ITC242

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ITC242 Introduction to Data CommunicationsWeek
10Topic 16 Data link control
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Last Week

• Data Transmission
• Describe the difference between analogue and
  digital signals
• Transmission impairments attenuation and noise
  affect signal quality
• Shannons formula provides a theoretical estimate
  of maximum channel capacity

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Last Week

• Data Communication Fundamentals
• The difference between analogue and digital
  transmission
• Digital and analogue encoding techniques
• ASK, FSK, PSK
• NRZ-L, NRZI, Manchester, Differential Manchester
• Asynchronous transmission
• Synchronous transmission

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Topic 16 Data link control

• Learning Objectives
• Describe flow control and error control

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Flow Control

• Necessary when data is being sent faster than it
  can be processed by receiver
• Computer to printer is typical setting
• Can also be from computer to computer, when a
  processing program is limited in capacity

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TCP Flow Control

• receive side of TCP connection has a receive
  buffer

• speed-matching service matching the send rate to
  the receiving apps drain rate

• app process may be slow at reading from buffer

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TCP Flow control how it works

• Rcvr advertises spare room by including value of
  RcvWindow in segments
• Sender limits unACKed data to RcvWindow
• guarantees receive buffer doesnt overflow

• (Suppose TCP receiver discards out-of-order
  segments)
• spare room in buffer
• RcvWindow
• RcvBuffer-LastByteRcvd - LastByteRead

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Error Control Process

• All transmission media have potential for
  introduction of errors
• All data link layer protocols must provide method
  for controlling errors
• Error control process has two components
• Error detection
• Error correction

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Error Detection Parity Bits

• Bit added to each character to make all bits add
  up to an even number (even parity) or odd number
  (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
  character12.5)

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Error Detection

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

otherwise
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Parity Checking
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
0
0
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Internet checksum (review)

• Goal detect errors (e.g., flipped bits) in
  transmitted packet (note used at transport layer
  only)

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless?

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts checksum value into UDP checksum
  field

Complement 1011010100111101
1011101101010101 1000111100001100 0100101011000010
0110011001100000 0101010101010101 1011101110110101
0110011001100000 0101010101010101 1000111100001100
1111111111111111
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Error Detection Cyclic Redundancy Check (CRC)

• Data in frame treated as a single binary number,
  divided by a unique prime binary, and remainder
  is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33-bit
  divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2-N
• Overhead is low (1-3)

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Checksumming Cyclic Redundancy Check

• view data bits, D, as a binary number
• choose r1 bit pattern (generator), G
• goal choose r CRC bits, R, such that
• ltD,Rgt exactly divisible by G (modulo 2)
• receiver knows G, divides ltD,Rgt by G. If
  non-zero remainder error detected!
• can detect all burst errors less than r1 bits
• widely used in practice (802.11 WiFi, ATM)

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CRC Example

• Want
• D.2r XOR R nG
• equivalently
• D.2r nG XOR R
• equivalently
• if we divide D.2r by G, want remainder R

D.2r G
R remainder
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Error Correction

• Two types of errors
• Lost frame
• Damaged frame
• Automatic Repeat reQuest (ARQ)
• Error detection
• Positive acknowledgment
• Retransmission after time-out
• Negative acknowledgment and retransmission