Chapter 4 Digital Transmission

4.1

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4-1 DIGITAL-TO-DIGITAL CONVERSION

In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding , block coding , and scrambling . Line coding is always needed; block coding and scrambling may or may not be needed.

Topics discussed in this section:

Line Coding Line Coding Schemes Block Coding Scrambling 4.2

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Figure 4.1

Line coding and decoding

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Figure 4.2

Signal element versus data element

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Figure 4.3

Effect of lack of synchronization

Figure 4.4

Line coding schemes

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Figure 4.5

Unipolar NRZ scheme

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Nonreturn to Zero-Level (NRZ-L)

 Two different voltages for 0 and 1 bits  Voltage constant during bit interval  no transition I.e. no return to zero voltage  e.g. Absence of voltage for zero, constant positive voltage for one  More often, negative voltage for one value and positive for the other  This is NRZ-L

Figure 4.6

Polar NRZ-L and NRZ-I schemes

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Nonreturn to Zero Inverted

 Nonreturn to zero inverted on ones  Constant voltage pulse for duration of bit  Data encoded as presence or absence of signal transition at beginning of bit time  Transition (low to high or high to low) denotes a binary 1  No transition denotes binary 0  An example of differential encoding

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Note

In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit.

Differential Encoding

 Data represented by changes rather than levels  More reliable detection of transition rather than level  In complex transmission layouts it is easy to lose sense of polarity

NRZ pros and cons

 Pros  Easy to engineer  Make good use of bandwidth  Cons  dc component  Lack of synchronization capability  Used for magnetic recording  Not often used for signal transmission

Multilevel Binary

  Use more than two levels Bipolar-AMI        zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity No loss of sync if a long string of ones (zeros still a problem) No net dc component Lower bandwidth Easy error detection

Pseudoternary

 One represented by absence of line signal  Zero represented by alternating positive and negative  No advantage or disadvantage over bipolar-AMI

Figure 4.9

Bipolar schemes: AMI and pseudoternary

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Trade Off for Multilevel Binary

 Not as efficient as NRZ  Each signal element only represents one bit  In a 3 level system could represent log 2 3 = 1.58 bits  Receiver must distinguish between three levels (+A, -A, 0)  Requires approx. 3dB more signal power for same probability of bit error

 

Biphase

Manchester   Transition in middle of each bit period Transition serves as clock and data   Low to high represents one High to low represents zero  Used by IEEE 802.3(ethernet) standard for baseband coaxial cable and twisted-pair CSMA/CD bus LANs Differential Manchester      Midbit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE 802.5 token ring LAN, using shielded twisted pair

Manchester Encoding

Differential Manchester Encoding

Biphase Pros and Cons

 Con  At least one transition per bit time and possibly two  Maximum modulation rate is twice NRZ  Requires more bandwidth  Pros  Synchronization on mid bit transition (self clocking)  No dc component  Error detection  Absence of expected transition

Figure 4.7

Polar RZ scheme

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Figure 4.8

Polar biphase: Manchester and differential Manchester schemes

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Note

In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization.

4-2 ANALOG-TO-DIGITAL CONVERSION

We have seen in Chapter 3 that a digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data. In this section we describe two techniques, and delta modulation .

pulse code modulation Topics discussed in this section:

Pulse Code Modulation (PCM) Delta Modulation (DM) 4.52

Figure 4.21

Components of PCM encoder

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Note

According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal.

Figure 4.24

Recovery of a sampled sine wave for different sampling rates

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Example 4.9

Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second.

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Figure 4.26

Quantization and encoding of a sampled signal

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Example 4.14

We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample?

Solution The human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:

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4-3 TRANSMISSION MODES

The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick.

While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

Topics discussed in this section:

Parallel Transmission Serial Transmission 4.74

Figure 4.31

Data transmission and modes

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Figure 4.32

Parallel transmission

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Figure 4.33

Serial transmission

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Note

In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.

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Note

Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.

Figure 4.34

Asynchronous transmission

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Note

In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.

Figure 4.35

Synchronous transmission

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Asynchronous Transmission

   Data transmitted 1 character at a time Character format is 1 start & 1+ stop bit, plus data of 5-8 bits Character may include parity bit     Timing needed only within each character Resynchronization each start bit Uses simple, cheap technology Wastes 20-30% of bandwidth

Synchronous Transmission

   Large blocks of bits transmitted without start/stop codes Synchronized by clock signal or clocking data Data framed by preamble and postamble bit patterns     More efficient than asynchronous Overhead typically below 5% Used at higher speeds than asynchronous Requires error checking, usually provided by HDLC

Synchronization Choices

  Low-speed terminals and PCs commonly use asynchronous transmission   inexpensive “burst” tendency of communication reduces impact of inefficiency Large systems and networks commonly use synchronous transmission   overhead too expensive; efficiency necessary error-checking more important