Definition of SIP Trunk

Of all the VoIP related verbiage, it seems to me that none is so thoroughly misconstrued as the term 'SIP trunk'. I have seen it used by various service providers and vendors to describe just about every possible interaction between a SIP client and a SIP server.  In practice, it is not uncommon to find two parties agreeing on the meaning they intend to use, like two old-fashioned modems negotiating a baud rate.

To illustrate the confusing state of affairs, one need only search for a working defintion.  The lack of one is so glaring that the grand-daddy of SIP, Jonathan Rosenberg, was compelled to author a best practices guide in February of 2008.  Rosenberg's document describes the technical requirements of four possible SIP trunking scenarios.  The SIP Forum has also published technical guidelines.  Not to be outdone, Microsoft has come up with at least two defintions of its own: one for its Office Communications Server and another for the Response Point small business PBX.  Naturally, the definitions differ. There are others as well, like Ingate's marketing effort here.   You get the picture.

My own defintion FWIW is broader than those mentioned above.   I think of a SIP trunk as describing the number of concurrent sessions that are allowed between two SIP agents (taken from the PSTN where a trunk referred to a physical TDM circuit with a set number of channels) regardless of the application's behavior.  A SIP trunk can refer to any relationship between two SIP agents that limits the communications channels to a single call session, 24 call sessions, or any other number of call sessions (but no more or less than the number of concurrent call sessions agreed to by the parties in advance).  The salient characteristic being the analogy of a SIP session to a TDM channel.  To me, the most practical definition of SIP trunk refers to capacity as opposed to techincal requirements.  But that's just me.

Confused?  Don't despair.  So is everyone else.  Perhaps, like the term 'reasonable man' to  members of a jury, a SIP trunk is destined to mean different things to different people.  So long as you understand that there is no definition set in stone, you can take comfort in the knowledge that the person you are talking to is probably just as confused as you are.
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This page contains a single entry by Eric Hernaez published on July 2, 2008 2:20 PM.

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