1. Lecture slides prepared for “Business Data Communications”, 7/e, by William Stallings and Tom Case, Chapter 5 “Data Communication Fundamentals”.

2. Data Communication Fundamentals
Chapter 5
Data Communication Fundamentals
The transmission of data across a transmission medium in a business network
Involves more than simply inserting a signal on the medium. A considerable degree
of cooperation between devices at either end of the medium is needed. This chapter
and the next discuss the essential mechanisms involved in the successful transmission
of data between two devices across a transmission medium. First, we discuss
the distinction between analog and digital transmission. Then we discuss the ways
in which signals can be encoded for effective and efficient communication. Next,
we look at the issue of synchronization: in order to correctly decode the incoming
signal, the receiver must know when each arriving bit begins and ends so that it
can keep pace with the transmitter. Several common techniques for synchronizing
the receiver with the transmitter are described. Finally, this chapter introduces the
concept of error detection.

3. Analog and Digital Data Communications
The way in which the electromagnetic signals are encoded to convey data determines the efficiency and reliability of the transmission
Data
Entities that convey meaning or information
Transmission
The communication of data across a computer network by the propagation and processing of signals
Signal
Electric or electromagnetic representation of data
Signaling
The physical propagation of the signal along a communication medium
Electromagnetic signals, which are capable of propagation on a variety of transmission
media, can be used to convey data. The exact way in which these signals are
encoded to convey data determines the efficiency and reliability of the transmission.
This section introduces some basic concepts that are essential to understanding how
data are passed from sender to receiver over a transmission medium.
The terms analog and digital correspond, roughly, to continuous and discrete ,
respectively. These two terms are frequently used in data communications in at
least three contexts: data, signaling, and transmission.
Signals and data are fundamental components in computer networks and it
is important to understand how they differ. Signals are used to represent data that
is passed across a computer network and data must be converted into appropriate
signals by transmitters that can be correctly interpreted by receivers.
The use of these terms in different contexts is often the source of confusion
in articles and books. In this section, we clarify the various uses of these two terms.
Briefly, we define data as entities that convey meaning, or information. Signals
are electric or electromagnetic representations of data. Signaling is the physical
propagation of the signal along a communication medium. Transmission is the
communication of data across a computer network by the propagation and
processing of signals.
Many different types of data are captured, transmitted, and stored in business
networks. As we have seen in the previous chapters, audio and image have joined
text and numbers as important types of business data. We have also observed that
some types of data are analog while others are digital.

4. Analog Data/Digital Data
Continuous values on some interval
Voice and video
Data collected by sensors, such as temperature and pressure
Digital data
Discrete values
Text, integers, binary data
Signals are used to encode and transmit data
Analog data take on continuous values on some interval. For example, voice
and video are continuously varying patterns of intensity. Most data collected by
sensors, such as temperature and pressure, also have continuous values. Digital data
take on discrete values; examples are text, integers, and binary data. To transmit
either type of data from one point to another over a wired or wireless medium, the
data has to be converted into signals. Hence, signals are used to encode and transmit
data and different types of equipment are used to convert data into signals, and
vice versa, depending on the type of data (analog or digital) and the type of transmission
facilities (analog or digital) in the network.

5. Analog Signals/Digital Signals
Continuously varying electromagnetic wave that may be transmitted over both guided and unguided media
Digital signal
Sequence of voltage pulses
Generally cheaper than analog signaling
Less susceptible to noise interference
Suffer more from attenuation than analog signals
Cannot be used on optical fiber or wireless media
One of the major takeaways from Chapter 4 is that electromagnetic signals
are the basis of data transmission in business and other communication systems. An
analog signal is a continuously varying electromagnetic wave that may be transmitted
over both guided and unguided media. The components of analog (sine) waveforms,
amplitude, frequency, and phase can be used to convey data. A digital signal
is a sequence of voltage pulses that may be transmitted over a wired medium; for
example, a constant positive voltage may represent binary 0, and a constant negative
value may represent binary 1. The principal advantages of digital signaling are
that it is generally cheaper than analog signaling and is less susceptible to noise
interference. The principal disadvantage is that digital signals suffer more from
attenuation than do analog signals. Note that digital signaling is possible only on
copper media and cannot be used on optical fiber or wireless media.

