SIP Trunking and Office Communications Server 2007 R2...

SIP Trunking is a generic means to connect multiple IP-PBXs across a wide area network (WAN) within the same enterprise. To know more about SIP Trunking... click here.

The purpose of a SIP Trunk in an Office Communications Server topology is to enable an enterprise to connect its on-premises voice network to a service provider offering Public Switched Telephone Network (PSTN) origination and termination. Relying on an independent server provider for PSTN connectivity eliminates the need to deploy and maintain IP-PSTN gateways.

The Office Communications Server 2007 R2 Session Initiation Protocol (SIP) Trunking capability enables the following scenarios:

An enterprise user inside or outside the corporate firewall can make a local or long-distance call specified by an E.164-compliant number that is terminated on the PSTN as a service of the corresponding service provider.

Any PSTN subscriber can contact an enterprise user inside or outside the corporate firewall by dialing a Direct Inward Dialing (DID) number associated with that enterprise user.

How does it work?

An IP Virtual Private Network (VPN) is used for connectivity between the enterprise network (OCS VOIP) and the PSTN service provider. The purpose of this private network is to provide IP connectivity, security, and (optionally) quality-of-service guarantees.

In this SIP trunking topology, the Mediation Server performs media and signaling translation, just as it does when connected to an enterprise IP-PSTN gateway.

All SIP traffic and media traffic between the enterprise network and the service provider network flows through the VPN and Mediation Server.

The Mediation Server can discover the service provider proxy (the Session Border Controller in the diagram) through:

Static provisioning, in which the Mediation Server is provisioned manually with a Fully Qualified Domain Name (FQDN) or IP address of the service provider proxy.

A Domain Name System (DNS) query against an FQDN published by the PSTN service provider.

The PSTN service provider can use either of these methods for discovering the list of enterprise proxies (Mediation Servers) to which it can connect.

Note: This article applies to Office Communications Server 2007 R2.

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