The IMS: IP Multimedia Concepts And Services, Second Edition

10.10. Access and location information

10.10.1. P-Access-Network-Info

The P-Access-Network-Info header is a 3GPP-specific header and indicates to the IMS network over which access technology the UE is attached to IMS. In our example the access technology is GPRS. It also includes the Cell Global ID (CGI), which indicates the location of the user.

Tobias's UE will include the P-Access-Network-Info header in every request (besides ACK and CANCEL requests) and every response (besides responses to the CANCEL request) that it sends out, but only if that request or response is sent integrity-protected (i.e., via an SA, see Section 10.7).

The first time this header is sent out is, therefore, within the second REGISTER request, which is sent after the 401 (Unauthorized) response has been received by the UE. The header looks like:

REGISTER sip:home1.fr SIP/2.0
P-Access-Network-Info: 3GPP-UTRAN-TDD
                       ;utran-cell-id-3gpp=234151D0FCE11

Tobias's S-CSCF will remove the P-Access-Network-Info header from every request or response that it sends toward another entity. The only exception from this rule are ASs that are in the same trust domain as the S-CSCF (see Section 10.5.10).

When the P-Access-Network-Info header is sent in an INVITE request that is sent for an emergency call, the P-CSCF and S-CSCF can determine from the Cell-ID which emergency centre is closest to the user and should be contacted. When writing this chapter the details for IMS emergency calls were still under discussion in 3GPP ...