Editing IP Multimedia Subsystem (section)

==== PSTN gateways ====
A PSTN/CS gateway interfaces with [[PSTN]] [[Circuit switching|circuit switched]] (CS) networks. For signalling, CS networks use [[ISDN User Part]] (ISUP) (or [[Bearer Independent Call Control|BICC]]) over [[Message Transfer Part]] (MTP), while IMS uses SIP over IP. For media, CS networks use [[Pulse-code modulation]] (PCM), while IMS uses [[Real-time Transport Protocol]] (RTP).
* A signalling gateway (SGW) interfaces with the signalling plane of the CS. It transforms lower layer protocols as [[Stream Control Transmission Protocol]] (SCTP, an IP protocol) into [[Message Transfer Part]] (MTP, a [[Signalling System 7]] (SS7) protocol), to pass [[ISDN User Part]] (ISUP) from the MGCF to the CS network. The SGW does call control protocol conversion between SIP and ISUP/BICC under the control of the MGCF.
* A ''media gateway controller function'' (MGCF) is a SIP endpoint that interfaces with the SGW over SCTP. It also controls the resources in a ''[[Media Gateway]]'' (MGW) across an [[Gateway Control Protocol|H.248]] interface.
* A ''[[media gateway]]'' (MGW) interfaces with the media plane of the CS network, by converting between [[Real-time Transport Protocol|RTP]] and [[PCM]]. It can also transcode when the [[codec]]s don't match (e.g., IMS might use [[Adaptive Multi-Rate|AMR]], PSTN might use [[G.711]]).