Breakout gateway control function
The IMS will have to coexist with the circuit-switched domain of the network operator for some time. For example, GSM will have to exist with the IMS as will cdma2000 and the IMS. This is not unusual since the EV-DO network of cdma2000 has to operate with the cdmaOne network that is now in place, which is the task faced by Verizon Wireless today. Cingular Wireless needs to operate traditional GSM with the High-speed Packet Access services that are rolling out as part of UMTS. Clearly the IMS and the PSTN will also have to coexist.
To move from the IMS to the circuit-switched domain, the IMS uses the breakout gateway control function (BGCF), which determines where the breakout to the circuit-switched domain occurs. The outcome of the selection is not obvious at first since the breakout can be in the same network or to another network. For example, Cingular Wireless will implement IMS within the structure of its current GSM and this represents one network from the IMS perspective. Thus, an IMS Cingular Wireless user could talk to a GSM Cingular Wireless user and the BGCF would select the network in which it currently resides. In this case, the BGCF retains control and selects a media gateway control function (MGCF) to handle the session further.
Suppose that the IMS Cingular Wireless user needs to talk to a Verizon Wireless user that is operating in the circuit-switched domain and not the IMS domain of the Verizon Wireless network. Now the BGCF selects a breakout that is to another network. In this case the BGCF hands control of the session to the BGCF in the selected network. Now the BGCF in the selected network chooses the MGCF that will handle the session since it selects a circuit-switched domain that is part of its network. This scenario assumes that the selected network has the IMS as part of its network. If the IMS is not in place, then the handoff goes directly through the media gateway to the circuit-switched domain. In this case, some functions of the IMS could be lost at the gateway.