AudioCodes Mediant 500 MSBR, User Manual

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Version 7.2 729 Mediant 500 MSBR
User's Manual 41. Advanced SBC Features
41 Advanced SBC Features
41.1 Configuring Call Preemption for SBC Emergency
Calls
The device supports emergency call preemption for SBC calls by prioritizing emergency
calls over regular calls. If the device receives an incoming emergency call when there are
unavailable resources to process the call, the device preempts one of the regular calls to
free up resources for sending the emergency call to its' destination (i.e., emergency service
provider), instead of rejecting it. The device may preempt more than one active call in order
to provide sufficient resources for processing the emergency call. Available resources
depends on the number of INVITE messages currently processed by the device.
If the device preempts a call, it disconnects the call as follows:

If the call is being setup (not yet established), it sends a SIP 488 response to the
incoming leg and a SIP CANCEL message to the outgoing leg.

If the call is already established, it sends a SIP BYE message to each leg. The device
includes in the SIP BYE message, the Reason header describing the cause as
"preemption".
Once the device terminates the regular call, it immediately sends the INVITE message of
the emergency call to its' destination without waiting for any response from the remote
sides (e.g., 200 OK after BYE). If the device is unable to preempt a call for the emergency
call, it rejects the emergency call with a SIP 503 "Emergency Call Failed" (instead of
"Service Unavailable") response.
For the device to identify incoming calls as emergency calls, you need to configure a
Message Condition rule in the Message Conditions table. Below are examples of Message
Condition rules for identifying emergency calls:
Figure 41-1: Examples of Message Conditions for Identifying Emergency Calls

Indices 0 and 1: SIP Resource-Priority header contains a string indicating an
emergency call.

Indices 2 to 4: Destination user-part contains the emergency provider's address.
The device applies the Message Condition rule only after call classification (but, before
inbound manipulation).
Note:
•
The device does not preempt established emergency calls.
•
The device does not monitor emergency calls with regards to Quality of
Experience (QoE).

To configure SBC emergency call preemption:
1. In the Message Conditions table (see Configuring Message Condition Rules on page
387), configure a Message Condition rule to identify incoming emergency calls. See