X.25 Protocol

The X.25 ProtocolCCITT Recommendation X.25First Published in 1976Revisions Every 4 Years -- 1980, 1984, 1988Interface Protocol for Packet Switched NetworksNetwork Providers Intended to Be Telcos

The X.25 ProtocolThe ModelNetwork Has Multiple Nodes (DCEs)Host Computers (DTEs) OutsideHosts Have Addresses Like Phone NumbersVirtual Call SetupVirtual Call ClearData TransferDTEDTEDCEDCEDCEDCEDCEX.25X.25X.25Intra-Network ProtocolDTE

The X.25 ProtocolThe X.25 Protocol LayersLayer NameDescription

The X.25 ProtocolPhysical Layer Sort ofFlag Character (01111110) at Beginning and EndData in between is LAPB FrameData Passed up to Frame LevelData Transparency 011111010 011111 10FlagCRCDataFrame Level (LAPB)Flag0

The X.25 ProtocolFrame Level LAPBFrame Level Header Has Frame TypeSequence and Acknowledgement NumbersError Recovery ProceduresEndpoints Are DTE and Local DCELAPB HdrInformation FieldPacket Level

The X.25 ProtocolLAPB HeaderAddressControlInformation FrameSupervisory FramesUnnumbered FramesN(R) is Ack Nr and Counts Modulo 8N(S) is Seq Nr and Counts Modulo 8RR = Receiver ReadyRNR = Receiver Not ReadyREJ = RejectSABM = Link Setup ReqUA = Unnumbered AckDM = Disconnected ModeFRMR = Frame Reject

The X.25 ProtocolLAPB Link Setup and DisconnectSABMUASABM = Set Asynchronous Balanced ModeUA Acknowledges SABMDISC Requests DisconnectUA Acknowledges DISCExchange on Local Link OnlyDISCUANow in Data Transfer ModeNow in Disconnected Mode

The X.25 ProtocolLAPB Data TransferI-Frame #1RR N(R)=2I-Frame Contains PacketSeq from 0 - 7 and back to 0RR Gives Next Expected I-FrameI-Frame Can also AcknowledgeI-Frame #2RR N(R)=3I-Frame #3I-Frame #0 N(R)=4

The X.25 ProtocolThe X.25 Packet LevelPacket Header Has Packet TypeChannel Number Identifies Logical ConnectionSequence and Acknowledgement NumbersNo Error Recovery -- Data Can Be LostAddressing across Multi-Node NetworkPkt HdrDataApplication Layer

The X.25 ProtocolPacket Level HeaderData PacketAcknowledgement PacketsOther PacketsChannel Nr Selects which ConnectionP(R) is Ack Nr and Counts Modulo 8P(S) is Seq Nr and Counts Modulo 8RR = Receiver ReadyRNR = Receiver Not Ready

The X.25 ProtocolCall SetupCall RequestIncoming CallCall AcceptedCall ConnectedInternal ProtocolEach Channel is DistinctSelect Unused ChannelDifferent Channel Numbers on Each EndEnd to End is Virtual CircuitVC = Local Chnl + Network Route + Remote ChnlInternal Network Protocol Not SpecifiedCall Setup is End to EndLocate Remote DCE

The X.25 ProtocolCall ClearingClear RequestClear IndicationClear ConfirmClear ConfirmInternal ProtocolEach Channel is DistinctChannels Become AvailableEnd to End is Virtual CircuitInternal Network Protocol Not SpecifiedClearing May be End to End or LocalClear Packet Used to Report Procedure ErrorsRemote DCE from Call Setup

The X.25 ProtocolData Transfer w/End to End AckData Packet #1Data Packet #1RR P(R)=2RR P(R)=2Internal ProtocolEach Channel is DistinctEnd to End is Virtual CircuitInternal Network Protocol Not SpecifiedEach Data Pkt Has Seq NrEach RR Has Next Expected Seq NrExample Shows End to End AcknowledgementRemote DCE from Call Setup

The X.25 ProtocolData Transfer w/Local AckData Packet #1Data Packet #1RR P(R)=2Internal ProtocolEach Channel is DistinctEnd to End is Virtual CircuitInternal Network Protocol Not SpecifiedEach Data Pkt Has Seq NrEach RR Has Next Expected Seq NrExample Shows Local AcknowledgementRR P(R)=2Data Packet #2RR P(R)=3Data Packet #2RR P(R)=3Remote DCE from Call Setup

The X.25 ProtocolX.25 Without a NetworkTwo DTEs Communicating DirectlyNo Intervening NetworkOne DTE Plays the Role of DCELAPB Ensures ReliabilityAcknowledgements End to EndDTE Addressing Immaterial

The X.25 Protocol

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