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Chapter 6

Chapter 6. Digital Data Communications Techniques. Asynchronous and Synchronous Transmission 1. We are concerned with serial transmission of data, I.e. transmission over a single path rather than parallel set of lines Signals are sent down the line one at a time each containing

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Chapter 6

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  1. Chapter 6 Digital Data Communications Techniques Data Communication

  2. Asynchronous and Synchronous Transmission 1 • We are concerned with serial transmission of data, I.e. transmission over a single path rather than parallel set of lines • Signals are sent down the line one at a time each containing • Less than one bit (Manchester) • One bit (NRZ-L) • More than one bit (QPSK) • In order for the receiver to sample the incoming bits properly, it must know the arrival time and duration of each bit that it receives • Typically, the receiver will attempt to sample the medium at the center of each bit time. Data Communication

  3. Asynchronous and Synchronous Transmission 2 • Timing problems require a mechanism to synchronize the transmitter and receiver • E.g. If there is 1 % clock drift in each bit period, assuming that bits are sampled at center of the bit time in the beginning, the receiver will start sampling incorrectly after 50 or more samples. • Two solutions • Asynchronous • Synchronous Data Communication

  4. Asynchronous • Data are transmitted one character at a time • 5 to 8 bits • Timing only needs maintaining within each character • Resynchronize with each character Data Communication

  5. Rules • When no characters are transmitted, the line is idle, that is equivalent to signaling element 1. • NRZ-L is common in asynchronous transmission • The beginning of a character is signaled by a start bit with a value of binary zero. • The bits of the character are sent beginning with the least significant bit. This is followed by the parity bit. • Then the stop bit is sent (binary 1). There is a minimum duration specified for the stop bit. Data Communication

  6. Asynchronous (diagram) Data Communication

  7. Asynchronous - Behavior • For 8 bit characters, maximum allowed clock drift is 5% • Figure 6.1c shows the effects of timing error • Data rate of 10 Kbps • Each bit is of 0.1 ms • Receiver is fast by 6%, samples the incoming characher every 94 s • Simple • Cheap • Overhead of 2 or 3 bits per char (~20%) • Good for data with large gaps (keyboard) Data Communication

  8. Async. Trans. Cont. • Twoerrors: • The character is incorrectly received • The bit count is out of alignment (framing error) • if bit 7 is a 1 and bit 8 is a 0, bit 8 could be mistakenfor a start bit • Framingerror can alsooccurifsomenoiseconditioncausesthefalseappearance of a start bit duringtheidlestate Data Communication

  9. Synchronous - Bit Level • Block of data transmitted without start or stop bits • Clocks must be synchronized • Can use separate clock line, one side pulses the line regularly with one short pulse per bit time • Good over short distances • Subject to impairments • Embed clock signal in data • Manchester encoding • Carrier frequency (analog): synchronize the receiver based on the phase of the carrier Data Communication

  10. Sync. Tran. Cont’d. • There should be a second level of synchronization to allow receiver recognize the beginning and end of block data. • To achieve this, a special bit pattern is used. Data Communication

  11. Synchronous - Block Level • Need to indicate start and end of block • Use preamble and postamble • The frame starts with a preamble called a flag, 8 bits long • Same flag is used as a postamble • More efficient (lower overhead) than async Data Communication

  12. Synchronous (diagram) Data Communication

  13. Example • HDLC contains 48 bits of control, preamble and postamble. • Thus, for 1000-character block of data, each frame consists of 48 bits of overhead, 1000x8=8000 bits of data yielding 48/8048=0.6% percentage overhead which is much lower than that of asynchronous transmission Data Communication

  14. 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 • For data rate of 10 Mbps, 1 s impulse noise event or fading event : error bursts of 10 bits • Effect greater at higher data rates Data Communication

  15. Error Detection Motivation Define Pb = probability of a single bit error P1 = probability that a frame arrives with no bit errors P2 = probability that a frame arrives with one or more undetected bit errors F = number of bits in a frame P1 = (1 - Pb) F and P2 = 1 - P1 Data Communication

  16. Example On 64 kbps ISDN channel Pb=10-6. At most one frame of length 1000 bits can be allowed to have undetected bit error per day. Let us see how this compares with the result of BER of 10-6: # frames in a day = 5.529  106 P2 = 1 / (5.529  106) = 0.18  10-6 = acceptable Compare this to P2 = 1- P1 = 1 – (1 – Pb)F = 1 – (1-10-6)1000 = 10-3 Conclusion:Unacceptable error rate! Do something Data Communication

  17. Error-DetectingCode • Additional bits added by transmitter for error detection code (error-detection code) • For a data block of k bits transmitter generates error detection code ofn-k bits, n-k<k • Transmit n bits • Receiver seperates frame into k bits data and (n-k) bits of error detection code, performs same calculation • A detected error occurs if there is a mismatch Data Communication

  18. Error Detection Process Data Communication

  19. Parity Check • Simplest error detection code is to append a parity bit • 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 Data Communication

