rfc3064.txt

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Network Working Group                                          B. Foster
Request for Comments: 3064                                 Cisco Systems
Category: Informational                                    February 2001


                           MGCP CAS Packages

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document contains a collection of media gateway Channel
   Associated Signaling (CAS) packages for R1 CAS, North American CAS,
   CAS PBX interconnect as well as basic FXO support.  Included are six
   packages.  The "MS" package covers MF single stage dialing trunks.
   This includes wink start and immediate start PBX DID/DOD trunks as
   well as basic R1 and Feature Group D (FGD) Terminating protocol [3].
   The "DT "package covers immediate start and basic DTMF and dial-pulse
   trunks and the "BL" package covers the interface to a basic PBX
   (digital or FXS interface).  In addition to the Terminating protocol,
   there are three other FGD protocols described in [3].  These include
   EAIN and EANA which is covered by the enclosed "MD" package and the
   Operator Service Signaling protocol which is handled by the "MO"
   package.  Support for basic FXO interconnect is provided by "DO"
   package.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119.

IESG Note:

   This document is being published for the information of the
   community.  It describes a protocol that is currently being deployed
   in a number of products.  Implementers should be aware of
   developments in the IETF Megaco Working Group and ITU SG16 who are
   currently working on a potential successor to this protocol.




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RFC 3064                   MGCP CAS Packages               February 2001


Table of Contents

   1.0.Introduction ................................................  3
   1.1. Functional Partitioning ....................................  3
   1.2. CAS Trunk Types ............................................  4
   1.2.1. "MS" Package .............................................  5
   1.2.2. "DT" Package .............................................  5
   1.2.3. "BL" Package .............................................  6
   1.2.4. "DO" Package .............................................  6
   1.2.5. "MD" Package .............................................  6
   1.2.6. "MO" Package .............................................  7
   2.0. Event Packages .............................................  7
   2.1. Events and Signals for the "MS" package ....................  9
   2.2. Events and Signals for the "DT" package .................... 10
   2.3. Events and Signals for the "BL" package (Basic PBX) ........ 10
   2.4. Events and Signals for the "DO" package .................... 11
   2.5. Events and Signals for the "MD" package .................... 12
   2.6. Events and Signals for the "MO" package .................... 13
   2.7. Event and Signal Descriptions .............................. 13
   3.0. Hook-State Signals and Events .............................. 23
   3.1. Overview of Approach ....................................... 23
   3.2. Suspend/Resume Processing .................................. 23
   3.3. Control over Disconnect for E911 ........................... 24
   3.3. Release and Release Complete ............................... 24
   3.4. Blocking CAS Trunks ........................................ 26
   3.5. Summary of Hook-State Events ............................... 26
   4.0. Glare Handling ............................................. 27
   4.1. Glare on MF Bi-directional Wink-start Trunks ............... 27
   4.2. Glare Handling - Basic PBX Trunks .......................... 27
   5.0. Example Call Flows ......................................... 28
   5.1. PBX to PBX ("MS", "DT, and "BL" packages)................... 28
   5.1.1. Call Setup Flows ......................................... 28
   5.1.2. Call Tear-Down ........................................... 34
   5.1.2.1. Origination End Initiates the Release .................. 35
   5.1.2.2. Termination End Initiates the Release .................. 38
   5.2. Example Call Flows - "DO" package .......................... 40
   5.2.1. Call Setup Flows ......................................... 40
   5.2.2. Call Tear-Down ........................................... 42
   5.3. Example Call Setup - "MD" Package .......................... 44
   5.4. Example Call Setup - "MO" Package .......................... 51
   Acknowledgements ................................................ 54
   References ...................................................... 55
   Author's Address ................................................ 55
   Full Copyright Statement ........................................ 56







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RFC 3064                   MGCP CAS Packages               February 2001


1.0.Introduction

1.1. Functional Partitioning

   There are a number of different possible approaches for partitioning
   the functional responsibility between the Call Agent and the Media
   Gateway:

    * The Call Agent takes all of the responsibility for the CAS state
      machine giving the media gateway detailed commands

    * The media gateway contains the CAS state machine and provides an
      abstract interface to the Call Agent

   Timing requirements of CAS protocols often involve reacting within
   time intervals measured in tens of milliseconds which makes direct
   control of timing impossible.  The approach used here is to allow the
   media gateway to handle low level CAS protocol and timing details
   where at all possible and have the Call Agent involved only whenever
   higher level processing is required.

   Taking this approach, the ideal situation would be to allow the Call
   Agent to treat as many CAS protocols in a similar way, leaving the
   details to the media gateway.  Example: for an incoming MF trunk that
   involves a single incoming digit string, the Call Agent should not
   care whether this is a wink start trunk or an immediate start trunk
   (media gateway should not have to provide the wink-start signal).

