used to shield the internal structure of a network from other networks.
4.13. Cell-ID
The identity of the cell through which the 3GPP UA is accessing the IMS (Cell-ID) may be used by the home network to provide localized services or information on the location of the terminal during an emergency call (when emergency calls are handled in IMS; see also the requirement stated in Section 4.16).
4.13.1. Cell-ID in Signaling from the UA to the Visited and Home Networks
Assuming that the Cell-ID is obtained by the UA by other mechanisms outside the scope of SIP, the Cell-ID must be transported at least in the following procedures:
o Registration o Session Establishment (Mobile Originated) o Session Establishment (Mobile Terminated) o Session Release
The Cell-ID is private information and only of interest in the UA home network. Therefore, the Cell-ID should be removed prior to sending the SIP signaling beyond the originating home network.
4.13.2. Format of the Cell-ID
The cell-ID must be sent in any of the formats described in the 3GPP Technical Specification 23.003 [26].
4.14. Release of Sessions
In addition to the normal mechanisms for releasing a SIP session (e.g., BYE), two cases are considered in this section: the ungraceful session release (e.g., the terminal moves to an out-of-coverage zone) and the graceful session release ordered by the network (e.g., prepaid caller runs out of credit).
We believe that this requirement is met by a SIP entity acting as a so-called transparent back-to-back UA.
4.14.1. Ungraceful Session Release
If an ungraceful session termination occurs (e.g., a flat battery or a mobile leaves coverage), when a call stateful SIP proxy server (such as the SIP serving proxy at home) is involved in a session, memory leaks and, eventually, server failure can occur due to hanging state machines. To ensure stable server operation and carrier grade
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