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FOREWORD
The CCITT (the International Telegraph and Telephone Consultative
Committee) is a permanent organ of the International Telecommunication
Union (ITU). CCITT is responsible for studying technical, operating and tar-
iff questions and issuing Recommendations on them with a view to standard-
izing 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 (Melbourne, 1988).
Recommendation I.255.4 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 Administration and a recog-
nized 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 photocopy-
ing and microfilm, without permission in writing from the ITU.
PAGE BLANCHE
Recommendation I.255.4
PRIORITY SERVICE
1 Definition
The Priority service provides for preferential treatment in the net-
work to calls originating from and/or addressed to certain numbers in the
order of path selection.
2 Description
2.1 General description
This service provides preferential treatment in the network for calls:
a) originating from subscribers' lines. This is designated as A-priority;
and/or
b) addressed to certain emergency numbers. This is designated as B-
priority and concerns the order of path selection in the ISDN.
For A-priority, subscribers' lines are classified into two A-priority
categories which are:
i) subscribers with first order priority (I) for all calls;
ii) subscribers with first order priority (II) on a per call basis.
Only one B-priority category is envisaged.
2.2 Specific terminology
The A-priority service meets the needs of a subscriber for whom it is
necessary to have calls completed urgently at any time when the service is
activated for his lines.
The B-priority service meets the needs of subscribers for calling
specified emergency numbers. This service also meets the needs of Adminis-
trations, communities and other organizations which provide this service.
2.3 Qualifications on the applicability to telecommunication services
The Priority supplementary service is applicable to all telecommuni-
cation services.
3 Procedures
3.1 Provision/withdrawal
The provision of this service requires arrangements between the sub-
scriber and the Administration.The service can also be established unilater-
ally by the Administration for administrative reasons. The subscriber has to
inform the Administration if he wants this service with A- or B-priority.
The Administration can withdraw the service at the request of the
subscriber or for administrative reasons.
3.2 Normal procedures
3.2.1 Activation/deactivation/interrogation procedures
Priority will be given to the subscriber by the Administration, which
has full control of the registration.
All changes to priority information are made by the Administration.
After provision of the A-priority service, the subscriber may activate
and deactivate the service by the use of a control procedure.
B-priority is activated upon provision.
3.2.2 Invocation and operation
When the service is provided for all calls, it will automatically be
invoked by the network at the call set-up.
For a call based request, it will be invoked by the user using a service
code as a part of a call set-up. For this service option, invocation is possible
only for the calling terminal in A-priority, category II.
For calls to B-priority numbers, this service may automatically be
invoked by the network depending on the address dialled, i.e. calls to fire,
rescue and ambulance services.
Calls originating from subscribers' lines with the appropriate A-pri-
ority category will be treated as priority calls at the sending exchange as well
as at the transit and the incoming exchange. This means that a priority call
will have preferential access to all network units necessary for a call set-up.
When a priority call meets congestion in the network the call is not released,
but waits for the first line to be free.
This supplementary service does not give priority in accessing a busy
subscriber. For this purpose other supplementary services may be used.
The preferential treatment of priority calls within the network should
include the following functions:
_ extended waiting time before time-out of network units;
_ waiting for a line on the route to become free.
3.3 Exceptional procedures
When a priority call is requested within an Administration that has
no agreement to provide this service, the call will proceed as for a normal
call.
If the priority call is not connected due to congestion, the call will be
cancelled after a time-out (e.g. five seconds).
(Further study required.)
3.4 Alternate procedures
None identified.
4 Network capabilities 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 ser-
vice.
5 Interworking consideration
The service will be provided whenever there is an ISDN User Part
(ISUP) available.
In networks where there is no ISUP available or where priority ser-
vice is not offered, the priority call will be handled as a normal call.
If a B-priority call originates in a network that does not support the
priority service, the priority will be assigned to the call upon entering the
network supporting the priority service.
6 Interaction with other supplementary services
6.1 Call Waiting
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.2 Call Transfer
Priority service is restricted only to A-B connections.
6.3 Connected Line Identification Presentation (COLP)
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.4 Connected Line Identification Restriction (COLR)
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.5 Calling Line Identification Presentation (CLIP)
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.6 Calling Line Identification Restriction (CLIR)
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.7 Closed User Group
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.8 Conference Calling
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.9 Direct Dialling-In
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.10 Diversion services
Priority service is restricted to A-B connections.
6.11 Line Hunting
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.12 Three Party Service
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.13 User-to-User Signalling
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.14 Multiple Subscriber Number
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.15 Call Hold
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.16 Advice of Charge
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.17 Multi-level precedence and preemption service (MLPP)
No impact, i.e. neither supplementary service affects the operation of
the other supplementary service.
6.18 Priority service
If at some point there is more than one priority connection waiting
for service, "first in, first out" principle is to be followed.
7 Overall SDL diagram
The overall SDL Diagram is shown in Figures 1/I.255.4 and 2/
I.255.4.
Fig.1 = 24cm
Fig. 2 = 24 cm