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RFC 3525

 
 
 

Gateway Control Protocol Version 1

Part 2 of 7, p. 26 to 58
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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
   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.

   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.

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   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 a 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 ServiceChange
      command.  The MGC may also use ServiceChange to instruct the MG to
      take a Termination or group of Terminations in or out of service.

   These commands are detailed in 7.2.1 through 7.2.8.

7.1   Descriptors

   The parameters to a command are termed Descriptors.  A descriptor
   consists of a name and a list of items.  Some items may have values.
   Many Commands share common descriptors.  This subclause enumerates
   these descriptors.  Descriptors may be returned as output from a
   command.  In any such return of descriptor contents, an empty
   descriptor is represented by its name unaccompanied by any list.
   Parameters and parameter usage specific to a given Command type are
   described in the subclause that describes the Command.

7.1.1 Specifying parameters

   Command parameters are structured into a number of descriptors.  In
   general, the text format of descriptors is
   DescriptorName=<someID>{parm=value, parm=value, ...}.

   Parameters may be fully specified, overspecified or underspecified:

   1) Fully specified parameters have a single, unambiguous value that
      the command initiator is instructing the command responder to use
      for the specified parameter.

   2) Underspecified parameters, using the CHOOSE value, allow the
      command responder to choose any value it can support.

   3) Overspecified parameters have a list of potential values.  The
      list order specifies the command initiator's order of preference
      of selection.  The command responder chooses one value from
      the offered list and returns that value to the command initiator.

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   If a required descriptor other than the Audit descriptor is
   unspecified (i.e., entirely absent) from a command, the previous
   values set in that descriptor for that Termination, if any, are
   retained.  In commands other than Subtract, a missing Audit
   descriptor is equivalent to an empty Audit descriptor.  The Behaviour
   of the MG with respect to unspecified parameters within a descriptor
   varies with the descriptor concerned, as indicated in succeeding
   subclauses.  Whenever a parameter is underspecified or overspecified,
   the descriptor containing the value chosen by the responder is
   included as output from the command.

   Each command specifies the TerminationId the command operates on.
   This TerminationId may be "wildcarded".  When the TerminationId of a
   command is wildcarded, the effect shall be as if the command was
   repeated with each of the TerminationIds matched.

7.1.2 Modem descriptor

   The Modem descriptor specifies the modem type and parameters, if any,
   required for use in e.g., H.324 and text conversation.  The
   descriptor includes the following modem types: V.18, V.22, V.22 bis,
   V.32, V.32 bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
   extensions.  By default, no Modem descriptor is present in a
   Termination.

7.1.3 Multiplex descriptor

   In multimedia calls, a number of media streams are carried on a
   (possibly different) number of bearers.  The multiplex descriptor
   associates the media and the bearers.  The descriptor includes the
   multiplex type:

   -  H.221;

   -  H.223;

   -  H.226;

   -  V.76;

   -  possible extensions,

   and a set of TerminationIDs representing the multiplexed bearers, in
   order.  For example:

      Mux = H.221{ MyT3/1/2, MyT3/2/13, MyT3/3/6, MyT3/21/22}

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7.1.4 Media descriptor

   The Media descriptor specifies the parameters for all the media
   streams.  These parameters are structured into two descriptors: a
   TerminationState descriptor, which specifies the properties of a
   Termination that are not stream dependent, and one or more Stream
   descriptors each of which describes a single media stream.

   A stream is identified by a StreamID.  The StreamID is used to link
   the streams in a Context that belong together.  Multiple streams
   exiting a Termination shall be synchronized with each other.  Within
   the Stream descriptor, there are up to three subsidiary descriptors:
   LocalControl, Local, and Remote.  The relationship between these
   descriptors is thus:

   Media descriptor
      TerminationState Descriptor
      Stream descriptor
         LocalControl descriptor
         Local descriptor
         Remote descriptor

   As a convenience, LocalControl, Local, or Remote descriptors may be
   included in the Media descriptor without an enclosing Stream
   descriptor.  In this case, the StreamID is assumed to be 1.

7.1.5 TerminationState descriptor

   The TerminationState descriptor contains the ServiceStates property,
   the EventBufferControl property and properties of a Termination
   (defined in Packages) that are not stream specific.

   The ServiceStates property describes the overall state of the
   Termination (not stream specific).  A Termination can be in one of
   the following states: "test", "out of service", or "in service".  The
   "test" state indicates that the Termination is being tested.  The
   state "out of service" indicates that the Termination cannot be used
   for traffic.  The state "in service" indicates that a Termination can
   be used or is being used for normal traffic.  "in service" is the
   default state.

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   Values assigned to Properties may be simple values
   (integer/string/enumeration) or may be underspecified, where more
   than one value is supplied and the MG may make a choice:

   -  Alternative Values - multiple values in a list, one of which must
      be selected

   -  Ranges - minimum and maximum values, any value between min and max
      must be selected, boundary values included

   -  Greater Than/Less Than - value must be greater/less than specified
      value

   -  CHOOSE Wildcard - the MG chooses from the allowed values for the
      property

   The EventBufferControl property specifies whether events are buffered
   following detection of an event in the Events descriptor, or
   processed immediately.  See 7.1.9 for details.

7.1.6 Stream descriptor

   A Stream descriptor specifies the parameters of a single
   bidirectional stream.  These parameters are structured into three
   descriptors: one that contains Termination properties specific to a
   stream and one each for local and remote flows.  The Stream
   Descriptor includes a StreamID which identifies the stream.  Streams
   are created by specifying a new StreamID on one of the Terminations
   in a Context.  A stream is deleted by setting empty Local and Remote
   descriptors for the stream with ReserveGroup and ReserveValue in
   LocalControl set to "false" on all Terminations in the Context that
   previously supported that stream.

   StreamIDs are of local significance between MGC and MG and they are
   assigned by the MGC.  Within a Context, StreamID is a means by which
   to indicate which media flows are interconnected: streams with the
   same StreamID are connected.

   If a Termination is moved from one Context to another, the effect on
   the Context to which the Termination is moved is the same as in the
   case that a new Termination were added with the same StreamIDs as the
   moved Termination.

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7.1.7 LocalControl descriptor

   The LocalControl descriptor contains the Mode property, the
   ReserveGroup and ReserveValue properties and properties of a
   Termination (defined in Packages) that are stream specific, and are
   of interest between the MG and the MGC.  Values of properties may be
   underspecified as in 7.1.1.

   The allowed values for the mode property are send-only, receive-only,
   send/receive, inactive and loop-back. "Send" and "receive" are with
   respect to the exterior of the Context, so that, for example, a
   stream set to mode=sendOnly does not pass received media into the
   Context.  The default value for the mode property is "Inactive".
   Signals and Events are not affected by mode.

   The boolean-valued Reserve properties, ReserveValue and ReserveGroup,
   of a Termination indicate what the MG is expected to do when it
   receives a Local and/or Remote descriptor.

