Transcript Chapter 4 Data Link Layer - Indiana State University

Chapter 4 Data Link Layer
MIS 430
Chapter 4
Data Link Layer Functions
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Media access control – when computers
can transmit
Detects and corrects transmission errors
Identifies the start and end of
messages
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I. Media Access Control
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Def’n – method to control when
computers can transmit on same circuit
(not needed with full duplex circuit)
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Controlled Access
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XON/XOFF:
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if ready to receive, sends XON character
If not ready, sends XOFF character
Hardware: DTR line in serial port
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Media Access Control, contd
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Polling: send a signal to a client that
allows it to transmit
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Roll-call: server works consecutively
through the clients
Hub polling (aka token passing): a
computer starts the poll, sends it message,
then passes the token to next computer;
can transmit only if it holds the token
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Media Access Control, contd
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Contention
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Wait until free, then broadcast
802.3 (Ethernet) uses contention
Can have collisions
Relative performance: response time
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Contention better for small networks (~20)
Polling better for larger networks with more
transmissions
See Fig 4.1 p. 118 for comparison chart
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II. Error Control
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Human errors
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Network errors
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Control thru the application program
Corrupted data
Lost data
Networks should prevent, detect, and
correct corrupted data
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Prevention is much preferred!
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Source of Errors
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Line noise/distortion
Line outages: cuts! Faulty equipment
White noise: background hiss
Impulse noise: spikes
Cross-talk: signal picked up in another circuit
Echo: poor connections
Attenuation: loss of signal power over distance
Intermodulation: several circuits combine
Jitter: phase changes can cause volume fluctuation
Harmonic distortion: amp doesn’t reproduce
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Error Prevention
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Shielding
Move cables away from noise
Change mux technology (crosstalk)
Replace repeater/amplifier or put closer
together
Purchase conditioned circuit (better
quality lines) from common carrier
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Error Detection
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Send extra data with each packet so that
receiving side checks message received with
message sent
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Parity checking
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Make number of ones an even or odd number by adding
another bit to byte (50% accuracy)
LRC: longitudinal redundancy checking
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Add block check character to packet
Similar to parity through packet
98% error detection rate
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Error Detection
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Polynomial Checking
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Based on mathematical algorithm
CRC: Cylic Redundancy Check adds 8, 16,
24, or 32 bits to message
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CRC-16: 99.998% error detection
CRC-32: 99.99999998% error detection!
For example, see next slide..
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UPC Code: CRC Check Digit
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UPC: 12345 67890 + check digit
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12345 = is manufacturer code
67890 = is product code for that manufacturer
Algorithm:
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Starting from right, add odd digits
Multiply sum by 3
Starting from right, add even digits
Add to previous number
Check digit is ten’s complement of 1s digit
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UPC Example: checksum
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Suppose UPC is: 12345 67890
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0+8+6+4+2=20
20*3=60
9+7+5+3+1=25
60+25=85
10-5=5 which is the check digit added to end
Scan product, calculate check digit
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If same as with packet, OK
If not same as with packet, need to rescan
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Correction: Retransmission
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If error is detected, correction occurs
when packet is retransmitted
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ARQ – Automatic Repeat reQuest
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Stop and Wait ARQ: ACK/NAK
Continuous ARQ: send next packet unless get a
NAK
Both of these are flow control techniques
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Forward Error Correction
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Add redundant bits to correct errors
without retransmitting packet
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Hamming Code (corrects 1-bit errors)
Reed-Solomon (corrects longer errors)
RAID: redundant array of inexpensive
disks uses forward error correction
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RAID 5: 3 drives = 2 drives
(18+18+18=36 GB with redundancy)
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III. Data Link Protocols
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Asynchronous transmission
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Start-stop: each character sent independently of
other characters
Start bit, 7 data bits, stop bit, parity or 10 bits per
character (2/8 overhead)
Asynchronous file transfer protocols
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Xmodem: CRC-8 with 132 char packets
Kermit: CRC-24 with 1000 char packets
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Data Link Protocols
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Synchronous Transmission
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Message sent in a block (packet) w/ checksum
Less overhead – sync characters at start and end
of packet rather than for each character
SDLC: Synchronous Data Link Control – IBM
3270 standard
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Bit-oriented protocol for 3270 terminals
3270 represents IBM mainframe connection
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PC can do this with 3270/IRMA card
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Synchronous Transmission
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HDLC: High-level data link control
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ISO standard, very similar to SDLC
Ethernet (802.3)
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Similar to SDLC but length is carried along
with other signal characters
CRC-32 plus up to 1492 byte packets:
Dest Addr|Source Addr|Length|Control|…message…|CRC-32
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Synchronous Transmission
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PPP: Point to Point Protocol
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1990s: dial-up networking to ISP
CRC-16 plus packet up to 1,500 bytes:
Flag|Address|Control|Protocol|…message…|CRC-16|Flag
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IV. Transmission Efficiency
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How many overhead bits are needed
beyond the information bits?
TE=total # information bits/total # bits
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Ex: 7-bit ASCII asynchronous
3 overhead bits, 7 data bits
TE=7/10=70%
Thus V.90 56K maximum is 43.6 Kbps
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Improving Efficiency
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Increase size of message in packet
Decrease number of overhead bits
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But CRC will detect errors
Problem:If a much longer packet has an error and must be
retransmitted, this reduces efficiency!
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TRIB Calculation
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TRIB=K(M-C)(1-P)
(M/R)+T
K=information bits/character 7
M=packet length in characters 400
R=data transmission rate in char/sec 600
C=avg number of noninformation char/block 10
P=probability that a block will require retransmission .01
T=time between in blocks in seconds .025 sec
Example using values above:
TRIB=7(400-10)(1-.01)/[400/600+.025]=3,908 bps
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Mgt Focus 4-2: Packet Size
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Standard Commercial tested packet size
from 500 to 32,000 bytes
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32,000 byte more 44% more efficient but
response time delays occurred
Ideal packet size was between 4,000 and
8,000 bytes
However, this depends on the application
and message pattern
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