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Voice Feature Guide 01

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What Is the SIP Protocol

What Is the SIP Protocol


Session Initiation Protocol (SIP), defined in RFC 3261, is used for setting up, modifying, and terminating sessions with one or more participants. The session can be a multimedia meeting, distance learning, or Internet telephony. SIP can be used for initiating sessions or inviting a member to join a session that has been set up otherwise. SIP transparently supports the mapping of names and the redirecting service, which facilitates the implementation of intelligent network, and personal mobile service. Once the session is set up, media streams are simply transmitted at the bearer layer through the Real-time Transport Protocol (RTP).

Position of the SIP Protocol on the Network

The SIP protocol is a signaling control protocol at the application layer. In the five-layer TCP/IP model, SIP is an application layer protocol. In the seven-layer OSI model, SIP is a session layer protocol. Figure 1-19 shows the position of the SIP protocol on the network.

The SIP protocol is independent from transmission protocols but is carried on different transmission protocols, such as, UDP, TCP, TLS, and SCTP. SIP is always carried on the UDP for its efficient transmission.

Figure 1-19 Position of the SIP protocol on the network
The SIP protocol must work together with other protocols to complete multimedia calls. Figure 1-20 shows positions of the SIP and other protocols.
Figure 1-20 Positions of the SIP and other protocols
The SIP protocol works with the Real-time Transmit Protocol (RTP), Real-Time Transport Control Protocol (RTCP), domain name server (DNS), Resource ReServation Protocol (RSVP), and Session Description Protocol (SDP) to complete multimedia calls.
  • RTP: A protocol defined by RFC 3550 for transmitting E2E real-time data. It provides the following functions for a series of E2E real-time data transmission services: payload type identification, sequence number arranging, timestamp, and transmission monitoring.
  • RTCP: A protocol that controls transmission of real-time media streams.
  • RSVP: A protocol that preserves network resources.
  • SDP: A text-based application layer protocol for describing multimedia sessions.
  • Sigcomp: A mechanism defined by RFC 3320 and used by application layer protocols to compress messages before they are sent to the network.

Advantages of the SIP Protocol

SIP will revolutionize the mode of communication service provisioning and the users' habit of communication consumption. An innovating communication mode integrating video phone service, messaging, Web service, e-mail, synchronous browsing, and conference call will be introduced to the telecommunication industry. Adopting SIP as the control layer protocol has the following advantages:

  1. Based on an open Internet standard, SIP has inherent benefits in the integration and interoperability of voice and data services. SIP can implement across-media and across-device call control, and supports various media formats. SIP also supports dynamic adding and deleting of media streams, which make it easier to support richer service features.
  2. SIP is intelligently extensible to the service and terminal side, reducing the network load and facilitating the provisioning of service.
  3. SIP supports mobile functions at the application layer, including the dynamic registering mechanism, location management mechanism, and redirecting mechanism.
  4. SIP supports features such as presence, fork, and subscription, which facilitates development of new services.
  5. As a simple protocol, SIP has generally acknowledged extensibility.
Updated: 2019-02-22

Document ID: EDOC1100067358

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