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RFC 3015

Megaco Protocol Version 1.0

Pages: 179

Obsoletes:  28852886
Obsoleted by:  3525
Part 2 of 6 – Pages 21 to 45
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   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

   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

   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

   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
      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 sections 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 subsection 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 subsection 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_.}.
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   Parameters may be fully specified, over-specified or under-specified:

   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. Under-specified parameters, using the CHOOSE value, allow the
      command responder to choose any value it can support.

   3. Over-specified 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.

   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.  A missing Audit descriptor is equivalent to an empty Audit
   Descriptor.  The behavior of the MG with respect to unspecified
   parameters within a descriptor varies with the descriptor concerned,
   as indicated in succeeding sections.  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.22bis, V.32,
   V.32bis, V.34, V.90, V.91, Synchronous ISDN, and allows for
   extensions.  By default, no modem descriptor is present in a

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:
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   *  H.221
   *  H.223,
   *  H.226,
   *  V.76,
   *  Possible Extensions

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

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

7.1.4 Media Descriptor

   The Media Descriptor specifies the parameters for all the media
   streams.  These parameters are structured into two descriptors, a
   Termination State 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
        Stream Descriptor
                LocalControl Descriptor
                Local Descriptor
                Remote Descriptor

   As a convenience a LocalControl, Local, or Remote descriptor 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 Termination State Descriptor

   The Termination State 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
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   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.

   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
   *  CHOOSE Wildcard: the MG chooses from the allowed values for the

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

7.1.6 Stream Descriptor

   A Stream descriptor specifies the parameters of a single bi-
   directional 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 section 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.  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 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, is 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
   section 7.1.8.

   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
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   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 (RFC2327).  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,
   *  the use of alternatives is allowed in place of a single parameter

   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
   RFC2327. 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.

   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.
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   Either the local or the remote descriptor or both may be

   *  unspecified (i.e., absent),
   *  empty,
   *  underspecified through use of CHOOSE in a property value,
   *  fully specified, or
   *  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 section 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.

   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
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   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.

   *  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

   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
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   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

   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 section 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 MG.
   Notification may be deferred if the event is absorbed into the
   current dial string of an active digit map (see section 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 this section.

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

   In the case that an embedded EventsDescriptor being activated, the MG
   continues event processing based on the newly activated
   EventsDescriptor (Note -  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
         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

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

   The default value of EventBufferControl is Off.

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

   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

   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.

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

   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.

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

   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
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   signal lists.  A SignalsDescriptor may contain zero signals and
   sequential signal lists.  Support of sequential signal lists is

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

   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, 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 section 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

   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 duration is so short that it will stop on its
      own unless a new signal is applied that causes it to stop; no
      timeout value is needed.

   If the signal type is specified in a SignalsDescriptor, it overrides
   the default signal type (see Section 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, a
   sequence of signals to be played sequentially, and a signal type.
   Only the trailing element of the sequence of signals in a sequential
   signal list may be an on/off signal.  If the trailing element of the
   sequence is an on/off signal, the signal type of the sequential
   signal list shall be on/off as well.  If the sequence of signals in a
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   sequential signal list contains signals of type timeout and the
   trailing element is not of type on/off, the type of the sequential
   signal list SHALL be set to timeout.  The duration of a sequential
   signal list with type timeout is the sum of the durations of the
   signals it contains.  If the sequence of signals in a sequential
   signal list contains only signals of type brief, the type of the
   sequential signal list SHALL be set to brief.  A signal list is
   treated as a single signal of the specified type when played out.

   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. 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 section 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 flagshall 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 a descriptor
   even if the descriptor in the command was not present, or had no
   underspecified parameters.  Possible items in the Audit Descriptor
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           | Modem          |
           | Mux            |
           | Events         |
           | Media          |
           | Signals        |
           | ObservedEvents |
           | DigitMap       |
           | Statistics     |
           | Packages       |
           | EventBuffer    |

   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 section 7.2.8.
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7.1.14 DigitMap Descriptor 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.

   *  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. 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

   1. The start timer (T) is used prior to any digits having been
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   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.  The Start
   timer is started at the beginning of every digit map use, but can be
   overridden. 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
   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)
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   timer or the long (L) timer for subsequent events, over-riding 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 results are

   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. 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

   *  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. 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.
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   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

   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 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

   6. If no completion event is reported out of step 5, processing
      returns to step 2.
ToP   noToC   Page 40 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 which
   itself contains a digit map parameter.  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. 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

   *  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. 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 if the event
   includes a digit map parameter.  Regardless of whether a digit map is
   activated, 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.
ToP   noToC   Page 41 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 Annex E (section E.6) is
   used to collect the dialled digits, then the dialling 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 parameter 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
   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 behavior 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.
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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 and the detection time(s). Detection times are reported with
   a precision of hundredths of a second.  Time is expressed in UTC.

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
   sections, 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.

   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.

   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 section 8) of which the topology descriptor is
      part contains an Add command in which a CHOOSE wildcard is used;
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   *  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. The default association when a termination is not
   mentioned in the Topology descriptor is bothway (if T3 is added to a
   context with T1 and T2 with topology (T3,T1,oneway) it will be
   connected bothway to T2).

   The figure below 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
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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

               Figure 4: A Sequence Of Example Topologies
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       | Topology |    Description                                   |
       |    1     | No topology descriptors                          |
       | When no topology descriptors are included, all terminations |
       | have a both way connection to all other terminations.       |
       |    2     | T1, T2, Isolate                                  |
       | Removes the connection between T1 and T2.                   |
       | T3 has a both way connection with both T1 and T2.  T1 and   |
       | T2 have bothway connection to T3.                           |
       |    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 bothway and T1,T3 bothway |
       |          | may be implied or explicit).                     |
       | All terminations have a bothway connection to all other     |
       | terminations.                                               |
       | A oneway connection must implemented in such a way that the |
       | other Terminations in the Context are not aware of the      |
       | change in topology.                                         |

(page 45 continued on part 3)

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