rfc3525.txt

OTP是开放电信平台的简称

   DTMF  Dual Tone Multi-Frequency




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RFC 3525                Gateway Control Protocol               June 2003


   FAS   Facility Associated Signalling

   GSM   Global System for Mobile communications

   GW    GateWay

   IANA  Internet Assigned Numbers Authority (superseded by Internet
         Corporation for Assigned Names and Numbers - ICANN)

   IP    Internet Protocol

   ISUP  ISDN User Part

   IVR   Interactive Voice Response

   MG    Media Gateway

   MGC   Media Gateway Controller

   NFAS  Non-Facility Associated Signalling

   PRI   Primary Rate Interface

   PSTN  Public Switched Telephone Network

   QoS   Quality of Service

   RTP   Real-time Transport Protocol

   SCN   Switched Circuit Network

   SG    Signalling Gateway

   SS7   Signalling System No. 7

5  Conventions

   In the H.248.1 Recommendation, "SHALL" refers to a mandatory
   requirement, while "SHOULD" refers to a suggested but optional
   feature or procedure.  The term "MAY" refers to an optional course of
   action without expressing a preference.  Note that these definition
   are overridden in the present document by the RFC 2119 conventions
   stated at the beginning of this document.  RFC 2119 has a more
   precise definition of "should" than is provided by the ITU-T.







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RFC 3525                Gateway Control Protocol               June 2003


6  Connection model

   The connection model for the protocol describes the logical entities,
   or objects, within the Media Gateway that can be controlled by the
   Media Gateway Controller.  The main abstractions used in the
   connection model are Terminations and Contexts.

   A Termination sources and/or sinks one or more streams.  In a
   multimedia conference, a Termination can be multimedia and sources or
   sinks multiple media streams.  The media stream parameters, as well
   as modem, and bearer parameters are encapsulated within the
   Termination.

   A Context is an association between a collection of Terminations.
   There is a special type of Context, the null Context, which contains
   all Terminations that are not associated to any other Termination.
   For instance, in a decomposed access gateway, all idle lines are
   represented by Terminations in the null Context.

   Following is a graphical depiction of these concepts.  The diagram of
   Figure 1 gives several examples and is not meant to be an
   all-inclusive illustration.  The asterisk box in each of the Contexts
   represents the logical association of Terminations implied by the
   Context.



























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RFC 3525                Gateway Control Protocol               June 2003


         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context                          +-------------+ |  |
         | |                                 | Termination | |  |
         | |                                 |-------------| |  |
         | |  +-------------+             +->| SCN Bearer  |<---+->
         | |  | Termination |   +-----+   |  |   Channel   | |  |
         | |  |-------------|   |     |---+  +-------------+ |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |---+  +-------------+ |  |
         | |  +-------------+   +-----+   |  | Termination | |  |
         | |                              |  |-------------| |  |
         | |                              +->| SCN Bearer  |<---+->
         | |                                 |   Channel   | |  |
         | |                                 +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         |                                                      |
         |                    +------------------------------+  |
         |   (NULL Context)   |Context                       |  |
         |  +-------------+   |              +-------------+ |  |
         |  | Termination |   | +-----+      | Termination | |  |
         |  |-------------|   | |     |      |-------------| |  |
         |  | SCN Bearer  |   | |  *  |------| SCN Bearer  |<---+->
         |  |   Channel   |   | |     |      |   Channel   | |  |
         |  +-------------+   | +-----+      +-------------+ |  |
         |                    +------------------------------+  |
         |                                                      |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context                                          |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Termination |   +-----+      | Termination | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         | ___________________________________________________  |
         +------------------------------------------------------+

            Figure 1: Examples of Megaco/H.248 Connection Model








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RFC 3525                Gateway Control Protocol               June 2003


   The example in Figure 2 shows an example of one way to accomplish a
   call-waiting scenario in a decomposed access gateway, illustrating
   the relocation of a Termination between Contexts.  Terminations T1
   and T2 belong to Context C1 in a two-way audio call.  A second audio
   call is waiting for T1 from Termination T3.  T3 is alone in Context
   C2.  T1 accepts the call from T3, placing T2 on hold.  This action
   results in T1 moving into Context C2, as shown in Figure 3.

         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T2    |   +-----+      | Term. T1    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| RTP Stream  |---|  *  |------| SCN Bearer  |<---+->
         | |  |             |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |                                 +-------------+ |  |
         | |                    +-----+      | Term. T3    | |  |
         | |                    |     |      |-------------| |  |
         | |                    |  *  |------| SCN Bearer  |<---+->
         | |                    |     |      |   Channel   | |  |
         | |                    +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

     Figure 2: Example Call Waiting Scenario / Alerting Applied to T1



















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RFC 3525                Gateway Control Protocol               June 2003


         +------------------------------------------------------+
         |Media Gateway                                         |
         | +-------------------------------------------------+  |
         | |Context C1                                       |  |
         | |  +-------------+                                |  |
         | |  | Term. T2    |   +-----+                      |  |
         | |  |-------------|   |     |                      |  |
       <-+--->| RTP Stream  |---|  *  |                      |  |
         | |  |             |   |     |                      |  |
         | |  +-------------+   +-----+                      |  |
         | +-------------------------------------------------+  |
         |                                                      |
         | +-------------------------------------------------+  |
         | |Context C2                                       |  |
         | |  +-------------+                +-------------+ |  |
         | |  | Term. T1    |   +-----+      | Term. T3    | |  |
         | |  |-------------|   |     |      |-------------| |  |
       <-+--->| SCN Bearer  |---|  *  |------| SCN Bearer  |<---+->
         | |  |   Channel   |   |     |      |   Channel   | |  |
         | |  +-------------+   +-----+      +-------------+ |  |
         | +-------------------------------------------------+  |
         +------------------------------------------------------+

          Figure 3. Example Call Waiting Scenario / Answer by T1

6.1   Contexts

   A Context is an association between a number of Terminations.  The
   Context describes the topology (who hears/sees whom) and the media
   mixing and/or switching parameters if more than two Terminations are
   involved in the association.

   There is a special Context called the null Context.  It contains
   Terminations that are not associated to any other Termination.
   Terminations in the null Context can have their parameters examined
   or modified, and may have events detected on them.

   In general, an Add command is used to add Terminations to Contexts.
   If the MGC does not specify an existing Context to which the
   Termination is to be added, the MG creates a new Context.  A
   Termination may be removed from a Context with a Subtract command,
   and a Termination may be moved from one Context to another with a
   Move command.  A Termination SHALL exist in only one Context at a
   time.







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RFC 3525                Gateway Control Protocol               June 2003


   The maximum number of Terminations in a Context is a MG property.
   Media gateways that offer only point-to-point connectivity might
   allow at most two Terminations per Context.  Media gateways that
   support multipoint conferences might allow three or more Terminations
   per Context.

6.1.1 Context attributes and descriptors

   The attributes of Contexts are:

   -  ContextID.

   -  The topology (who hears/sees whom).

      The topology of a Context describes the flow of media between the
      Terminations within a Context.  In contrast, the mode of a