Internet Standards/CD2.mdf/ccitt/1992/i/i255_3.asc

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FOREWORD

    The CCITT (the International Telegraph and Telephone Consul-
tative Committee) is a permanent organ of the International Tele-
communication Union (ITU). CCITT is responsible for studying
technical, operating and tariff questions and issuing Recommen-
dations on them with a view to standardizing telecommunications
on a worldwide basis.

    The Plenary Assembly of CCITT which meets every four years,
establishes the topics for study and approves Recommendations
prepared by its Study Groups. The approval of Recommendations
by the members of CCITT between Plenary Assemblies is covered
by the procedure laid down in CCITT Resolution No. 2 (Mel-
bourne, 1988).

    Recommendation I.255.3 was prepared by Study Group I and
was approved under the Resolution No. 2 procedure on 2 July
1990.





___________________





CCITT  NOTE

    In this Recommendation, the expression "Administration" is
used for shortness to indicate both a telecommunication Adminis-
tration and a recognized private operating agency.



















πITU1990

All rights reserved. No part of this publication may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying and microfilm, without permission in
writing from the ITU.

PAGE BLANCHE

Recommendation I.255.3

MULTI-LEVEL  PRECEDENCE  AND  PREEMPTION  SER-
VICE  (MLPP)

1    Definition

    The multi-level precedence and preemption (MLPP) service provides
prioritized call handling service. This service has two parts _ pre-
cedence 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 prece-
dence call in the absence of idle resources. Users in networks that
do not support this service will not be affected by this service.

2    Description

2.1    General description

    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 sub-
scriber may select a precedence level up to and including the max-
imum 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 pre-
empted 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 prece-
dence 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 notifi-
cation;

_    any called party of an active call that is being preempted by a
higher precedence call should be required to acknowledge the pre-
emption before being connected to the new calling party (see _2.2 for
definition of active call); 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.

    The MLPP service is not intended to provide preemption of
users that do not subscribe to the MLPP service. The service pro-
vides for preemption of calls within the MLPP domain, which
consists of the resources belonging to the users that subscribe to
the MLPP service. In other words, calls that are originated by, or
made to, non-MLPP users will not be preempted. Calls that are
originated by MLPP subscribers may be preempted by calls of
higher precedence only in networks that support this service.

    2.2    Specific terminology

    Precedence is the priority associated with a call.

    A precedence call is a call with precedence level higher than the
lowest level of precedence.

    An MLPP call is a call that has a precedence level established
and is either being set up (i.e. before alerting) or is set up.

     User A is a party of the MLPP call and is not the called party of
the precedence call.

    User B is the other party of the MLPP call and is the called party
of the precedence call.

    User C is the party who initiates the precedence call to user B.

    An active call is a call that has the connection established and the
calling and called parties are active on the call.

    Response timer TK is started when the service provider notifies
the called user of a precedence call (for example, this would be the
preemption notification if preemption occurs at the user inter-
face). This timer establishes the time that userB has to accept the
precedence call from userC. The length of this timer is in the
range of 4-30seconds.

    An alternate party is the party to which a precedence call will be
diverted. Diversion will occur either when the response timer TK
expires, when the called party is busy on a call of equal or higher
precedence, or when

the called party is busy with access resources non-preemptable. Alter-
nate party diversion is an optional terminating feature that is sub-
scribed to by the called party; thus, the alternate party to which a
precedence call is diverted is specified by the called party at the
time of subscription.

2.3    Qualifications on the applicability to telecommunications ser-
vices

    This supplementary service is considered meaningful when
applied to the telephony teleservice, speech, 3.1kHz audio, 7 kHz
audio, and 64 kbit/s unrestricted bearer services. Furthermore, it
may be meaningful when applied to other services.

3    Procedures

3.1    Provision/withdrawal

    For a given ISDN number, a maximum authorized precedence
level may be subscribed to for each service or collectively for all
services.

    The service provider should provide the capability of combining
3.1 kHz audio and speech for the purpose of assigning subscrip-
tion options. Subscriber options are summarized below:

include 255-3t1e

Subscriber optionsValue

Maximum authorized prece-
dence levela)









Alternate party







Access resource non-preemptableb)_    0
(Flash Override: highest)

_    1    (Flash)

_    2    (Immediate)

_    3    (Priority)

_    4    (Routine: lowest)

_    No

_    Yes

    _    network operating position

    _    alternate party directory number

_    No

_    Yes

a)    A call of higher precedence level can preempt calls of lower
precedence. For example, a Flash call can preempt Immediate,
Priority, or Routine calls.

b)    A user having this option will not experience preemption of
calls by higher precedence calls, if the cause for preemption
would be due to called party busy condition. However, the user
may still experience preemption of calls due to a lack of service
provider resources other than the user's own access resources.

3.2    Normal procedures

3.2.1    Activation/deactivation/registration

    (None identified.)

3.2.2    Invocation and operation

    The precedence level is selected by the subscriber on a per call
basis. The subscriber may select any precedence level up to and
including his maximum authorized precedence level. The service
provider at the subscriber's originating interface ensures that the
selected precedence level does not exceed the maximum level
assigned to that ISDN number. Once set for a call, this precedence
level cannot be changed.

