SIP Trunk Service Provider

The Emergency Services Service Provider is a third-party service that acts as a liaison between a Lync deployment and the PSAP via SIP trunk. It is important to clarify that Lync Server 2013 does not natively contact a Public Safety Access Point directly. Instead, it is the responsibility of the Emergency Services Service Provider to route the emergency calls to the correct PSAP.

Figure 32.12 displays the process that occurs when an emergency call is placed from a Lync endpoint. Lync 2013 provides an endpoint’s location to the Emergency Services Service Provider using the PIDF-LO format, which is really an XML blob containing all the detailed address information.

Figure 32.12. SIP trunk E911 service provider.

When planning for Enhanced 911 services, an organization must first identify where E911 will be deployed. This might be only within a primary site, in multiple sites, or extended to branch sites. When planning E911 for branch sites, be sure to consider scenarios in which a WAN link is unavailable. It is possible that branch or remote sites will not be able to provide location information or even contact an Emergency Services Service Provider without a resilient WAN link.

The connection to the Emergency Services Service Provider is accomplished through a dedicated SIP trunk. When the SIP trunk is being provisioned, a VPN tunnel to the Emergency Services Service Provider is created using an existing Internet connection, or a dedicated connection can be provisioned to separate and isolate the emergency calls.

An advantage to the service provider method is that it can also serve remote users signed in through an Edge Server. The addresses entered by remote users won’t be validated in advance through a master database, but the service provider can answer the call, confirm the address delivered by the Lync client’s PIDF-LO data, and then route to an appropriate PSAP for the remote user.

The final consideration with the SIP trunk is to recognize that it does not bypass Call Admission Control policies. If a bandwidth policy is exceeded by an emergency call, the call will not succeed. When planning for E911, be sure to consider the effects of Call Admission Control on where SIP trunks to an Emergency Services Service Provider are placed. For example, in a site where WAN bandwidth is constrained, it might make sense to deploy a local Mediation Server and SIP trunk to a provider to ensure that Call Admission Controls never prevent an emergency call across the WAN link.