Mapping of Integrated Services Digital Network (ISDN) User Part (ISUP) Overlap Signalling to the Session Initiation Protocol (SIP)
RFC 3578

Document Type RFC - Proposed Standard (August 2003; No errata)
Last updated 2015-10-14
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Network Working Group                                       G. Camarillo
Request for Comments: 3578                                      Ericsson
Category: Standards Track                                    A. B. Roach
                                                             dynamicsoft
                                                             J. Peterson
                                                                 NeuStar
                                                                  L. Ong
                                                                   Ciena
                                                             August 2003

         Mapping of Integrated Services Digital Network (ISDN)
                  User Part (ISUP) Overlap Signalling
                to the Session Initiation Protocol (SIP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes a way to map Integrated Services Digital
   Network User Part (ISUP) overlap signalling to Session Initiation
   Protocol (SIP).  This mechanism might be implemented when using SIP
   in an environment where part of the call involves interworking with
   the Public Switched Telephone Network (PSTN).

Camarillo, et al.           Standards Track                     [Page 1]
RFC 3578             ISUP Overlap Signalling to SIP          August 2003

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conversion of ISUP Overlap Signalling into SIP en-bloc
       Signalling . . . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.1.  Waiting for the Minimum Amount of Digits . . . . . . . .  4
       2.2.  The Minimum Amount of Digits has been Received . . . . .  4
   3.  Sending Overlap Signalling to a SIP Network. . . . . . . . . .  5
       3.1.  One vs. Several Transactions . . . . . . . . . . . . . .  5
       3.2.  Generating Multiple INVITEs. . . . . . . . . . . . . . .  6
       3.3.  Receiving Multiple Responses . . . . . . . . . . . . . .  8
       3.4.  Canceling Pending INVITE Transactions. . . . . . . . . .  9
       3.5.  SIP to ISUP. . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 10
   5.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   7.  Intellectual Property Statement. . . . . . . . . . . . . . . . 11
   8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
   9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

Camarillo, et al.           Standards Track                     [Page 2]
RFC 3578             ISUP Overlap Signalling to SIP          August 2003

1.  Introduction

   A mapping between the Session Initiation Protocol (SIP) [1] and the
   ISDN User Part (ISUP) [2] of SS7 is described in RFC 3398 [3].
   However, RFC 3398 only takes into consideration ISUP en-bloc
   signalling.  En-bloc signalling consists of sending the complete
   telephone number of the callee in the first signalling message.
   Although modern switches always use en-bloc signalling, some parts of
   the PSTN still use overlap signalling.

   Overlap signalling consists of sending only some digits of the
   callee's number in the first signalling message.  Further digits are
   sent in subsequent signalling messages.  Although overlap signalling
   in the PSTN is the source of much additional complexity, it is still
   in use in some countries.

   Like modern switches, SIP uses en-bloc signalling.  The Request-URI
   of an INVITE request always contains the whole address of the callee.
   Native SIP end-points never generate overlap signalling.

   Therefore, the preferred solution for a gateway handling PSTN overlap
   signalling and SIP is to convert the PSTN overlap signalling into SIP
   en-bloc signalling using number analysis and timers.  The gateway
   waits until all the signalling messages carrying parts of the
   callee's number arrive, and only then, it generates a SIP INVITE
   request.  Section 2 describes how to convert ISUP overlap signalling
   into en-bloc SIP this way.

   However, although it is the preferred solution, conversion of overlap
   to en-bloc signalling sometimes results in unacceptable (multiple
   second) call setup delays to human users.  In these situations, some
   form of overlap signalling has to be used in the SIP network to
   minimize the call setup delay.  However, introducing overlap
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