Version 6.4 315 March 2012
SIP User's Manual 18. GW and IP to IP
• 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. The device sends a Q.931 DISCONNECT over the ISDN MLPP PRI to the
partner switch to preempt the remote end instrument.
b. The device sends a 488 (Not Acceptable Here) response with this Reason
cause code.
Cause=5: The Reason cause code "Network Preemption" 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. The softswitch sends the device a SIP BYE request with this Reason cause
code.
b. 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.
c. 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 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. 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’.
b. 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
Notes:
• If required, you can exclude the "resource-priority” tag from the SIP
Require header in INVITE messages for Tel-to-IP calls when MLPP
priority call handling is used. This is configured using the RPRequired
parameter.
• For a complete list of the MLPP parameters, see 'MLPP Parameters' on
page 676.
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