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Chapter 4 Data Link Layer. MIS 430. Data Link Layer Functions. Media access control – when computers can transmit Detects and corrects transmission errors Identifies the start and end of messages. I. Media Access Control. - PowerPoint PPT Presentation
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Chapter 4
Chapter 4 Data Link Layer
MIS 430
Chapter 4 2
Data Link Layer Functions
Media access control – when computers can transmit
Detects and corrects transmission errors
Identifies the start and end of messages
Chapter 4 3
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
Chapter 4 4
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
Chapter 4 5
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
Chapter 4 6
II. Error Control Human errors
Control thru the application program Network errors
Corrupted data Lost data
Networks should prevent, detect, and correct corrupted data Prevention is much preferred!
Chapter 4 7
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
Chapter 4 8
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
Chapter 4 9
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
Chapter 4 10
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..
Chapter 4 11
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
Chapter 4 12
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
Chapter 4 13
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
Chapter 4 14
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)
Chapter 4 15
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
Chapter 4 16
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
Chapter 4 17
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
Chapter 4 18
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
Chapter 4 19
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
Chapter 4 20
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!
Chapter 4 21
TRIB Calculation TRIB=K(M-C)(1-P) (M/R)+TK=information bits/character 7M=packet length in characters 400R=data transmission rate in char/sec 600C=avg number of noninformation char/block 10P=probability that a block will require retransmission .01T=time between in blocks in seconds .025 secExample using values above:
TRIB=7(400-10)(1-.01)/[400/600+.025]=3,908 bps
Chapter 4 22
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