   If the value of a Reserve property is True, the MG SHALL reserve
   resources for all alternatives specified in the Local and/or Remote
   descriptors for which it currently has resources available.  It SHALL
   respond with the alternatives for which it reserves resources.  If it
   cannot not support any of the alternatives, it SHALL respond with a
   reply to the MGC that contains empty Local and/or Remote descriptors.
   If media begins to flow while more than a single alternative is
   reserved, media packets may be sent/received on any of the
   alternatives and must be processed, although only a single
   alternative may be active at any given time.

   If the value of a Reserve property is False, the MG SHALL choose one
   of the alternatives specified in the Local descriptor (if present)
   and one of the alternatives specified in the Remote descriptor (if
   present).  If the MG has not yet reserved resources to support the
   selected alternative, it SHALL reserve the resources.  If, on the
   other hand, it already reserved resources for the Termination
   addressed (because of a prior exchange with ReserveValue and/or
   ReserveGroup equal to True), it SHALL release any excess resources it
   reserved previously.  Finally, the MG shall send a reply to the MGC
   containing the alternatives for the Local and/or Remote descriptor
   that it selected.  If the MG does not have sufficient resources to
   support any of the alternatives specified, it SHALL respond with
   error 510 (insufficient resources).

   The default value of ReserveValue and ReserveGroup is False.  More
   information on the use of the two Reserve properties is provided in
   7.1.8.

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   A new setting of the LocalControl Descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus, to retain
   information from the previous setting, the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.8 Local and Remote descriptors

   The MGC uses Local and Remote descriptors to reserve and commit MG
   resources for media decoding and encoding for the given Stream(s) and
   Termination to which they apply.  The MG includes these descriptors
   in its response to indicate what it is actually prepared to support.
   The MG SHALL include additional properties and their values in its
   response if these properties are mandatory yet not present in the
   requests made by the MGC (e.g., by specifying detailed video encoding
   parameters where the MGC only specified the payload type).

   Local refers to the media received by the MG and Remote refers to the
   media sent by the MG.

   When text encoding the protocol, the descriptors consist of session
   descriptions as defined in SDP (RFC 2327).  In session descriptions
   sent from the MGC to the MG, the following exceptions to the syntax
   of RFC 2327 are allowed:

   -  the "s=", "t=" and "o=" lines are optional;

   -  the use of CHOOSE is allowed in place of a single parameter value;
      and

   -  the use of alternatives is allowed in place of a single parameter
      value.

   A Stream Descriptor specifies a single bi-directional media stream
   and so a single session description MUST NOT include more than one
   media description ("m=" line).  A Stream Descriptor may contain
   additional session descriptions as alternatives.  Each media stream
   for a termination must appear in distinct Stream Descriptors.  When
   multiple session descriptions are provided in one descriptor, the
   "v=" lines are required as delimiters; otherwise they are optional in
   session descriptions sent to the MG.  Implementations shall accept
   session descriptions that are fully conformant to RFC 2327.  When
   binary encoding the protocol the descriptor consists of groups of
   properties (tag-value pairs) as specified in Annex C.  Each such
   group may contain the parameters of a session description.

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   Below, the semantics of the Local and Remote descriptors are
   specified in detail.  The specification consists of two parts.  The
   first part specifies the interpretation of the contents of the
   descriptor.  The second part specifies the actions the MG must take
   upon receiving the Local and Remote descriptors.  The actions to be
   taken by the MG depend on the values of the ReserveValue and
   ReserveGroup properties of the LocalControl descriptor.

   Either the Local or the Remote descriptor or both may be:

   1) unspecified (i.e., absent);

   2) empty;

   3) underspecified through use of CHOOSE in a property value;

   4) fully specified; or

   5) overspecified through presentation of multiple groups of
      properties and possibly multiple property values in one or more of
      these groups.

   Where the descriptors have been passed from the MGC to the MG, they
   are interpreted according to the rules given in 7.1.1, with the
   following additional comments for clarification:

   a) An unspecified Local or Remote descriptor is considered to be a
      missing mandatory parameter.  It requires the MG to use whatever
      was last specified for that descriptor.  It is possible that there
      was no previously specified value, in which case the descriptor
      concerned is ignored in further processing of the command.

   b) An empty Local (Remote) descriptor in a message from the MGC
      signifies a request to release any resources reserved for the
      media flow received (sent).

   c) If multiple groups of properties are present in a Local or Remote
      descriptor or multiple values within a group, the order of
      preference is descending.

   d) Underspecified or overspecified properties within a group of
      properties sent by the MGC are requests for the MG to choose one
      or more values which it can support for each of those properties.
      In case of an overspecified property, the list of values is in
      descending order of preference.

   Subject to the above rules, subsequent action depends on the values
   of the ReserveValue and ReserveGroup properties in LocalControl.

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   If ReserveGroup is True, the MG reserves the resources required to
   support any of the requested property group alternatives that it can
   currently support.  If ReserveValue is True, the MG reserves the
   resources required to support any of the requested property value
   alternatives that it can currently support.

   NOTE - If a Local or Remote descriptor contains multiple groups of
   properties, and ReserveGroup is True, then the MG is requested to
   reserve resources so that it can decode or encode the media stream
   according to any of the alternatives.  For instance, if the Local
   descriptor contains two groups of properties, one specifying
   packetized G.711 A-law audio and the other G.723.1 audio, the MG
   reserves resources so that it can decode one audio stream encoded in
   either G.711 A-law format or G.723.1 format.  The MG does not have to
   reserve resources to decode two audio streams simultaneously, one
   encoded in G.711 A-law and one in G.723.1.  The intention for the use
   of ReserveValue is analogous.

   If ReserveGroup is true or ReserveValue is True, then the following
   rules apply:

   -  If the MG has insufficient resources to support all alternatives
      requested by the MGC and the MGC requested resources in both Local
      and Remote, the MG should reserve resources to support at least
      one alternative each within Local and Remote.

   -  If the MG has insufficient resources to support at least one
      alternative within a Local (Remote) descriptor received from the
      MGC, it shall return an empty Local (Remote) in response.

   -  In its response to the MGC, when the MGC included Local and Remote
      descriptors, the MG SHALL include Local and Remote descriptors for
      all groups of properties and property values it reserved resources
      for.  If the MG is incapable of supporting at least one of the
      alternatives within the Local (Remote) descriptor received from
      the MGC, it SHALL return an empty Local (Remote) descriptor.

   -  If the Mode property of the LocalControl descriptor is RecvOnly,
      SendRecv, or LoopBack, the MG must be prepared to receive media
      encoded according to any of the alternatives included in its
      response to the MGC.

   If ReserveGroup is False and ReserveValue is False, then the MG
   SHOULD apply the following rules to resolve Local and Remote to a
   single alternative each:

   -  The MG chooses the first alternative in Local for which it is able
      to support at least one alternative in Remote.

