Presentation on theme: "An Introduction to Megaco/H.248 Tom Taylor"— Presentation transcript:
An Introduction to Megaco/H.248 Tom Taylor
PTT Megaco/H Media Gateway Control References for Megaco/H.248 RFC requirements RFC 3015 (Megaco) or ITU-T Rec. H.248 H.248 Annexes F to K History and tutorial: T. Taylor, "Megaco/H.248: A New Standard For Media Gateway Control", IEEE Communications Magazine, October 2000.
PTT Megaco/H Media gateway control vs. call signalling SG MG MGC SG MG MGC SIP User Agent H.323 Endpoint PSTN Call signalling Media gateway control signalling Media flows SIP-T, ISUP in H.323, Q.BICC SIP H.323 call signalling Gateway control protocol Gateway control protocol
PTT Megaco/H Gateway control family tree IPDC SGCP MGCP (I-RFC 2705) Megaco/H.248 MDCP Early 1998 Late (Standard in 2000) ASPEN
PTT Megaco/H Gateway control functions Connection Bearer Mediation Media Transcoding Context Terminations Events and Signals The context supports multiple media streams if required for multimedia service.
PTT Megaco/H Basic concepts Connection model: terminations, streams, and the context Termination properties: descriptors Context properties Message structure: transactions, actions, and commands Event and signal processing Packages
PTT Megaco/H Media flow connections Connections achieved by placing two or more terminations into a common context. Context viewed as mixing bridge Termination = source or sink of media flows –provisioned vs. ephemeral terminations –provisioned terminations reside in NULL context when not active Flows specified by stream –common streamID implies connected flows –default streamID = 1 to simplify audio-only operation Transport, medium, encoding/decoding specified per stream at each termination
PTT Megaco/H Context example: multimedia Stream=1, medium=audio Stream=1, medium=audio Stream=2, medium=video Stream=2, medium=video T1 T2 T3
PTT Megaco/H Descriptors Properties of terminations are organized syntactically into descriptors –basic ones are Termination State, Media, Events, and Signals descriptors –Media descriptor actually composed of other descriptors: Stream descriptors, which in turn contain LocalControl, Local, and Remote descriptors Default property values can be configured in the MG
PTT Megaco/H Context properties currently three properties can be specified for a context –more could be added in the future topology descriptor allows detailed specification of connectivity between individual pairs of terminations –useful for legal interception priority flag can guide MG's allocation of scarce resources emergency flag can indicate contexts which must be maintained and restored in the event of failures
PTT Megaco/H Null context and ROOT termination Null context introduced as a convention –where persistent terminations are held when they are not in a real context When terminations are returned to the null context, they take on their configured default property values. ROOT termination represents the MG itself –useful for specifying properties of the MG as a device, and to refer to the MG in commands like AuditCapability and ServiceChange
PTT Megaco/H Message structure Megaco/H.248 message Header Transaction Transaction... Transaction Req or Reply Req or Reply Req or Reply Trans Hdr Action... Action Ctx Hdr Ctx Properties Command... Command Cmd Hdr Descriptor... Descriptor
PTT Megaco/H Commands Megaco/H.248 provides the following commands For termination manipulation: Add, Subtract, Move, Modify For event reporting: Notify For management: AuditCapability, AuditValue, ServiceChange
PTT Megaco/H Events Events are detected at the MG and reported to the MGC –example: in-band signalling MGC controls what events it wants to learn about at any given time –sets the termination Events descriptor Events can have side effects –stop playout of signals –start new signals –automatically update the set of events of interest
PTT Megaco/H Signals Signals cause things to happen on terminations –play a tone, display text,... Specified in the Signals descriptor for a termination MGC can specify duration of signal ahead of time or signal can play until explicitly stopped Signals stop playing when any event is detected unless MGC says otherwise.
PTT Megaco/H Packages Add detailed content to the protocol –all events, signals, and statistics are specified in packages –can also specify additional properties Package definition a continuing process –being created by multiple standards bodies –private packages also allowed Packages can inherit from and extend other packages.