6. Analog and Digital Signaling of Analog and Digital Data
Both analog and digital data can be represented by either analog or digital
signals; this is illustrated in Figure 5.1. The trend today is decidedly toward digital
transmission, wherever this can be used. Local area networks, which are designed
for transmitting computer data, have always supported digital signals. LANs have
made the use of digital signals to transmit both digital and analog data commonplace
in businesses of all sizes. However, there is still a lot of communications
equipment in place that is only capable of transmitting data as analog signals so
it remains important to have a fundamental understanding about both analog and
digital data transmission.

7. Analog Data  Analog Signal
Digital Data  Analog Signal
Analog Data  Analog Signal
Modem (modulator/demodulator)
Coverts binary voltage pulses by modulating a carrier frequency
Demodulates the signal to recover the original data at the other end
Codec (coder/decoder)
Takes an analog signal and approximates that signal by a bit stream
At the other end of a line the bit stream is used to reconstruct the analog data
Transmitter technologies use modulation techniques that enable sound and/or video waveforms to be conveyed as electromagnetic waveforms over wires or airwaves
Converting analog data to analog signals is quite common. Transmitter technologies
in traditional telephones, broadcast television, analog cable television,
and AM and FM radio use modulation techniques that enable sound and/or video
waveforms to be conveyed as electromagnetic waveforms over wires or airwaves. In
some instances, the electromagnetic waves that are transmitted are nearly identical
to the waveforms for the original analog waveforms; the transmitted frequency
range (spectrum) may be the same and in most respects, these are electromagnetic
equivalents of the original analog waveform. This is essentially what happens when
transducers in analog telephone handsets convert voice sounds to electromagnetic
sine waves that are carried over the telephone network. In other instances,
the analog data are converted into an analog signal whose frequency range is different
from that for the original data. An example is the transmission of an FM radio
broadcast over an analog cable TV channel.
Converting digital data to analog signals involves the use of a modem
(modulator/demodulator). The modem converts a series of incoming binary voltage
pulses, which represent binary 0s and 1s, into an analog signal by modulating
a carrier frequency . The resulting signal occupies a certain spectrum of frequency
centered about the carrier. The most common modems, dial-up modems, represent
digital data in the voice spectrum and hence enable digital data to be propagated
over ordinary voice-grade telephone lines. At the other end of the line, a modem
demodulates the signal to recover the original data. The major varieties of modulation
techniques used by modems are discussed later in this section.
In an operation very similar to that performed by a modem, analog data
can be represented by a digital signal. The device that performs this function is a
codec (coder/decoder). In essence, the codec takes an analog signal that directly
represents voice, audio data, or video and approximates that signal by a bit
stream. At the other end of a line, the bit stream is used to reconstruct the analog
data. The basic process used by codecs to perform this conversion is discussed in
Section 5.2.
Finally, digital data can be represented directly, in binary form, by two voltage
levels, one to represent a binary 1 and the other to represent a binary 0. To improve
propagation characteristics and synchronization between transmitters and receivers,
however, the binary data may be encoded into digital signals that use voltage level
transitions, rather than constant-voltage levels, to represent the binary data.
The major types of digital data encoding are outlined in Section 5.2.

8. Combination Benefits Digital data, digital signal
The equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog equipment
Analog data, digital signal
Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment
Digital data, analog signal
Some transmission media, such as optical fiber and satellite, only propagate analog signals
Analog data, analog signal
Analog data are easily converted to an analog signal
Each of the four combinations just described is in widespread use. The reasons
why businesses choose a particular combination for any given communications task
vary. Some of the representative reasons include:
• Digital data, digital signal: In general, the equipment for encoding digital data
into a digital signal is less complex and less expensive than digital-to-analog
equipment.
• Analog data, digital signal: Conversion of analog data to digital form permits
the use of modern digital transmission and switching equipment.
• Digital data, analog signal: Some transmission media, such as optical fiber and
satellite, only propagate analog signals.
• Analog data, analog signal: Analog data are easily converted to an analog signal.