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   -  If the MG is unable to support at least one Local and one Remote
      alternative, it returns Error 510 (Insufficient Resources).

   -  The MG returns its selected alternative in each of Local and
      Remote.

   A new setting of a Local or Remote descriptor completely replaces the
   previous setting of that descriptor in the MG.  Thus, to retain
   information from the previous setting, the MGC must include that
   information in the new setting.  If the MGC wishes to delete some
   information from the existing descriptor, it merely resends the
   descriptor (in a Modify command) with the unwanted information
   stripped out.

7.1.9 Events descriptor

   The EventsDescriptor parameter contains a RequestIdentifier and a
   list of events that the Media Gateway is requested to detect and
   report.  The RequestIdentifier is used to correlate the request with
   the notifications that it may trigger.  Requested events include, for
   example, fax tones, continuity test results, and on-hook and off-hook
   transitions.  The RequestIdentifier is omitted if the
   EventsDescriptor is empty (i.e., no events are specified).

   Each event in the descriptor contains the Event name, an optional
   streamID, an optional KeepActive flag, and optional parameters.  The
   Event name consists of a Package Name (where the event is defined)
   and an EventID.  The ALL wildcard may be used for the EventID,
   indicating that all events from the specified package have to be
   detected.  The default streamID is 0, indicating that the event to be
   detected is not related to a particular media stream.  Events can
   have parameters.  This allows a single event description to have some
   variation in meaning without creating large numbers of individual
   events.  Further event parameters are defined in the package.

   If a digit map completion event is present or implied in the
   EventsDescriptor, the EventDM parameter is used to carry either the
   name or the value of the associated digit map.  See 7.1.14 for
   further details.

   When an event is processed against the contents of an active Events
   Descriptor and found to be present in that descriptor ("recognized"),
   the default action of the MG is to send a Notify command to the MGC.
   Notification may be deferred if the event is absorbed into the
   current dial string of an active digit map (see 7.1.14).  Any other
   action is for further study.  Moreover, event recognition may cause
   currently active signals to stop, or may cause the current Events
   and/or Signals descriptor to be replaced, as described at the end of

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   this subclause.  Unless the Events Descriptor is replaced by another
   Events Descriptor, it remains active after an event has been
   recognized.

   If the value of the EventBufferControl property equals LockStep,
   following detection of such an event, normal handling of events is
   suspended.  Any event which is subsequently detected and occurs in
   the EventBuffer descriptor is added to the end of the EventBuffer (a
   FIFO queue), along with the time that it was detected.  The MG SHALL
   wait for a new EventsDescriptor to be loaded.  A new EventsDescriptor
   can be loaded either as the result of receiving a command with a new
   EventsDescriptor, or by activating an embedded EventsDescriptor.

   If EventBufferControl equals Off, the MG continues processing based
   on the active EventsDescriptor.

   In the case of an embedded EventsDescriptor being activated, the MG
   continues event processing based on the newly activated
   EventsDescriptor.

     NOTE 1 - For purposes of EventBuffer handling, activation of an
     embedded EventsDescriptor is equivalent to receipt of a new
     EventsDescriptor.

   When the MG receives a command with a new EventsDescriptor, one or
   more events may have been buffered in the EventBuffer in the MG.  The
   value of EventBufferControl then determines how the MG treats such
   buffered events.

   Case 1

   If EventBufferControl equals LockStep and the MG receives a new
   EventsDescriptor, it will check the FIFO EventBuffer and take the
   following actions:

   1) If the EventBuffer is empty, the MG waits for detection of events
      based on the new EventsDescriptor.

   2) If the EventBuffer is non-empty, the MG processes the FIFO queue
      starting with the first event:

      a) If the event in the queue is in the events listed in the new
         EventsDescriptor, the MG acts on the event and removes the
         event from the EventBuffer.  The time stamp of the Notify shall
         be the time the event was actually detected.  The MG then waits
         for a new EventsDescriptor.  While waiting for a new
         EventsDescriptor, any events detected that appear in the

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         EventsBufferDescriptor will be placed in the EventBuffer.  When
         a new EventsDescriptor is received, the event processing will
         repeat from step 1.

      b) If the event is not in the new EventsDescriptor, the MG SHALL
         discard the event and repeat from step 1.

   Case 2

   If EventBufferControl equals Off and the MG receives a new
   EventsDescriptor, it processes new events with the new
   EventsDescriptor.

   If the MG receives a command instructing it to set the value of
   EventBufferControl to Off, all events in the EventBuffer SHALL be
   discarded.

   The MG may report several events in a single Transaction as long as
   this does not unnecessarily delay the reporting of individual events.

   For procedures regarding transmitting the Notify command, refer to
   the appropriate annex or Recommendation of the H.248 sub-series for
   specific transport considerations.

   The default value of EventBufferControl is Off.

     NOTE 2 - Since the EventBufferControl property is in the
     TerminationStateDescriptor, the MG might receive a command that
     changes the EventBufferControl property and does not include an
     EventsDescriptor.

   Normally, recognition of an event shall cause any active signals to
   stop.  When KeepActive is specified in the event, the MG shall not
   interrupt any signals active on the Termination on which the event is
   detected.

   An event can include an Embedded Signals descriptor and/or an
   Embedded Events descriptor which, if present, replaces the current
   Signals/Events descriptor when the event is recognized.  It is
   possible, for example, to specify that the dial-tone Signal be
   generated when an off-hook Event is recognized, or that the dial-tone
   Signal be stopped when a digit is recognized.  A media gateway
   controller shall not send EventsDescriptors with an event both marked
   KeepActive and containing an embedded SignalsDescriptor.

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   Only one level of embedding is permitted.  An embedded
   EventsDescriptor SHALL NOT contain another embedded EventsDescriptor;
   an embedded EventsDescriptor MAY contain an embedded
   SignalsDescriptor.

   An EventsDescriptor received by a media gateway replaces any previous
   Events descriptor.  Event notification in process shall complete, and
   events detected after the command containing the new EventsDescriptor
   executes, shall be processed according to the new EventsDescriptor.

   An empty Events Descriptor disables all event recognition and
   reporting.  An empty EventBuffer Descriptor clears the EventBuffer
   and disables all event accumulation in LockStep mode: the only events
   reported will be those occurring while an Events Descriptor is
   active.  If an empty Events Descriptor is activated while the
   Termination is operating in LockStep mode, the events buffer is
   immediately cleared.

7.1.10   EventBuffer descriptor

   The EventBuffer descriptor contains a list of events, with their
   parameters if any, that the MG is requested to detect and buffer when
   EventBufferControl equals LockStep (see 7.1.9).

