Topic Last Modified: 2010-11-09

Session Initiation Protocol (SIP) is used to initiate and manage voice over IP (VoIP) communications sessions for basic telephone service and for 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 an Internet telephony service provider (ITSP) beyond your firewall. Typically, a SIP trunk is used to connect your organization’s central site to an ITSP. In some cases, you may also opt to use SIP trunking to connect your branch site to an ITSP.

SIP Trunks vs. Direct SIP Connections

The term trunk is derived from circuit-switched technology. It refers to a dedicated physical line that connects telephone switching equipment. Like the predecessor time division multiplexing (TDM) trunks, SIP trunks are connections between two separate SIP networks, the Microsoft Lync Server 2010 enterprise and the ITSP. Unlike circuit-switched trunks, SIP trunks are virtual connections that can be established over any of the supported SIP trunking connection types. For details about the supported connection types, see How Do I Implement SIP Trunking?.

Direct SIP connections, on the other hand, are SIP connections that do not cross the local network boundary (that is, they connect to a PSTN gateway or PBX within your internal network). For details about how you can use direct SIP connections with Lync Server 2010, see Direct SIP Connections.

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