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Chapter 4 Data Link Layer

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.

heidi-farmer
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Chapter 4 Data Link Layer

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  1. 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)+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 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 Chapter 4

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