9. Table 5.1 Analog and Digital Transmission
A final distinction remains to be made. Both analog and digital signals may be
transmitted on suitable transmission media. The way these signals are treated is a
function of the transmission system. Table 5.1 summarizes the methods of transmission.

10. Transmission Choices Analog transmission Digital transmission
Only transmits analog signals, without regard for data content
Attenuation overcome with amplifiers
Signal is not evaluated or regenerated
Digital transmission
Transmits analog or digital signals
Uses repeaters rather than amplifiers
Switching equipment evaluates and regenerates signal
Analog transmission is a means of transmitting analog signals without regard
to their content; the signals may represent analog data (such as voice) or digital
data (such as data that pass through a modem). In either case, the analog signal suffers
attenuation, which limits the length of the transmission link. To achieve longer
distances, the analog transmission system includes amplifiers that boost the energy
in the signal. Unfortunately, the amplifier also boosts the noise components of any
signal. With amplifiers cascaded to achieve long distance, the signal has the potential
to become more and more distorted. For analog data, such as voice, quite a
bit of distortion can be tolerated and the data (spoken words) remain intelligible.
However, for digital data transmitted as analog signals, cascaded amplifiers are
likely to increase the number of errors.
Digital transmission , by contrast, is concerned with the content of the signal.
In general, a digital signal can be propagated only a limited distance before attenuation
endangers the integrity of the data represented by the signal. To achieve
greater distances, repeaters are used. A repeater receives the digital signal, recovers
the pattern of ones and zeros, and retransmits a new signal. Thus the attenuation is
overcome.
The same technique may be used with an analog signal if it is assumed that
the signal carries digital data. At appropriately spaced points, the transmission system
has retransmission devices rather than amplifiers. The retransmission device
recovers the digital data from the analog signal and generates a new, clean analog
signal. Thus noise is not cumulative. This process is illustrated in Figure 4.9.

11. Advantages of Digital Transmission
Cost – large scale and very large scale integration has caused continuing drop in cost
Data Integrity – effect of noise and other impairments is reduced
Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division
Security and Privacy – Encryption possible
Integration – All signals (voice, video, image, data) treated the same
The question naturally arises as to which is the preferred method of transmission
of business data. The answer being supplied by the telecommunications
industry and its business customers is digital, despite the enormous investments
that were made in analog communications facilities during the last century. Today,
both long-haul and intra-building networks are being converted to digital transmission
and, where possible, digital signaling techniques. The most important reasons
for this are summarized in Table 5.2.

12. Data Encoding Techniques
Analog or digital data must be converted into a signal for purposes of transmission
The mapping from binary digits to signal elements is the encoding scheme for transmission
The basis for analog encoding is a continuous constant frequency signal known as the carrier signal
Modulation
The conversion of digital signals to analog form
Demodulation
The conversion of analog data signals back to digital form
As we have pointed out, data, either analog or digital, must be converted into a signal
for purposes of transmission.
In the case of digital data, different signal elements are used to represent
binary 1 and binary 0. The mapping from binary digits to signal elements is the
encoding scheme for transmission. Encoding schemes are designed to minimize
errors in determining the start and end of each bit and errors in determining
whether each bit is a 1 or a 0.
For analog data, the encoding scheme is designed to enhance the quality, or
fidelity, of transmission. That is, we would like the received analog data to be an
accurate and exact reproduction of the transmitted data.
The basis for analog encoding is a continuous constant-frequency signal known
as the carrier signal . Digital information is encoded by means of a modem that
modulates one of the three characteristics of the carrier: amplitude, frequency, or
phase, or some combination of these.

