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Chapter 6 Digital Data Communication Techniques. 1/26. Asynchronous and Synchronous Transmission. Timing problems require a mechanism to synchronize the transmitter and receiver timing (rate, duration, spacing) of the data bits must be the same at transmitter & receiver

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Chapter 6

Digital Data Communication Techniques



Asynchronous and Synchronous Transmission

  • Timing problems require a mechanism to synchronize the transmitter and receiver
    • timing (rate, duration, spacing) of the data bits must be the same at transmitter & receiver
    • receiver samples stream of data bits at bit intervals
    • if clocks not aligned and drifting, the receiver will sample at wrong time after sufficient bits are sent
    • Example: for 1Mbps data stream, one bit will be transmitted every 1μs. With 1% clock drift at the receiver (faster or slower than transmitter), then wrong sampling will occur after 50 bit (50*0.01μs=0.5 μs).
  • Two solutions to synchronizing clocks
    • asynchronous transmission
    • synchronous transmission



Asynchronous Transmission

  • Avoid timing problem by not sending long stream of bits
  • Data is transmitted one character at a time, where each character is five or eight bits in length
  • Receiver can synchronize at the beginning of each new character
  • idle state: no transmission,
  • NRZ-L signalling is common for asynchronous transmission
  • The beginning of the character is signalled by a start bit
  • This is followed by a character of 5 or 8 bits long
  • The bits of the character are transmitted beginning with the least significant bit
  • A parity bit is then added for the purpose of error detection
  • The end of the character is a stop element.



Effect of timing error in asynchronous transmission

  • Example: The figure below shows the effects of a timing error of sufficient magnitude to cause error in reception. In this example, we assume a data rate of 10Kbps; therefore each bit is 100μs duration. Assume that the receiver is fast by 6%, or 6μs per bit time. Thus, the receiver samples the incoming character every 94μs. As we can see, the last sample is erroneous.



Asynchronous Transmission- Behavior

  • simple
  • cheap
  • overhead of 2 or 3 bits per char (~20%)
  • good for data with large gaps (keyboard)



Synchronous Frame format

Synchronous Transmission

  • Block of data bits are transmitted as a frame
  • Clocks must be synchronized
    • can use separate clock line between transmitter & receiver
      • one side send one short pulse and the other side uses this pulse for clocking; problem with long distances
    • or embed the clocking information in the data signal
      • Manchester encoding for digital signals
      • carrier frequency for analog transmission
  • Need to indicate start and end of block of data
    • use preamble (8bit flag) and postamble (8bit flag)
  • Control fields contain data link control protocol information
  • More efficient (lower overhead) than asynchronous



Synchronous Transmission

  • More efficient (lower overhead) than asynchronous transmission (two start and stop bits for every 8 bit character, (2/(2+8))*100%=20%).
  • Example: A frame in one of the standard schemes contains 48 bits of control, preamble, and postamble. Thus, for a 1000 character block of data, each frame consists of 48 bits of overhead and 1000*8=8000 bits of data, for a percentage overhead of only (48/(8000+48))*100%=0.6%.



Types of Errors

  • An error occurs when a bit is altered between transmission and reception (1 is transmitted and 0 is received, and visa versa)
  • Single bit errors
    • only one bit altered
    • caused by white noise
  • Burst errors
    • contiguous sequence of B bits in which first, last, and any number of intermediate bits in error
    • caused by impulse noise or by fading in wireless
    • effect greater at higher data rates
  • Example: An impulse noise event or fading event of 1μs occurs. At a data rate of 10Mbps, there is a resulting error burst of 10 bits. At a data rate of 100Mbps, there is an error burst of 100bits.



Error Detection

  • Define the following probabilities:

Pb: probability that a bit is received in error, known as Bit Error Rate (BER)

P1: probability that a frame arrives with no bit error

P2: probability that, with an error detecting algorithm in use, a frame arrives with one or more undetected errors

P1=(1-Pb)Fwhere F is the number of bits per frame


  • Example: a defined objective for the ISDN (Integrated Service Digital Network) is that the BER on a 64-Kbps channel should be less than 10-6. Suppose that one frame with undetected bit error occur per day, and the frame length is 1000bits. Determine P1and P2.



