A comparison of SIP and ISUP

Or: SIP for telephone engineers...

The Session Initiation Protocol (SIP) is an application-layer control protocol that can establish, modify and terminate multimedia sessions or calls. These multimedia sessions include multimedia conferences, distance learning, Internet telephony and similar applications. SIP can invite both persons and "robots", such as a media storage service. SIP can invite parties to both unicast and multicast sessions; the initiator does not necessarily have to be a member of the session to which it is inviting. Media and participants can be added to an existing session.

Resource reservation isn't done with SIP

Since not all phone calls require resource reservation and not all flows with resource reservation will use SIP, the Internet architecture separates the establishment of a session from reserving resources for it. A session could well consist of only text chat, for example, or a distributed chess match, neither of which would call for resource reservation. Other applications, such as media-on-demand or VPN, require resource reservation, but are not likely to use SIP. See also the FAQ.

SIP supports service language, priority, ...

SIP supports more descriptors for a call, including

• service language (Accept-Language, Content-Language);
• call priority (Priority);
• caller/callee organization;
• subject of call;
• time to call back (Retry-After).

SIP can introduce conference calls but doesn't support voting, management of conference calls

See general FAQ.

How is "alerting" handled?

The 1xx (provisional response) messages deal with this. See FAQ.

Last updated by Henning Schulzrinne