Session Management and PDP Context

To exchange data packets with external PDNs after a successful GPRS attach, a mobile station must apply for an address used in the PDN. In general, this address is called PDP

address (Packet Data Protocol address). In case the PDN is an IP network, this will be an IP address.

For each session, a so-called PDP context is created, which describes the characteristics of the session. It contains the PDP type (e.g. IPv4), the PDP address assigned to the mobile station (e.g. 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 the MS, the SGSN, and the GGSN. Once a mobile station has an active PDP context, it is "visible" for the external network and can send and receive data packets. The mapping between the two addresses (PDP ~ GSM address) makes the transfer of data packets between MS and GGSN possible.

The allocation of a PDP address can be static or dynamic. In the first case, the mobile station permanently owns a PDP address, which has been assigned by the network operator of the user's home-PLMN. Using a dynamic addressing concept, a PDP address is assigned upon activation of a PDP context; i.e., each time a mobile station attaches to the network it will in general get a new PDP address, and after its GPRS detach this PDP address will be again available to other MSs. The PDP address can be assigned by the user's home-PLMN operator (Dynamic Home-PLMN PDP Address) or by the operator of the visited network (Dynamic Visited-PLMN PDP Address). The GGSN is responsible for the allocation and deactivation of the addresses.

Figure 11.3 shows the PDP context activation procedure initialized by the MS. Using the message activate pdp context request, the MS informs the SGSN about the requested PDP context. If a dynamic address is requested, the parameter pdp address will be left empty. Afterward, the usual GSM security functions (e.g. authentication of the user) are performed. If access is granted, the SGSN will send a create pdp context request to the affected GGSN. The GGSN creates a new entry in its PDP context table, which enables the GGSN to route data packets between the SGSN and the external PDN. It confirms this 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

MS SGSN GGSN

Activate pdp Context Request

[PDP Type, PDP AdrlrBK, Qu5 RaquBslad. Access Point, ...I

Security functions

m Activate PDP Context Accept

Create PDP Context Request

(PDP Type, PDP Address, QdS Negatived. Agce^e P^IIKU ...]

__ Create pdp Context Response

[PDP Address. QgS Negotiated. ..]

[PDP Type, PDP Address, QdS Negatieled ]

Figure 11.3: PDP context activation

Figure 11.3: PDP context activation confirms the activation of the new PDP context to the MS (activate pdp context accept).

It is also worth mentioning that the GPRS standard supports anonymous PDP context activation, which is useful for special applications such as pre-paid services. In such a session, the user (i.e. the IMSI) using the PDP context remains unknown to the network. Security functions as shown in Figure 11.3 are skipped. Only dynamic address allocation is possible in this case.

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