7.1.11   Signals descriptor

   Signals are MG generated media such as tones and announcements as
   well as bearer-related signals such as hookswitch.  More complex
   signals may include a sequence of such simple signals interspersed
   with and conditioned upon the receipt and analysis of media or
   bearer-related signals.  Examples include echoing of received data as
   in Continuity Test package.  Signals may also request preparation of
   media content for future signals.

   A SignalsDescriptor is a parameter that contains the set of signals
   that the Media Gateway is asked to apply to a Termination.  A
   SignalsDescriptor contains a number of signals and/or sequential
   signal lists.  A SignalsDescriptor may contain zero signals and
   sequential signal lists.  Support of sequential signal lists is
   optional.

   Signals are defined in packages.  Signals shall be named with a
   Package name (in which the signal is defined) and a SignalID.  No
   wildcard shall be used in the SignalID.  Signals that occur in a
   SignalsDescriptor have an optional StreamID parameter (default is 0,
   to indicate that the signal is not related to a particular media
   stream), an optional signal type (see below), an optional duration
   and possibly parameters defined in the package that defines the

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   signal.  This allows a single signal to have some variation in
   meaning, obviating the need to create large numbers of individual
   signals.

   Finally, the optional parameter "notifyCompletion" allows a MGC to
   indicate that it wishes to be notified when the signal finishes
   playout.  The possible cases are that the signal timed out (or
   otherwise completed on its own), that it was interrupted by an event,
   that it was halted when a Signals descriptor was replaced, or that it
   stopped or never started for other reasons.  If the notifyCompletion
   parameter is not included in a Signals descriptor, notification is
   generated only if the signal stopped or was never started for other
   reasons.  For reporting to occur, the signal completion event (see
   E.1.2) must be enabled in the currently active Events descriptor.

   The duration is an integer value that is expressed in hundredths of a
   second.

   There are three types of signals:

   -  on/off - the signal lasts until it is turned off;

   -  timeout - the signal lasts until it is turned off or a specific
      period of time elapses;

   -  brief - the signal will stop on its own unless a new Signals
      descriptor is applied that causes it to stop; no timeout value is
      needed.

   If a signal of default type other than TO has its type overridden to
   type TO in the Signals descriptor, the duration parameter must be
   present.

   If the signal type is specified in a SignalsDescriptor, it overrides
   the default signal type (see 12.1.4).  If duration is specified for
   an on/off signal, it SHALL be ignored.

   A sequential signal list consists of a signal list identifier and a
   sequence of signals to be played sequentially.  Only the trailing
   element of the sequence of signals in a sequential signal list may be
   an on/off signal.  The duration of a sequential signal list is the
   sum of the durations of the signals it contains.

   Multiple signals and sequential signal lists in the same
   SignalsDescriptor shall be played simultaneously.

   Signals are defined as proceeding from the Termination towards the
   exterior of the Context unless otherwise specified in a package.

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   When the same Signal is applied to multiple Terminations within one
   Transaction, the MG should consider using the same resource to
   generate these Signals.

   Production of a Signal on a Termination is stopped by application of
   a new SignalsDescriptor, or detection of an Event on the Termination
   (see 7.1.9).

   A new SignalsDescriptor replaces any existing SignalsDescriptor.  Any
   signals applied to the Termination not in the replacement descriptor
   shall be stopped, and new signals are applied, except as follows.
   Signals present in the replacement descriptor and containing the
   KeepActive flag shall be continued if they are currently playing and
   have not already completed.  If a replacement signal descriptor
   contains a signal that is not currently playing and contains the
   KeepActive flag, that signal SHALL be ignored.  If the replacement
   descriptor contains a sequential signal list with the same identifier
   as the existing descriptor, then

   -  the signal type and sequence of signals in the sequential signal
      list in the replacement descriptor shall be ignored; and

   -  the playing of the signals in the sequential signal list in the
      existing descriptor shall not be interrupted.

7.1.12   Audit descriptor

   The Audit descriptor specifies what information is to be audited.
   The Audit descriptor specifies the list of descriptors to be
   returned.  Audit may be used in any command to force the return of
   any descriptor containing the current values of its properties,
   events, signals and statistics even if that descriptor was not
   present in the command, or had no underspecified parameters.
   Possible items in the Audit descriptor are:

      Modem
      Mux
      Events
      Media
      Signals
      ObservedEvents
      DigitMap
      Statistics
      Packages
      EventBuffer

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   Audit may be empty, in which case, no descriptors are returned.  This
   is useful in Subtract, to inhibit return of statistics, especially
   when using wildcard.

7.1.13   ServiceChange descriptor

   The ServiceChangeDescriptor contains the following parameters:

      .  ServiceChangeMethod
      .  ServiceChangeReason
      .  ServiceChangeAddress
      .  ServiceChangeDelay
      .  ServiceChangeProfile
      .  ServiceChangeVersion
      .  ServiceChangeMGCId
      .  TimeStamp
      .  Extension

   See 7.2.8.

7.1.14   DigitMap descriptor

7.1.14.1 DigitMap definition, creation, modification and deletion

   A DigitMap is a dialing plan resident in the Media Gateway used for
   detecting and reporting digit events received on a Termination.  The
   DigitMap descriptor contains a DigitMap name and the DigitMap to be
   assigned.  A digit map may be preloaded into the MG by management
   action and referenced by name in an EventsDescriptor, may be defined
   dynamically and subsequently referenced by name, or the actual
   digitmap itself may be specified in the EventsDescriptor.  It is
   permissible for a digit map completion event within an Events
   descriptor to refer by name to a DigitMap which is defined by a
   DigitMap descriptor within the same command, regardless of the
   transmitted order of the respective descriptors.

   DigitMaps defined in a DigitMapDescriptor can occur in any of the
   standard Termination manipulation Commands of the protocol.  A
   DigitMap, once defined, can be used on all Terminations specified by
   the (possibly wildcarded) TerminationID in such a command.  DigitMaps
   defined on the root Termination are global and can be used on every
   Termination in the MG, provided that a DigitMap with the same name
   has not been defined on the given Termination.  When a DigitMap is
   defined dynamically in a DigitMap descriptor:

   -  A new DigitMap is created by specifying a name that is not yet
      defined.  The value shall be present.

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   -  A DigitMap value is updated by supplying a new value for a name
      that is already defined.  Terminations presently using the
      digitmap shall continue to use the old definition; subsequent
      EventsDescriptors specifying the name, including any
      EventsDescriptor in the command containing the DigitMap
      descriptor, shall use the new one.

   -  A DigitMap is deleted by supplying an empty value for a name that
      is already defined.  Terminations presently using the digitmap
      shall continue to use the old definition.

7.1.14.2 DigitMap Timers

   The collection of digits according to a DigitMap may be protected by
   three timers, viz. a start timer (T), short timer (S), and long timer
   (L).

   1) The start timer (T) is used prior to any digits having been
      dialed.  If the start timer is overridden with the value set to
      zero (T=0), then the start timer shall be disabled.  This implies
      that the MG will wait indefinitely for digits.

