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i2553sum.txt
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INTERNATIONAL TELECOMMUNICATION UNION
ITU-T RECOMMENDATION SUMMARY
Rec. No. : I.255.3
Title : Multi-level precedence and preemption service (MLPP)
Study Group : I - Services
Version : New
Date of adoption : 1990
Notes :
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The multi-level precedence and preemption (MLPP) service provides
prioritized call handling service. This service has two parts -
precedence and preemption. Precedence involves assigning a priority
level to a call. Preemption involves the seizing of resources, which
are in use by a call of a lower precedence, by a higher level
precedence call in the absence of idle resources. Users in networks
that do not support this service will not be affected by this service.
The MLPP service is provided as a network provider's option to a
domain of a network. The domain can be the whole network or a subset of
the network. The MLPP service applies to all network resources in the
domain that is in common use. The maximum precedence level of a
subscriber is set at the subscription time by the service provider,
based on the subscriber's need. The subscriber may select a
precedence level up to and including the maximum precedence level
subscribed to, on a per call basis.
Precedence calls (MLPP calls that have a higher precedence than the
lowest level of precedence) that are not responded to by the called
party (e.g. call unanswered and/or unacknowledged, called party busy
with call of equal or higher precedence, or called party busy and
non-preemptable) are diverted to a predetermined alternate party.
This alternate party may be another subscriber or a network
operating position.
Preemption may take one of two forms. First the called party may be
busy with a lower precedence call which must be preempted in favour
of completing the higher precedence call from the calling party.
Second, the network resources may be busy with calls, some of which
are of lower precedence than the call requested by the calling
party. One or more of these lower precedence calls must be preempted
to complete the higher precedence call. There are three
characteristics of preemption:
- any party whose connection was terminated (whether that
resource is reused or not) must receive a distinctive
preemption notification;
- any called party of an active call that is being
preempted by a higher precedence call should be required to
acknowledge the preemption before being connected to the new
calling party; and
- when there are no idle resources, preemption of the
lowest lower level of precedence resources shall occur.
A call can be preempted any time after the precedence level of the
call has been established and before call clearing has begun.
To order the complete text of this Recommendation, please use the
Order Form for ITU-T Recommendations. An electronic version of this
form is available on ITUDOC (Winword 2.0: UPI=ITU-5265;
ASCII: UPI=ITU-2488).