SIP Basics Explained

On this page you can find information on what SIP is, what and how you can use it for in a few short sentences. If you are interested in how the future's way of communication works, then I would suggest you to read this article, which will turn out to be very useful for you.

SIP, or Session Initiation Protocol was invented to establish sessions in the Internet, to transport session description from callers to callees, to allow parameter changes in mid-session and it also terminates the session.

sip basics explained
Figure 1 - SIP Basics explained

Aim of SIP applications

(as shown by Figure 1):
  • To set up VoIP calls
  • To set up multimedia conferences
  • To signal Transport
  • For text messaging and general messaging
  • To notify events.

Facts about SIP Syntax

  • You can fold lines.
  • White space only matters in the first line
  • Field names and media types are not case-sensitive, but everything else is
  • You can combine multi-valued header fields as a comma-list

There are certain methods that are used with SIP:

  • INFO: if you need information at the middle of a call
  • COMET: preconditions are met
  • REFER: to ask the callee to issue SIP request (call transfer)
  • NOTIFY: so that your subscribers get notified
  • SUBSCRIBE: to subscribe to an event
  • PRACK: for provisional acknowledgement
  • INVITE: to initiate a call
  • BYE: to end and transfer the call
  • ACK: so that a final response can be acknowledged
  • CANCEL: to cancel ringing and searches
  • REGISTER: to register with location service
  • OPTIONS: to show features supported by the other side

SIP building blocks:

  • SIP registrar: it can maintain mappings between names and addresses
  • SIP stateless proxy: it routes call requests
  • SIP (forking) proxy: it also routes call requests
  • SIP redirect server: it returns new location for requests
  • SIP user agent (UA): PC, IP Phone (which can convert voice into digital data and back), conference bridge and so on.

SIP is part of the latest development of modern telecommunication technologies, which Ozeki Phone System uses. The intent of the Ozeki Phone System is to use all the available resources to their most, so it can provide you the highest quality, flexibility and advancement during your communication sessions.

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