Mlpp Preemption Events In Sip Reason Header - AudioCodes Mediant 800B MSBR User Manual

30.12.1 MLPP Preemption Events in SIP Reason Header

The device sends the SIP Reason header (as defined in RFC 4411) to indicate the reason
and type of a preemption event. The device sends a SIP BYE or CANCEL request, or SIP
480, 486, 488 response (as appropriate) with a Reason header whose Reason-params can
includes one of the following preemption cause classes:

Reason: preemption ;cause=1 ;text="UA Preemption"

Reason: preemption ;cause=2 ;text="Reserved Resources Preempted"

Reason: preemption ;cause=3 ;text="Generic Preemption"

Reason: preemption ;cause=4 ;text="Non-IP Preemption"
This Reason cause code indicates that the session preemption has occurred in a non-
IP portion of the infrastructure. The device sends this code in the following scenarios:
•
The device performs a network preemption of a busy call (when a high priority
call is received), the device sends a SIP BYE or CANCEL request with this
Reason cause code.
•
The device performs a preemption of a B-channel for a Tel-to-IP outbound call
request from the softswitch for which it has not received an answer response
(e.g., Connect), and the following sequence of events occurs:
a.
b.

Reason: preemption; cause=5; text="Network Preemption"
This Reason cause code indicates preempted events in the network. Within the
Defense Switched Network (DSN) network, the following SIP request messages and
response codes for specific call scenarios have been identified for signaling this
preemption cause:
•
SIP:BYE - If an active call is being preempted by another call
•
CANCEL - If an outgoing call is being preempted by another call
•
480 (Temporarily Unavailable), 486 (User Busy), 488 (Not Acceptable Here) -
Due to incoming calls being preempted by another call.
The device receives SIP requests with preemption reason cause=5 in the following
cases:
•
The softswitch performs a network preemption of an active call - the following
sequence of events occurs:
a.
b.
c.
•
The softswitch performs a network preemption of an outbound call request for the
device that has not received a SIP 2xx response - the following sequence of
events occur:
a.
b.
User's Manual
The device sends a Q.931 DISCONNECT over the ISDN MLPP PRI to the
partner switch to preempt the remote end instrument.
The device sends a 488 (Not Acceptable Here) response with this Reason
cause code.
The softswitch sends the device a SIP BYE request with this Reason cause
code.
The device initiates the release procedures for the B-channel associated
with the call request and maps the preemption cause to PRI Cause = #8
'Preemption'. This value indicates that the call is being preempted. For PRI,
it also indicates that the B-channel is not reserved for reuse.
The device sends a SIP 200 OK in response to the received BYE, before the
SIP end instrument can proceed with the higher precedence call.
The softswitch sends the device a SIP 488 (Not Acceptable Here) response
code with this Reason cause code. The device initiates the release
procedures for the B-channel associated with the call request and maps the
preemption cause to PRI Cause = #8 'Preemption'.
The device deactivates any user signaling (e.g., ringback tone) and when
the call is terminated, it sends a SIP ACK message to the softswitch.
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Mediant 800B MSBR
Document #: LTRT-12813