March 2006
Intermediate to advanced
464 pages
12h 16m
English
book
The IMS: IP Multimedia Concepts And Services, Second Edition
by Miikka Poikselka, Georg Mayer, Hisham Khartabil, Aki Niemi
Content preview from The IMS: IP Multimedia Concepts And Services, Second Edition
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3.8. Discovering the IMS entry point
In order to communicate with the IMS, an item of UE has to know at least one IP address of the P-CSCF. The mechanism by which the UE retrieves these addresses is called "P-CSCF discovery". Two dynamic mechanisms for P-CSCF discovery have been standardized in 3GPP: the Dynamic Host Configuration Protocol's (DHCP) DNS procedure and the GPRS procedure. Additionally, it is possible to configure either the P-CSCF name or the IP address of the P-CSCF in the UE.
In the GPRS procedure (Figure 3.7), the UE includes the P-CSCF address request flag in the PDP context activation request (or secondary PDP context activation request) and receives the IP address(es) of the P-CSCF in the response. This information is transported in the protocol configuration options information element [3GPP TS 24.008]. The mechanism the Gateway GPRS Support Node (GGSN) used to get the IP address(es) of the P-CSCF(s) is not standardized. This mechanism does not work with pre-Release 5 GGSNs.
Figure 3.7. A GPRS-specific mechanism for discovering the P-CSCF.
Figure 3.8. A generic mechanism for discovering the P-CSCF.
In the DHCP DNS procedure (Figure 3.8), the UE sends a DHCP query to the IP connectivity access network (e.g., GPRS), which relays the request to a DHCP server. ...
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