Chapter 4 Chapter 4 Data Link Layer MIS 430

2 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

3 Chapter 4 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

4 Chapter 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

5 Chapter 4 Media Access Control, contd Contention Wait until free, then broadcast (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

6 Chapter 4 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!

7 Chapter 4 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

8 Chapter 4 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

9 Chapter 4 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

10 Chapter 4 Error Detection Polynomial Checking Based on mathematical algorithm CRC: Cylic Redundancy Check adds 8, 16, 24, or 32 bits to message CRC-16: % error detection CRC-32: % error detection! For example, see next slide..

11 Chapter 4 UPC Code: CRC Check Digit UPC: check digit = is manufacturer code = 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

12 Chapter 4 UPC Example: checksum Suppose UPC is: =20 20*3= = = =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

13 Chapter 4 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

14 Chapter 4 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 ( =36 GB with redundancy)

15 Chapter 4 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

16 Chapter 4 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

17 Chapter 4 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

18 Chapter 4 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

19 Chapter 4 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

20 Chapter 4 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!

21 Chapter 4 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/ ]=3,908 bps

22 Chapter 4 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