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SIP Trunking Topology

Communications Server 2007 R2

Topic Last Modified: 2009-01-21

The term SIP trunking has various meanings within the telecommunications industry, including:

  • A generic means to connect one vendor’s IP-PBX with another vendor’s equipment
  • A generic means to connect multiple IP-PBXs across a wide area network (WAN) within the same enterprise
  • A generic means for an IP-PBX to connect to a remote hosted service for the purpose of originating and terminating calls to the PSTN network

This final meaning is the sense in which the term SIP trunking is used in Office Communications Server 2007 R2.

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 service provider for PSTN connectivity eliminates the need to deploy and maintain IP-PSTN gateways.

The following figure depicts the SIP trunking topology in Office Communications Server 2007 R2.

Figure 1. SIP trunking topology
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As shown in the diagram, an IP virtual private network (VPN) is used for connectivity between the enterprise network and the PSTN service provider. The purpose of this private network is to provide IP connectivity, security, and (optionally) quality-of-service guarantees. In such an environment, you do not need to additionally secure the SIP signaling traffic (with TLS) or the media traffic (with SRTP). Connections between the enterprise and the service provider therefore consist of plain TCP connections for SIP and plain RTP (over UDP) for media tunneled through an IP VPN. Be sure that all firewalls between the VPN routers have ports open to allow the VPN routers to communicate, and that the IP addresses on the external edges of the VPN routers are publicly routable.

It is expected that the demarcation point in most service providers’ networks will be a session border controller, although this is not required. Likewise, whether to use an IP-PSTN gateway or a soft switch in the service provider network is up to the service provider.

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 the Mediation Server.

The Mediation Server discovers 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.

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