rfc3525.txt
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Termination (send/receive/...) describes the flow of the media at the ingress/egress of the media gateway. - The priority is used for a Context in order to provide the MG with information about a certain precedence handling for a Context. The MGC can also use the priority to control autonomously the traffic precedence in the MG in a smooth way in certain situations (e.g., restart), when a lot of Contexts must be handled simultaneously. Priority 0 is the lowest priority and a priority of 15 is the highest priority. - An indicator for an emergency call is also provided to allow a preference handling in the MG.6.1.2 Creating, deleting and modifying Contexts The protocol can be used to (implicitly) create Contexts and modify the parameter values of existing Contexts. The protocol has commands to add Terminations to Contexts, subtract them from Contexts, and to move Terminations between Contexts. Contexts are deleted implicitly when the last remaining Termination is subtracted or moved out.6.2 Terminations A Termination is a logical entity on a MG that sources and/or sinks media and/or control streams. A Termination is described by a number of characterizing Properties, which are grouped in a set of Descriptors that are included in commands. Terminations have unique identities (TerminationIDs), assigned by the MG at the time of their creation.Groves, et al. Standards Track [Page 17] RFC 3525 Gateway Control Protocol June 2003 Terminations representing physical entities have a semi-permanent existence. For example, a Termination representing a TDM channel might exist for as long as it is provisioned in the gateway. Terminations representing ephemeral information flows, such as RTP flows, would usually exist only for the duration of their use. Ephemeral Terminations are created by means of an Add command. They are destroyed by means of a Subtract command. In contrast, when a physical Termination is Added to or Subtracted from a Context, it is taken from or to the null Context, respectively. Terminations may have signals applied to them (see 7.1.11). Terminations may be programmed to detect Events, the occurrence of which can trigger notification messages to the MGC, or action by the MG. Statistics may be accumulated on a Termination. Statistics are reported to the MGC upon request (by means of the AuditValue command, see 7.2.5) and when the Termination is taken out of the call it is in. Multimedia gateways may process multiplexed media streams. For example, Recommendation H.221 describes a frame structure for multiple media streams multiplexed on a number of digital 64 kbit/s channels. Such a case is handled in the connection model in the following way. For every bearer channel that carries part of the multiplexed streams, there is a physical or ephemeral "bearer Termination". The bearer Terminations that source/sink the digital channels are connected to a separate Termination called the "multiplexing Termination". The multiplexing termination is an ephemeral termination representing a frame-oriented session. The MultiplexDescriptor for this Termination describes the multiplex used (e.g., H.221 for an H.320 session) and indicates the order in which the contained digital channels are assembled into a frame. Multiplexing terminations may be cascades (e.g., H.226 multiplex of digital channels feeding into a H.223 multiplex supporting an H.324 session). The individual media streams carried in the session are described by StreamDescriptors on the multiplexing Termination. These media streams can be associated with streams sourced/sunk by Terminations in the Context other than the bearer Terminations supporting the multiplexing Termination. Each bearer Termination supports only a single data stream. These data streams do not appear explicitly as streams on the multiplexing Termination and they are hidden from the rest of the context. Figures 4, 5, 6, and 6a illustrate typical applications of the multiplexing termination and Multiplex Descriptor.Groves, et al. Standards Track [Page 18] RFC 3525 Gateway Control Protocol June 2003 +-----------------------------------+ | Context +-------+ | +----+ | | | Circuit 1 -|--| TC1|---------+ Tmux | | | +----+ (Str 1) | | Audio +-----+ | | | +-----*-----+ |----- | +----+ | H.22x | Stream 1 | | Circuit 2 -|--| TC2|---------+ multi-| | TR1 | | +----+ (Str 1) | plex | |(RTP)| | | | | Video | | | +----+ | +-----*-----+ |----- Circuit 3 -|--| TC3|---------+ | Stream 2 | | / +----+ (Str 1) | | +-----+ / | +-------+ | / +-----------------\-----------------+ Audio, video, and control \ signals are carried in frames Tmux is an ephemeral with two spanning the circuits. explicit Stream Descriptors and a Multiplex Descriptor. Figure 4: Multiplexed Termination Scenario - Circuit to Packet (Asterisks * denote the centre of the context) Context +--------------------------------------+ | +-------+ +-------+ | +----+ | | | | +----+ Circuit 1 ----| TC1|---+ Tmux1 | Audio | Tmux2 +---| TC4|--- +----+ | +---*----+ | +----+ | | | Str 1 | | | +----+ | H.22x | | H.22x | +----+ Circuit 2 ----| TC2|---+ multi-| | multi-+---| TC5|--- +----+ | plex | | plex | +----+ | | | Video | | | +----+ | +---*----+ | +----+ Circuit 3 ----| TC3|---+ | Str 2 | +---| TC6|--- +----+ | | | | +----+ | +-------+ +-------+ | +-----------------\-----/--------------+ \ / Tmux1 and Tmux2 are ephemerals each with two explicit Stream Descriptors and a Multiplex Descriptor. Figure 5: Multiplexed Termination Scenario - Circuit to Circuit (Asterisks * denote the centre of the context)Groves, et al. Standards Track [Page 19] RFC 3525 Gateway Control Protocol June 2003 +-----------------------------------+ | Context +-------+ | +----+ | | | Circuit 1 -|--| TC1|---------+ Tmux | | | +----+ (Str 1) | | Audio +-----+ | | | +-----*-----+ TR1 |----- | +----+ | H.22x | Stream 1 |(RTP)| Circuit 