[This is pre-release documentation and subject to change in future releases. This topic's current status is: Milestone-Ready]

Topic Last Modified: 2010-03-09

Session Initiation Protocol (SIP) is used to initiate and manage voice over IP (VoIP) communications sessions for basic telephone service as well as many additional real-time communication services, such as instant messaging, conferencing, presence detection, and multimedia. This section provides planning information for implementing SIP trunks, a type of SIP connection that extends beyond the boundary of your local network.

What is SIP Trunking?

A SIP trunk is an IP connection that establishes a SIP communications link between your organization and a switching entity beyond your firewall. Typically, you would use a SIP trunk to connect your enterprise central site or data center to an Internet telephony service provider (ITSP). In some cases, you may also opt to use SIP trunking to connect a data center to a branch site.

SIP Trunks vs. Direct SIP Links

The term trunk, derived from legacy circuit-switched technology, refers to a dedicated physical line that connects telephone switching equipment. Similar to the predecessor TDM trunks, SIP trunks are connections between telecommunications systems, but unlike circuit-switched trunks, SIP trunks are virtual connections that can be established over any of the supported SIP trunking connection types.

Direct SIP links, on the other hand, are SIP connections that do not cross the local network boundary. For more information about how you can use direct SIP links with Communications Server 2010, see Direct SIP Links.