What does the Mobility Management (MM) layer do?
The main function of the Mobility Management sublayer is to support the mobility of user terminals, such as informing the network of its present location and providing user identity confidentiality. A further function of the MM sublayer is to provide connection management services to the different entities of the upper Connection Management (CM) sublayer.
Sometimes MM is referred to as GMM; are they the same?
MM sub-layer consists of two types of procedures; MM procedures for non-GPRS services and GMM procedures for GPRS services. The G in GMM stands for GPRS.
What are MM Procedures?
Depending on how they can be initiated, three types of MM procedures can be distinguished:
- MM common procedures: A MM common procedure can always be initiated whilst a RR connection exists. The procedures belonging to this type are:
Initiated by the network:
- TMSI reallocation procedure;
- authentication procedure;
- identification procedure;
- MM information procedure;
- abort procedure.
However, abort procedure is used only if an MM connection is being established or has already been established i.e. not during MM specific procedures or during IMSI detach procedure, see subclause 4.3.5.
Initiated by the mobile station:
- IMSI detach procedure (with the exceptions specified in subclause 4.3.4).
- MM specific procedures: A MM specific procedure can only be initiated if no other MM specific procedure is running or no MM connection exists. The procedures belonging to this type are:
- normal location updating procedure;
- periodic updating procedure;
- IMSI attach procedure.
- MM connection management procedures: These procedures are used to establish, maintain and release a MM connection between the mobile station and the network, over which an entity of the upper CM layer can exchange information with its peer. A MM connection establishment can only be performed if no MM specific procedure is running. More than one MM connection may be active at the same time.
What are GMM procedures?
Depending on how they can be initiated, two types of GMM procedures can be distinguished:
- GMM common procedures: In Iu mode, a GMM common procedure can always be initiated whilst a PS signalling connection exists.
The procedures belonging to this type are:
Initiated by the network when a GMM context has been established:
- P-TMSI (re-) allocation;
- GPRS authentication and ciphering;
- GPRS identification;
- GPRS information.
- GMM specific procedures: Initiated by the network and used to detach the IMSI in the network for GPRS services and/or non-GPRS services and to release a GMM context:
Initiated by the MS and used to attach or detach the IMSI in the network for GPRS services and/or non-GPRS services and to establish or release a GMM context:
- GPRS attach and combined GPRS attach;
- GPRS detach and combined GPRS detach.
Initiated by the MS when a GMM context has been established:
- normal routing area updating and combined routing area updating;
- periodic routing area updating.
In UMTS, initiated by the MS and used to establish a secure connection to the network and/or to request the resource reservation for sending data:
Do i need to have RR (RRC) connection to perform Mobility Management functions?
Yes. If you do not have a RR connection than you need to establish one to perform MM functions. You cannot perform any MM/GMM procedures without RRC conection.
How is MM signalling done?
After RRC connection has been setup, the direct transfer messages are used to perform MM signalling. Look at the tutorial on message sequence for Registration for details. It could be seen that after RRC connection has been setup
 3GPP TS 24.008 - Mobile radio interface Layer 3 specification; Core network protocols; Stage 3
 3GPP TS 24.007 - Mobile radio interface signalling layer 3; General aspects
 3GPP TS 44.018 - Mobile radio interface layer 3 specification, Radio Resource Control (RRC) protocol
 3GPP TS 23.060 - General Packet Radio Service (GPRS); Service description; Stage 2
 3GPP TS 23.012 - Location Management Procedures
 3GPP TS 29.002 - Mobile Application Part (MAP) specification
 3GPP TS 25.331 - RRC Protocol Specification
 3GPP TS 25.304 - User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode
 3GPP TS 23.122 - NAS Functions related to Mobile Station (MS) in idle mode