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

ibahceci@etu.edu.tr

Data Link Layer

Functions of DLL Frame transmission

Reliable: Error free Efficient: Transmission Rate vs.

Channel condition What to do?

Error correction/detection Channel impairements

DLL Design Issues Service interface to network layer Dealing with transmission errors Flow control Frame management: Network

packets encapsulated

Data Path

1. Unacknowledged connectionless service -> Reliable channel ->Ethernet2. Acknowledged connectionless service -> Unreliable channel -> Wi-Fi 3. Acknowledged connection-oriented service

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

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

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

Byte FLAGing

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

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

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

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

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

FEC: error correction codes Hamming Binary convolutional Reed-Solomon Low-density parity check codes

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)

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

Hamming Code Linear block code Error syndrome

Convolutional Code

Decoding Soft-decision Hard-decision

Cyclic codes A subset of linear block codes Non-binary cyclic codes

BCH codes Reed-Solomon codes

Error Detection Codes Linear systematic block codes

Parity Checksums Cyclic redundancy checks (CRCs)