rfc2885.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   example, Recommendation H.221 describes a frame structure for
   multiple media streams multiplexed on a number of digital 64 kbit/s
   channels.  Such a case is handled in the connection model in the
   following way.  For every bearer channel that carries part of the
   multiplexed streams, there is a Termination.  The Terminations that
   source/sink the digital channels are connected to a separate
   Termination called the multiplexing Termination. This Termination
   describes the multiplex used (e.g. how the H.221 frames are carried
   over the digital channels used).  The MuxDescriptor is used to this
   end.  If multiple media are carried, this Termination contains
   multiple StreamDescriptors. The media streams can be associated with
   streams sourced/sunk by other Terminations in the Context.

   Terminations may be created which represent multiplexed bearers, such
   as an ATM AAL2.  When a new multiplexed bearer is to be created, an
   ephemeral termination is created in a context established for this
   purpose.  When the termination is subtracted, the multiplexed bearer
   is destroyed.

6.2.1 Termination Dynamics

   The protocol can be used to create new Terminations and to modify
   property values of existing Terminations.  These modifications
   include the possibility of adding or removing events and/or signals.
   The Termination properties, and events and signals are described in
   the ensuing sections. An MGC can only release/modify terminations and
   the resources that the termination represents which it has previously
   seized via, e.g., the Add command.





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6.2.2 TerminationIDs

   Terminations are referenced by a TerminationID, which is an arbitrary
   schema chosen by the MG.

   TerminationIDs of physical Terminations are provisioned in the Media
   Gateway. The TerminationIDs may be chosen to have structure.  For
   instance, a TerminationID may consist of trunk group and a trunk
   within the group.

   A wildcarding mechanism using two types of wildcards can be used with
   TerminationIDs.  The two wildcards are ALL and CHOOSE.  The former is
   used to address multiple Terminations at once, while the latter is
   used to indicate to a media gateway that it must select a Termination
   satisfying the partially specified TerminationID.  This allows, for
   instance, that a MGC instructs a MG to choose a circuit within a
   trunk group.

   When ALL is used in the TerminationID of a command, the effect is
   identical to repeating the command with each of the matching
   TerminationIDs.  Since each of these commands may generate a
   response, the size of the entire response may be large.  If
   individual responses are not required, a wildcard response may be
   requested.  In such a case, a single response is generated, which
   contains the UNION of all of the individual responses which otherwise
   would have been generated, with duplicate values suppressed.
   Wildcard response may be particularly useful in the Audit commands.

   The encoding of the wildcarding mechanism is detailed in Annexes A
   and B.

6.2.3 Packages

   Different types of gateways may implement Terminations that have
   widely differing characteristics.  Variations in Terminations are
   accommodated in the protocol by allowing Terminations to have
   optional Properties, Events, Signals and Statistics implemented by
   MGs.

   In order to achieve MG/MGC interoperability, such options are grouped
   into Packages, and a Termination realizes a set of such Packages.
   More information on definition of packages can be found in section
   12.  An MGC can audit a Termination to determine which Packages it
   realizes.

   Properties, Events, Signals and Statistics defined in Packages, as
   well as parameters to them, are referenced by identifiers (Ids).
   Identifiers are scoped. For each package, PropertyIds, EventIds,



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   SignalIds, StatisticsIds and ParameterIds have unique name spaces and
   the same identifier may be used in each of them.  Two PropertyIds in
   different packages may also have the same identifier, etc.

6.2.4 Termination Properties and Descriptors

   Terminations have properties.  The properties have unique
   PropertyIDs.  Most properties have default values.  When a
   Termination is created, properties get their default values, unless
   the controller specifically sets a different value.  The default
   value of a property of a physical Termination can be changed by
   setting it to a different value when the Termination is in the null
   Context.  Every time such a Termination returns to the null Context,
   the values of its properties are reset to this default value.

   There are a number of common properties for Terminations and
   properties specific to media streams. The common properties are also
   called the termination state properties.  For each media stream,
   there are local properties and properties of the received and
   transmitted flows.

   Properties not included in the base protocol are defined in Packages.
   These properties are referred to by a name consisting of the
   PackageName and a PropertyId.  Most properties have default values
   described in the Package description. Properties may be read- only or
   read/write.  The possible values of a property may be audited, as can
   their current values.  For properties that are read/write, the MGC
   can set their values.  A property may be declared as "Global" which
   has a single value shared by all terminations realizing the package.
   Related properties are grouped into descriptors for convenience.