   2) If the Media Gateway can determine that at least one more digit is
      needed for a digit string to match any of the allowed patterns in
      the digit map, then the interdigit timer value should be set to a
      long (L) duration (e.g., 16 seconds).

   3) If the digit string has matched one of the patterns in a digit
      map, but it is possible that more digits could be received which
      would cause a match with a different pattern, then instead of
      reporting the match immediately, the MG must apply the short timer
      (S) and wait for more digits.

   The timers are configurable parameters to a DigitMap.  Default values
   of these timers should be provisioned on the MG, but can be
   overridden by values specified within the DigitMap.

7.1.14.3 DigitMap Syntax

   The formal syntax of the digit map is described by the DigitMap rule
   in the formal syntax description of the protocol (see Annex A and
   Annex B).  A DigitMap, according to this syntax, is defined either by
   a string or by a list of strings.  Each string in the list is an
   alternative event sequence, specified either as a sequence of digit
   map symbols or as a regular expression of digit map symbols.  These
   digit map symbols, the digits "0" through "9" and letters "A" through
   a maximum value depending on the signalling system concerned, but
   never exceeding "K", correspond to specified events within a package

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   which has been designated in the Events descriptor on the Termination
   to which the digit map is being applied.  (The mapping between events
   and digit map symbols is defined in the documentation for packages
   associated with channel-associated signalling systems such as DTMF,
   MF, or R2.  Digits "0" through "9" MUST be mapped to the
   corresponding digit events within the signalling system concerned.
   Letters should be allocated in logical fashion, facilitating the use
   of range notation for alternative events.)

   The letter "x" is used as a wildcard, designating any event
   corresponding to symbols in the range "0"-"9".  The string may also
   contain explicit ranges and, more generally, explicit sets of
   symbols, designating alternative events any one of which satisfies
   that position of the digit map.  Finally, the dot symbol "." stands
   for zero or more repetitions of the event selector (event, range of
   events, set of alternative events, or wildcard) that precedes it.  As
   a consequence of the third timing rule above, inter-event timing
   while matching a terminal dot symbol uses the short timer by default.

   In addition to these event symbols, the string may contain "S" and
   "L" inter-event timing specifiers and the "Z" duration modifier.  "S"
   and "L" respectively indicate that the MG should use the short (S)
   timer or the long (L) timer for subsequent events, overriding the
   timing rules described above.  If an explicit timing specifier is in
   effect in one alternative event sequence, but none is given in any
   other candidate alternative, the timer value set by the explicit
   timing specifier must be used.  If all sequences with explicit timing
   controls are dropped from the candidate set, timing reverts to the
   default rules given above.  Finally, if conflicting timing specifiers
   are in effect in different alternative sequences, the long timer
   shall be used.

   A "Z" designates a long duration event: placed in front of the
   symbol(s) designating the event(s) which satisfy a given digit
   position, it indicates that that position is satisfied only if the
   duration of the event exceeds the long-duration threshold.  The value
   of this threshold is assumed to be provisioned in the MG.

7.1.14.4 DigitMap Completion Event

   A digit map is active while the Events descriptor which invoked it is
   active and it has not completed.  A digit map completes when:

   -  a timer has expired; or

   -  an alternative event sequence has been matched and no other
      alternative event sequence in the digit map could be matched
      through detection of an additional event (unambiguous match); or

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   -  an event has been detected such that a match to a complete
      alternative event sequence of the digit map will be impossible no
      matter what additional events are received.

   Upon completion, a digit map completion event as defined in the
   package providing the events being mapped into the digit map shall be
   generated.  At that point the digit map is deactivated.  Subsequent
   events in the package are processed as per the currently active event
   processing mechanisms.

7.1.14.5 DigitMap Procedures

   Pending completion, successive events shall be processed according to
   the following rules:

   1) The "current dial string", an internal variable, is initially
      empty.  The set of candidate alternative event sequences includes
      all of the alternatives specified in the digit map.

   2) At each step, a timer is set to wait for the next event, based
      either on the default timing rules given above or on explicit
      timing specified in one or more alternative event sequences.  If
      the timer expires and a member of the candidate set of
      alternatives is fully satisfied, a timeout completion with full
      match is reported.  If the timer expires and part or none of any
      candidate alternative is satisfied, a timeout completion with
      partial match is reported.

   3) If an event is detected before the timer expires, it is mapped to
      a digit string symbol and provisionally added to the end of the
      current dial string.  The duration of the event (long or not long)
      is noted if and only if this is relevant in the current symbol
      position (because at least one of the candidate alternative event
      sequences includes the "Z" modifier at this position in the
      sequence).

   4) The current dial string is compared to the candidate alternative
      event sequences.  If and only if a sequence expecting a
      long-duration event at this position is matched (i.e., the event
      had long duration and met the specification for this position),
      then any alternative event sequences not specifying a long
      duration event at this position are discarded, and the current
      dial string is modified by inserting a "Z" in front of the symbol
      representing the latest event.   Any sequence expecting a long-
      duration event at this position but not matching the observed
      event is discarded from the candidate set.  If alternative event
      sequences not specifying a long duration event in the given

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      position remain in the candidate set after application of the
      above rules, the observed event duration is treated as irrelevant
      in assessing matches to them.

   5) If exactly one candidate remains and it has been fully matched, a
      completion event is generated indicating an unambiguous match.  If
      no candidates remain, the latest event is removed from the current
      dial string and a completion event is generated indicating full
      match if one of the candidates from the previous step was fully
      satisfied before the latest event was detected, or partial match
      otherwise.  The event removed from the current dial string will
      then be reported as per the currently active event processing
      mechanisms.

   6) If no completion event is reported out of step 5, processing
      returns to step 2.

7.1.14.6 DigitMap Activation

   A digit map is activated whenever a new Event descriptor is applied
   to the Termination or embedded Event descriptor is activated, and
   that Event descriptor contains a digit map completion event.  The
   digit map completion event contains an eventDM field in the requested
   actions field.  Each new activation of a digit map begins at step 1
   of the above procedure, with a clear current dial string.  Any
   previous contents of the current dial string from an earlier
   activation are lost.

   A digit map completion event that does not contain an eventDM field
   in its requested actions field is considered an error.  Upon receipt
   of such an event in an EventsDescriptor, a MG shall respond with an
   error response, including Error 457 - Missing parameter in signal or
   event.

7.1.14.7 Interaction Of DigitMap and Event Processing

   While the digit map is activated, detection is enabled for all events
   defined in the package containing the specified digit map completion
   event.  Normal event behaviour (e.g., stopping of signals unless the
   digit completion event has the KeepActive flag enabled) continues to
   apply for each such event detected, except that:

   -  the events in the package containing the specified digit map
      completion event other than the completion event itself are not
      individually notified and have no side-effects unless separately
      enabled; and

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   -  an event that triggers a partial match completion event is not
      recognized and therefore has no side effects until reprocessed
      following the recognition of the digit map completion event.

