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.

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