  20. Parity Check - Example • Transmitter transmits IRA character G (1110001) • Using odd parity, it will append a 1 and transmit 11110001 • The receiver assumes no error has occured if the total no of 1s is odd • if one bit (or any odd no of bits) is erroneously converted, the receiver will detect an error Data Communication

  21. Cyclic Redundancy Check • For a block of k bits transmitter generates n-k bit sequence known as Frame Check Sequence (FCS) • Transmit n bits which is exactly divisible by some number • Receiver divides frame by that number • If no remainder, assume no error • For math, see Stallings chapter 6 Data Communication

  22. Modulo 2 Arithmetic Addition and Subtraction are exclusive OR operation. Ex: 1111+1010=0101, 1111-0101=1010 Multiplication uses addition Ex: 11001  11 11001 110010 101011 Data Communication

  23. CRC Motivation Define T= n bit frame to be transmitted D = k-bit message, the first k bits of T F = (n-k) bit FCS, the last n bits of T P = pattern of n-k+1 bits, predetermined divisor We want T/P to have no remainder. Clearly, T=2n-kD+F. Then (2n-kD)/P= Q+R/P where R is the remainder. Now, choose F=R and see what happens: T/P=(2n-kD+R)/P=Q+R/P+R/P=Q (modulo 2 binary number added to itself is zero) Conclusion: With F=R, T is exactly divisible by P Data Communication

  24. CRC Example D =1010001101 (10 bits) P=110101 (n-k+1=6 bits) FCS = to be calculated (5 bits) • D is multiplied by 25 2n-kD=101000110100000 and 2n-kD/P is computed as Data Communication

  25. CRC Example Data Communication

  26. CRC Example (cont’d) Hence T=2n-kD+R=101000110101110 which is transmitted. If there are no errors, the receiver receives T intact. Dividing T by P yields no remainder. If there are errors, what receiver receives is not T and dividing by P yields a remainder and receiver concludes that an error has occurred. Data Communication

  27. CRC Polynomials • Each binary number can be viewed as coefficients of a polynomial with a dummy variable X. • E.g. For M=110011 M(X)=X5+X4+X+1 • Popular polynomials used in practice are • CRC-12 = X12 + X11 + X3 + X + 1 • CRC-16 = X16 + X15 + X2 + 1 • CRC-CCITT = X16 + X12 + X5 + 1 • CRC-32 = too long, check the book Data Communication

  28. Error Correction • Correction of detected errors usually requires data block to be retransmitted (see chapter 7) • 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 Data Communication

  29. Error Correction Process Diagram Data Communication

  30. 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 Data Communication

  31. 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 Data Communication

  32. Block Code Principles • Hamming distance d(v1,v2): between two n-bit binary sequences v1 and v2 : no of bits in which v1 and v2 disagree • v1 = 011011 , v2 = 110001, d(v1,v2)=3 • Instead of transmitting each block as k bits, map each bit k-bit sequence into a unique n-bit codeword Data Communication

  33. Example • Suppose a codeword is received with bit pattern 00100 • In terms of Hamming distances, we have • If an invalid codeword is received, the valid code with min distance is selected • What if there is more than one valid code at a minimum distance? Data Communication

  34. Example (cont’d) • The code can correct single bit error • Also can detect a double-bit error Data Communication

  35. Block Error Correcting Codes • (n,k) block code encodes k data bits into n bit codewords • 2k valid codewords out of a total of 2n possible codewords • For a code consisting of the codewords w1, w2, …, ws, s=2n , min distance of the code: Data Communication

  36. Block Error Correcting Codes • Maksimum no of guaranteed correctable errors per codeword satisfies: • No of errors that can be detected, t = dmin -1 • If dmin errors occur, this change one valid code into another Data Communication

  37. Block Code Design • Design of a block code involves some considerations: • Largest possible value of dmin • Relatively easy encode and decode • (n-k) to be small, to reduce bandwidth • (n-k) to be large, to reduce error rate Data Communication

  38. Line Configuration • Topology • Physical arrangement of stations on medium • Point to point • Multi point • Computer and terminals, local area network • Advantages of multipoint configuration • the computer needs only a single I/O port and • a single transmission line, which saves cost Data Communication

  39. Traditional Configurations Data Communication

  40. Line Configuration • Half duplex • Only one station may transmit at a time • Requires one data path • Often used for terminal-to-computer interaction • Full duplex • Simultaneous transmission and reception between two stations • Requires two data paths (or echo canceling) • Used for computer-to-computer data exchange Data Communication

  41. Line Conf. Cont’d • For analog signaling, if station operates on only one frequency, then it must use half duplex. • If it uses two frequencies, then it may send on one frequency and receive on the other (either guided or unguided media) • Digital signaling requires guided transmission. Hence, full duplex operation requires two different transmission paths, while half duplex transmission requires only one transmission path. Data Communication

  42. Foreground Reading • Stallings chapter 6 Data Communication

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