2 -|--| TC2|---------+ multi-| +-----+ | +----+ (Str 1) | plex | | | | | | Video +-----+ | +----+ | +-----*-----+ TR2 |----- Circuit 3 -|--| TC3|---------+ | Stream 2 |(RTP)| / +----+ (Str 1) | | +-----+ / | +-------+ | / +-----------------\-----------------+ Audio, video, and control \ Tmux is an ephemeral with two signals are carried in frames explicit Stream Descriptors and spanning the circuits. and a Multiplex Descriptor. Figure 6: Multiplexed Termination Scenario - Single to Multiple Terminations (Asterisks * denote the centre of the context) Context +---------------------------------------------+ | +-------+ +-------+ | Cct 1 +----+ | | | | Audio +-----+ ----| TC1|---+ Tmux1 | | Tmux2 +-----*-----| TR1 |----- +----+ | | | | Stream 1 |(RTP)| | | | Data | | +-----+ Cct 2 +----+ | H.226 +-------+ H.223 | | ----| TC2|---+ multi-|(Str 1)| multi-| Control +-----+ +----+ | plex | | plex +-----*-----+ Tctl|----- | | | | | Stream 3 +-----+ Cct 3 +----+ | | | | | ----| TC3|---+ | | | +-----+ +----+ | | | +-----*-----+ TR2 |----- | +-------+ | | Video |(RTP)| | +-------+ Stream 2 +-----+ | | +---------------------------------------------+ Tmux1 has a Multiplex Descriptor and a single data stream. Tmux2 has a Multiplex Descriptor with a single bearer and three explicit Stream Descriptors. Figure 6a: Multiplexed Termination Scenario - Cascaded Multiplexes (Asterisks * denote the centre of the context) Note: this figure does not appear in Rec. H.248.1Groves, et al. Standards Track [Page 20] RFC 3525 Gateway Control Protocol June 2003 Terminations may be created which represent multiplexed bearers, such as an ATM AAL Type 2 bearer. When a new multiplexed bearer is to be created, an ephemeral Termination is created in a Context established for this purpose. When the Termination is subtracted, the multiplexed bearer is destroyed.6.2.1 Termination dynamics The protocol can be used to create new Terminations and to modify property values of existing Terminations. These modifications include the possibility of adding or removing events and/or signals. The Termination properties, and events and signals are described in the ensuing subclauses. An MGC can only release/modify Terminations and the resources that the Termination represents which it has previously seized via, e.g., the Add command.6.2.2 TerminationIDs Terminations are referenced by a TerminationID, which is an arbitrary schema chosen by the MG. TerminationIDs of physical Terminations are provisioned in the Media Gateway. The TerminationIDs may be chosen to have structure. For instance, a TerminationID may consist of trunk group and a trunk within the group. A wildcarding mechanism using two types of wildcards can be used with TerminationIDs. The two wildcards are ALL and CHOOSE. The former is used to address multiple Terminations at once, while the latter is used to indicate to a media gateway that it must select a Termination satisfying the partially specified TerminationID. This allows, for instance, that a MGC instructs a MG to choose a circuit within a trunk group. When ALL is used in the TerminationID of a command, the effect is identical to repeating the command with each of the matching TerminationIDs. The use of ALL does not address the ROOT termination. Since each of these commands may generate a response, the size of the entire response may be large. If individual responses are not required, a wildcard response may be requested. In such a case, a single response is generated, which contains the UNION of all of the individual responses which otherwise would have been generated, with duplicate values suppressed. For instance, given a Termination Ta with properties p1=a, p2=b and Termination Tb withGroves, et al. Standards Track [Page 21] RFC 3525 Gateway Control Protocol June 2003 properties p2=c, p3=d, a UNION response would consist of a wildcarded TerminationId and the sequence of properties p1=a, p2=b,c and p3=d. Wildcard response may be particularly useful in the Audit commands. The encoding of the wildcarding mechanism is detailed in Annexes A and B.6.2.3 Packages Different types of gateways may implement Terminations that have widely differing characteristics. Variations in Terminations are accommodated in the protocol by allowing Terminations to have optional Properties, Events, Signals and Statistics implemented by MGs. In order to achieve MG/MGC interoperability, such options are grouped into Packages, and typically a Termination realizes a set of such Packages. More information on definition of packages can be found in clause 12. An MGC can audit a Termination to determine which Packages it realizes. Properties, Events, Signals and Statistics defined in Packages, as well as parameters to them, are referenced by identifiers (Ids). Identifiers are scoped. For each package, PropertyIds, EventIds, SignalIds, StatisticsIds and ParameterIds have unique name spaces and the same identifier may be used in each of them. Two PropertyIds in different packages may also have the same identifier, etc. To support a particular package the MG must support all properties, signals, events and statistics defined in a package. It must also support all Signal and Event parameters. The MG may support a subset of the values listed in a package for a particular Property or Parameter. When packages are extended, the properties, events, signals and statistics defined in the base package can be referred to using either the extended package name or the base package name. For example, if Package A defines event e1, and Package B extends Package A, then B/e1 is an event for a termination implementing Package B. By definition, the MG MUST also implement the base Package, but it is optional to publish the base package as an allowed interface. If it does publish A, then A would be reported on the Package Descriptor⌨️ 快捷键说明
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