7.1.14.8 Wildcards

   Note that if a package contains a digit map completion event, then an
   event specification consisting of the package name with a wildcarded
   ItemID (Property Name) will activate a digit map; to that end, the
   event specification must include an eventDM field according to
   section 7.1.14.6.  If the package also contains the digit events
   themselves, this form of event specification will cause the
   individual events to be reported to the MGC as they are detected.

7.1.14.9 Example

   As an example, consider the following dial plan:

   0                      Local operator

   00                     Long-distance operator

   xxxx                   Local extension number (starts with 1-7)

   8xxxxxxx               Local number

   #xxxxxxx               Off-site extension

   *xx                    Star services

   91xxxxxxxxxx           Long-distance number

   9011 + up to 15 digits International number



   If the DTMF detection package described in E.6 is used to collect the
   dialed digits, then the dialing plan shown above results in the
   following digit map:

    (0| 00|[1-7]xxx|8xxxxxxx|Fxxxxxxx|Exx|91xxxxxxxxxx|9011x.)

7.1.15   Statistics descriptor

   The Statistics Descriptor provides information describing the status
   and usage of a Termination during its existence within a specific
   Context.  There is a set of standard statistics kept for each
   Termination where appropriate (number of octets sent and received for

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   example).  The particular statistical properties that are reported
   for a given Termination are determined by the Packages realized by
   the Termination.  By default, statistics are reported when the
   Termination is Subtracted from the Context.  This behaviour can be
   overridden by including an empty AuditDescriptor in the Subtract
   command.  Statistics may also be returned from the AuditValue
   command, or any Add/Move/Modify command using the Audit descriptor.

   Statistics are cumulative; reporting Statistics does not reset them.
   Statistics are reset when a Termination is Subtracted from a Context.

7.1.16   Packages descriptor

   Used only with the AuditValue command, the PackageDescriptor returns
   a list of Packages realized by the Termination.

7.1.17   ObservedEvents descriptor

   ObservedEvents is supplied with the Notify command to inform the MGC
   of which event(s) were detected.  Used with the AuditValue command,
   the ObservedEventsDescriptor returns events in the event buffer which
   have not been Notified.  ObservedEvents contains the
   RequestIdentifier of the EventsDescriptor that triggered the
   notification, the event(s) detected, optionally the detection time(s)
   and any parameters of the observed event.  Detection times are
   reported with a precision of hundredths of a second.

7.1.18   Topology descriptor

   A Topology descriptor is used to specify flow directions between
   Terminations in a Context.  Contrary to the descriptors in previous
   subclauses, the Topology descriptor applies to a Context instead of a
   Termination.  The default topology of a Context is that each
   Termination's transmission is received by all other Terminations.
   The Topology descriptor is optional to implement.  An MG that does
   not support Topology descriptors, but receives a command containing
   one, returns Error 444 Unsupported or unknown descriptor, and
   optionally includes a string containing the name of the unsupported
   Descriptor ("Topology") in the error text in the error descriptor.

   The Topology descriptor occurs before the commands in an action.  It
   is possible to have an action containing only a Topology descriptor,
   provided that the Context to which the action applies already exists.

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   A Topology descriptor consists of a sequence of triples of the form
   (T1, T2, association).  T1 and T2 specify Terminations within the
   Context, possibly using the ALL or CHOOSE wildcard.  The association
   specifies how media flows between these two Terminations as follows.

   -  (T1, T2, isolate) means that the Terminations matching T2 do not
      receive media from the Terminations matching T1, nor vice versa.

   -  (T1, T2, oneway) means that the Terminations that match T2 receive
      media from the Terminations matching T1, but not vice versa.  In
      this case use of the ALL wildcard such that there are Terminations
      that match both T1 and T2 is not allowed.

   -  (T1, T2, bothway) means that the Terminations matching T2 receive
      media from the Terminations matching T1, and vice versa.  In this
      case it is allowed to use wildcards such that there are
      Terminations that match both T1 and T2.  However, if there is a
      Termination that matches both, no loopback is introduced.

   CHOOSE wildcards may be used in T1 and T2 as well, under the
   following restrictions:

   -  the action (see clause 8) of which the topology descriptor is part
      contains an Add command in which a CHOOSE wildcard is used;

   -  if a CHOOSE wildcard occurs in T1 or T2, then a partial name SHALL
      NOT be specified.

   The CHOOSE wildcard in a Topology descriptor matches the
   TerminationID that the MG assigns in the first Add command that uses
   a CHOOSE wildcard in the same action.  An existing Termination that
   matches T1 or T2 in the Context to which a Termination is added, is
   connected to the newly added Termination as specified by the Topology
   descriptor.

   If a termination is not mentioned within a Topology Descriptor, any
   topology associated with it remains unchanged.  If, however, a new
   termination is added into a context its association with the other
   terminations within the context defaults to bothway, unless a
   Topology Descriptor is given to change this (e.g., if T3 is added to
   a context with T1 and T2 with topology (T3, T1, oneway) it will be
   connected bothway to T2).

   Figure 7 and the table following it show some examples of the effect
   of including topology descriptors in actions.  In these examples it
   is assumed that the topology descriptors are applied in sequence.

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     +------------------+  +------------------+  +------------------+
     |      +----+      |  |      +----+      |  |      +----+      |
     |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
     |      +----+      |  |      +----+      |  |      +----+      |
     |       ^  ^       |  |          ^       |  |          ^       |
     |       |  |       |  |          |       |  |          |       |
     |    +--+  +--+    |  |          +---+   |  |          +--+    |
     |    |        |    |  |              |   |  |             |    |
     |    v        v    |  |              v   |  |             |    |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     +------------------+  +------------------+  +------------------+
     1. No Topology Desc.   2. T1, T2, Isolate    3. T3, T2, Oneway

     +------------------+  +------------------+  +------------------+
     |      +----+      |  |      +----+      |  |      +----+      |
     |      | T2 |      |  |      | T2 |      |  |      | T2 |      |
     |      +----+      |  |      +----+      |  |      +----+      |
     |          |       |  |          ^       |  |       ^  ^       |
     |          |       |  |          |       |  |       |  |       |
     |          +--+    |  |          +---+   |  |    +--+  +--+    |
     |             |    |  |              |   |  |    |        |    |
     |             v    |  |              v   |  |    v        v    |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |  | | T1 |<-->| T3 | |
     | +----+    +----+ |  | +----+    +----+ |  | +----+    +----+ |
     +------------------+  +------------------+  +------------------+
     4. T2, T3 oneway      5. T2, T3 bothway     6. T1, T2 bothway

     Note: the direction of the arrow indicates the direction of flow.

