rfc3054.txt

最新的RFC

Network Working Group                            P. Blatherwick (Editor)
Request for Comments: 3054                               Nortel Networks
Category: Informational                                         R. Bell
                                                           Cisco Systems
                                                              P. Holland
                                                    Circa Communications
                                                   (Chair TIA TR-41.3.4)
                                                            January 2001


           Megaco IP Phone Media Gateway Application Profile

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 specifies a particular application of the Megaco/H.248
   Protocol for control of Internet telephones and similar appliances:
   the Megaco IP Phone Media Gateway.  The telephone itself is a Media
   Gateway (MG), controlled by the Megaco/H.248 Protocol, with
   application control intelligence located in the Media Gateway
   Controller (MGC).  To achieve a high degree of interoperability and
   design efficiency in such end-user devices, a consistent
   architectural approach, a particular organization of Terminations and
   Packages, and a Protocol Profile are described.  The approach makes
   use of existing Protocol features and user interface related
   Packages, and is thus a straight-forward application of the
   Megaco/H.248 Protocol.

1.  Introduction

   This document represents the current view from the TIA working group
   on VoIP (Voice over IP) telephone specification [1], TIA TR-41.3.4,
   with the intent of using this as part of its "whole device"
   specification as an optional method of device control.

   Industry feedback has made it clear that interoperability and
   acoustic performance of Internet telephones is key to the rapid and
   extensive commercialization of these products.  To facilitate this,
   the TIA has established working group TR-41.3.4 to develop a standard



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   for VoIP telephones.  The TR-41.3.4 working group has included the
   "whole device" within the scope of the standard, so a full range of
   requirements including acoustic performance, protocols, methods for
   powering and safety are provided.  Where possible, the requirements
   are based on existing standards, which are included by reference.

   The TIA TR-41.3.4 working group has also recognized that its proposed
   standard must enable creative application of the equipment, encourage
   the development of new capabilities and allow for high levels of
   product customization.  To achieve this, peer to peer architectures
   that are based on protocols such as H.323 or SIP and master/slave
   architectures such as Megaco/H.248 Protocol are both necessary and
   complementary.

   In support of the Megaco/H.248 Protocol development effort, the TR-
   41.3.4 working group has considered product enabling issues and
   requirements, and has developed an approach to use the Megaco/H.248
   Protocol for Internet telephone device control.  This document
   represents the working group's current view.

   This document covers the general requirements of the Megaco IP Phone
   application (section 3), architectural approach and MG organization
   (section 4), details of specific Termination types used and Packages
   supported by each (section 5), and the Megaco IP Phone Protocol
   Profile (section 6).

2.  Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in RFC 2119 [5].

3.  General Requirements

   The following general requirements were identified to drive the
   Megaco IP Phone design [1]:

   1. The Megaco IP Phone must meet the basic needs of the business user
      from day one;

   2. Provide a path for rapid expansion to support sophisticated
      business telephony features;

   3. Flexibility to allow for a very wide range of telephones and
      similar devices to be defined, from very simple to very feature
      rich;

   4. Simple, minimal design;



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   5. Allow device cost to be appropriate to capabilities provided;

   6. Packages and Termination types must have characteristics that
      enable reliability;

   7. The IP Phone MG shall meet the appropriate Megaco/H.248 Protocol
      requirements as provided in the Megaco Requirements document [2]
      and be a straight-forward application of the Megaco/H.248 Protocol
      [3].

4.  Architecture Description

   The following subsections describe the general design approach and
   organization of the Megaco IP Phone MG.

4.1.  Design Approach

   Design intent of the Megaco IP Phone is to keep it determinedly
   simple while providing required support for fully featured business
   telephones and the flexibility to allow for a very wide range of
   telephone configurations and similar appliances.

   The approach to achieve this goal is to provide a very simple and
   direct master/slave control model in which very little feature
   intelligence is required in the end device.  This design intent
   matches the Megaco/H.248 Protocol approach well.

