1. Introduction

   SIP [1] is an application layer protocol for establishing,
   terminating and modifying multimedia sessions.  It is typically
   carried over IP.  Telephone calls are considered a type of multimedia
   sessions where just audio is exchanged.

   Integrated Services Digital Network (ISDN) User Part (ISUP) [12] is a
   level 4 protocol used in Signaling System No. 7 (SS7) networks.  It
   typically runs over Message Transfer Part (MTP) although it can also
   run over IP (see SCTP [19]).  ISUP is used for controlling telephone
   calls and for maintenance of the network (blocking circuits,
   resetting circuits etc.).

   A module performing the mapping between these two protocols is
   usually referred to as Media Gateway Controller (MGC), although the
   terms 'softswitch' or 'call agent' are also sometimes used.  An MGC
   has logical interfaces facing both networks, the network carrying
   ISUP and the network carrying SIP.  The MGC also has some
   capabilities for controlling the voice path; there is typically a
   Media Gateway (MG) with E1/T1 trunking interfaces (voice from Public
   Switched Telephone Network - PSTN) and with IP interfaces (Voice over
   IP - VoIP).  The MGC and the MG can be merged together in one
   physical box or kept separate.

   These MGCs are frequently used to bridge SIP and ISUP networks so
   that calls originating in the PSTN can reach IP telephone endpoints
   and vice versa.  This is useful for cases in which PSTN calls need to
   take advantage of services in IP world, in which IP networks are used
   as transit networks for PSTN-PSTN calls, architectures in which calls
   originate on desktop 'softphones' but terminate at PSTN terminals,
   and many other similar next-generation telephone architectures.

   This document describes logic and procedures which an MGC might use
   to implement the mapping between SIP and ISUP by illustrating the
   correspondences, at the message level and parameter level, between
   the protocols.  It also describes the interplay between parallel
   state machines for these two protocols as a recommendation for
   implementers to synchronize protocol events in interworking
   architectures.