                       Figure 7: Example topologies

   Topology Description

   1 No topology descriptors    When no topology descriptors are
                                included, all Terminations have a
                                bothway connection to all other
                                Terminations.

   2 T1, T2 Isolate             Removes the connection between T1 and
                                T2.  T3 has a bothway connection with
                                both T1 and T2.  T1 and T2 have bothway
                                connection to T3.

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   3 T3, T2 oneway              A oneway connection from T3 to T2 (i.e.,
                                T2 receives media flow from T3).  A
                                bothway connection between T1 and T3.

   4 T2, T3 oneway              A oneway connection between T2 to T3.
                                T1 and T3 remain bothway connected.

   5 T2, T3 bothway             T2 is bothway connected to T3.  This
                                results in the same as 2.

   6 T1, T2 bothway (T2, T3     All Terminations have a bothway
     bothway and T1, T3         connection to all other Terminations.
     bothway may be implied or
     explicit).

   A oneway connection must be implemented in such a way that the other
   Terminations in the Context are not aware of the change in topology.

7.1.19   Error Descriptor

   If a responder encounters an error when processing a transaction
   request, it must include an error descriptor in its response.  A
   Notify request may contain an error descriptor as well.

   An error descriptor consists of an IANA-registered error code,
   optionally accompanied by an error text.  H.248.8 contains a list of
   valid error codes and error descriptions.

   An error descriptor shall be specified at the "deepest level" that is
   semantically appropriate for the error being described and that is
   possible given any parsing problems with the original request.  An
   error descriptor may refer to a syntactical construct other than
   where it appears.  For example, Error descriptor 422 - Syntax Error
   in Action, could appear within a command even though it refers to the
   larger construct - the action - and not the particular command within
   which it appears.

7.2   Command Application Programming Interface

   Following is an Application Programming Interface (API) describing
   the Commands of the protocol.  This API is shown to illustrate the
   Commands and their parameters and is not intended to specify
   implementation (e.g., via use of blocking function calls).  It
   describes the input parameters in parentheses after the command name
   and the return values in front of the Command.  This is only for
   descriptive purposes; the actual Command syntax and encoding are

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   specified in later subclauses.  The order of parameters to commands
   is not fixed.  Descriptors may appear as parameters to commands in
   any order.  The descriptors SHALL be processed in the order in which
   they appear.

   Any reply to a command may contain an error descriptor; the API does
   not specifically show this.

   All parameters enclosed by square brackets ([. . .]) are considered
   optional.

7.2.1 Add

   The Add Command adds a Termination to a Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
       Add( TerminationID
        [, MediaDescriptor]
        [, ModemDescriptor]
        [, MuxDescriptor]
        [, EventsDescriptor]
        [, EventBufferDescriptor]
        [, SignalsDescriptor]
        [, DigitMapDescriptor]
        [, AuditDescriptor]
        )

   The TerminationID specifies the Termination to be added to the
   Context.  The Termination is either created, or taken from the null
   Context.  If a CHOOSE wildcard is used in the TerminationID, the
   selected TerminationID will be returned.  Wildcards may be used in an
   Add, but such usage would be unusual.  If the wildcard matches more
   than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.

   The optional MediaDescriptor describes all media streams.

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   The optional ModemDescriptor and MuxDescriptor specify a modem and
   multiplexer if applicable.  For convenience, if a Multiplex
   descriptor is present in an Add command and lists any Terminations
   that are not currently in the Context, such Terminations are added to
   the Context as if individual Add commands listing the Terminations
   were invoked. If an error occurs on such an implied Add, error 471 -
   Implied Add for Multiplex failure shall be returned and further
   processing of the command shall cease.

   The EventsDescriptor parameter is optional.  If present, it provides
   the list of events that should be detected on the Termination.

   The EventBufferDescriptor parameter is optional.  If present, it
   provides the list of events that the MG is requested to detect and
   buffer when EventBufferControl equals LockStep.

   The SignalsDescriptor parameter is optional.  If present, it provides
   the list of signals that should be applied to the Termination.

   The DigitMapDescriptor parameter is optional.  If present, it defines
   a DigitMap definition that may be used in an EventsDescriptor.

   The AuditDescriptor is optional.  If present, the command will return
   descriptors as specified in the AuditDescriptor.

   All descriptors that can be modified could be returned by MG if a
   parameter was underspecified or overspecified.  ObservedEvents,
   Statistics, and Packages, and the EventBuffer descriptors are
   returned only if requested in the AuditDescriptor.

   Add SHALL NOT be used on a Termination with a serviceState of
   "OutofService".

7.2.2 Modify

   The Modify Command modifies the properties of a Termination.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]

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      Modify( TerminationID
         [, MediaDescriptor]
         [, ModemDescriptor]
         [, MuxDescriptor]
         [, EventsDescriptor]
         [, EventBufferDescriptor]
         [, SignalsDescriptor]
         [, DigitMapDescriptor]
         [, AuditDescriptor]
         )

   The TerminationID may be specific if a single Termination in the
   Context is to be modified.  Use of wildcards in the TerminationID may
   be appropriate for some operations.  If the wildcard matches more
   than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.  The CHOOSE option is an
   error, as the Modify command may only be used on existing
   Terminations.

   For convenience, if a Multiplex Descriptor is present in a Modify
   command, then:

   -  if the new Multiplex Descriptor lists any Terminations that are
      not currently in the Context, such Terminations are added to the
      context as if individual commands listing the Terminations were
      invoked.

   -  if any Terminations listed previously in the Multiplex Descriptor
      are no longer present in the new Multiplex Descriptor, they are
      subtracted from the context as if individual Subtract commands
      listing the Terminations were invoked.

   The remaining parameters to Modify are the same as those to Add.
   Possible return values are the same as those to Add.

7.2.3 Subtract

   The Subtract Command disconnects a Termination from its Context and
   returns statistics on the Termination's participation in the Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]

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     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
      Subtract(TerminationID
         [, AuditDescriptor]
         )

   TerminationID in the input parameters represents the Termination that
   is being subtracted.  The TerminationID may be specific or may be a
   wildcard value indicating that all (or a set of related) Terminations
   in the Context of the Subtract Command are to be subtracted.  If the
   wildcard matches more than one TerminationID, all possible matches
   are attempted, with results reported for each one.  The order of
   attempts when multiple TerminationIDs match is not specified.

   The use of CHOOSE in the TerminationID is an error, as the Subtract
   command may only be used on existing Terminations.

   ALL may be used as the ContextID as well as the TerminationId in a
   Subtract, which would have the effect of deleting all Contexts,
   deleting all ephemeral Terminations, and returning all physical
   Terminations to Null Context.  Subtract of a termination from the
   Null Context is not allowed.

   For convenience, if a multiplexing Termination is the object of a
   Subtract command, then any bearer Terminations listed in its
   Multiplex Descriptor are subtracted from the context as if individual
   Subtract commands listing the Terminations were invoked.