13. Figure 5.2 Modulation of Analog Signals for Digital Data
Figure 5.2 illustrates the three basic forms of
modulation of analog signals for digital data:
• Amplitude-shift keying
• Frequency-shift keying
• Phase-shift keying
In all these cases, the resulting signal contains a range of frequencies on both sides
of the carrier frequency; this range is the bandwidth of the transmitted signal.

14. Amplitude Shift Keying (ASK)
Digital data are represented as variations in the different amplitudes of the carrier wave
The amplitude (or height) of the sine wave varies to transmit the ones and zeros
Inexpensive
Is susceptible to sudden gain changes from noise, distortions, and other signal impairments making it a rather inefficient modulation technique
Also used to transmit digital data over optical fiber
In amplitude-shift keying (ASK) , digital data are represented as variations
in the different amplitudes of the carrier wave. In its most simple form, one binary
digit (typically a 1) is represented by the presence of the carrier wave, the other
(typically a 0) by the absence of the carrier. This is illustrated in Figure 5.2a. In
other cases, one amplitude shift represents a binary 1 and a second amplitude shift
represents a binary 0. ASK is susceptible to sudden gain changes from noise, distortions,
and other signal impairments. This makes it a rather inefficient modulation
technique. However, ASK modulation and demodulation techniques are inexpensive
and this can increase its attractiveness for business networks.
The ASK technique is also commonly used to transmit digital data over optical
fiber. For LED transmitters, binary 1 is represented by a short pulse of light and
binary 0 by the absence of light. Laser transmitters normally have a fixed “bias”
current that causes the device to emit a low light level. This low level represents
binary 0, while a higher-amplitude lightwave represents binary 1.

15. Frequency Shift Keying (FSK)
Digital information is transmitted through discrete frequency changes of the carrier wave
Simplest form is binary FSK (BFSK) in which the two binary values are represented by two different frequencies near the carrier frequency
Advantages:
Less susceptible to noise than ASK
Is easy to decode
Often has a better signal-to-noise ratio than ASK
Is a signaling option supported by most dial-up modems
Is commonly used for high frequency radio transmission
In frequency-shift keying (FSK) , digital information is transmitted through
discrete frequency changes of the carrier wave. The simplest form of FSK is called
binary FSK (BFSK), in which the two binary values are represented by two different
frequencies near the carrier frequency. BSFK is illustrated in Figure 5.2b. FSK
is less susceptible to noise and other error sources than ASK; it is easy to decode
and often has a better signal-to-noise ratio than ASK. FSK is a signaling option
supported by most dial-up modems. It is also commonly used for high-frequency
(4–30 MHz) radio transmission.

16. Phase Shift Keying (PSK)
The phase of the carrier signal is shifted to encode data
Is more noise resistant and efficient than ASK or FSK
Is widely used in business networks, especially wireless networks
A four-phase system (quadrature phase-shift keying, QPSK) could encode two bits with each signal burst
QFSK can be used to double the data rate while maintaining the same bandwidth
In phase-shift keying (PSK) , the phase of the carrier signal is shifted to encode
data. Figure 5.2c is an example of a two-phase system. In this system, a 0 is represented
by sending a signal burst of the same phase as the preceding signal burst
sent. A 1 is represented by sending a signal burst of opposite phase to the preceding
one. Figure 5.2c represents binary phase-shift keying (BPSK) which uses two shifts
separated by 1800
Phase-shift keying can use more than two phase shifts. A four-phase system
could encode two bits with each signal burst. Such systems are examples of quadrature-
phase-shift keying (QPSK) . Relative to BFSK, QFSK can be used to double the
data rate while maintaining the same bandwidth. The phase-shift keying technique
is more noise resistant and efficient than either ASK or FSK.
PSK is widely used in business networks, especially wireless networks. It is
used in wireless LANs and RFID (radio frequency identification) applications.
It is also used in Bluetooth and ZigBee systems as well as in modems used for
satellite communications.

