Version 6.2 267 February 2011
SIP User's Manual 8. IP Telephony Capabilities
MLPP Precedence Level Precedence Level in Resource-
Priority SIP Header
DSCP Configuration Parameter
8 flash-override MLPPFlashOverRTPDSCP
9 (highest) flash-override-override MLPPFlashOverOverRTPDSCP
Precedence Ring Tone: You can assign a ring tone (in the CPT file) that is played
when a Precedence call is received from the IP side. This is configured by the
parameter PrecedenceRingingType.
Emergency Telecommunications Services calls (e.g., E911): ETS calls can be
configured to be regarded as having a higher priority than any MLPP call (default),
using the E911MLPPBehavior parameter.
MLPP Preemption Events in SIP Reason Header: The device sends the SIP
Reason header (as defined in RFC 4411) to indicate the reason a preemption event
occurred and the type of preemption event. The device sends a SIP BYE or CANCEL
request, or 480, 486, 488 responses (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”
• Reason: preemption; cause=5; text=”Network Preemption”
Cause=4: The Reason cause code "Non-IP Preemption" 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. 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.
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