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Lecture 5

Lecture 5. Review of Signal encoding techniques Digital data into digital signals Digital data into analog signals Analog data into digital signals Analog data into analog signal Synchronous and asynchronous communication Error correction and detection. Analog data into analog signal.

rahim-freeman
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Lecture 5

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  1. Lecture 5 • Review of Signal encoding techniques • Digital data into digital signals • Digital data into analog signals • Analog data into digital signals • Analog data into analog signal • Synchronous and asynchronous communication • Error correction and detection ECS 152A Computer Networks

  2. Analog data into analog signal • Why modulate analog signals? • Higher frequency can give more efficient transmission • Permits frequency division multiplexing (chapter 8) • Types of modulation • Amplitude • Frequency • Phase ECS 152A Computer Networks

  3. Spectrum of AM Signal ECS 152A Computer Networks

  4. Asynchronous and Synchronous Transmission • Timing problems require a mechanism to synchronize the transmitter and receiver • Two solutions • Asynchronous • Synchronous ECS 152A Computer Networks

  5. Asynchronous • Data transmitted on character at a time • 5 to 8 bits • Timing only needs maintaining within each character • Resynchronize with each character ECS 152A Computer Networks

  6. Asynchronous (diagram) ECS 152A Computer Networks

  7. Asynchronous - Behavior • In a steady stream, interval between characters is uniform (length of stop element) • In idle state, receiver looks for transition 1 to 0 • Then samples next seven intervals (char length) • Then looks for next 1 to 0 for next char • Advantages • Simple • Cheap • Overhead of 2 or 3 bits per char (~20%) • Good for data with large gaps (keyboard) ECS 152A Computer Networks

  8. Synchronous - Bit Level • Block of data transmitted without start or stop bits • Clocks must be synchronized • Can use separate clock line • Good over short distances • Subject to impairments • Embed clock signal in data • Manchester encoding • Carrier frequency (analog) ECS 152A Computer Networks

  9. Synchronous - Block Level • Need to indicate start and end of block • Use preamble and postamble • e.g. series of SYN (hex 16) characters • e.g. block of 11111111 patterns ending in 11111110 • More efficient (lower overhead) than async ECS 152A Computer Networks

  10. Synchronous (diagram) ECS 152A Computer Networks

  11. Types of Error • An error occurs when a bit is altered between transmission and reception • Single bit errors • One bit altered • Adjacent bits not affected • White noise • Burst errors • Length B • Contiguous sequence of B bits in which first last and any number of intermediate bits in error • Impulse noise • Fading in wireless • Effect greater at higher data rates ECS 152A Computer Networks

  12. Error Detection Process ECS 152A Computer Networks

  13. Error Detection • Additional bits added by transmitter for error detection code • Parity • Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones • Even number of bit errors goes undetected ECS 152A Computer Networks

  14. 1 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 0 1 1 0 1 1 0 1 0 0 1 1 1 Last column consists of check bits for each row Bottom row consists of check bit for each column Two dimensional parity check codes ECS 152A Computer Networks Figure 3.52

  15. Cyclic Redundancy Check • For a block of k bits transmitter generates n bit sequence • Transmit k+n bits which is exactly divisible by some number • Receive divides frame by that number • If no remainder, assume no error ECS 152A Computer Networks

  16. Addition: Multiplication: = q(x) quotient x3 + x2 + x Division: x3 + x+ 1 ) x6 + x5 x6 + x4 + x3 dividend divisor x5 + x4 + x3 x5 + x3 + x2 3 35 ) 122 x4 + x2 105 x4 + x2 + x 17 x = r(x) remainder Cyclic Redunancy check ECS 152A Computer Networks Figure 3.55

  17. Steps: 1) Multiply i(x) by xn-k(puts zeros in (n-k) low order positions) 2) Divide xn-k i(x) by g(x) 3) Add remainder r(x) to xn-k i(x) (puts check bits in the n-k low order positions): Cyclic Redundancy Check quotient remainder xn-ki(x) = g(x) q(x) + r(x) b(x) = xn-ki(x) + r(x) transmitted codeword ECS 152A Computer Networks

  18. Generator polynomial: g(x)= x3 + x + 1 Information: (1,1,0,0) i(x) = x3 + x2 Encoding: x3i(x) = x6 + x5 x3 + x2 + x 1110 x3 + x+ 1 ) x6 + x5 1011 ) 1100000 x6 + x4 + x3 1011 x5 + x4 + x3 1110 1011 x5 + x3 + x2 1010 x4 + x2 1011 x4 + x2 + x x 010 • Transmitted codeword: • b(x) = x6 + x5 + x • b= (1,1,0,0,0,1,0) Cyclic Redundancy Check ECS 152A Computer Networks

  19. Error Correction • Correction of detected errors usually requires data block to be retransmitted Not appropriate for wireless applications • Bit error rate is high • Lots of retransmissions • Propagation delay can be long (satellite) compared with frame transmission time • Would result in retransmission of frame in error plus many subsequent frames • Need to correct errors on basis of bits received ECS 152A Computer Networks

  20. Error Correction Process Diagram ECS 152A Computer Networks

  21. Error Correction Process • Each k bit block mapped to an n bit block (n>k) • Codeword • Forward error correction (FEC) encoder • Codeword sent • Received bit string similar to transmitted but may contain errors • Received code word passed to FEC decoder • If no errors, original data block output • Some error patterns can be detected and corrected • Some error patterns can be detected but not corrected • Some (rare) error patterns are not detected • Results in incorrect data output from FEC ECS 152A Computer Networks

  22. Working of Error Correction • Add redundancy to transmitted message • Can deduce original in face of certain level of error rate • E.g. block error correction code • In general, add (n – k ) bits to end of block • Gives n bit block (codeword) • All of original k bits included in codeword • Some FEC map k bit input onto n bit codeword such that original k bits do not appear • Again, for math, see chapter 6 ECS 152A Computer Networks

  23. Line Configuration • Topology • Physical arrangement of stations on medium • Point to point • Multi point • Computer and terminals, local area network • Half duplex • Only one station may transmit at a time • Requires one data path • Full duplex • Simultaneous transmission and reception between two stations • Requires two data paths (or echo canceling) ECS 152A Computer Networks

  24. Traditional Configurations ECS 152A Computer Networks

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