PLMN Network Intelligence 2 © Manzur Ashraf

Switching

Switching elements in GSM

Most mobile networks consist of two hierarchical levels: the MSC and the GMSC levels. Transit nodes can also be found in large mobile networks.Small networks can have GMSC functions integrated into their MSCs, which results in a "flat", meshed structure. The traffic channels of the access and core networks differ. The access network handles 13 kbit/s traffic channels, and the core network 64 kbit/s channels. Voice recoding between 13 and 64 kbit/s is performed in the BSC. Encryption equipment is normally located in the base stations.

Switching is performed at four levels of the structure: in the BTS, BSC, MSC and GMSC. Concentration is performed in the air interface, because the number of traffic channels is limited. This concentration is controlled by the BSC, which assigns idle traffic channels to mobiles. After decryption in the BTS, four 13 kbit/s traffic channels are multiplexed onto one 64 kbit/s channel - normally a time slot of a PCM link between the BTS and the BSC. Ordinarily, an MSC switches 64 kbit/s traffic channels between the GMSC and one of its BSCs. It must continuously keep track of (or be able to find out) which section of its own service area a called mobile is in. It must also be capable of switching to another MSC whenever a mobile moves to a cell in another MSC's service area during an ongoing call

A GMSC switches 64 kbit/s traffic channels between external networks and the MSCs of its own PLMN as well as between external fixed networks and interworking PLMNs. As mentioned earlier, GMSCs are connected to external networks at the national or international hierarchical network levels. When a GMSC receives a call from an external network, it must determine the MSC (or the interworking PLMN) that is currently able to reach the called mobile - in other words, which MSC service area or which interworking PLMN it must connect to. This is where network intelligence enters the picture. The HLR informs the GMSC of which MSC to use, and because the calling mobile does not have a fixed extension to that MSC, the HLR will also provide the GMSC with a temporary routing number to be used for signalling. This number was retrieved by the HLR from the VLR involved

Hand-over

Switching in MSCs during handover

There are five types of handover: Intra-cell handover: A new channel is selected in the same cell due to interference or other disturbance on the channel being used by the mobile. In the case of GSM, the resultant re-switching only involves the BSC. Intra-BSC handover (in GSM): A new channel is selected in a cell that is managed by the same BSC. The resultant re-switching only involves one BSC. Intra-MSC handover: A new channel is selected in a cell managed by another BSC but by the same MSC. In the case of GSM, the resultant re-switching involves two BSCs and one MSC. Inter-MSC handover: A new channel is selected in a cell managed by another MSC in the same mobile network. The resultant re- switching involves several MSCs. Inter-system handover: A new channel is selected in a cell managed by another mobile network, with which "our" PLMN interworks.

Architecture of large mobile networks

Trunk & Access Network

GSM Access Network

Traffic channels in GSM

In-depth Hand-over

1. BSC11 informs its MSC that the mobile needs handover. It also indicates the cells that are suited for handover and gives priority to BTS After analysing the situation, MSC1 sends a signal to MSC2, requesting handover to cell BTS211. MSC1 also sends the mobile's subscriber data which is stored in its VLR. 3. MSC2 orders BSC21 to allocate an idle traffic channel to the mobile. 4. BSC21 allocates an idle channel (provided such a channel is available). 5. MSC2 indicates the channel in BTS211 to which the mobile should be connected. Then MSC1 and MSC2 reserve new paths for the connection through their switches.

6. MSC1 orders BSC11 to check that the mobile is connected to the new traffic channel in BTS211, and BSC11 orders the mobile to change channels. 7. After the mobile has changed channels, it must acknowledge this order. The acknowledgement is received by BSC21 and sent on to MSC2. If no acknowledgement appears within a preset time, the connection is cleared. 8. MSC2 now makes a through-connection from MSC1 to BSC21 in its own switch and sends the acknowledgement on to MSC1. MSC1 sets up the new path between the GMSC and MSC2 in its switch and disconnects the previous one between the GMSC and BSC11. Then BSC11 is ordered to release the original traffic channel in BTS112.

Network Intelligence & Value- added service

registration and paging in a GSM mobile network

Registration process STEP 1: a. Mobile is ON (activated in LA1). b. Mobile reads Local area Identity (LAI) through cell’s control channel c. Mobile contact the MSC and report its position (LA1) d. MSC1 enters mobile’s position into VLR e. MSC1 sends a signal to HLR reporting the mobile active in its(MSC’s) service area. f. Mobile receives an ACK (it concludes registration process)

STEP 2. a) The mobile enters an LA2 cell, reads a new LAI on the control channel and makes a new registration. b) This information, too, is received by MSC1, which changes the LAI in its VLR. Note that the change need not be reported to the HLR, because the mobile is still in the same MSC service area.

STEP 3. a)The mobile enters an LA3 cell, and now the registration is received by MSC2. Since the mobile is "new" in MSC2's service area, it is reported to the HLR, which informs MSC1 that the mobile has entered another MSC service area. b) MSC1 deletes it from its VLR.

paging sequence Assume that the GMSC receives a call addressed to the mobile after it has been registered in LA3. In answer to a question to the HLR, the GMSC is informed (indirectly through a routing number) that the mobile is in MSC2's service area. From its VLR, MSC2 finds out that the mobile was last registered in LA3. Provided LA3 only consists of cells within BSC21's service area, MSC2 requests paging from BSC21, which responds by sending a paging call to all base stations in LA3. If the mobile is still in an LA3 cell, it will answer the call.

Notes on Registration The example illustrates the importance of the mobile being able not only to register when it is turned on and when it enters a new location area, but also to report when it is turned off. This procedure is called detachment in GSM. Information showing that the mobile is turned off is stored in the MSC that receives the information and in the HLR. Thus, if a mobile that is currently off is called from another network, the call will stop at the GMSC level. Instead of automatic registration, a manual procedure can be initiated (by means of a smart card in GSM). The user gains access by inserting the smart card into his mobile, and registration then proceeds as in step 1 above.

Setting up and releasing a mobile call

1. The subscriber in the fixed network dials the B-subscriber's mobile number. The PSTN identifies the number and sets up a connection to the called network (GMSC). 2. The GMSC does not know through which MSC (or in which interworking mobile network) the mobile can be reached; nor does it know whether the mobile is free, busy, turned on or turned off. To be able to continue, the GMSC must therefore request a routing number from the HLR. 3. The registration function continuously updates the HLR on the location of the mobile (that is, in which MSC service area it is to be found). Provided the mobile is turned on and free, the HLR will call that service area's VLR and request a free routing number. The mobile's subscriber data is sent along with this request.

4. The GMSC receives the routing number and uses it to select a route in the PLMN. In our example, the GMSC forwards the call to MSC1. 5. MSC1 consults its VLR to find out in which group of cells (location area) the mobile is at the moment. (Keeping the VLR informed of the mobile's location is also part of the registration function.) MSC1 then orders BSC11 to find the mobile. 6. BSC11 sends a paging call to all cells in its service area that can be visited by the mobile at the moment. When the call is answered, BSC11 allocates the mobile a control channel for signalling with MSC1. A traffic channel over radio access and between MSC1 and BSC11 is also reserved for the call. 7. Signalling between MSC1 and the mobile concludes with the set- up of a traffic channel through the switches in BSC11 and MSC1. A connection has now been established between the telephone in the fixed network and the mobile.