   By default, the Statistics parameter is returned to report
   information collected on the Termination or Terminations specified in
   the Command.  The information reported applies to the Termination's
   or Terminations' existence in the Context from which it or they are
   being subtracted.

   The AuditDescriptor is optional.  If present, the command will return
   only those descriptors as specified in the AuditDescriptor, which may
   be empty.  If omitted, the Statistics descriptor is returned, by
   default.  Possible return values are the same as those to Add.

   When a provisioned Termination is Subtracted from a Context, its
   property values shall revert to:

   -  the default value, if specified for the property and not
      overridden by provisioning;

   -  otherwise, the provisioned value.

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7.2.4 Move

   The Move Command moves a Termination to another Context from its
   current Context in one atomic operation.  The Move command is the
   only command that refers to a Termination in a Context different from
   that to which the command is applied.  The Move command shall not be
   used to move Terminations to or from the null Context.

     TerminationID
     [,MediaDescriptor]
     [,ModemDescriptor]
     [,MuxDescriptor]
     [,EventsDescriptor]
     [,SignalsDescriptor]
     [,DigitMapDescriptor]
     [,ObservedEventsDescriptor]
     [,EventBufferDescriptor]
     [,StatisticsDescriptor]
     [,PackagesDescriptor]
      Move( TerminationID
         [, MediaDescriptor]
         [, ModemDescriptor]
         [, MuxDescriptor]
         [, EventsDescriptor]
         [, EventBufferDescriptor]
         [, SignalsDescriptor]
         [, DigitMapDescriptor]
         [, AuditDescriptor]
         )

   The TerminationID specifies the Termination to be moved.  It may be
   wildcarded, but CHOOSE shall not be used in the TerminationID.  If
   the wildcard matches more than one TerminationID, all possible
   matches are attempted, with results reported for each one.  The order
   of attempts when multiple TerminationIDs match is not specified.  The
   Context to which the Termination is moved is indicated by the target
   ContextId in the Action.  If the last remaining Termination is moved
   out of a Context, the Context is deleted.

   The Move command does not affect the properties of the Termination on
   which it operates, except those properties explicitly modified by
   descriptors included in the Move command.  The AuditDescriptor with
   the Statistics option, for example, would return statistics on the
   Termination just prior to the Move.  Possible descriptors returned
   from Move are the same as for Add.

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   For convenience, if a multiplexing Termination is the object of a
   Move command, then any bearer Terminations listed in its Multiplex
   Descriptor are also moved as if individual Move commands listing the
   Terminations were invoked.

   Move SHALL NOT be used on a Termination with a serviceState of
   "OutofService".

7.2.5 AuditValue

   The AuditValue Command returns the current values of properties,
   events, signals and statistics associated with Terminations.

   TerminationID
   [,MediaDescriptor]
   [,ModemDescriptor]
   [,MuxDescriptor]
   [,EventsDescriptor]
   [,SignalsDescriptor]
   [,DigitMapDescriptor]
   [,ObservedEventsDescriptor]
   [,EventBufferDescriptor]
   [,StatisticsDescriptor]
   [,PackagesDescriptor]
     AuditValue(TerminationID,
      AuditDescriptor
      )

   TerminationID may be specific or wildcarded.  If the wildcard matches
   more than one TerminationID, all possible matches are attempted, with
   results reported for each one.  The order of attempts when multiple
   TerminationIDs match is not specified.  If a wildcarded response is
   requested, only one command return is generated, with the contents
   containing the union of the values of all Terminations matching the
   wildcard.  This convention may reduce the volume of data required to
   audit a group of Terminations.  Use of CHOOSE is an error.

   The appropriate descriptors, with the current values for the
   Termination, are returned from AuditValue.  Values appearing in
   multiple instances of a descriptor are defined to be alternate values
   supported, with each parameter in a descriptor considered
   independent.

   ObservedEvents returns a list of events in the EventBuffer.  If the
   ObservedEventsDescriptor is audited while a DigitMap is active, the
   returned ObservedEvents descriptor also includes a digit map
   completion event that shows the current dial string but does not show
   a Termination method.

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   EventBuffer returns the set of events and associated parameter values
   currently enabled in the EventBufferDescriptor.  PackagesDescriptor
   returns a list of packages realized by the Termination.
   DigitMapDescriptor returns the name or value of the current DigitMap
   for the Termination.  DigitMap requested in an AuditValue command
   with TerminationID ALL returns all DigitMaps in the gateway.
   Statistics returns the current values of all statistics being kept on
   the Termination.   Specifying an empty Audit descriptor results in
   only the TerminationID being returned.  This may be useful to get a
   list of TerminationIDs when used with wildcard.  Annexes A and B
   provide a special syntax for presenting such a list in condensed
   form, such that the AuditValue command tag does not have to be
   repeated for each TerminationID.

   AuditValue results depend on the Context, viz. specific, null, or
   wildcarded.  (Note that ContextID ALL does not include the null
   Context.)  The TerminationID may be specific, or wildcarded.

   The following are examples of what is returned in case the context
   and/or the termination is wildcarded and a wildcarded response has
   been specified.

   Assume that the gateway has 4 terminations: t1/1, t1/2, t2/1 and
   t2/2.  Assume that terminations t1/* have implemented packages aaa
   and bbb and that terminations t2/* have implemented packages ccc and
   ddd.  Assume that Context 1 has t1/1 and t2/1 in it and that Context
   2 has t1/2 and t2/2 in it.

   The command:

     Context=1{AuditValue=t1/1{Audit{Packages}}}

   Returns:

     Context=1{AuditValue=t1/1{Packages{aaa,bbb}}}

   The command:

     Context=*{AuditValue=t2/*{Audit{Packages}}}

   Returns:

     Context=1{AuditValue=t2/1{Packages{ccc,ddd}}},
     Context=2{AuditValue=t2/2{Packages{ccc,ddd}}}

   The command:

     Context=*{W-AuditValue=t1/*{Audit{Packages}}}

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   Returns:

     Context=*{W-AuditValue=t1/*{Packages{aaa,bbb}}}

   Note: A wildcard response may also be used for other commands such as
   Subtract.

   The following illustrates other information that can be obtained with
   the AuditValue Command:

   ContextID TerminationID Information Obtained

   Specific  wildcard      Audit of matching Terminations in a Context

   Specific  specific      Audit of a single Termination in a Context

   Null      Root          Audit of Media Gateway state and events

   Null      wildcard      Audit of all matching Terminations in the
                            null Context

   Null      specific      Audit of a single Termination outside of any
                            Context

   All       wildcard      Audit of all matching Terminations and the
                            Context to which they are associated

   All       Root          List of all ContextIds (the ContextID list
                            should be returned by using multiple action
                            replies, each containing a ContextID from
                            the list)

   All       Specific      (Non-null) ContextID in which the
                            Termination currently exists


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