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TOBB ETÜ ELE46/ELE563 Communications Networks Lecture 01 May 6, 2014 Fall 2011 Tuesday 10:30 – 12:20 (310) Thursday 15:30 – 17:20 (372) İsrafil Bahçeci Office: 168 ibahceci@etu.edu.tr
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Data Link Layer
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Functions of DLL Frame transmission Reliable: Error free Efficient: Transmission Rate vs. Channel condition What to do? Error correction/detection Channel impairements
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DLL Design Issues Service interface to network layer Dealing with transmission errors Flow control Frame management: Network packets encapsulated
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Data Path 1.Unacknowledged connectionless service -> Reliable channel ->Ethernet 2.Acknowledged connectionless service -> Unreliable channel -> Wi-Fi 3.Acknowledged connection-oriented service
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ACK/NACK issue DLL ACK/NACK is an optimization, not requirements Higher layers may take care of errors PHY medium determines this optimization Fiber, ethernet Wireless Connection-oriented service: reliable streaming Appropriate for unreliable, long channels (satellite, long- distance telephone circuit, etc..) Establish a connection before data exchange Numbered frames Each frames received exactly once, all in order 3 phases Connection establishment, initialize counters Frame transmission Connection release
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Framing Bit stream from PHY layer may contain errors DLL splits bit streams in to discrete frames Checksum: CRC = cyclic redundancy check bits Receiver calculates the checksum bits and compares it with the one contained in the frame Bad frames detection
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Frame generation Must alleviate the frame reception (e.g., sync) at the receiver side Byte-count Number of bytes in the header Errors in this info is catastrpohic Flag bytes with byte stuffing Frame start at certain slots indicate by a known flag byte (start and end with FLAGs) If FLAG exists in paylod, insert ESC charcater; ESC is removed before forwarding to network layer PPP: point-to-point protocol Flag bits with bit stuffing Physical layer coding violation
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Byte FLAGing
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Bit FLAGing Bit level flagging Bit stuffing Ex. HDLC – high level data link control Frame begins with 0x7E : 01111110 Bit sutffing: a consecutive 5 1s stuffed with 0
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Frame Length Data based frame length ~12.5% with bit stuffing Ethernet Preamble: all frames start with a known bit pattern 72-bits for 802.11
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Error Control ACK/NACK ACK: correct reception NACK: error!, retransmit Complete ACK/frame loss Timers: Time to wait for ACK/NACK Timer cancellation if ACK arrives before Multiple transmission of frames: Frame numbering Overall goal: Each frame is only passed only once to the network layer
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Flow Control How to control frame rate so that receiver can accept transmissions Sender: fast processors, receiver: slow processor HD Video to a smartphone with small computing power! Feedback-based flow control Rx to Tx: send more/less data Rate-based flow control Tx-built in flow control without feedback Usually feed-back based in DLL: Rx side determines the transmission of additional data until it processes the previously received ones
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Error Detection/Correction Fiber, cable: small error Wireless links: larger errors Add redundancy for correction FER: forward error correction Enough redundancy for detection Depends on channel reliability Fiber: error detection Wireless: error correction + detection Error types Single-bit errors, few-bit errors Burst errors Erasure channel: easier to correct than flip-errors FEC/ED can be used in PHY,DLL and network layers as well
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FEC: error correction codes Hamming Binary convolutional Reed-Solomon Low-density parity check codes
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FEC Frame length: m bits Redundancy: r bits Linear block code: r bits from m information bits via linear combination (e.g., in mod-2 for binary codes) of information block Systematic code: m bits are sent directly Code rate = m/(m+r)
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Hamming distance Distance between two sequences Binary: seq_d = seq1 XOR seq2 Number of 1s in seq_d Hamming distance of d -> d single bit errors
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Hamming Code Linear block code Error syndrome
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Convolutional Code
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Decoding Soft-decision Hard-decision
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Cyclic codes A subset of linear block codes Non-binary cyclic codes BCH codes Reed-Solomon codes
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Error Detection Codes Linear systematic block codes Parity Checksums Cyclic redundancy checks (CRCs)
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