Topic Last Modified: 2011-01-28
Microsoft Lync Server 2010 uses a SIP trunk, routed through the Mediation Server, to connect an emergency call to the Emergency Services Service Provider. You can either configure a SIP trunk for E9-1-1 at the central site only or one at each branch site. However, if the WAN link between a central site and a branch site is unavailable and there is a SIP trunk configured for E9-1-1 only at the central site, then a call placed by a user at the branch site will need a special phone usage record in the user’s voice policy that will route the call through the local PSTN gateway. The same is true if call admission control is applied to the call.
For details about deploying a SIP trunk in Lync Server 2010, see How Do I Implement SIP Trunking?. To help deploy the SIP trunks for E9-1-1, you should first answer the following questions.
- Should you deploy the SIP trunk over a dedicated leased or a shared internet connection?
It is important that emergency calls always connect. A dedicated line will provide a connection that will not be pre-empted by other traffic on the network, and the ability to implement QoS. If connecting over the public internet, a VPN is required.
- Is your E9-1-1 deployment designed for disaster tolerance?
Because this is an emergency solution, resiliency is important. Deploy your primary and secondary Mediation Servers and SIP trunks in disaster tolerant locations. It is a good idea to deploy your primary Mediation Server closest to the users that it is supporting, and route failover calls through the secondary Mediation Server (located in a different geographic location).
- Should you deploy a separate SIP trunk for each branch office?
Lync Server provides several strategies for handling Voice resiliency in branch offices including: having resilient data networks, deploying a SIP trunk at each branch, or pushing calls out to the local gateway during outages. For details, see Branch Site SIP Trunking.
- Is call admission control (CAC) enabled?
Lync Server does not handle emergency calls any differently than an ordinary call. For this reason, bandwidth management, or call admission control (CAC) can have a negative impact on an E9-1-1 configuration. Emergency calls can be blocked or routed to the local PSTN gateway if a CAC is enabled and the configured limit is exceeded on a link where emergency calls are being routed.