Communications Server 2010 supports implementing E9-1-1 for clients connected located both inside and outside of the network. The main difference in the two scenarios is the way that the client location is determined. If the client is located within the network, the location information is automatically provided by the Location Information Server. If the client is located outside of the network, or the network location is not available in the location database, the user must manually enter the location. For more information about how a client obtains location information, see Determining Location Information.

The following diagram shows how an emergency call is routed from Communications Server to the PSAP.

DIAGRAM

When an emergency call is placed from a compatible Communications Server client:

1. An Invite is routed to Communications Server 2010 containing the location and the callback number.
   
   
2. Communications Server matches the number type (specifying that it is an emergency call), and routes the call, based on the PSTN Usage value defined in the location policy, through the Mediation Server to the Emergency Services Provider using a SIP Trunk.
   
   
3. The Emergency Services Provider routes the emergency call to the correct PSAP based on the location that is associated with the call.
   
   
4. If configured, onsite security is sent an IM notification and conferenced into the call.
   
   
5. If necessary, the PSAP uses the callback number to contact the client directly.
   
   

For E9-1-1, it is important to maintain a consistent connection from the client, through the Mediation Server, to the Emergency Services Provider. If a user places an emergency call using a client, and the client cannot connect to the Emergency Services Provider, or cannot connect to the Location Information Server to obtain a location, than the call may not be routed to the correct PSAP. This is particularly important for branch offices which depend on a WAN connection to the main office for access to the Location Information Server and the SIP trunk connection to the Emergency Services Provider (in a Centralized model). Communications 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 more information, see Branch-Site Voice Resiliency.

To help decide if a SIP trunk is needed for a branch site, review the Centralized vs. Distributed SIP trunking section of How Do I Implement SIP Trunking?.

Emergency calls can be blocked or routed to the local PSTN gateway if a Call Admission Control (CAC) is enabled and the configured limit is exceeded on a link where emergency calls are being routed. In such an instance, the PSAP must determine the location from the phone number sent with the call. This behavior may not provide location information to the PSAP if:

• There is not a location record in the emergency database for the number making the emergency call.
  
  
• The location associated with the number making the emergency call is not specific enough to route emergency responders to the caller. This is also true for emergency calls originating from both the user’s number and the main office’s number.
  
  

You can avoid having CAC block emergency calls by:

• Not implementing CAC on link connecting the network site to the SIP trunk used by E9-1-1.
  
  
• Deploying a separate SIP trunk from the network site to the Emergency Services Provider.
  
  

For more information about CAC, see Overview of Call Admission Control.