Mobility management
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This page covers Mobility Management in GSM and UMTS networks including those functions related to GPRS.
Mobility Management is one of the major functions of a GSM or UMTS Network. The aims of mobility management are as follows.
- to track where the subscribers are so that calls can be sent to them
- to allow contact to the subscribers to be established
- to record what the subscriber has paid for and provide those, and only those services
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Location Update
The Location update procedure allows a mobile device which has moved from one area to the next to inform the network. The responsibility for this lies with the mobile which should detect the Location Area Code (see below) of any new cell that it moves into. If this is different from the mobile's previous cell, then the mobile should contact the network and perform a location update. The mobile sends a message ( location update request) to the network containing the mobiles previous location and TMSI.
Paging
Paging is one of the fundamental mobility management procedures of a GSM network (and also other cellular networks). It allows a mobile phone for which there is an incoming message, or call, to be contacted whilst it is roaming around.
Essentially, all the cells where the subscriber could be (those belonging to one location area are instructed to broadcast a message (with the TMSI; see below) telling the mobile phone to contact the network. Once contact has been established, the mobile will be told why the network contacted it and will act appropriately (e.g. getting a radio channel and starting ringing in the case of a call). The process involves cooperation between the NSS, BSS and the Mobile Station.
Paging requires that mobile phones regularly listen to a cell broadcast channel. The exact configuration of this may vary from network to network and is one of the key determining factors in determining the standby lifetime of the mobile.
Paging is initiated by the VLR which normally knows the current location of the subscriber to the level of a location area. The VLR also knows which BSCs control cells in that location area. The VLR sends a paging request message to each of the relevant BSCs. The BSCs then send a paging request to every single cell within the location area. This paging request is broadcast on the cell broadcast channel to which the mobile is listening.
Assuming that the mobile is functioning and is in the coverage area of the network, the paging message will be received and the mobile becomes aware that it has a pending message. It is now the job of the mobile to contact the network. This it does by sending a random access request and getting a dedicated communications channel allocated. At this point the BSC informs the VLR that the mobile is in communication with the network and the VLR forwards the communication to the mobile.
Location Area (Location Area Code)
A location area is a set of cells which are grouped together in order to optimise signalling. To each such group a number called a location area code is assigned which is unique within the current mobile network. The location area code is broadcast by each base station (known as a BTS in GSM or a Node-B in UMTS at regular intervals.
If location areas are very large then there will be many mobiles in each one. This means that there will be very much paging traffic going on since every paging request has to be broadcast to every cell in the location area, which would waste bandwidth and waste power on the mobile by requiring it to listen for broadcast messages too much of the time. If on the other hand, there are many small location areas, the mobile must contact the network very often for changes of location which will also drain the mobile's battery.
Routing Area
A routing area is a subdivision of a location area. Routing areas are for use by mobiles which are GPRS attached. The bursty nature of packet traffic means that more paging messages are expected per mobile and so it is worth knowing the location of the mobile more accurately than it would be with circuit switched traffic. A change from routing area to routing area (called a Routing Area Update) is done in an almost identical way to a change from location area to location area. The main differences is that the SGSN is the element involved and that
Roaming Number Allocation
When a call is routed to a mobile network subscriber, that call is initially routed toward the subscriber's home network using the subscriber's own number, just like a normal phone call.
Once the call reaches the subscriber's home network, and arrives in the Gateway MSC, the issue becomes different. It is perfectly possible for the subscriber to be roaming in a different country. For that reason, a number belonging to the Visited MSC where the subscriber is currently located is allocated on a temporary basis (maximum for a few seconds). This number is called the Mobile Subscriber Roaming Number or MSRN.
The number is allocated by the VLR where the subscriber is located, sent in a signalling message to the HLR and from there to the Gateway MSC. The Gateway MSC then routes the call just as a normal telephone call where the dialled number is the MSRN. The call travels through the international networks until it arrives at the Visited MSC. The Visited MSC checks in the VLR which subscriber the call is for and in which Location Area they are located, pages the subscriber and delivers the call.
TMSI
The TMSI, or the Temporary Mobile Subscriber Identity is the identity most commonly sent between the mobile and the network. It is a randomly allocated number which is given to the mobile the first time that it is switched on. The TMSI is local to a location area so must be updated every time the mobile does a location update procedure. The network can also force the mobile to accept a new TMSI at any time. These procedures make it difficult to trace which mobile is which except briefly, the first time the mobile is switched on or when the data in the mobile becomes invalid for some reason. At that point the global IMSI must be sent to the network.
A key use of the TMSI is in paging of the mobile.