03.543GPPDescription for the use of a Shared Inter Working Function (SIWF) in a GSM PLMNStage 2TS
An IWF provides specific functions associated with the visited MSC for the interworking with other networks. It comprises signalling and traffic channel related functions. The traffic channel related functions are provided by an Inter Working Unit (IWU).
The SIWF concept is that it provides specific functions for the interworking with other networks. It comprises signalling and traffic channel related functions. Whereas the signalling related functions are associated with the visited MSC, the IWU providing the traffic channel related functions has another physical location.
The entity that contains all additional functions needed in the visited MSC to provide the SIWF is called SIWF Controller (SIWFC). The entity where the IWU is located is called SIWF Server (SIWFS). The Interface between a visited MSC and a SIWFS is called the K Interface.
SIWFS can be provided by a MSC (MSC/SIWFS) or by another network entity (stand alone SIWFS).
Figure 1 shows the network configuration.
Figure 1: Network configuration
4.2 Routeing principles
The call is routed according to the following principles:
– a loop i.e. DTE – MS – BSS – visited MSC – SIWF Server – visited MSC – Fixednetwork. This is applicable for both mobile originating or mobile terminating calls. This is called the Loop Method. (Figure 2)
– a non-loop i.e. DTE – MS – BSS – visited MSC – SIWF Server – Fixed network. This is only applicable for mobile originating calls. This is called the Non Loop Method. (Figure 3).
The signalling link between the MSC with SIWFC and the SIWFS is used for normal circuit switched signalling and for conveying additional controlling information via MAP signalling.
Figure 2: The Loop Method. The traffic circuits ‘a’ and ‘n’, where ‘a’ is used to send and receive data between MS and SIWF Server and ‘n’ is used to send and receive data of the fixed network
Figure 3: The Non Loop Method. The traffic circuit ‘a’ is used to send and receive data between MS and SIWF Server
4.3 Visited MSC with additional SIWF functionality (SIWFC)
The visited MSC is responsible for allocation of an IWF or a SIWFS. In the case the visited MSC has the additional functions required to provide a SIWF, this additional functionality in the visited MSC is called SIWFC. SIWFC is a logical part of the MSC.
The visited MSC examines IWF Capabilities based on the service request and available resources. The visited MSC comprises of functions for selection of an appropriate SIWFS and the allocation of the traffic channels depending on the SIWF type. In addition to the GSM phase 2 MSC responsibility, the visited MSC including SIWFC is responsible for:
– The analysis for the selection of an appropriate SIWFS is based on the contents in the information received during call set-up. For this purpose the Visited MSC has the knowledge of routeing information to SIWFS and the service Capabilities on each SIWFS.
– In addition the operator may specify conditions for redirection.
– The examination results in that either the MSC/IWF is used (if such exist) or that the call is redirected to a SIWFS.
– In case the analysis results in that the call shall be routed to a SIWFS, the visited MSC allocate ‘a’- circuits for mobile originating calls and ‘n’ circuits for mobile terminating calls to be used between the MSC and the SIWFS, and establishing a connection to the SIWFS.
– The MSC determines via the signalling to the SIWFS, how the call shall be routed i. e. either back to the visited MSC (loop method) or to the final destination (non loop method). This indication is sent in a MAP Procedure.
– By-passing the SIWFS connection in the Speech part of Alternate services.
The visited MSC may or may not contain an IWF.
4.4 SIWFS – Shared Inter Working Function Server
The logical entity that contains the IWU is called SIWF Server (SIWFS). It offers IWU to remotely located MSCs. The IWU provides the traffic channel related functions. SIWFS contains also the DRM which is needed for the additional signalling between entities involved i.e. the visited MSC and the SIWFS. See figure 4.
SIWFS is responsible for:
– allocation of ‘n’-circuit for mobile originating calls and ‘a’-circuit for mobile terminating calls to be used to the MSC.
– signalling interworking to ISDN, PSTN and (PS)PDN, allocation of resources towards the fixed network in case the non Loop Method is used.
An operator can decide to concentrate all the data/fax traffic to one specific SIWFS.
The physical location of the SIWFS can be an MSC or in another network entity. In the case the SIWFS is implemented in an MSC it is denominated MSC/SIWFS.
Figure 4: The logical entity SIWFS
The IWUs in SIWFS have the same functions as the existing IWFs The responsibility by the IWU concerns the traffic handling functions e.g.
– Interworking between the rate adapted A interface frames carrying the user data and the signals on the fixed network circuits.
– Data interworking in the transparent and non transparent mode e.g. modems, V.110 frames, RLP, the handling of fax protocol and handling of network based datacompression.
4.4.3 Data call Routeing Mechanism (DRM)
The DRM is a logical function which supports the handling of calls in SIWFS.
DRM is used for examination of service request and available resources for the support of data/fax call in SIWFS. The examination results in that either the call is accepted and that the IWU is connected or that the call is rejected.
The DRM receives information whether the call is to be routed in a Loop (back to the visited MSC) or as Non Loop (directly to the final destination) via the signalling from the visited MSC. The SIWFS shall have routeing cababilities but it has no own routeing alternatives.
4.5 K Interface and Signalling
The link between the visited MSC and the SIWFS supports signalling functions and functions for the transport of traffic. In addition the link supports signalling functions to control the SIWFS i.e. IWU located in the selected SIWFS.
The traffic channel supports that a circuit will be used to send and receive data in the A interface format and will be called ‘a’ circuit. Another circuit will be used to send and receive data in the format of the fixednetwork and will be called ‘n’ circuit. The circuits ‘a’ and ‘n’ consist of one 64kbps channel each.
The signalling channel supports e. g. ISUP i.e. the signalling between the visited MSC and the SIWFS by using e. g. ISUP. The checking and the mapping of information element parameters of the GSM 04.08 signalling take place in the visited MSC in accordance with GSM 09.07.
The signalling on the K Interface requires additional controlling signalling. This signalling is handled by a MAP procedure. The MAP Procedures supports at least submission of all GSM-BC, all ISDN-BC (09.07), A-Number, B-Number and CMM. The MAP Dialogue is open throughout the call.
4.6 Fixed Network
Fixed network can be any network with which GSM PLMN is interworking for data services. Requirements on fixed networks are outside the scope of the present document.