The present invention relates to the transmission of messages between network entities in a wireless communications network. The invention is particularly but not exclusively concerned with the transmission of messages across the A-interface between a base station sub-system (BSS) and a mobile switching centre (MSC), and over the E-interface between two mobile switching centres.
FIG. 1 represents a known GSM architecture. A user terminal in the form of a mobile station MS communicates across a radio interface Um with a base station sub-system BSS. The base station sub-system BSS comprises a plurality of base stations BTS and a set of base station controllers BSC. In FIG. 1, two BTSs are shown in communication with one base station controller, and a further base station controller is shown although its associated base stations are not shown. It will readily be appreciated that in practice there is a large number of base stations and base station controllers forming part of any base station sub-system. As is well known, the base stations are geographically arranged in a cellular network, with each base station communicating with mobile stations in one or more adjacent cell. In the GSM architecture, the interface between the base stations and their associated base station controller is referred to as the Abis interface. The base station sub-system BSS is in communication with a network sub-system or core network CN. The network subsystem comprises a number of network entities, such as the home location register (HLR), virtual location register (VLR), etc which are well known in the art and are not discussed further herein. The entities in core network CN support the operations of a mobile switching centre MSC which is in communication over the A-interface with one or more base station controller that it is managing. The A-interface can be any suitable communication path, whether wired or wireless and there is a well-known layered protocol for communicating information between the BSCs and the MSC across the A-interface. The mobile switching centre MSC is during inter-MSC handover (here referred as MSC-A i.e. the controlling MSC on which the call was originally established) in communication with another mobile switching centre MSC (here referred as MSC-B i.e. the MSC to which the MS is handed over) across an interface which is referred to as the E-interface. The gateway mobile switching centre GMSC which provides an edge function with a public land mobile network (PLMN). Once again, a standard protocol allows communication between the MSC of the core network and the GMSC.
The present invention is concerned with information transfer from the base station sub-system BSS and the mobile switching centre MSC over the A-interface, and information transfer between the mobile switching centre MSC and another MSC over the E-interface. One particular type of information transferred between these entities is a base station sub-system application part (BSSAP) message. The message is transferred as part of the signalling connection control part (SCCP) of the information transfer protocol which is used across the A-interface. The BSSAP message is transferred over the E-interface as the contents of the signalling information in a AN-APDU parameter (access network-application protocol data unit).
The standard 3GPP TS 48.006 (signalling transport mechanism specification for the BSC-MSC interface) specifies the base station sub-system application part (BSSAP) user func
tion of the signalling connection control part SCCP. The BSSAP user function is sub-divided into two separate subfunctions:
1. A direct transfer application sub-part (DTAP) which is used to transfer messages between the mobile switching centre MSC and the user terminal MS.
2. The BSS management application sub-part (BSSMAP) which supports other procedures between the mobile switching centre MSC and the base station sub-system BSS which are related to the user terminal MS, such as resource management, handover or control, or to a cell within the base station sub-system or to the whole base station sub-system. The standard 3GPP TS 48.008 sets out a description of the layer-3 protocol for the BSSMAP information exchange.
The DTAP and BSSMAP layer-3 messages between the MSC and BSS are contained in the user data field of exchanged SCCP frames. The user data field is a mandatory parameter of data frames and always contains either a DTAP or BSSMAP message.
A distribution function located in an intermediate layer of protocol between the SCCP layer and layer-3 (the distribution sub-layer) performs discrimination between data related to the DTAP and BSSMAP sub-parts. The protocol for this layer consists of management of a distribution data unit, in which a discrimination parameter D indicates whether the message is a DTAP or BSSMAP message.
FIG. 2 shows the user data field structure of a BSSMAP message. The message comprises a distribution data unit octet DDU labelled DISCRIMINATION, and length indicator octet LI labelled LENGTH IND L and a number of different octets labelled LAYER-3 and MESSAGE (With octets in between which are not shown) which comprise the actual layer-3 message L-3m. The distribution data unit DDU comprises a discrimination parameter D which is coded on one octet as shown in FIG. 3. That is, the octet DISCRIMINATION comprises a discrimination bit D which, when the message is a BSSMAP message is set to a “not transparent” value zero (DTAP messages have the discrimination parameter D set to the “transparent” value one, which allows direct transfer of this part of the message from the BSS to the user terminal MS).
The length indicator L1 is coded in one octet and is the binary representation of the number of octets of the subsequent layer-3 message parameter L-3m.
With a length indicator of one octet, it is possible to indicate a BSSMAP message having a maximum length of 255 octets. This represents a limitation on the length of the BSSMAP message, and as requirements on information transfer across the A and E interfaces increase, this is seen as a limiting factor for the introduction of new information to be transmitted for features which could otherwise be transmitted in the BSSMAP messages.
For example, it would be desirable to consider incorporating the following types of data into BSSMAP messages, and the incorporation of such data could cause the BSSMAP messages to exceed the currently restricted message length.
Rel-5 SNA [Shared Network Area] Access Information
information element IE (in HANDOVER REQUEST and COMMON ID messages at E-interface). Proposed Rel-6 SNA Access Information IE to BSS at A-interface also and related connectionless information, e.g. possible Information Transfer procedure.
Rel-5 Cell Load Information within Old BSS to New BSS Information IE.
Old BSS to New BSS Information IE future additions in