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
Data Link Layer
Functions of DLL
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Frame transmission
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Reliable: Error free
Efficient: Transmission Rate vs. Channel
condition
What to do?
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Error correction/detection
Channel impairements
DLL Design Issues
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Service interface to network layer
Dealing with transmission errors
Flow control
Frame management: Network packets
encapsulated
Data Path
1.
2.
3.
Unacknowledged connectionless service -> Reliable channel ->Ethernet
Acknowledged connectionless service -> Unreliable channel -> Wi-Fi
Acknowledged connection-oriented service
ACK/NACK issue
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DLL ACK/NACK is an optimization, not requirements
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PHY medium determines this optimization
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Higher layers may take care of errors
Fiber, ethernet
Wireless
Connection-oriented service: reliable streaming
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Appropriate for unreliable, long channels (satellite, longdistance telephone circuit, etc..)
Establish a connection before data exchange
Numbered frames
Each frames received exactly once, all in order
3 phases
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Connection establishment, initialize counters
Frame transmission
Connection release
Framing
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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
Frame generation

Must alleviate the frame reception (e.g., sync) at
the receiver side
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Byte-count
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Flag bytes with byte stuffing
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Number of bytes in the header
Errors in this info is catastrpohic
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
Byte FLAGing
Bit FLAGing
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Bit level flagging
Bit stuffing
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Ex. HDLC – high level data link control
Frame begins with 0x7E : 01111110
Bit sutffing: a consecutive 5 1s stuffed with
0
Frame Length
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Data based frame length
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~12.5% with bit stuffing
Ethernet
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Preamble: all frames start with a known bit
pattern
72-bits for 802.11
Error Control

ACK/NACK
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Complete ACK/frame loss
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ACK: correct reception
NACK: error!, retransmit
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
Flow Control
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How to control frame rate so that receiver
can accept transmissions
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Sender: fast processors, receiver: slow processor
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Feedback-based flow control
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Rx to Tx: send more/less data
Rate-based flow control
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HD Video to a smartphone with small computing
power!
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
Error Detection/Correction
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Fiber, cable: small error
Wireless links: larger errors
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Add redundancy for correction
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Fiber: error detection
Wireless: error correction + detection
Error types
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Enough redundancy for detection
Depends on channel reliability
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FER: forward error correction
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
FEC: error correction codes
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Hamming
Binary convolutional
Reed-Solomon
Low-density parity check codes
FEC
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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
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Systematic code: m bits are sent directly
Code rate = m/(m+r)
Hamming distance
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Distance between two sequences
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Binary: seq_d = seq1 XOR seq2
Number of 1s in seq_d
Hamming distance of d -> d single bit
errors
Hamming Code
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Linear block code
Error syndrome
Convolutional Code
Decoding
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Soft-decision
Hard-decision
Cyclic codes
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A subset of linear block codes
Non-binary cyclic codes
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BCH codes
Reed-Solomon codes
Error Detection Codes
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Linear systematic block codes
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Parity
Checksums
Cyclic redundancy checks (CRCs)