Definition of Events for Channel-Oriented Telephony Signalling
RFC 5244

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Network Working Group                                     H. Schulzrinne
Request for Comments: 5244                                   Columbia U.
Updates: 4733                                                  T. Taylor
Category: Standards Track                                         Nortel
                                                               June 2008

     Definition of Events for Channel-Oriented Telephony Signalling

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.

Abstract

   This memo updates RFC 4733 to add event codes for telephony signals
   used for channel-associated signalling when carried in the telephony
   event RTP payload.  It supersedes and adds to the original assignment
   of event codes for this purpose in Section 3.14 of RFC 2833.  As
   documented in Appendix A of RFC 4733, some of the RFC 2833 events
   have been deprecated because their specification was ambiguous,
   erroneous, or redundant.  In fact, the degree of change from Section
   3.14 of RFC 2833 is such that implementations of the present document
   will be fully backward compatible with RFC 2833 implementations only
   in the case of full ABCD-bit signalling.  This document expands and
   improves the coverage of signalling systems compared to RFC 2833.

Schulzrinne & Taylor        Standards Track                     [Page 1]
RFC 5244           Channel-Oriented Signalling Events          June 2008

Table of Contents
   1. Introduction ....................................................2
      1.1. Overview ...................................................2
      1.2. Terminology ................................................3
   2. Event Definitions ...............................................4
      2.1. Signalling System No. 5 ....................................6
           2.1.1. Signalling System No. 5 Line Signals ................6
           2.1.2. Signalling System No. 5 Register Signals ............7
      2.2. Signalling System R1 and North American MF .................8
           2.2.1. Signalling System R1 Line Signals ...................8
           2.2.2. Signalling System R1 Register Signals ...............8
      2.3. Signalling System R2 ......................................10
           2.3.1. Signalling System R2 Line Signals ..................10
           2.3.2. Signalling System R2 Register Signals ..............10
      2.4. ABCD Transitional Signalling for Digital Trunks ...........12
      2.5. Continuity Tones ..........................................14
      2.6. Trunk Unavailable Event ...................................14
      2.7. Metering Pulse Event ......................................15
   3. Congestion Considerations ......................................15
   4. Security Considerations ........................................16
   5. IANA Considerations ............................................17
   6. Acknowledgements ...............................................20
   7. References .....................................................20
      7.1. Normative References ......................................20
      7.2. Informative References ....................................21

1.  Introduction

1.1.  Overview

   This document extends the set of telephony events defined within the
   framework of RFC 4733 [4] to include signalling events that can
   appear on a circuit in the telephone network.  Most of these events
   correspond to signals within one of several channel-associated
   signalling systems still in use in the PSTN.

   Trunks (or circuits) in the PSTN are the media paths between
   telephone switches.  A succession of protocols have been developed
   using tones and electrical conditions on individual trunks to set up
   telephone calls using them.  The events defined in this document
   support an application where such PSTN signalling is carried between
   two gateways without being signalled in the IP network: the "RTP
   trunk" application.

   In the "RTP trunk" application, RTP is used to replace a normal
   circuit-switched trunk between two nodes.  This is particularly of
   interest in a telephone network that is still mostly
   circuit-switched.  In this case, each end of the RTP trunk encodes

Schulzrinne & Taylor        Standards Track                     [Page 2]
RFC 5244           Channel-Oriented Signalling Events          June 2008

   audio channels into the appropriate encoding, such as G.723.1 [13] or
   G.729 [14].  However, this encoding process destroys in-band
   signalling information that is carried using the least-significant
   bit ("robbed bit signalling") and may also interfere with in-band