网络实验计算机网络transmission v.pptx

Computer NetworksLecture 6 Jing Xu (徐晶)Dept. of Electronics and Information Eng.Huazhong University of Science and TechnologyJuly. 2012Problem in Chapter2:Physically Connecting Hosts-2-Review: Direct link networkKey components in protocol designEncoding FramingError DetectionReliable CommunicationMultiple Access ControlProtocol samplesComponentsProtocolsBISYNCDDCMPPPPHDLCError Detection2D ParityCRCFramingByte-orientedBit-orientedPHY link characteristicPoint-to-point-3-Lecture 5Chapter 2. Direct Link NetworksProblem: Physically Connecting Hosts 2.1 Hardware Building Blocks 2.2 Encoding (NRZ, NRZI, Manchester, 4B/5B) 2.3 Framing 2.4 Error Detection 2.5 Reliable Transmission 2.5.1 Stop-and-Wait 2.5.2 Sliding Window 2.5.3 Concurrent Logical Channels2.6 Ethernet (802.3)2.7 Rings (802.5, FDDI) 2.8 Wireless 2.9 Summary-4-Problems in Direct Link Networks… … … … 11001101110011011111110111001101Error in frame… … … … 11001101110011011111110111111101Frame lostReliable delivery Make a link appear reliable in spite of the fact that it corrupts frames from time to time-5-Reliable TransmissionProblemFrames may be corrupted by bit errors, and be thus discarded/lostFrames may be corrupted totallyBasic solutionACK (acknowledgement): a small control frame that a protocol sends back to its peer for acknowledgement. ACK = Confirm receive, NAK = Confirm lossTimeout: If the sender does not receive an ACK after a reasonable amount of time, then it retransmits the original frameSequence Number: Identify the framesDemo-6-Automatic Repeat reQuest (ARQ)ARQReliable delivery mechanism using ACKs and timeoutsLink layer assumptionsSequential channel – no frame reordering in transitAll frames experience the same propagation delayReceiverAcknowledges correct framesDetects and ignores frames with errorsDetects and drops out-of-bounds framesSenderSend the original frames Retransmit dropped and damaged frames2.5.1 Stop-and-Wait Stop-and-wait algorithm The simplest ARQ schemeAfter transmitting one frame,