17. Multilevel Signaling Techniques may be combined
A common combination is PSK and ASK, where some or all of the phase shifts may occur at one or two amplitudes
Is commonly used in today’s networks
Examples:
56 Kbps dial-up modems
Digital subscriber line (DSL) modems
Gigabit Ethernet networks
Finally, the basic techniques just discussed may be combined. A common
combination is PSK and ASK, where some or all of the phase shifts may occur at
one or two amplitudes. These techniques are referred to as multilevel signaling
because each signal element represents multiple bits. Note that four-phase phase-shift
keying is an example of multilevel signaling. Multilevel signaling is in common
use in today’s networks. Examples include 56 Kbps dial-up modems, digital subscriber
line (DSL) modems, and Gigabit Ethernet networks.

18. Modems
Continue to be one of the most widely used pieces of communications gear
Is a device that modulates an analog carrier wave to encode digital information
Also demodulates the signals it receives to decode transmitted information
Direct broadcast satellite, Wi-Fi, and mobile phones use modems to communicate
Three popular types are:
Voice-grade
Cable
ADSL
Although both public and private telecommunications facilities are
becoming increasingly digital, the use of analog transmission is still widespread.
Thus, the modem continues to be one of the most widely used pieces of communications
gear. A modem is essentially a device that modulates an analog carrier wave
to encode digital information; it also demodulates the signals it receives to decode
transmitted information.
Modems are offered in several different forms for use in different applications.
Standalone modems, for instance, are self-contained, with internal power supplies,
and are used with separate information products. Where a number of circuits
come together—such as at the interface to an Internet service provider’s computer
system—rack-mounted modems are often used, sharing power supplies and
packaging. Modems can also be packaged inside another system (such as personal
computer). Such integrated modems usually lower overall cost but increase the
complexity of the computing device and the cost of designing it. Integrated modems
are usually offered as an option, because to standardize on a particular modem type
with a product might restrict the usefulness of the product.
Modems are pervasive in today’s computer networks. For example, direct
broadcast satellite, Wi-Fi, and mobile phones use modems to communicate. Modems
are also found in WiMax systems and some home networking systems. In this subsection,
we provide a brief introduction to three popular types of modems: voice-grade,
cable, and ADSL.

19. Table 5.3 Modem Specifications
Voice-grade modems are designed for the transmission of digital data over
ordinary telephone lines. Thus, the modems make use of the same 4-kHz bandwidth
available for voice signals. Because modems are used in pairs for communications,
and because this use often occurs over the public telephone network, standards are
essential to allow different voice-grade modems to be paired. Table 5.3 lists the most
popular voice-grade modem types, as designated by the ITU-T Recommendation
that defines them.

20. Figure 5.3 Cable Modems
A cable modem permits Internet access over cable television networks.
The cable television industry has been the early leader in providing high-speed
access to the home. Figure 5.3 shows a typical layout for cable delivery. At the
cable central location, or linked by a high-speed line, is the Internet service provider
(ISP). Typically, the cable company is an ISP but it may also provide links to other
ISPs. From the central location, the cable company lays out a network of above
or underground fiber and coaxial cable lines that can reach every home and office
in its region of operation. Traditionally, this system has been used to deliver one-way
transmission of television channels, using 6 MHz per channel. The same cable
layout, with appropriate electronics at both ends, can also be used to deliver a data
channel to the subscriber and to provide a reverse channel from subscriber to the
central location. Both upstream and downstream channels used for data transmission
are shared among a number of subscribers, using time-division multiplexing
a line-sharing technique described in Chapter 6. Within the subscriber’s home or
office, a splitter is employed to direct ordinary television signals to a television and
the data channel to a cable modem, which can serve one or a network of PCs.
Because cable modems enable data and television transmission to occur simultaneously
over the same cable, they are an example of a broadband modem.
Some cable TV companies offer VoIP telephone service. This enables cable
TV customers to eliminate connections to the public telephone network should they
choose to do so. Bundled voice, TV, and Internet services are often very competitively
priced. This makes them attractive to consumers and small businesses.

