📄 rfc3015.txt
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5. CONVENTIONS The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119.6. CONNECTION MODEL The connection model for the protocol describes the logical entities, or objects, within the Media Gateway that can be controlled by the Media Gateway Controller. The main abstractions used in the connection model are Terminations and Contexts. A Termination sources and/or sinks one or more streams. In a multimedia conference, a Termination can be multimedia and sources or sinks multiple media streams. The media stream parameters, as well as modem, and bearer parameters are encapsulated within the Termination.Cuervo, et al. Standards Track [Page 11] RFC 3015 Megaco Protocol Version 1.0 November 2000 +------------------------------------------------------+ |Media Gateway | | +-------------------------------------------------+ | | |Context +-------------+ | | | | | Termination | | | | | |-------------| | | | | +-------------+ +->| SCN Bearer |<---+-> | | | Termination | +-----+ | | Channel | | | | | |-------------| | |---+ +-------------+ | | <-+--->| RTP Stream |---| * | | | | | | | | |---+ +-------------+ | | | | +-------------+ +-----+ | | Termination | | | | | | |-------------| | | | | +->| SCN Bearer |<---+-> | | | Channel | | | | | +-------------+ | | | +-------------------------------------------------+ | | | | | | +------------------------------+ | | |Context | | | +-------------+ | +-------------+ | | | | Termination | | +-----+ | Termination | | | | |-------------| | | | |-------------| | | <-+->| SCN Bearer | | | * |------| SCN Bearer |<---+-> | | Channel | | | | | Channel | | | | +-------------+ | +-----+ +-------------+ | | | +------------------------------+ | | | | | | +-------------------------------------------------+ | | |Context | | | | +-------------+ +-------------+ | | | | | Termination | +-----+ | Termination | | | | | |-------------| | | |-------------| | | <-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+-> | | | Channel | | | | Channel | | | | | +-------------+ +-----+ +-------------+ | | | +-------------------------------------------------+ | | ___________________________________________________ | +------------------------------------------------------+ Figure 1: Example of H.248 Connection ModelCuervo, et al. Standards Track [Page 12] RFC 3015 Megaco Protocol Version 1.0 November 2000 A Context is an association between a collection of Terminations. There is a special type of Context, the null Context, which contains all Terminations that are not associated to any other Termination. For instance, in a decomposed access gateway, all idle lines are represented by Terminations in the null Context. Figure 1 above is a graphical depiction of these concepts. The diagram of Figure 1 gives several examples and is not meant to be an all-inclusive illustration. The asterisk box in each of the Contexts represents the logical association of Terminations implied by the Context. The example below shows an example of one way to accomplish a call- waiting scenario in a decomposed access gateway, illustrating the relocation of a Termination between Contexts. Terminations T1 and T2 belong to Context C1 in a two-way audio call. A second audio call is waiting for T1 from Termination T3. T3 is alone in Context C2. T1 accepts the call from T3, placing T2 on hold. This action results in T1 moving into Context C2, as shown below. +------------------------------------------------------+ |Media Gateway | | +-------------------------------------------------+ | | |Context C1 | | | | +-------------+ +-------------+ | | | | | Term. T2 | +-----+ | Term. T1 | | | | | |-------------| | | |-------------| | | <-+--->| RTP Stream |---| * |------| SCN Bearer |<---+-> | | | | | | | Channel | | | | | +-------------+ +-----+ +-------------+ | | | +-------------------------------------------------+ | | | | +-------------------------------------------------+ | | |Context C2 | | | | +-------------+ | | | | +-----+ | Term. T3 | | | | | | | |-------------| | | | | | * |------| SCN Bearer |<---+-> | | | | | Channel | | | | | +-----+ +-------------+ | | | +-------------------------------------------------+ | +------------------------------------------------------+ Figure 2: Example Call Waiting Scenario / Alerting Applied to T1Cuervo, et al. Standards Track [Page 13] RFC 3015 Megaco Protocol Version 1.0 November 2000 +------------------------------------------------------+ |Media Gateway | | +-------------------------------------------------+ | | |Context C1 | | | | +-------------+ | | | | | Term. T2 | +-----+ | | | | |-------------| | | | | <-+--->| RTP Stream |---| * | | | | | | | | | | | | | +-------------+ +-----+ | | | +-------------------------------------------------+ | | | | +-------------------------------------------------+ | | |Context C2 | | | | +-------------+ +-------------+ | | | | | Term. T1 | +-----+ | Term. T3 | | | | | |-------------| | | |-------------| | | <-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+-> | | | Channel | | | | Channel | | | | | +-------------+ +-----+ +-------------+ | | | +-------------------------------------------------+ | +------------------------------------------------------+ Figure 3. Example Call Waiting Scenario / Answer by T16.1 Contexts A Context is an association between a number of Terminations. The Context describes the topology (who hears/sees whom) and the media mixing and/or switching parameters if more than two Terminations are involved in the association. There is a special Context called the null Context. It contains Terminations that are not associated to any other Termination. Terminations in the null Context can have their parameters examined or modified, and may have events detected on them. In general, an Add command is used to add Terminations to Contexts. If the MGC does not specify an existing Context to which the Termination is to be added, the MG creates a new Context. A Termination may be removed from a Context with a Subtract command, and a Termination may be moved from one Context to another with a Move command. A Termination SHALL exist in only one Context at a time.Cuervo, et al. Standards Track [Page 14] RFC 3015 Megaco Protocol Version 1.0 November 2000 The maximum number of Terminations in a Context is a MG property. Media gateways that offer only point-to-point connectivity might allow at most two Terminations per Context. Media gateways that support multipoint conferences might allow three or more terminations per Context.6.1.1 Context Attributes and Descriptors The attributes of Contexts are: * ContextID. * The topology (who hears/sees whom). The topology of a Context describes the flow of media between the Terminations within a Context. In contrast, the mode of a 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. * 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.Cuervo, et al. Standards Track [Page 15] RFC 3015 Megaco Protocol Version 1.0 November 2000 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. Signals are MG generated media streams such as tones and announcements as well as line signals such as hookswitch. 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 section 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 Termination. The Terminations that source/sink the digital channels are connected to a separate Termination called the multiplexing Termination. This Termination describes the multiplex used (e.g. how the H.221 frames are carried over the digital channels used). The MuxDescriptor is used to this end. If multiple media are carried, this Termination contains multiple StreamDescriptors. The media streams can be associated with streams sourced/sunk by other Terminations in the Context. Terminations may be created which represent multiplexed bearers, such as an ATM 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.⌨️ 快捷键说明
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