APS审核—数据通信.doc

数据通信Data CommunicationContent1 Overview, introduction to computer networks, signal encoding methods2 Data link control protocols: HDLC, ARQ, Error Detection (Parity, CRC)3 Error correction (CRC Digital Logic, Hamming, PacketBased FEC, Convolutional Codes)4 Medium Access Control Protocols5 Ethernet and IEEE 802.116 Hubs/Bridges/SwitchesApplication Layer Web browsers Firefox, IE, Chrome, Opera, etc Web servers Email MSN Messenger, Skype, BitTorrent, etc (TCP/UDP) IP Telephony FTP/SSH/Telnet/etcTransport Layer Connection setup and teardown Ensure all packets arrive and inorder Ensure all packets are error free error control Fragment data into packets Controls transmission rate or flow controlNetwork Layer Logical addressing Routing Determine topology Compute best route to destination Multicast Best effort packet forwarding Handles delivery errorsData Link Layer Framing Packaging packets into frames Physical addressing Flow control Error control Access control Arbitrate access to a shared mediumPhysical Layer Bits representation Physical characteristics of interfaces and medium Synchronization Data rate Physical topology Transmission mode: simplex, half duplex, full duplexEncoding Schemes Non Return to Zero Level (NRZL) Non Return to Zero Inverted (NRZI) Manchester Differential Manchester 4B/5B 2B/1QNonReturn to Zero Level (NRZL)Rule: 0 (high level), 1 (low level)NonReturn to Zero Inverted (NRZI)Rule: 1 (transition), 0 (stay at current signal) It solves the problem of consecutive 1s, but the problem of consecutive 0s is still thereManchesterDifferential ManchesterRules:1. Always transition in middle of interval.2. If 0, transition at beginning of interval.3. If 1, no transition at beginning of intervalAmplitude, Phase, and Frequency Modulation of a digital baseband signalCyclic Redundancy Check Block based error detection commonly used in link layer networks, e.g., Ethernet Idea: Given a kbit message, generate an nbit frame sequence so that a combined k+n bit frame is evenly divisible by some predefined number. On receipt, no remainder means no error.General Steps1. Choose a width W, and a polyG2. Add W zero bits to the message D, called D3. Divide D by G using CRC arithmetic.4. Remainder is the CRC,5. XOR with D, and send resulting message (D).D” = 101110011Hamming (7,4)Generat