2Media Gateway Control References for Megaco/H.248 RFC requirementsRFC 3015 (Megaco) or ITU-T Rec. H.248H.248 Annexes F to KHistory and tutorial:T. Taylor, "Megaco/H.248: A New Standard For Media Gateway Control", IEEE Communications Magazine, October 2000.
3Media gateway control vs. call signalling SIP-T, ISUP in H.323, Q.BICCSGMGCMGCSGSIPSIPUser AgentPSTNPSTNH.323 callsignallingGatewaycontrolprotocolGatewaycontrolprotocolH.323EndpointMGMGCall signallingMedia gateway control signallingMedia flows
4Gateway control family tree SGCPEarly 1998IPDCASPENLate 1998MGCP(I-RFC 2705)MDCP1999(Standard in 2000)Megaco/H.248
5Gateway control functions ConnectionBearer MediationMedia TranscodingEvents and SignalsContextTerminationsThe context supportsmultiple media streamsif required for multimediaservice.
6Basic conceptsConnection model: terminations, streams, and the contextTermination properties: descriptorsContext propertiesMessage structure: transactions, actions, and commandsEvent and signal processingPackages
7Media flow connections Connections achieved by placing two or more terminations into a common context.Context viewed as mixing bridgeTermination = source or sink of media flowsprovisioned vs. ephemeral terminationsprovisioned terminations reside in “NULL context” when not activeFlows specified by streamcommon streamID implies connected flowsdefault streamID = 1 to simplify audio-only operationTransport, medium, encoding/decoding specified per stream at each terminationFlows are uni-directional. For a given termination, the convention is that the "send" direction is from the context outwards and the "receive" direction is from the exterior into the context.Streams are bidirectional, thus support up to one active flow in each direction.
10DescriptorsProperties of terminations are organized syntactically into descriptorsbasic ones are Termination State, Media, Events, and Signals descriptorsMedia descriptor actually composed of other descriptors: Stream descriptors, which in turn contain LocalControl, Local, and Remote descriptorsDefault property values can be configured in the MGThe TerminationState descriptor describes termination properties which are independent of any media streams the termination supports at a given time.The Media descriptor describes the streams currently supported by the termination. Streams can be added or deleted at any time. Each stream is described by a separate Stream descriptor. If only one stream is supported, the Stream level of syntax may optionally be omitted, and the StreamID is assumed to be 1.Each Stream descriptor contains a LocalControl, a Local, and a Remote descriptor.The LocalControl descriptor contains properties which relate to the stream as a whole rather than individual directions.The Local descriptor specifies the media flow in the "send" direction, while the Remote descriptor does the same for the media flow in the "receive" direction. When text encoding of the Megaco/H.248 message is being used, the syntax of both descriptors is based on the Session Description Protocol (SDP).Default property values apply to a termination between the time it is created and or it is returned to the NULL context through subtraction and the time the MGC specifies over-riding values. However, the MGC may modify these properties at any time, even in the same transaction
11Context properties more could be added in the future currently three properties can be specified for a contextmore could be added in the futuretopology descriptor allows detailed specification of connectivity between individual pairs of terminationsuseful for legal interceptionpriority flag can guide MG's allocation of scarce resourcesemergency flag can indicate contexts which must be maintained and restored in the event of failuresOne possible future addition is a video bridging property, indicating the algorithm used for mixing video in multi-party contexts. Examples: current speaker only, mosaic of N most recent speakers, etc.
12Null context and ROOT termination Null context introduced as a conventionwhere persistent terminations are held when they are not in a real contextWhen terminations are returned to the null context, they take on their configured default property values.ROOT termination represents the MG itselfuseful for specifying properties of the MG as a device, and to refer to the MG in commands like AuditCapability and ServiceChange
13Message structure Megaco/H.248 message Header Transaction Transaction TransactionReq or Reply Req or Reply Req or ReplyTrans Hdr Action ActionCtx Hdr Ctx Properties Command CommandCmd Hdr Descriptor Descriptor
14Commands Megaco/H.248 provides the following commands For termination manipulation: Add, Subtract, Move, ModifyFor event reporting: NotifyFor management: AuditCapability, AuditValue, ServiceChange
15Events Events are detected at the MG and reported to the MGC example: in-band signallingMGC controls what events it wants to learn about at any given timesets the termination Events descriptorEvents can have side effectsstop playout of signalsstart new signalsautomatically update the set of events of interest
16Signals Signals cause things to happen on terminations play a tone, display text, ...Specified in the Signals descriptor for a terminationMGC can specify duration of signal ahead of time or signal can play until explicitly stoppedSignals stop playing when any event is detected unless MGC says otherwise.
17Packages Add detailed content to the protocol all events, signals, and statistics are specified in packagescan also specify additional propertiesPackage definition a continuing processbeing created by multiple standards bodiesprivate packages also allowedPackages can inherit from and extend other packages.