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q951_1s.txt
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INTERNATIONAL TELECOMMUNICATION UNION
ITU-T RECOMMENDATION SUMMARY
Rec. No. : Q.951.1
Title : Stage 3 description for number identification supplementary
services using DSS 1
Study Group : XI - Switching and Signalling
Version : New
Date of adoption : 1992
Notes :
DIRECT-DIALLING-IN (DDI)
This service enables a user to call directly to another user on an
ISDN private branch exchange (ISPBX) or other private systems, without
attendant intervention. This supplementary service is based on the use
of the ISDN number and does not include Sub-addressing.
DDI numbers form part of the international numbering plan according to
Recommendation E.164.
Recommendation Q.951.1 provides the flexibility to Administrations to
use national numbering plans of fixed or variable lengths. This
flexibility also applies to DDI numbers, i.e. even within a given
ISPBX or other private system, DDI numbers of different lengths may
appear.
In networks with an open numbering plan, the length of the DDI number
is not necessarily known by the servicing local exchange nor by any
other entity of the public network.
DDI is provided when at least a part of the ISDN number which is
significant to the called user is passed to that user. These last
digits (fixed or variable length) are sent from the exchange and
received by the ISPBX or other private system by en-bloc or overlap
receiving, which finally and automatically establish a call to the
destination without the assistance of an operator. These last digits
are sent in accordance with Recommendation Q.931, º 5.2.
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).