EGPRS: GPRS/EDGE
8.4 GPRS Mobility Management
The GPRS is an additional service provided by the mobile network. The mobile user can be registered in the network with user’s location known to the VLR and HLR but still unknown to the packet core. In order to provide packet services, the GPRS network
106 Introduction to Mobile Network Engineering
Idle
Standby
Ready GPRS Attach/Detach
Standby timer expired
Ready timer expired
Packet Transmit/Receive MS Location is
known at cell level
MS Location is known at RA level, MS can be paged.
MS Location is not known to
GPRS.
Figure 8.4 GPRS mobility management states.
introduces three specific mobility management states, namely, IDLE, STANDBY and READY, as shown in Figure 8.4.
8.4.1 Mobility Management States 8.4.1.1 IDLE State
The IDLE state is used when the subscriber (MS) is passive (not GPRS attached). The subscriber is not reachable by the GPRS network. The packet core network elements hold no valid context for the subscriber and the subscriber is not attached to the mobility management. In order to change state, the MS has to perform a GPRS attach procedure.
8.4.1.2 READY State
The subscriber is attached to the mobility management and the location of an MS is known on a cell level. The MS is capable of receiving PTM (Point To Multi-point) and PTP (Point-To-Point) data. The SGSN can send data to the MS without paging at any time and the MS can send data to the SGSN at any time. The network holds a validmobil- ity management contextfor the subscriber. If the READY timer expires, the MS moves to STANDBY state. If the MS performs a GPRS Detach procedure, the MS moves to the IDLE state and the mobility management context is removed. An MS in the READY state does not necessarily have radio resources reserved.
8.4.1.3 STANDBY State
The subscriber isattached to the mobility management and the location of an MS is known on a routing area level. The MS can be paged from the network. The network holds a valid mobility management context for the subscriber. If the MS sends data, the MS moves to the READY state. The MS or the network can initiate the GPRS Detach procedure to move to the IDLE state. After expiry of the MS reachable timer, the network can detach the MS.
8.4.2 PDP Context Activation
In the process of PDP context activation, the network provides an IP address for the MS.
Two different options are possible:
• static allocation: the MS has a permanent IP address and
• dynamic allocation: the corresponding GGSN assigns a temporary IP address to the MS.
A PDP context is normally activated by the MS. In the case of mobile terminating packet transfer, the GGSN must initiate a paging to the MS that requests the PDP context activation in order to receive the packets: that is, the GGSN can only request a PDP context activation but not initiate one.
It is important to note here that a PDP context activation request from the GGSN is only possible with a static IP address has been allocated to the MS. This address must then be stored in the HLR’s profile record for the respective MS.For each data trans- fer session, a PDP context must be created. It contains the PDP type (e.g. IPv4), the PDP address assigned to the MS (i.e. an IP address), the requested QoS class and the address of a GGSN that serves as the access point to the external network. This context is stored in all participating network entities: the MS, the SGSN and the GGSN. The call flow in Figure 8.5 shows the PDP context activation procedure initialized by the MS.
As observed from Figure 8.5, while MS initiates the PDP context activation, the func- tion of allocation and release of the PDP context belongs to GGSN. The GGSN creates an entry in PDP context table, which enables the GGSN to route data packets between the SGSN and the external PDN. It confirms that to the SGSN with a message CREATE PDP CONTEXT RESPONSE, which also contains the dynamic PDP address (if needed).
Finally, the SGSN updates its PDP context table and confirms the activation of the new PDP context to the MS (ACTIVATE PDP CONTEXT ACCEPT).
PDP Context request
Security procedures
Create PDP Context request
Activate PDP Context (PDP type, PDP address, QoS
parameter, APN , ..)
SGSN GGSN
MS
Create PDP Context response
Figure 8.5 PDP context activation procedure.
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INACTIVE
Transfer to ACTIVE
when MS in STANDBY/READY PDP not activated
No routing context for MS, SGSN,GGSN Data transfer not possible
ACTIVE PDP activated
Routing context for MS, SGSN, GGSN established Data transfer possible
GPRS Detach/PDP Deactivation/STANDBY
timer expired
PDP Context Activation
Figure 8.6 PDP states/phases.
One PDP context includes one address (usually a dynamic IP address) and one set of QoS attributes. Several PDP contexts may be activated simultaneously. When the subscriber has finished the use of the activated addresses, they have to be deactivated.
As long as PDPs are activated, the MS has to be in the STANDBY or READY state.
A return to GMM ‘IDLE’ automatically deactivates all active PDPs. Figure 8.6 shows the relation between phases of the packet services in relation to the states of the mobile station and respective procedures.
8.4.3 Location Management
Just as in circuit-switched GSM, the main task of location management is to keep track of the user’s current location, so that incoming packets can be routed to their MS. For this purpose, the MS has to send location update messages to its SGSN. The location update frequency depends on the state in which the MS currently is. In IDLE state, no location update is performed; that is, the current location of the MS is unknown. If a MS is in READY state, it will inform its SGSN of every movement to a new cell. The GSM Location Area is usually divided into several Routing Areas (RAs) for packet ser- vices. When the MS is in a STANDBY state it must inform the SGSN when moving into new RA.