SIP

SIP is the call-control language of IMS — a text-based, HTTP-like request/response protocol that sets up, modifies and tears down media sessions. The verbs are readable: INVITE to start a session, ACK to confirm, BYE to end it, REGISTER to bind a user to their current location, plus responses like the familiar "200 OK" and "180 Ringing." It deliberately handles only the signalling; the actual voice or video runs on a separate media path, and the media details (codecs, ports, addresses) are carried inside SIP messages as SDP.

That separation is the thing to internalise: SIP negotiates, RTP carries. For an engineer, SIP's text format is a gift — a packet capture of a failing call is largely human-readable, and you can often spot the problem (a rejected codec offer, a 4xx error code, a missing route header) by reading the ladder diagram of messages. In IMS the same protocol underpins VoLTE, VoNR, video calling and messaging; only the application servers behind it differ.