   It is important to note that additional functionality, built-in
   feature capability or system-specific optimization can easily be
   provided, at the option of the implementer, by defining additional
   Termination types, Event/Signal Packages, or providing built-in
   application capability.  This document defines the minimal design
   only.

4.2.  General Structure

   As shown in Figure 1 below, the Megaco IP Phone is organized as a
   Media Gateway (MG) that consists of a User Interface Termination and
   a set of Audio Transducer Terminations.

   Several - potentially thousands - of Megaco IP Phone MGs may be
   controlled by a single Media Gateway Controller (MGC).  This is
   distinguished from the organization between traditional analog or PBX
   telephones behind an IP network, where the MGC would control an MG
   which in turn controls the collection of telephone devices in
   question.  In the case of a Megaco IP Phone MG, the MG directly





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   implements the media terminations like handset, handsfree and
   headset, as well as the user interface.  In this case, the Megaco IP
   Phone itself is the MG.

                             +---------------+
                             |               |
                             |      MGC      |
                             |               |
                             +---------------+
                                     ^ \ \ \
                                     |
                                     v
               +---------------------------------------------+
               |                                             |
               |   Megaco IP Phone MG                        |
               |   ==================      Audio Transducer  |
               |                           Terminations:     |
               | Audio context(s):         + - - - - - - - + |
               | +---------------------+     +-----------+   |
               | |     Context A       |   | | Handset   | | |
               | |                     |     +-----------+   |
          RTP  | |  +-----+   +-----+  |   | +-----------+ | |
      <--------+-+->| Tr  |   | Ta2 |<-+-----| Handsfree |   |
        audio  | |  +-----+   +-----+  |   | +-----------+ | |
       stream  | |                     |     +-----------+   |
               | +---------------------+   | | Headset   | | |
               |                             +-----------+   |
               |                           |               | |
               |                              ETC.           |
               |                           + - - - - - - - + |
               |                                             |
               |  +----------------------------------------+ |
               |  | User Interface Termination             | |
               |  | +--------------+      +--------------+ | |
               |  | | Text Display |      | Keypad       | | |
               |  | +--------------+      +--------------+ | |
               |  | +--------------+      +--------------+ | |
               |  | | Softkeys     |      | Indicators   | | |
               |  | +--------------+      +--------------+ | |
               |  | +--------------+                       | |
               |  | | Function Keys|       ETC.            | |
               |  | +--------------+                       | |
               |  +----------------------------------------+ |
               +---------------------------------------------+

             Figure 1: Megaco IP Phone Termination / Package Model





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4.3.  Termination / Package Organization

   As shown in Figure 1, each Audio Transducer Termination represents an
   individually controllable audio input/output element of the telephone
   device, such as Handset, Handsfree, Headset, etc.  By separating each
   audio element as a distinct Termination, more flexible applications
   can be easily implemented, such as paging, group listening, and so
   on. Since this is actually only the logical view of the device,
   represented by protocol, it is also quite possible to simplify
   representation of the device by hiding all available audio
   input/outputs behind a single Audio Transducer Termination, for
   example the Handset, and implement control of multiple real
   input/outputs locally inside the device.

   All non-audio user interface elements are associated with the User
   Interface Termination.  This special Termination supports Packages to
   implement all user interaction with the telephone user interface,
   including Function Keys, Indicators, the Dialpad, etc, as appropriate
   for the specific device capabilities (within constraints given in the
   section on User Interface Termination).  The User Interface
   Termination cannot be placed in any Context.  This grouping of user
   interface elements behind a well-know Termination greatly simplifies
   audits to determine actual device configuration, and reduces the
   number of Terminations involved in representing user interface.

   In addition, TerminationID naming conventions are provided to
   identify specific Terminations within the Megaco IP Phone MG and
   group them into related sets.  These conventions use a set of well
   known identifier names to specify the individual Terminations, for
   example the User Interface Termination ("ui"), the Handset Audio
   Transducer ("at/hs"), or the Handsfree Audio Transducer ("at/hf").
   This specific naming is important in this application, especially for
   the Audio Transducer Terminations, since the real input/output