Chapter 4 Digital Transmission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Topics discussed in this section: 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

Figure 4.1 Line coding and decoding

Figure 4.2 Signal element versus data element

Figure 4.3 Effect of lack of synchronization

Figure 4.4 Line coding schemes

Figure 4.5 Unipolar NRZ scheme

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

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

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 Cons Used for magnetic recording 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

Trade Off for Multilevel Binary Not as efficient as NRZ Each signal element only represents one bit In a 3 level system could represent log23 = 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 Pros 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

Figure 4.8 Polar biphase: Manchester and differential Manchester schemes

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

There is another factor that can be used to improve performance, and that is the encoding scheme. The encoding scheme is simply the mapping from data bits to signal elements. A variety of approaches have been tried. In what follows, we describe some of the more common ones; they are defined in Table 5.2

Figure 5.2 shows the signal encoding for the binary sequence 01001100011 using six different signal encoding schemes.

Figure 4.18 AMI used with scrambling

Figure 4.19 Two cases of B8ZS scrambling technique

B8ZS substitutes eight consecutive zeros with 000VB0VB. Note B8ZS substitutes eight consecutive zeros with 000VB0VB.

Figure 4.20 Different situations in HDB3 scrambling technique

HDB3 substitutes four consecutive zeros with 000V or B00V depending Note HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.

Topics discussed in this section: 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, pulse code modulation and delta modulation. Topics discussed in this section: Pulse Code Modulation (PCM) Delta Modulation (DM)

Figure 4.21 Components of PCM encoder

According to the Nyquist theorem, the sampling rate must be 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

Example 4.9 Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second.

Figure 4.26 Quantization and encoding of a sampled signal

Example 4.12 What is the SNRdB in the example of Figure 4.26? Solution We can use the formula to find the quantization. We have eight levels and 3 bits per sample, so SNRdB = 6.02(3) + 1.76 = 19.82 dB Increasing the number of levels increases the SNR.

Example 4.13 A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample? Solution We can calculate the number of bits as Telephone companies usually assign 7 or 8 bits per sample.

Topics discussed in this section: 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

Figure 4.31 Data transmission and modes

Figure 4.32 Parallel transmission

Figure 4.33 Serial transmission

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.

Asynchronous here means “asynchronous at the byte level,” 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

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

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 3

Example An asynchronous transmission scheme uses 8 data bits, an even parity bit, and a stop element of length 2 bits. What percentage of clock inaccuracy can be tolerated at the receiver with respect to the framing error? Assume that the bit samples are taken at the middle of the clock period. Also assume that at the beginning of the start bit the clock and incoming bits are in phase.

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 4

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