Mobility management
Mobility management states. The mobility management (MM) function is to support the mobility of user terminals. The MM activities related to a GPRS terminal are characterized by one of the three different states, i.e., IDLE, STANDBY, and READY. Figures 4-9 and 4-10 illustrate the MM state models of the MS and the SGSN, respectively.
In GPRS IDLE state, the MS camps onto the GSM network. The MS can receive circuit-switched paging and perform location area updates. In this stage, the MS behaves like any other GSM phone. It is not attached to GPRS mobility management yet. The MS and the SGSN contexts hold no valid location or routing information for the subscriber.
Figure 4-9 MM state model of an MS.
Figure 4-10 MM state model of an SGSN.
Data transmission to and from the MS is not possible. The GPRS MS is not reachable, as paging is not possible. The MS makes the transition to the STANDBY state by attaching itself with the network using the GPRS attach procedure.
In the GPRS READY state, the network is aware of cell location as a result of the successful mobility management procedure. The MS can send or receive PDP PDUs and activate and deactivate PDP contexts. The MS makes the transition to the STANDBY state if no data transmission occurs for a settable timer period. A GPRS detach procedure brings the MS back to the IDLE state.
In GPRS STANDBY, the MS is attached to the network. The MS and the SGSN have established MM contexts. The MS can be paged for PS and CS call via SGSN. The MS can initiate activate and deactivate PDP context. The MS makes the transition to the READY state by transmitting or receiving an LLC PDU. The transition to the IDLE state happens when the MS implicit detach from the network occurs or the SGSN receives a MAP cancel location from the HLR.
GPRS attach. The mobile station must perform a GPRS attach in order to be known to the network and move to the READY state. Following the successful GPRS attach, an MM context is said to be active at the MS and the SGSN. The MS can activate PDP context only after a successful attach.
Figure 4-11 GPRS attach procedure (part 1 of 2).
Figures 4-11 and Figure 4-12 show the steps for the GPRS attach procedure.
1. The MS sends the attach request message to the serving SGSN. The key parameters are:
■ IMSI or P-TMSI
■ Routing area identifier (RAI)
■ Cipher key sequence number
■ Attach type
■ DRX
2. If the MS is known to the SGSN, i.e., the SGSN has not changed since the MS was last attached to the network, then step 3 is not required.
3. If this MS is unknown to the serving SGSN, the SGSN takes additional steps to get the IMSI from the old SGSN. In case the old SGSN failed to provide the IMSI, the new SGSN requests the MS to provide the IMSI. The identity request message is used in both cases.
Figure 4-12 GPRS attach procedure (part 2 of 2).
4. Once the IMSI is known, the serving SGSN initiates the authentication procedure by sending an authentication and ciphering request’ to the MS. The procedure is described in Section 4.6.3.
5. If the SGSN has changed since the MS was last attached to the network, the new serving SGSN initiates the update location procedure with the HLR.
6. The HLR sends a cancel location to the old SGSN and sends subscription data to the new SGSN by sending an insert subscriber data message.
7. The SGSN sends an attach accept to the MS. The MS acknowledges by sending an attach complete message.
In the case of GPRS attach failure, one of the following errors is returned in the attach reject message.
■ Illegal MS
■ GPRS service not allowed
■ GPRS and non-GPRS services not allowed
■ PLMN not allowed
■ Location area not allowed
■ Roaming not allowed in this location area
■ GPRS services not allowed in the PLMN
■ No suitable cells in location area
The Gs interface between the SGSN and the MSC/VLR is not a mandatory interface. In case this interface is active, the MS can perform combined GPRS/IMSI attach. In addition to action shown in step 6, the HLR also performs a cancel location with the old MSC/VLR and updates the new MSC/VLR with the subscription data.
GPRS detach. GPRS detach can be performed either by the MS or the network. The MS uses this procedure to inform the network that it does not want GPRS services anymore. The SGSN or the HLR initiates a detach procedure to inform the MS that GPRS services are no more accessible to the MS. The three different types of detach are:
■ IMSI detach
■ GPRS detach
■ Combined IMSI/GPRS detach
The detach request could be explicit or implicit. The explicit detach can be initiated by the MS or the network.
In case of implicit detach, it is the network that initiates the detach procedure, without informing the MS. This may happen in the following cases: (1) after the mobile reachable timer expiry and (2) once the logical link disconnects because of irrecoverable radio error causes.
Figure 4-13 shows the MS-initiated detach procedure. The information elements included in the detach request message are:
■ Detach type (GPRS detach only, IMSI detach only, combined GPRS/IMSI detach)
■ P-TMSI
■ Switch off (detach because of switch off)
Figure 4-14 shows the SGSN-initiated detach procedure. The main parameter in the detach request message is detach type. This parameter indicates whether the MS has been requested to perform a new attach and PDP context activation for the previously activated PDP contexts. If yes, then the attach procedure will be initiated when the detach procedure is completed.
Figure 4-13 MS-initiated detach procedure.
Figure 4-15 shows the HLR-initiated detach procedure. The HLR initiates this procedure as a result of operator action to invoke barring or withdrawing the subscription.
Paging. Figure 4-16 shows the GPRS paging procedure. On receiving a PDU from GGSN/SGSN meant for a MS in its serving area, the SGSN sends a BSSGP paging request to the serving BSS.
Figure 4-14 SGSN-initiated detach procedure.
Figure 4-15 HLR-initiated detach procedure.
Figure 4-16 GPRS paging procedure.
The main parameters included in this request are:
■ IMSI
■ P-TMSI
■ Routing area
■ Channel needed (indicating GPRS paging)
■ Negotiated QoS
■ DRX parameters
The BSS checks for the routing area in the paging request message and pages the MS in each cell belonging to RA. The MS responds with any LLC frame, e.g., RR or info frame, other than NULL LLC. The BSS, on receiving the LLC frame, adds cell global identity (CGI) and sends the LLC frame to the SGSN. The SGSN considers the LLC frame a successful paging response.
