Without gaps and start and stop bit

Without gaps and start and stop bits, there is no built-in mechanism to help the
receiving device adjust its bit synchronization midstream. Timing becomes very important,
therefore, because the accuracy of the received information is completely dependent
on the ability ofthe receiving device to keep an accurate count of the bits as they come in.
SECTION 4.5 KEY TERMS 135
The advantage of synchronous transmission is speed. With no extra bits or gaps to
introduce at the sending end and remove at the receiving end, and, by extension, with
fewer bits to move across the link, synchronous transmission is faster than asynchronous
transmission. For this reason, it is more useful for high-speed applications such as
the transmission of data from one computer to another. Byte synchronization is accomplished
in the data link layer.
We need to emphasize one point here. Although there is no gap between characters
in synchronous serial transmission, there may be uneven gaps between frames.
Isochronous
In real-time audio and video, in which uneven delays between frames are not acceptable,
synchronous transmission fails. For example, TV images are broadcast at the rate
of 30 images per second; they must be viewed at the same rate. If each image is sent
by using one or more frames, there should be no delays between frames. For this type
of application, synchronization between characters is not enough; the entire stream of
bits must be synchronized. The isochronous transmission guarantees that the data
arrive at a fixed rate.
4.4 RECOMMENDED READING
For more details about subjects discussed in this chapter, we recommend the following
books. The items in brackets [...] refer to the reference list at the end of the text.
Books
Digital to digital conversion is discussed in Chapter 7 of [Pea92], Chapter 3 of
[CouOl], and Section 5.1 of [Sta04]. Sampling is discussed in Chapters 15, 16, 17, and
18 of [Pea92], Chapter 3 of [CouO!], and Section 5.3 of [Sta04]. [Hsu03] gives a good
mathematical approach to modulation and sampling. More advanced materials can be
found in [Ber96].
4.5 KEY TERMS
adaptive delta modulation
alternate mark inversion (AMI)
analog-to-digital conversion
asynchronous transmission
baseline
baseline wandering
baud rate
biphase
bipolar
bipolar with 8-zero substitution (B8ZS)
bit rate
block coding
companding and expanding
data element
data rate
DC component
delta modulation (DM)
differential Manchester
digital-to-digital conversion
digitization
136 CHAPTER 4 DIGITAL TRANSMISSION
eight binary/ten binary (8B/lOB)
eight-binary, six-ternary (8B6T)
four binary/five binary (4B/5B)
four dimensional, five-level pulse
amplitude modulation (4D-PAM5)
high-density bipolar 3-zero (HDB3)
isochronous transmission
line coding
Manchester
modulation rate
multilevel binary
multiline transmission, 3 level (MLT-3)
nonreturn to zero (NRZ)
nonreturn to zero, invert (NRZ-I)
nonreturn to zero, level (NRZ-L)
Nyquist theorem
parallel transmission
polar
pseudoternary
pulse amplitude modulation (PAM)
pulse code modulation (PCM)
pulse rate
quantization
quantization error
return to zero (RZ)
sampling
sampling rate
scrambling
self-synchronizing
serial transmission
signal element
signal rate
start bit
stop bit
synchronous transmission
transmission mode
two-binary, one quaternary (2B I Q)
unipolar
4.6 SUMMARY
o Digital-to-digital conversion involves three techniques: line coding, block coding,
and scrambling.
o Line coding is the process of converting digital data to a digital signal.
o We can roughly divide line coding schemes into five broad categories: unipolar,
polar, bipolar, multilevel, and multitransition.
o Block coding provides redundancy to ensure synchronization and inherent error
detection. Block coding is normally referred to as mB/nB coding; it replaces each
m-bit group with an n-bit group.
o Scrambling provides synchronization without increasing the number of bits. Two
common scrambling techniques are B8ZS and HDB3.
o The most common technique to change an analog signal to digital data (digitization)
is called pulse code modulation (PCM).
o The first step in PCM is sampling. The analog signal is sampled every Ts s, where Ts
is the sample interval or period. The inverse of the sampling interval is called the
sampling rate or sampling frequency and denoted by fs, where fs =lITs. There are
three sampling methods-ideal, natural, and flat-top.
o According to the Nyquist theorem, to reproduce the original analog signal, one
necessary condition is that the sampling rate be at least twice the highest frequency
in the original signal.
SECTION 4.7 PRACTICE SET 137
o Other sampling techniques have been developed to reduce the complexity of PCM.
The simplest is delta modulation. PCM finds the value of the signal amplitude for
each sample; DM finds the change from the previous sample.
o While there is only one way to send parallel data, there are three subclasses of
serial transmission: asynchronous, synchronous, and isochronous.
o In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or
more stop bits (1 s) at the end of each byte.
o 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.
o The isochronous mode provides synchronized for the entire stream of bits must. In
other words, it guarantees that the data arrive at a fixed rate.
4.7 PRACTICE SET
Review Questions
1. List three techniques of digital-to-digital conversion.
2. Distinguish between a signal element and a data element.
3. Distinguish between data rate and signal rate.
4. Define baseline wandering and its effect on digital transmission.
5. Define a DC component and its effect on digital transmission.
6. Define the characteristics of a self-synchronizing signal.
7. List five line coding schemes discussed in this book.
8. Define block coding and give its purpose.
9. Define scrambling and give its purpose.
10. Compare and contrast PCM and DM.
11. What are the differences between parallel and serial transmission?
12. List three different techniques in serial transmission and explain the differences.
Exercises
13. Calculate the value of the signal rate for each case in Figure 4.2 if the data rate is
1 Mbps and c = 1/2.
14. In a digital transmission, the sender clock is 0.2 percent faster than the receiver clock.
How many extra bits per second does the sender send if the data rate is 1 Mbps?
15. Draw the graph of the NRZ-L scheme using each of the following data streams,
assuming that the last signa11evel has been positive. From the graphs, guess the
bandwidth for this scheme using the average number of changes in the signal level.
Compare your guess with the corresp.onding entry in Table 4.1.
a. 00000000
b. 11111111
c. 01010101
d. 00110011
138 CHAPTER 4 DIGITAL TRANSMISSION
16. Repeat Exercise 15 for the NRZ-I scheme.
17. Repeat Exercise 15 for the Manchester scheme.
18. Repeat Exercise 15 for the differential Manchester scheme.
19. Repeat Exercise 15 for the 2B1Q scheme, but use the following data streams.
a. 0000000000000000
b. 1111111111111111
c. 0101010101010101
d. 0011001100110011
20. Repeat Exercise 15 for the MLT-3 scheme, but use the following data streams.
a. 00000000
b. 11111111
c. 01010101
d. 00011000
21. Find the 8-bit data stream for each case depicted in Figure 4.36.