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The study is concluded in 3GPP Rel-10 timeframe, even though there were more than 10 alternative<br />
solutions proposed in study phase. Those alternatives were narrowed down to two alternatives: SIP level<br />
anchor and MGW anchor. The SIP level anchor solution was finally chosen as the specification solution<br />
because it has less impact on CS entities, and is easier to deploy.<br />
Figure 7.13 provides the reference architecture for SRVCC using the Access Transfer Control Function<br />
(ATCF) enhancements (non-emergency session). The figure only depicts the specific reference points for<br />
the ATCF.<br />
Figure 7.13. IMS Service Centralization and Continuity Reference Architecture when using ATCF<br />
Enhancements.<br />
7.3.5.1 ACCESS TRANSFER CONTROL FUNCTION<br />
The Access Transfer Control Function (ATCF) is a function in the serving (visited if roaming) network.<br />
When SRVCC enhanced with ATCF is used, the ATCF is included in the session control plane for the<br />
duration of the call before and after Access Transfer. It should be noted that it is recommended for the<br />
ATCF be co-located with one of the existing functional entities within the serving network (e.g. P-CSCF,<br />
IBCF, or MSC Server).<br />
The ATCF shall:<br />
� Based on operator policy, decide to<br />
o allocate a STN-SR<br />
o include itself for the SIP sessions<br />
o instruct the ATGW to anchor the media path for originating and terminating sessions<br />
www.4gamericas.org February 2011 Page 73
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