    An MLPP call is automatically established with the lowest prece-
dence unless a higher precedence is specified.

3.2.3    Operation

    During a call set-up, if there is shortage of some resource, then
the network can determine if resources are held by calls of lower
precedence. The network can then release the lowest lower prece-
dence call(s) and seize the necessary resources that are required
to set up the higher precedence call. These resources can include
inter-office circuits, channels, conference bridges, and circuit-
switched data circuits.

    The preemption operation depends on whether the network
needs to preempt a common network facility such as an inter-
switch trunk which is currently being used by a different sub-
scriber than the intended called subscriber or whether it needs to
preempt a channel on the user access of the desired called sub-
scriber.

    If a common network facility is preempted, both existing parties
concerned are notified of the preemption and the existing connec-
tion is immediately disconnected. The new call is then set up using
the preempted facility in the normal manner without any special
notification to the new called party.

     If a called user access channel is to be preempted, both the called
and non-called parties shall be notified of the preemption and the
existing MLPP call shall be immediately cleared. The called party
must acknowledge the preemption before the higher precedence
call is completed. The called party is then notified that a new
MLPP call is available.

3.3    Exceptional procedures

3.3.1    Activation/deactivation/registration

    (None identified.)

3.3.2    Invocation and operation

    If the service provider cannot comply with a precedence call
request, the calling party should receive a notification that the
precedence call is unsuccessful. Possible causes are:

a)    the requested precedence level is not subscribed to;

b)    equal or higher precedence calls have prevented completion;

c)    the dialled number is non-preemptable; and

d)    there are no idle network resources to make a connection to the
dialled number and the called subscriber belongs to a network
that does not support preemption.

    A precedence call should be diverted to a predetermined alter-
nate party, which can be another subscriber or a network operat-
ing position, if the called party (userB) does not acknowledge
preemption or does not answer a precedence call (a call of prece-
dence level 0-3) before the response timerTKexpires. To prevent
infinite diversions, an alternate party diversion is limited to a
total of five alternate party diversion attempts. Once this limit
isreached, no additional diversion attempts will be made. In the
case of preemption at the user interface, if no alternate party has
been specified by the called party, an unacknowledged indication
will be returned for the precedence call.

    In addition, a precedence call should be diverted to a predeter-
mined alternate party if the called party is busy on a call of equal
or higher precedence or is busy and non-preemptable. To prevent
infinite diversions, an alternate party diversion is limited to a
total of five alternate party diversion attempts. Once this limit is
reached, no additional diversion attempts will be made. If no
alternate party has been specified by the called party, a prece-
dence call blocked notification will be returned to the calling
party for the precedence call.

4    Network capability for charging

    This Recommendation does not cover charging principles.
Future Recommendations in the D-Series are expected to contain
that information.

    It shall be possible to charge the subscriber accurately for the
service.

5    Interworking considerations

    In public networks that support the MLPP service, the network
shall ensure that only MLPP calls from the same domain as the
preempting call are a subject of preemption and that the connec-
tions of non-MLPP users are not preempted. The precedence level
of a call may not be changed when interworking with other
MLPP networks.

    A network that does not support the MLPP service is required, if
bilaterally agreed, to convey the parameters of the MLPP service
intact. If the parameters are received from another network, the
network should pass them

on with no action taken, if bilaterally agreed, and with no effect on the
network that does not support the MLPP service.

6    Interaction with other supplementary services

    The following interactions with other supplementary services
apply within the MLPP domain.

6.1    Call Waiting

a)    The incoming call is of the lowest precedence level (precedence
level4):

1)    one or more MLPP calls are of the lowest precedence level:

no interaction; call waiting service is invoked.

    2)    all MLPP calls are precedence calls:

Call Waiting service is invoked. If the in-band call waiting
tone is being applied as a network provider option, then the
tone should not be provided in this case since it would dis-
rupt a higher precedence call.

b)    The incoming call is a precedence call:

1)    One or more MLPP calls are of lower precedence than the
incoming call:

An MLPP call of the lowest precedence level is preempted
unless the called subscriber is non-preemptable. If the called
subscriber is non-preemptable, Call Waiting service is
invoked and the precedence level of the incoming call is pro-
vided to the called user along with call waiting indication.

2)    one or more MLPP calls are of the same precedence and the rest
of the MLPP calls are of higher precedence than the incom-
ing call:

Call Waiting service is invoked. The precedence level of the
incoming call is provided, along with call waiting indication,
to the called users on MLPP calls at the same precedence
level as the incoming call.

If the called user is on an MLPP call at a higher precedence
level, Call Waiting service is invoked. If the in-band call
waiting tone is being applied as a network provider option,
then the tone should not be provided in this case, since it
would disrupt a higher precedence call.

 3)    All the MLPP calls are of higher precedence:

Call Waiting service is invoked. If in-band call waiting tone
is being applied as a network provider option, then the tone
should not be provided in this case since it would disrupt a
higher precedence call.

