network protocols handbook.pdf

127Protocols GuideVoice Over IP(VOIP) - SignallingProtocol NameMegaco/H.248: Media GatewayControl ProtocolProtocol DescriptionMegaco/H.248, the Media Gateway Control Protocol, is forthe control of elements in a physically decomposed multimediagateway, enabling the separation of call control from mediaconversion. The Media Gateway Control Protocol (Megaco) isa result of joint efforts of the IETF and the ITU-T Study Group16. Therefore, the IETF defined Megaco is the same as ITU-TRecommendation H.248.Megaco/H.248 addresses the relationship between the MediaGateway (MG), which converts circuit-switched voice to packetbasedtraffic, and the Media Gateway Controller (MGC, sometimescalled a call agent or softswitch, which dictates the servicelogic of that traffic). Megaco/H.248 instructs an MG to connectstreams coming from outside a packet or cell data network ontoa packet or cell stream such as the Real-Time Transport Protocol(RTP). Megaco/H.248 is essentially quite similar to MGCPfrom an architectural standpoint and the controller-to-gatewayrelationship, but Megaco/H.248 supports a broader range ofnetworks, such as ATM.There are two basic components in Megaco/H.248: terminationsand contexts. Terminations represent streams enteringor leaving the MG (for example, analog telephone lines, RTPstreams, or MP3 streams). Terminations have properties, suchas the maximum size of a jitter buffer, which can be inspectedand modified by the MGC.Terminations may be placed into contexts, which are defined asoccuring when two or more termination streams are mixed andconnected together. The normal, “active” context might have aphysical termination (say, one DS0 in a DS3) and one ephemeralone (the RTP stream connecting the gateway to the network).Contexts are created and released by the MG under commandof the MGC. A context is created by adding the first termination,and is released by removing (subtracting) the last termination.A termination may have more than one stream, and therefore acontext may be a multistream context. Audio, video, and datastreams may exist in a context among several terminations.Protocol StructureAll Megaco/H.248 messages are in the format of ASN.1 textmessages. Megaco/H.248 uses a series of commands to manipulateterminations, contexts, events, and signals. The followingis a list of the commands:1. Add. - The Add command adds a termination to a context.The Add command on the first Termination in aContext is used to create a Context.2. Modify - The Modify command modifies the properties,events and signals of a termination.3. Subtract - The Subtract command disconnects a Terminationfrom its Context and returns statistics on theTermination’s participation in the Context. The Subtractcommand on the last Termination in a Context deletesthe Context.4. Move - The Move command atomically moves a Terminationto another context.5. AuditValue - The AuditValue command returns the currentstate of properties, events, signals and statistics ofTerminations.6. AuditCapabilities - The AuditCapabilities command returnsall the possible values for Termination properties,events and signals allowed by the Media Gateway.7. Notify - The Notify command allows the Media Gatewayto inform the Media Gateway Controller of the occurrenceof events in the Media Gateway.8. ServiceChange - The ServiceChange Command allowsthe Media Gateway to notify the Media GatewayController that a Termination or group of Terminationsis about to be taken out of service or has just been returnedto service. ServiceChange is also used by theMG to announce its availability to an MGC (registration),and to notify the MGC of impending or completed restartof the MG. The MGC may announce a handover to theMG by sending it ServiceChange command. The MGCmay also use ServiceChange to instruct the MG to takea Termination or group of Terminations in or out of service.All of these commands are sent from the MGC to the MG, althoughServiceChange can also be sent by the MG. The Notifycommand, with which the MG informs the MGC that one of theevents the MGC was interested in has occurred, is sent by theMG to the MGC.Related protocolsRTP, RTSP, SIP, H.323, MGCPSponsor SourceMegaco/H.248 v1 is defined by IETF (www.ietf.org ) and ITU-T.Megaco/H.248 version 2 is in drafting status.Referencehttp://www.javvin.com/protocol/rfc3525.pdfGateway Control Protocol Version 1http://www.javvin.com/protocol/megaco-h248v2.pdfThe Megaco/H.248 Gateway Control Protocol, version 2