CCN Digital Communication Slides Chapter Seven

CCN Slide 1

1. Chapter Seven:
Digital Communication

2. Introduction
•Many signals in modern communication systems
are digital
•Additionally, analog signals are transmitted
digitally
•Digitizing a signal results in reduced distortion
and improvement in signal-to-noise ratios

3. Types of
Signal
Transmission

4. Channels and Information Capacity
•All practical communication channels are band-
limited
•There are theoretical limits to the rate at which
data may be transmitted
•The relationship between time, information
capacity, and channel bandwidth is given by
Hartley’s Law:
IktB

5. Shannon-Hartley Theorem
•There is a limit to the amount of data that can be
sent in a given bandwidth:
C2Blog
2
M

6. Pulse Modulation
•Nyquist showed that it is possible to reconstruct a band-limited signal
from periodic samples, as long as the sampling rate is at least twice
the frequency of the of highest frequency component of the signal
•Several types of sampling are available for pulse modulation

7. Sampling Rate Errors
•Sampling rates that
are too low result
in aliasingor
foldover
•The figures
illustrate correct
and incorrect
sampling rates:

8. Sampling
•Sampling alone is not a digital technique
•The immediate result of sampling is a pulse-amplitude
modulation (PAM)signal
•PAM is an analog scheme in which the amplitude of the
pulse is proportional to the amplitude of the signal at the
instant of sampling
•Another analog pulse-forming technique is known as
pulse-duration modulation (PDM). This is also known as
pulse-width modulation (PWM)
•Pulse-position modulation is closely related to PDM

9. Analog Pulse-Modulation Techniques

10. Pulse-Code Modulation
•Pulse-Code Modulation (PCM) is the most commonly used
digital modulation scheme
•In PCM, the available range of signal voltages is divided
into levels and each is assigned a binary number
•Each sample is represented by a binary number and
transmitted serially
•The number of levels available depends upon the number
of bits used to express the sample value
•The number of levels is given by: N = 2
m

11. Quantizing
•The process of converting analog signals to PCM is called
quantizing
•Since the original signal can have an infinite number of
signal levels, the quantizing process will produce errors
called quantizing errors or quantizing noise
•The dynamic range of a system is the ratio of the strongest
possible signal that can be transmitted and the weakest
discernible signal
•In a linear PCM system, the maximum dynamic range is
found by:
DR = (1.76 + 6.02m) dB

12. Companding
•Compandingis used to improve dynamic range
•Compression is used on the transmitting end and
expanding is used on the receiving end, hence
companding

13. Coding and Decoding
•The process of converting an analog signal into
PCM is called coding, the inverse operation is
called decoding
•Both procedures are accomplished in a CODEC

14. PCM Coding

15. Delta Modulation
•In Delta Modulation, only one bit is transmitted
per sample
•That bit is a one if the current sample is more
positive than the previous sample, and a zero if it
is more negative
•Since so little information is transmitted, delta
modulation requires higher sampling rates than
PCM for equal quality of reproduction

16. Line Codes
•Line codes are methods of converting binary numbers back
into analog voltages or currents
•The simplest line code is to use the presence or absence of
a voltage/current to indicate the logic state
•Unipolar NRZ (non-return-to-zero) means that there is no
requirement for a signal to return to zero at the end of each
element
•RZ (return-to-zero) methods are used to eliminate low-
frequency ac components and dc components

17. Bipolar NRZ Code

18. Time-Division Multiplexing
•There are two basic types of multiplexing:
–Frequency-division multiplexing (FDM
–Time-division multiplexing
•In TDM, each information signal is allowed to use all
available bandwidth
•In theory, it is possible to to divide the bandwidth or the
time among the users of a channel
•Continuously variable signals, such as analog, are not well
adapted to TDM because the signal is present all the time

19. TDM in Telephony
•TDM is used extensively in telephony
•The most common standard is the DS-1 signal, which
consists of 24 PCM voice channels, multiplexed using
TDM
•Each channel is sampled at 8 kHz with 8 bits per sample,
which gives a bit rate of 64 kb/s for each voice channel
•The samples must be transmitted at the rate they were
obtained to be reconstructed
•The overall bit rate is 1.544 Mb/s
•The whole system is known as a T1 Carrier

20. Digital Signal Hierarchy
Coax, fiber-optic274.1764032DS-4T4
Fiber optics560.168064DS-5T5
Coax, microwave44.736672DS-3T3
Low-capacitance
twisted-pair
microwave
6.31296DS-2T2
Twisted-pair3.15248DS-1CT1C
Twisted-pair1.54424DS-1T1
Typical
Medium
Bit Rate
(Mb/s)
Voice
Channels
SignalCarrier

21. Data Compression
•Data compression is a technique used to reduce
the bandwidth to transmit an analog signal in a
digital form
•The exact bandwidth necessary is dependent upon
the modulation scheme

22. Lossy and Lossless Compression
•There are two main categories of data
compression:
–Lossless compression involves transmitting all of the
data in the original signal but using fewer bits. Lossless
compression generally looks for redundancies in the
data
–Lossy compression allows for some reduction in the
quality of the transmitted signal. Lossy compression
involves reducing the number of bits per sample or
reducing the sampling rate

23. Vocoders
•A vocoder (voice coder)is an example of lossy
compression applied to human speech
•A typical vocoder reduces the amount of data that
needs to be transmitted by constructing a model of
the human vocal system

24. Vocoder Types
•There are two main
ways of generating
the excitation signal
in a linear predictive
vocoder:
–Pulse Excited Linear
Predictive (PELP)
–Residual Excited
Linear Predictive
(RELP)