Error Detection

  • Will have errors
  • Detect using error-detecting code
  • This code is added by the transmitter
  • Recalculated and checked by the receiver
  • Still chance of undetected errors
  • Parity
    • parity bit set so character has even (even parity) or odd (odd parity) number of ones
    • even number of bit errors goes undetected



Parity Check

  • Example: If the transmitter is transmitting an IRA character G (1110001) and using an odd parity, it will append a 1 and transmit 11110001. The receiver examines the received character and, if the total number of 1’s is odd, assumes that no error has occurred. If one bit (or any odd number of bits) is erroneously inverted during transmission (for example, 11100001), then the receiver will detect an error.



Cyclic Redundancy Check (CRC)

  • CRC is one of most common and powerful error detection code
  • for block of k data bits, the transmitter generates an n-k bit sequence called Frame Check Sequence (FCS), such as the resulting frame length is n bits
  • transmits the n bit frame 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



Cyclic Redundancy Check (CRC)

  • CRC can be clarified by three equivalent ways:
    • Modulo 2 Arithmetic
    • Polynomials
    • Digital Logic
  • Modulo 2 Arithmetic: binary addition with no carry (exclusive-OR (XOR) operation)








CRC: Modulo 2 Arithmetic



CRC: Polynomials

  • A second way of viewing the CRC process is to express all values as polynomials in a dummy variable X, with binary coefficients
  • The coefficients corresponds to the bits in the binary number



Error Correction Process

FEC: Forward Error Correction



Error Correction Process

  • On the transmission, each k-bit block of data is mapped into an n-bit block (n > k) called a codeword, using an FEC (Forward Error Correction) encoder.
  • At the receiver, the FEC decoder has four possible outcomes:

1. If there are no bit errors, the decoder produces the original data block as output.

2. For certain error patterns, it is possible for the decoder to detect and correct those errors

3. For certain error patterns, the decoder can detect but not correct the errors, the decoder simply reports an uncorrectable error.

4. For certain, typically rare, error patterns, the decoder does not detect that any errors have occurred



Block Code Principles

  • The Hamming distance d(v1, v2) between two n-bit binary sequences v1 and v2 is the number of bits in which v1 and v2 disagree
  • If v1=011011 and v2=110001, the d(v1, v2)=3
  • Consider the following assignment:
  • Suppose that a codeword block is received with the bit pattern 00100. This is a not valid code word, so an error is detected.
  • The Hamming distance d(00000, 00100) =1

d(00111, 00100) =2

d(11001, 00100) =4

d(11110, 00100) =3

  • Most probably one bit in error (minimum distance): correct 00100 → 00000



Block Code Principles

  • Consider the following assignment:
  • The Hamming distance between the code words

d(00000, 00111) =3; d(00000, 11001) =3; d(00000, 11110) =4

d(00111, 11001) =4; d(00111, 11110) =3; d(11001, 11110) =3

 The minimum hamming distance = dmin=3

  • The maximum number of guaranteed correctable errors per code word is:
  • The number of errors that can be detected satisfies:



Block Code Principles

  • With an (n , k) block code, there are 2k, valid code words and a total of 2npossible codewords
  • The ratio of the redundant bits to data bits (n-k)/k is called the redundancy of the code
  • The ratio of the data bits to the total bits k/n is called the code rate



How Coding Improves System Performance

  • For BER=10-6, the coding gain = 2.77dB



Line Configuration - Topology

  • Physical arrangement of stations on medium
    • point to point - two stations
      • such as between two routers / computers
    • multi point - multiple stations
      • traditionally mainframe computer and terminals
      • now typically a local area network (LAN)



Line Configuration – Duplex

  • Classify data exchange half or full duplex
  • Half duplex (two-way alternate)
    • only one station may transmit at a time
    • requires one data path
  • Full duplex (two-way simultaneous)
    • simultaneous transmission and reception between two stations
    • requires two data paths
      • separate media or frequencies used for each direction