21. Figure 5.4 ADSL Modem Application
In the implementation and deployment of a high-speed wide area public
digital network, the most challenging part is the link between subscriber and network:
the digital subscriber line. With billions of potential endpoints worldwide,
the prospect of installing new cable for each new customer is daunting. Instead,
network designers have sought ways of exploiting the installed base of twisted-pair
wire that links virtually all residential and business customers to telephone networks.
These links were installed to carry voice-grade signals in a bandwidth from
0 to 4 kHz. However, the wires are capable of transmitting signals over a far broader
spectrum—1 MHz or more. The asymmetric digital subscriber line (ADSL) is the
most widely publicized of a family of modem technologies designed to provide high-speed
digital data transmission over ordinary telephone wire. ADSL is available from
most carriers in the U.S. and is supported by ISPs in most countries around the world.
Figure 5.4 depicts the ADSL configuration for Internet access. The telephone
central office can provide support for a number of ISPs, each of which must support
the ADSL modem technology. At the central office, the ISP data signal is
combined with a voice signal from the ordinary telephone voice switch. The combined
signal can then be transmitted to/from a local subscriber over the subscriber
line. At the subscriber’s site, the twisted pair is split and routed to both a PC and
a telephone. At the PC, an ADSL modem demodulates the data signal for the PC.
At the telephone, a microfilter passes the 4-kHz voice signal. The data and voice
signals are combined on the twisted-pair line using frequency-division multiplexing
techniques, as described in Chapter 6. Because they enable both voice and data to
be transmitted simultaneously over the same link, ADSL modems are a type of
broadband modem.

22. Figure 5.5 Fiber to the Home
Currently, the best performance is available with the use of optical fiber from
the carrier directly to the home or business premises, referred to as fiber to the
home (FTTH) or fiber to the premises (FTTP). FTTH/FTTP has become a feasible
alternative to cable and DSL technologies for residential and small business customers.
Figure 5.5 depicts a typical configuration for residential service. An optical
fiber cable runs from the central office to a neighborhood. The cable’s bandwidth
is shared by up to 32 end users by splitting the signal at the neighborhood end.
The central office provides a broadcast service in one frequency band for television
and video on demand service. Other frequency bands are assigned separately to
each end user for two-way Internet access and two-way voice service to the ordinary
public telephone network. Thus, unlike cable, with FTTH, each subscriber has dedicated
channels for data and voice transmission. At the end user, a modem converts
the optical signal to an electrical signal for access by devices such as computers,
telephones, and televisions.

23. Speeds for Internet Access Methods
Table 5.4 compares the performance of the various modems we have discussed
and with ISDN (integrated services digital network) access, which uses a
digital signaling technique.

