tobb et Ü ele46/ele563 com munications networks lecture 01 may 6, 2014
DESCRIPTION
TOBB ET Ü ELE46/ELE563 Com munications Networks Lecture 01 May 6, 2014. Fall 20 11 Tuesday 10: 30 – 1 2:2 0 ( 310 ) Thursday 15: 30 – 1 7:2 0 ( 372 ) İsrafil Bahçeci Office : 168 ibahcec [email protected]. Data Link Layer. Functions of DLL. Frame transmission Reliable: Error free - PowerPoint PPT PresentationTRANSCRIPT
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TOBB ETÜ ELE46/ELE563Communications Networks
Lecture 01May 6, 2014
Fall 2011
Tuesday 10:30 – 12:20 (310)Thursday 15:30 – 17:20 (372)
İsrafil BahçeciOffice: 168
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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 ->Ethernet2. 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)