network protocols handbook.pdf

126Protocols GuideVoice Over IP(VOIP) - SignallingProtocol NameH.245: Control Protocol forMultimedia CommunicationProtocol DescriptionH.245, a control signaling protocol in the H.323 multimedia communicationarchitecture, is for of the exchange of end-to-endH.245 messages between communicating H.323 endpoints/terminals. The H.245 control messages are carried over H.245control channels. The H.245 control channel is the logical channel0 and is permanently open, unlike the media channels. Themessages carried include messages to exchange capabilities ofterminals and to open and close logical channels.After a connection has been set up via the call signaling procedure,the H.245 call control protocol is used to resolve the callmedia type and establish the media flow, before the call can beestablished. It also manages the call after it has been established.The steps involved are:• Master-slave determination process. This is used to determinethe master of the call and is useful for avoidingconflicts during call control operations.• Capability exchange procedure. Each endpoint notifiesthe other what kind of information it is capable of receivingand transmitting through the receive and transmit capabilities.• Logical channel procedures. Used for opening and closinglogical channels, which are multiplexed paths betweenthe endpoints used for data transfer.• Request mode command. Using this command, at anypoint during the conference, the receiving endpoint canrequest a change in mode of the transmitted informationprovided the mode is in the transmit capability of thetransmitter.• Control flow command. This can be used by the receiverto fix an upper limit for the transmitter bit rate on any logicalchannel.• Communication mode messages. Used by the multipointcontroller for selecting a common mode of operation in amultipoint conference.• Conference request and response messages. Used forcontrolling a multipoint conference, e.g. password requests,conference chair control.• Round trip delay commands. Used for measuring theround-trip delay between two endpoints on the controlchannel.• Video fast update command. Used for requesting updatesfor video frames, in case of data loss.• End session command. After this command the endpointsclose all logical channels, drop the call and informthe gatekeeper about the end of the call.Protocol StructureH.245 messages are in ASN.1 syntax. MultimediaSystemControlMessagetypes can be defined as request, response, commandand indication messages. Key H.245 messages are asfollows:MessageFunctionMaster-Slave DeterminationTerminal Capability SetOpen Logical ChannelClose Logical ChannelRequest ModeSend Terminal CapabilitySetEnd Session CommandDetermines which terminal is the masterand which is the slave. Possible replies:Acknowledge, Reject, Release (in case of atime out).Contains information about a terminal’scapability to transmit and receive multimediastreams. Possible replies: Acknowledge,Reject, Release.Opens a logical channel for transport ofaudiovisual and data information. Possiblereplies: Acknowledge, Reject, Confirm.Closes a logical channel between two endpoints.Possible replies: AcknowledgeUsed by a receive terminal to requestparticular modes of transmission from atransmit terminal. General mode typesinclude VideoMode, AudioMode, DataModeand Encryption Mode. Possible replies:Acknowledge, Reject, Release.Commands the far-end terminal to indicateits transmit and receive capabilities by sendingone or more Terminal Capability Sets.Indicates the end of the H.245 session. Aftertransmission, the terminal will not send anymore H.245 messages.Related protocolsRTP, RTSP, SIP, Megaco, H.248, Q.931, H.323, H.225, H.235Sponsor SourceH.245 is an ITU-T (http://www.itu.int/ITU-T/ ) standard.Referencehttp://www.javvin.com/protocol/H245v9.pdfControl Protocol for Multimedia Communication (version 9)http://www.h323forum.org/papers/H.323 papers and documents