Advanced Sbc Features; Configuring Call Preemption For Sbc Emergency Calls - AudioCodes Mediant 3000 User Manual

User's Manual
36

Advanced SBC Features

36.1 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 active regular
calls to free up resources for sending the emergency call to its' destination (i.e., emergency
service provider), and not reject 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 being 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. Below are examples of Message Condition rules, configured in
the Message Condition table, 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.
Figure 36-1: Examples of Message Condition Rules for Identifying SBC Emergency Calls
The device applies the Message Condition rule only after call classification (but, before
inbound manipulation).

To configure SBC emergency call preemption:
1. In the Message Condition table, configure a Message Condition rule to identify
incoming emergency calls. See above for examples. For more information on
Message Conditions, see ''Configuring Message Condition Rules'' on page 602.
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36. Advanced SBC Features
Mediant 3000