   Some goals in partitioning responsibility between the media gateway
   and media gateway:

    * Minimize the number of interactions between the Call Agent and the
      media gateway.

    * The media gateway should not have to do digit analysis (e.g., to
      determine that the incoming digits contain carrier access
      information).  This is a Call Agent's responsibility.

    * Provide some reasonable level of abstraction for the Call Agent so
      that it can reuse call flows when possible (e.g., Call Agent
      should not have to differentiate between wink start and immediate
      start interfaces when only one digit string is involved).

    * The media gateway should take care of the CAS protocol (and
      timeouts) where possible with the Call Agent taking over
      responsibility where the media gateway leaves off.





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RFC 3064                   MGCP CAS Packages               February 2001


   Use of Embedded Notifications: Rather than depending on the use of
   embedded notifications, signals and events were defined that had the
   specific semantics required.  There are two reasons for this:

   a) It allows an abstract interface for the Call Agent so that for
   example, the same incoming call-setup event can be used in the case
   of MF wink start and MF immediate start trunks, presenting a common
   interface to the Call Agent even though the semantics at the CAS
   state-machine level are slightly different (i.e., in the MF wink
   start case, a wink-start signal is provided reflexively as a result
   of an incoming seizure, where as in the immediate start case, this is
   not required).

   b) Potential events that might trigger an embedded notification
   (e.g., the incoming seizure mentioned above) typically needed to be
   visible to the Call Agent for billing anyway.

   This does not say that embedded notifications cannot be used.  It
   simply does not necessitate their use.

   Out-pulsing Approach: In order to provide the semantics for
   outpulsing, special higher level signals (e.g., "sup" for call set-up
   and "inf" for information) are included that contain the necessary
   semantics.

   Off-hook and On-hook Signals and Events: A higher level view of off-
   hook and on-hook events is taken in order to make the interface
   Q.931-like.  This provides the advantage that:

    * Similar call flows result when dealing with Q.931-based interfaces
      (e.g., PRI)

    * It's more evident (for ease in debug) when looking at message as
      to exactly what is going on without having to refer to previous
      events

1.2. CAS Trunk Types

   The following describes the types of trunks supported by the various
   packages.  Configuration of the specific trunk type (e.g., wink start
   versus immediate start) is done within the Media Gateway (MG) via
   provisioning facilities outside the scope of MGCP.  The Call Agent's
   responsibility is to support the particular package (i.e., in general
   the Call Agent does not have to differentiate between wink start and
   immediate start, since those differences are taken care of by the
   MG).  However, the Call Agent needs to know which trunks are
   incoming, outgoing or bi-directional.




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RFC 3064                   MGCP CAS Packages               February 2001


1.2.1. "MS" Package

   The "MS" package is used for PBX DID/DOD trunks as indicated in the
   following table.  It is also used for incoming or outgoing MF wink
   start trunks (R1 and FGD Terminating protocol [6]).

           Table 1 MF PBX Trunks

        --------------------------------------------------
       |  Trunk Type    |  Direction (w.r.t. the gateway) |
        --------------------------------------------------
       |MF, wink start  |Incoming - originate from PBX    |
       |                |(the same as FGD terminating     |
       |                | protocol)                       |
       |MF, wink start  |Outgoing - terminate on PBX      |
       |MF, wink start  |Bi-directional                   |
       |MF, Immediate   |Incoming (originate from PBX)    |
       |    start       |                                 |
       |MF, Immediate   |Outgoing (terminate on PBX)      |
       |    start       |                                 |
        --------------------------------------------------

1.2.2. "DT" Package

   DTMF and dial-pulse (DP) trunks (except basic PBX) are covered by the
   "DT" package along with the DTMF "D" package:

        Table 2 DTMF and DP Wink Start and Immediate Start Trunks

        --------------------------------------------------
       |  Trunk Type    |  Direction (w.r.t. the gateway) |
        --------------------------------------------------
       |DTMF, Immediate |Incoming (originate from PBX)    |
       | start, wink    |                                 |
       | start          |                                 |
       |DTMF, Immediate |Outgoing (terminate on PBX)      |
       | start, wink    |                                 |
       | start          |                                 |
        --------------------------------------------------












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RFC 3064                   MGCP CAS Packages               February 2001


1.2.3. "BL" Package

   DTMF and dial-pulse (DP) basic PBX trunks are covered by the "BL"
   package - along with the DTMF "D" package (essentially this is like a
   "basic line with no features") - either digital or FXS trunk
   interface:

          Table 3 Basic FXS Interface

         --------------------------------------
        | Trunk Type    |  Direction           |
        |               | (w.r.t. the gateway) |
         --------------------------------------
        |Basic, DTMF and |Bi-directional       |
        |DP, Loop Start  |                     |