Mobile radio systems

Abstract: The efficient operation of mobile systems having asynchronous transmission mode (ATM) networks require rapid changes in mobile affiliation. If such changes in affiliation were to require network switching the response times could be too long. An alternative technique is used whereby permanent virtual paths are set up across the ATM network between base stations within a defined area and the interface point of a fixed network. Within this area the virtual channel identifier (VCI) is given global significance and is used to designate the call. The ATM network may be replaced by a network of dedicated lines.

Description: The present invention relates to mobile radio asynchronous transmission mode (ATM) systems. In mobile systems it is important that the radio spectrum is used efficiently. Known techniques by which this efficiency can be improved requires the mobile terminals to frequently change the base station or base stations to which they are affiliated (i.e. through which they communicate). This can impose stringent requirements on switching times in the fixed network infrastructure if breaks in communication due to re-affiliation are to be kept short. Typically, the network part of a mobile system consists of base stations connected to a mobile network interface unit (MNIU). The MNIU is the point of interconnection between the mobile network and the fixed network for as long as a call is being handled by one of the base stations in its area of responsibility. It will be appreciated that the term "call" used throughout the specification also includes the set up of a connection, e.g. for signalling purposes. In networks which use or handle ATM data, the data transmitted in such networks is sub-divided into small packets of data, known as cells. The cells consist of 48 octets of user data and 5 octets of header information. European Patent application EP 0426269A discloses a mobile radio system comprising two networks of interconnected base stations. Communication of packetised data between each of these networks is effected by interface blocks connected to each network, and ATM switching nodes which serve to convey packetised data in a form of ATM cells between the interface blocks. A further known mobile radio system is disclosed in an article 'Packet Communication Technology for Wireless Networks' by Goodman and Holtzman, published at 1990 IEEE Military Communications Conference, Monterey CA, 30 September 1990, pages 627 to 632. This article teaches an arrangement of a mobile radio system comprising a plurality of base stations linked together by a metropolitan area network, which is arranged to communicate packetised data with the base stations and with public switches and cellular controllers. Furthermore, the article teaches that the cellular controllers may be arranged to assign a virtual circuit identifier to each cellular call, which serves to identify packets or cells associated with a particular mobile. An aim of the present invention is to provide an asynchronous transmission mode ATM system in which the network within such a system is not required to take any action to support mobility of a mobile terminal within the area controlled by a mobile network interface unit. According to the present invention there is provided an asynchronous transmission mode (ATM) system adapted for use with mobile terminals, the system comprising a mobile network interface unit connected to a network, a plurality of base stations connected to the network, and at least one mobile terminal arranged to communicate with at least one of the base stations over a radio link, the network being arranged to connect the mobile network interface unit and the base stations, said connections being defined by a virtual path identified, characterised in that a unique virtual channel identifier is allocated at call set up by the mobile network interface unit, for identifying a call associated with a mobile terminal which is being handled within an area controlled by the mobile network interface unit, which virtual channel identifier remains the same throughout the call and without dependence on which of the said base stations the mobile terminal communicates, the virtual channel identifier being placed in a virtual channel identifier field of ATM cells carrying user data associated with the call, wherein the mobile network interface unit further operates to use the virtual path identifier so that the network routes ATM cells to different ones of said base stations, in dependence upon which of the base stations said mobile terminal communicates. An embodiment of the present invention will now be described with reference to the accompanying drawings, in which: FIGURE 1 shows a virtual path across an ATM network and the virtual channels within the virtual path; and, FIGURE 2 shows an ATM system and the transmission paths between a mobile terminal and a mobile network interface unit. Referring to Figure 1, an ATM network 2 is shown having a virtual path 4 across the network. The virtual path comprises a number of virtual channels 6. The user data from individual transmission logical bursts is transported over the ATM network in one or more ATM cells. The data transported includes all types of information passing between the user and the network, including speech data. Each ATM cell includes a header which contains control information including the virtual path identifier (VPI) and the virtual channel identifier (VCI). The virtual path identifier and the virtual channel identifier are held within fields. The two fields are used to route the cells across the ATM network. The VPI is used to define a logical virtual path between two points in the network and in general there will be no direct physical connection between these two points. A virtual path will contain logical channels and the VCI value defines individual logic channels within the virtual path. The VCI only has significance within the virtual path and the same VCI value may be re-used on other virtual paths. The switching node that routes on the basis of the VPI will never need to examine the VCI field. The foregoing description represents the standard operation of an ATM network in accordance with CCITT Recommendations, eg, I. 121 Broadband Aspects of ISDN; I. 150 Broadband ATM Functional Characteristics CCITT Blue Book; I. 211 Overall Network Aspects and Functions, ISDN User Network Interfaces; I. 321 B-ISDN Protocol Reference Model and its Applications; I. 361 B-ISDN ATM Layer Specification. An explanation of the basic principles of ATM switching is given in a book entitled "Asynchronous Transfer Mode Solution for Broadband ISDN" by Martin de Prycker, published by Ellis Horwood 1991. ISBN 0-13-053513.3. The present invention as defined with reference to Figure 2, redefines the VCI so that it identifies the call and gives that call global significance within the area of a particular mobile network interface unit. As already stated, switching nodes which route on the basis of the VPI field do not examine the VCI field, and therefore this use of the VCI field does not conflict with normal ATM network operation. The connections between the mobile network interface unit 8 and the base stations 10a and 10b are re-defined by the VPI. The call within the whole mobile network interface unit area is represented by the VCI which has global significance across all such VPI's. Each call in progress has a unique VCI allocated by the mobile network interface unit 8 at the time the call is set up. The mobile network interface unit 8 can then identify the cell relating to a particular call by reference to the VCI alone. The network is configured so that the mobile network interface unit 8 has a virtual path to each of the base stations 10a and 10b. In Figure 2, a virtual path designated VPI A is shown connected to base station 10a, and a virtual path designated VPI B is shown connecting to base station 10b. These designations apply at the mobile network interface unit 8. Within the ATM network 2 the VPI may be translated resulting in different values being used at the base stations. Regardless of the values which are used they relate to the same virtual path. At the time the call to or from a mobile terminal 12 is set up, the call is allocated an unique VCI which is placed in the VCI field of all ATM cells carrying user data associated with that call from the particular base station 10a or 10b to the mobile network interface unit 8, and from the mobile interface network unit 8 to the particular base station 10a or 10b. The same VCI will generally be used in both directions of transmission, but this is not essential. In Figure 2, call X is allocated channel identifier VCI X . When the mobile terminal 12 is affiliated to a particular base station 10a or 10b the user data is transmitted across the network using the VPI associated with the connection from that base station to the mobile network interface unit, and the VCI allocated at the time of call set up. When the mobile terminal 12 is affiliated to base station 10a, the path identifier VPI A and channel identifier VCI X are used, and when the mobile terminal 12 is affiliated to base station 10b, the path identifier VPI B and the channel identifier VCI X are used. It will be seen from the above that no action is required by the ATM network to support mobility of the mobile terminal 12 within the area of the mobile network interface unit 8. Frequent and rapid reaffiliation of the mobile terminal is therefore possible, enabling a macrodiversity to be readily supported. It will readily be appreciated by those skilled in the art, that while the above embodiment is described with reference to an ATM network, the invention is not limited thereto, the network could comprise dedicated lines by way of which ATM type data is transmitted.