6.2    Call Transfer

    For a single step or a normal call transfer, the precedence level of
calls is preserved during the transfer process. For an explicit call
transfer (when two calls are involved), each connection of the
transferred call maintains the precedence level that it was
assigned when that connection of the call was established. Thus, a
call that is established through a normal or an explicit call trans-
fer may consist of two connections at different precedence levels.

6.3    Connected Line Identification Presentation

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.4    Connected Line Identification Restriction

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.5    Calling Line Identification Presentation

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.6    Calling Line Identification Restriction

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.7    Closed User Group

    No interaction _ MLPP does not override CUG restrictions.

    6.8    Conference Calling

    All connections to the conferees are processed at a precedence
equal to the conference precedence selected by the conference
controller for all connections to conferees on the conference call.
If a conferee is preempted,

the conference controller is notified of the preemption. When the con-
troller is preempted, all conferees are notified and the procedure
is followed as though the controller issued a _disconnect_ request.
Each connection of a call resulting from a split operation will
maintain the precedence level that it was assigned upon being
added to the conference call.

6.9    Direct Dialling-In

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.10    Call Diversion services

6.10.1    Call Forwarding Busy

    If the incoming call is of higher precedence than one or more
calls at user B, a call of the lowest precedence will be preempted
and the incoming call will be established, i.e. the Call Forwarding
service will not be invoked.

    If the incoming call is of equal or lower precedence than the
established calls, the Call Forwarding service will be invoked.

    If the called subscriber is non-preemptable, the Call Forwarding
service will be invoked regardless of the precedence levels of
incoming call and established calls.

    The precedence level of calls is preserved during the forwarding
process, and the forwarded-to user may be preempted.

    If call forwarding busy is activated by the called party and the
called party has specified an alternate party, the forwarding pro-
cedure will be performed prior to the alternate party diversion. If
a precedence call is forwarded (including possible multiple for-
wardings) and is not responded to by any forwarded-to party (e.g.
call unanswered or unacknowledged; called party busy with a call
of equal or higher precedence; or called party busy and
non-preemptable) within a specified period of time (typically
30seconds), the call will be diverted to the alternate party of the
original called subscriber. If no alternate party is specified, the
call will be forwarded in the normal manner.

6.10.2    Call Forwarding No Reply

    The precedence level of calls is preserved during the forwarding
process, and the forwarded-to user may be preempted.

    Call Forwarding No Reply will not apply to a precedence call if
an alternate party is specified by the called party. Unanswered
precedence calls will be diverted to the alternate party if that
option is subscribed to by the called user. Calls of the lowest level
of precedence will be forwarded in the normal manner.

6.10.3    Call Forwarding Unconditional

    The Call Forwarding Unconditional service takes precedence
over the MLPP service.

    The precedence level of calls is preserved during the forwarding
process, and the forwarded-to user may be preempted.

    If Call Forwarding Unconditional is activated by the called party
and the called party has specified an alternate party, the forward-
ing procedure will be performed prior to the alternate party
diversion. If a precedence call is forwarded (including possible
multiple forwardings) and is not responded to by any forwarded-
to party (e.g. call unanswered or unacknowledged; called party
busy with a call of equal or higher precedence; or called party
busy and non-preemptable) within a specified period of time (typ-
ically 30seconds), the call will be diverted to the alternate party
of the original called subscriber. If no alternate party is specified,
the call will be forwarded in the normal manner.

     6.11    Line Hunting

    If no interface is available and one or more MLPP calls are of
lower precedence than that of the incoming call, an MLPP call of
the lowest precedence should be preempted.

6.12    Three-Party service

    In minimal Three-Party service, each call will have its own pre-
cedence level. When a three-way conversation is established, each
connection maintains its assigned precedence level. Each connec-
tion of a call resulting from a split operation will maintain the
precedence level that it was assigned upon being added to the
three-way conversation. In the full Three-Party service, when one
of two original calls is preempted, the remaining parties of the
three-way conversation should be alerted of the preemption.

6.13    User-to-User Signalling

    When a connection is preempted, the service provider should
make certain that User-to-User Information (UUI) of the pre-
empted connection is not delivered to the users of the new connec-
tion.

6.14    Multiple Subscriber Number

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.15    Call Hold

    A held call may be preempted due to lack of network resources
or channels at the held party's interface. The call is cleared and
the served user, who invoked the Call Hold service, will be notified
of the preemption.

    For the case of multiple terminals on an interface, an idle chan-
nel that is reserved for a call held by another terminal may be
seized in order to complete a higher precedence call. In addition,
an active channel that is reserved for a held call may be pre-
empted and seized in order to complete a higher precedence call.
In both cases, the held call is not preempted and may be retrieved
when a channel becomes available.

6.16    Advice of Charge

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

6.17    Multi-Level Precedence and Preemption Service (MLPP)

    MLPP does not alter the basic call procedures under glare con-
ditions (i.e. the condition when two entities simultaneously
attempt to seize the same line).

6.18    Priority service

    No impact, i.e. neither supplementary service affects the opera-
tion of the other supplementary service.

7    Dynamic description

    The overall SDL is shown in Figures 1/I.255.3 and 2/I.255.3.

Figure 1/I.255.3 = 22.5 cm



Figure 2/I.255.3 = 22.5 cm