. 
     The bus 12 is a wide bandwidth bus for carrying control information, data and speech simultaneously or in a time division multiplexed manner. The control information, data and speech are communicated and controlled separately between the elements 10, 11 and 13 of the GSM radio on the GSM control data and speech channels. Other functional commands internal to the GSM radio are supported by the bus 12. 
     Under control of the ISDN controller 25, the ISDN S interface 26 maps decoded speech from the speech coder 16 at a net bit rate of 64 kB/s to the ISDN B 1 channel at a net bit rate of 12 kB/s (in bursts of 192 kB/s). Likewise, the ISDN S interface 26 maps data received from the transceiver 10 at a net bit rate of 12 kB/s, or less depending on which data rate is used, on the GSM data channel to a net bit rate of 64 kB/s (in bursts of 192 Kb/s) on the ISDN B2 channel. The channels B1 and B2 can indpendently be used for either speech or data. 
     The mapping is shown in greater detail in FIG. 2. In that figure, the left hand side represents the ISDN telephone or other equipment and the right hand side represents the GSM transceiver and the GSM network. 
     Referring to the control channels of the two systems, these are represented as the D channel in the ISDN system and the Dm channel in the GSM system. Each of these channels comprises three layers, as described in the respective specifications of the systems. These layers are represented as L1, L2 and L3. Control information is mapped directly between these layers in the respective channels. Look-up tables and bit rate converters are used as necessary by the ISDN controller 25 to perform the mapping. 
     Data and speech are mapped between the respective layers of the data and speech channels of the two systems. Thus, where information is present on the B2 channel of the ISDN network, this is presented as data on the Bm channel of the GSM network and vice versa. Where information is present on the B1 of the ISDN network, this is presented as digitised speech on the Bm of the GSM network and vice versa. 
     FIG. 3 represents the mapping between the GSM and ISDN channels shown in FIG. 2 with additional mapping in accordance with the invention. The information represented by the shaded portions of FIG. 3 is layer 2. 
     Where a packet of data is presented on the ISDN D channel, the packet is recognised by the ISDN controller 25 by virtue of the operation code at the start of the packet. A data packet is recognized at layer 2 by the address field in the layer 2 frame. Connection is then established to a packet handler (of the users choice, pre-programmed in the IDS). The ISDN controller 25 maps this data onto the GSM Bm channel in the form of a packet having an appropriate GSM operation code indicating that the information is a packet of data. Since the Bm channel of the GSM network has a bandwidth of 12 or 9.6 kB/s, it can support a packet of data received from the 16 kB/s wide D channel of the ISDN network, provided there is sufficient memory in the ISDN module 13 to store the packet and re-transmit it at the reduced data rate. 
     At layer 1 there is no way of distinguishing control from data. This distinction is made at layer 2, by means of the address field in the LAPD frames. There is no need to interpret and map the layer 3 information, which is passed as is. The mapping is done at layer 2 by extracting the information from the layer 2 (LAPD) frames, and packing this into GSM layer 2 frames (RLP frames). The layer 3 information merely has to be monitored to see when a disconnection is made (disconnection is done by means of a layer 3 disconnection message). 
     Referring to FIG. 4, a communications system is shown as an example of an application for the invention. 
     This system comprises a GSM network 40 connected to an ISDN network 41 and a non-ISDN network 42. Each of the networks 41 and 42 has interfaces 43, 44, 45 and 46 for connection to telephones 47, 48, 49, personal computers 50, 51 and 52 etc. Communicating with the GSM network 40 is a GSM portable radio 55. Connectable to the radio 55 via a system bus is an ISDN module 56. The module 56 could be an add-on board inserted within the housing of the radio and telephone 55. The module 56 has an S interface 57, to which is connected a telephone 58 and a computer 59. 
     The portable radio telephone 55 can make calls via the GSM network 40 to any one of the telephones 47 to 49. In addition, by means of an ordinary modem link, the radio telephone 55 can communicate with the computer 52. 
     In addition to the above communications, the computer 50 is arranged to periodically generate data messages on the ISDN control channel. These messages are in packet form and are recognised by the ISDN system 41, which routes them to their destination, for example computer 51, according to the address contained in the packet. Thus, a message can be passed from computer 50 to computer 51 without a communications channel being set up between those elements. 
     In addition, the computer 59 is similarly arranged to generate data messages on the control channel. These messages are recognised by the ISDN module 56 and the radio telephone 55 requests allocation of a channel on the GSM network 40 by transmission of the appropriate GSM channel request signal. Handshake is established between the radio telephone 55 and the GSM system 40 and the data message is transmitted to this system 40 on the GSM traffic channel. The GSM system 40 routes the data message to the destination identified in the packet, for example a unit connected to system 41 or system 42. Where the packet of data is sent on to other ISDN equipment, a further mapping may be carried out between the GSM system 40 and the ISDN system 41, mapping the message back to the ISDN control channel. This message may then be transmitted to, for example, computer 50 without setting up a traffic channel between the GSM system 40 and the computer 50. 
     Conversely, when one of the items of equipment 47, 48, 50, 51 connected to ISDN network 41 generates a data message on the control channel with an address which identifies an item of GSM subscriber equipment, (eg radio telephone 55), the message is passed from the ISDN network 41 to the GSM network 42 and in so doing it is mapped from the ISDN control channel to the GSM traffic channel and the GSM system 40 polls the radio telephone 55 for establishing a call. The radio telephone 55 responds (if it is capable of doing so) and handshake is established, whereby the message is passed to the radio telephone 55 by means of the traffic channel. The ISDN module 56 may map the message across again to the ISDN control channel and on that channel the data can be passed to the ISDN peripheral equipment 58 or 59. 
     Whenever a connection is established, this is a duplex channel. Both the GSM air interface connection and the ISDN D channel connection are duplex, so data may be mapped in both directions. Also in the case of an incoming data call, this is offered to all devices. If one of the devices wishes to accept the call it can do so, and also indicate that it wishes the data to be transferred on the D-channel.
This invention relates to an interface for interfacing between two different types of networks, such as a GSM cellular radio network and an integrated services digital network (ISDN). The interface communicates between channels of one such network with channels of the other such network by mapping control information between the first (D) and second (Dm) control channels. In a first state of operation, the interface maps data between the first (B1, B2) and the second (Bm) data channels under the control of control information on the control channels. In a second state of operation, the interface maps data between the first control channel (D) and the second data channel (Bm) under the control of control information on the control channel.