Claims: An asynchronous transmission mode (ATM) system adapted for use with mobile terminals, the system comprising a mobile network interface unit (8) connected to a network (2), a plurality of base stations (10a, 10b) connected to the network (2), and at least one mobile terminal (12) arranged to communicate with at least one of the base stations (10a, 10b) over a radio link, the network (2) being arranged to connect the mobile network interface unit (8) and the base stations (10a, 10b), said connections being defined by a virtual path identified, characterised in that a unique virtual channel identifier is allocated at call set up by the mobile network interface unit (8), for identifying a call associated with a mobile terminal (12) which is being handled within an area controlled by the mobile network interface unit (8), which virtual channel identifier remains the same throughout the call and without dependence on which of the said base stations (10a, 10b) the mobile terminal (12) communicates, the virtual channel identifier being placed in a virtual channel identifier field of ATM cells carrying user data associated with the call, wherein the mobile network interface unit (8) further operates to use the virtual path identifier so that the network routes ATM cells to different ones of said base stations (10a, 10b), in dependence upon which of the base stations (10a, 10b) said mobile terminal (12) communicates. An asynchronous transmission mode system as claimed in Claim 1, wherein the mobile network interface unit (8), by reference to a virtual channel identifier, identifies the cells relating to a particular call. An asynchronous transmission mode (ATM) system as claimed in Claims 1 or 2, wherein the mobile network interface unit (8) further operates to send said ATM cells to more than one of said plurality of base stations (10a, 10b) contemporaneously, or to receive ATM cells from more than one of said base stations (10a, 10b) contemporaneously, said ATM cells being routed to or from said more than one of said base stations (10a, 10b) by using the virtual path identifier, thereby facilitating support of macro-diversity. An asynchronous transmission mode system as claimed in any preceding Claim, wherein the network (2) is an ATM network. An asynchronous transmission mode system as claimed in any preceding Claim, wherein the network (2) comprises a plurality of dedicated lines.

Inventors: MCTIFFIN MICHAEL JOHN, MCTIFFIN, MICHAEL JOHN
Applicants: ROKE MANOR RESEARCH, ROKE MANOR RESEARCH LIMITED
Application year: 1993
Main classification: H04Q7
IPC codes: H04L12, H04Q11, H04W76