 a handoff: Data connection setup that involves the execution of two procedures known as GPRS attach and the PDP context activation. This estab- lishes the data path required to carry the SIP-related messages to the proxy-CSCF through the GGSN, which acts as the gateway for the proxy-CSCF. The messages involved in the GPRS attach and the PDP context acti- vation procedures are shown in Figure 17.5 and Figure 17.6. The steps are described as follows: as a part of the GPRS attach procedure, the MH sends an attach message (1) to the SGSN (responsible for mobility management, logical link management, and authentication and charg- ing functions in a UMTS network) with the MH's International Mobile Subscriber Identity (IMSI). The SGSN uses the IMSI to authenticate (messages 2, 3, 4 and 5) the MH with its HLR. Successful authentica- tion is followed by the SGSN sending a location update to the HLR (messages 6 and 7). The SGSN finally completes the attach procedure by sending an attach complete message (8) to the MH. Thus, a logical association is established between the MH and the SGSN. Once an MH is attached to an SGSN, it must activate a PDP address (or IP address) to begin packet data communication. Activation of the PDP address creates an association between the MH's current SGSN and the GGSN (acting as the interface between the GPRS/UMTS backbone network C 2009 by Taylor & Francis Group, LLC SIP-Based Mobility Management and Its Performance Evaluation Attach request (1) Identity request (2) Identity response (3) Authentication (4) IMEI Check (5) Location update (6, 7) Attach accept (8) FIGURE 17.5 GPRS attach procedure. Activate PDP context request (9) Create PDP context request (10) Create PDP context response (11) Radio access bearer setup (12, 13) Update PDP context request (14) Update PDP context response (15) Activate PDP context accept (16) FIGURE 17.6 PDP aontext activation procedure. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security and the external packet data network