Chapter 4 Data Link Layer - Indiana State University
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Transcript Chapter 4 Data Link Layer - Indiana State University
Chapter 4 Data Link Layer
MIS 430
Chapter 4
Data Link Layer Functions
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
Def’n – method to control when
computers can transmit on same circuit
(not needed with full duplex circuit)
Controlled Access
XON/XOFF:
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
Polling: send a signal to a client that
allows it to transmit
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
Contention
Wait until free, then broadcast
802.3 (Ethernet) uses contention
Can have collisions
Relative performance: response time
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
Human errors
Network errors
Control thru the application program
Corrupted data
Lost data
Networks should prevent, detect, and
correct corrupted data
Prevention is much preferred!
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Source of Errors
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
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
Send extra data with each packet so that
receiving side checks message received with
message sent
Parity checking
Make number of ones an even or odd number by adding
another bit to byte (50% accuracy)
LRC: longitudinal redundancy checking
Add block check character to packet
Similar to parity through packet
98% error detection rate
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Error Detection
Polynomial Checking
Based on mathematical algorithm
CRC: Cylic Redundancy Check adds 8, 16,
24, or 32 bits to message
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
UPC: 12345 67890 + check digit
12345 = is manufacturer code
67890 = is product code for that manufacturer
Algorithm:
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
Suppose UPC is: 12345 67890
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
If same as with packet, OK
If not same as with packet, need to rescan
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Correction: Retransmission
If error is detected, correction occurs
when packet is retransmitted
ARQ – Automatic Repeat reQuest
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
Add redundant bits to correct errors
without retransmitting packet
Hamming Code (corrects 1-bit errors)
Reed-Solomon (corrects longer errors)
RAID: redundant array of inexpensive
disks uses forward error correction
RAID 5: 3 drives = 2 drives
(18+18+18=36 GB with redundancy)
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III. Data Link Protocols
Asynchronous transmission
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
Xmodem: CRC-8 with 132 char packets
Kermit: CRC-24 with 1000 char packets
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Data Link Protocols
Synchronous Transmission
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
Bit-oriented protocol for 3270 terminals
3270 represents IBM mainframe connection
PC can do this with 3270/IRMA card
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Synchronous Transmission
HDLC: High-level data link control
ISO standard, very similar to SDLC
Ethernet (802.3)
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
PPP: Point to Point Protocol
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
How many overhead bits are needed
beyond the information bits?
TE=total # information bits/total # bits
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
Increase size of message in packet
Decrease number of overhead bits
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
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
Standard Commercial tested packet size
from 500 to 32,000 bytes
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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