   When a Termination is Added to a Context, the value of its read/write
   properties can be set by including the appropriate descriptors as
   parameters to the Add command.  Properties not mentioned in the
   command retain their prior values.  Similarly, a property of a
   Termination in a Context may have its value changed by the Modify
   command.  Properties not mentioned in the Modify command retain their
   prior values. Properties may also have their values changed when a
   Termination is moved from one Context to another as a result of a
   Move command.  In some cases, descriptors are returned as output from
   a command.

   The following table lists all of the possible Descriptors and their
   use.  Not all descriptors are legal as input or output parameters to
   every command.






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   Descriptor Name           Description

   Modem                     Identifies modem type and properties when
                             applicable.
   Mux                       Describes multiplex type for multimedia
                             terminations (e.g. H.221, H.223, H.225.0)
                             and Terminations forming the input mux.
   Media                     A list of media stream specifications (see
                             7.1.4).
   TerminationState          Properties of a Termination (which can be
                             defined in Packages) that are not stream
                             specific.
   Stream                    A list of remote/local/localControl
                             descriptors for a single stream.
   Local                     Contains properties that specify the media
                             flows that the MG receives from the remote
                             entity.
   Remote                    Contains properties that specify the media
                             flows that the MG sends to the remote
                             entity.
   LocalControl              Contains properties (which can be defined
                             in packages) that are of interest between
                             the MG and the MGC.
   Events                    Describes events to be detected by the MG
                             and what to do when an event is detected.
   EventBuffer               Describes events to be detected by the MG
                             when Event Buffering is active.
   Signals                   Describes signals and/or actions to be
                             applied (e.g. Busy Tone) to the
                             Terminations.
   Audit                     In Audit commands, identifies which
                             information is desired.
   Packages                  In AuditValue, returns a list of Packages
                             realized by Termination.
   DigitMap                  Instructions for handling DTMF tones at
                             the MG.
   ServiceChange             In ServiceChange, what, why service change
                             occurred, etc.
   ObservedEvents            In Notify or AuditValue, report of events
                             observed.
   Statistics                In Subtract and Audit, Report of
                             Statistics kept on a Termination.









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6.2.5 Root Termination

   Occasionally, a command must refer to the entire gateway, rather than
   a termination within it.  A special TerminationID, "Root" is reserved
   for this purpose.  Packages may be defined on Root.  Root thus may
   have properties and events (signals  are not appropriate for root).
   Accordingly, the root TerminationID may appear in:

    .  a Modify command - to change a property or set an event
    .  a Notify command - to report an event
    .  an AuditValue return - to examine the values of properties
       implemented on root
    .  an AuditCapability - to determine what properties of root are
       implemented
    .  a ServiceChange - to declare the gateway in or out of service.

   Any other use of the root TerminationID is an error.

7. COMMANDS

   The protocol provides commands for manipulating the logical entities
   of the protocol connection model, Contexts and Terminations.
   Commands provide control at the finest level of granularity supported
   by the protocol.  For example, Commands exist to add Terminations to
   a Context, modify Terminations, subtract Terminations from a Context,
   and audit properties of Contexts or Terminations. Commands provide
   for complete control of the properties of Contexts and Terminations.
   This includes specifying which events a Termination is to report,
   which signals/actions are to be applied to a Termination and
   specifying the topology of a Context (who hears/sees whom).

   Most commands are for the specific use of the Media Gateway
   Controller as command initiator in controlling Media Gateways as
   command responders.  The exceptions are the Notify and ServiceChange
   commands: Notify is sent from Media Gateway to Media Gateway
   Controller, and ServiceChange may be sent by either entity.  Below is
   an overview of the commands; they are explained in more detail in
   section 7.2.

   1. Add. The Add command adds a termination to a context.  The Add
      command on the first Termination in a Context is used to create a
      Context.

   2. Modify. The Modify command modifies the properties, events and
      signals of a termination.






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   3. Subtract. The Subtract command disconnects a Termination from its
      Context and returns statistics on the Termination's participation
      in the Context.  The Subtract command on the last Termination in a
      Context deletes the Context.

   4. Move. The Move command atomically moves a Termination to another
      context.

   5. AuditValue. The AuditValue command returns the current state of
      properties, events,  signals and statistics of Terminations.

   6. AuditCapabilities. The AuditCapabilities command returns all the
      possible values for Termination properties, events and signals
      allowed by the Media Gateway.

   7. Notify. The Notify command allows the Media Gateway to inform the
      Media Gateway Controller of the occurrence of events in the Media
      Gateway.

   8. ServiceChange. The ServiceChange Command allows the Media Gateway
      to notify the Media Gateway Controller that a Termination or group
      of Terminations is about to be taken out of service or has just
      been returned to service.   ServiceChange is also used by the MG
      to announce its availability to an MGC (registration), and to
      notify the MGC of impending or completed restart of the MG.  The
      MGC may announce a handover to the MG by sending it a