The IMS Bearer Plane
Figure 5.1 shows the positioning of the IMS-MGW with respect to its controller, the MGCF. The figure also situates the IMS-MGW and the MGCF with respect to the circuit-switched and the packet-switched (IP) domains. The protocol between the MGCF and the IMS-MGW (i.e., across the Mn reference point) is H.248. H.248 is a packet-switched protocol that runs over IP. Therefore, the Mn reference point is shown as inhabiting the IP domain. At one time, the Internet Engineering Task Force (IETF) and the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) worked jointly on protocol development in this area. Some documents still refer to Megaco, which is the IETF name for H.248.