24. Figure 5.6 Pulse-Code Modulation Example
The evolution of public telecommunications networks and private branch exchanges
to digital transmission and switching requires that voice data be represented in digital
form. The best-known technique for voice digitization is pulse code modulation
(PCM) . PCM is based on the sampling theorem, which states that if a signal is sampled
at regular intervals of time and at a rate higher than twice the highest significant
signal frequency, then the samples contain all the information of the original signal.
If voice data were limited to frequencies below 4000 Hz, as is done in the
analog telephone network, then 8000 samples per second would be sufficient to characterize
completely the voice signal. Note, however, that these are analog samples.
To convert to digital, each of these analog samples must be assigned a binary code.
Figure 5.6 shows an example in which the original signal is assumed to be band-limited
with a bandwidth of B . Analog samples are taken at a rate of 2B , or once every
Ts = 1/2B seconds. Each analog sample is approximated by being quantized into
one of 16 different levels. Each sample can then be represented by 4 bits, ranging
from 0000 (to represent 0) to 1111 (to represent 15). But because the quantized
values are only approximations, it is impossible to recover the original signal
exactly. By using an 8-bit sample, which allows 256 quantizing levels, the quality of
the recovered voice signal is comparable with that achieved via analog transmission.
Note that this implies that a data rate of 8000 samples per second * 8 bits per
sample = 64 kbps is needed to represent the voice data as a digital signal.
PCM can, of course, be used for applications other than voice signals. For
example, a color TV signal has a useful bandwidth of 4.6 MHz. According to the
sampling theorem, its analog waveform would have to be sampled 9.2 million times
per second to reasonably approximate the original signal. With this sampling rate
and the use of 10-bit samples (representing 1024 quantization levels), the data rate
would be 92 Mbps.
In recent times, variations on the PCM technique, as well as other encoding
techniques, have been used to reduce the digital data rate required to carry
voice. By using compression techniques, such as those discussed in Chapter 2, good quality
voice transmission can be achieved with a data rate as low as 8 Kbps. With
video, advantage can be taken of the fact that from frame to frame most picture elements
do not change. Inter-frame coding techniques low the video requirement to be
reduced to about 15 Mbps, and for slowly changing scenes, such as those in a video
teleconference, down to 1.5 Mbps or less. Indeed, recent advances have resulted in
commercial videoconference products with data rates as low as 64 Kbps.

25. Figure 5.7 Examples of Digital Signal Encoding Schemes
The most common, and easiest, way to transmit digital signals is to use two different
voltage levels for the two binary digits. Typically, a negative voltage represents
binary 1 and a positive voltage represents binary 0 (Figure 5.7a). This code is known
as Non-return-to-Zero-Level (NRZ-L) (meaning the signal never returns to zero
voltage, and a constant-voltage level is used for the duration of the signal pulse that
represents the binary digit). The amount of time a digital signal is left at a particular
voltage level to indicate a binary value is called a bit interval or bit time . NRZ-L is
often used for very short connections, such as between a personal computer and an
external modem or a terminal and a nearby host computer.
A variation of NRZ is NRZI (NRZ, Invert on Ones) . As with NRZ-L, NRZI
maintains a constant-voltage pulse for the duration of a bit interval. The data themselves
are encoded as the presence or absence of a signal transition at the beginning
of the bit interval. A transition (low to high or high to low) at the beginning of a
bit interval denotes a binary 1 for that bit time; no transition indicates a binary 0
(Figure 5.7b). NRZI is used in 100 Mbps Ethernet networks.

26. Differential NRZ Differential version is NRZI (NRZ, invert on ones)
Change=1, no change=0
Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level
NRZI is an example of differential encoding . In differential encoding, the signal
is decoded by comparing the polarity of adjacent signal elements rather than determining
the absolute value of a signal element. One benefit of this scheme is that it
may be more reliable to detect a transition in the presence of noise than to compare
a value to a threshold. Another benefit is that, with a complex cabling layout, it is
easy to lose the sense of the polarity of the signal. For example, if the leads from an
attached device to a twisted-pair cable are accidentally inverted, all ones and zeros
are inverted for NRZ-L. This does not happen with differential encoding.

27. Problems With NRZ
Difficult to determine where one bit ends and the next begins
In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
Timing is critical because any drift results in lack of synchronization and incorrect bit values being transmitted
A significant disadvantage of NRZ transmission is that it is difficult to determine
where one bit ends and another begins. To picture the problem, consider that
with a long string of ones or zeros for NRZ-L, the output is a constant voltage over
a long period of time. Under these circumstances, any drift between the timing of
transmitter and receiver results in the loss of synchronization between the two.

28. Biphase Alternatives to NRZ
Require at least one transition per bit time, and may even have two
Modulation rate is greater, so bandwidth requirements are higher
Maximum modulation rate is twice NRZ
Advantages
Synchronization due to predictable transitions
Error detection based on absence of a transition
There is a set of alternative coding techniques, grouped under the term
biphase , that overcomes this problem. Two of these techniques, Manchester and
differential Manchester, are the most well-known. All of the biphase techniques
require at least one transition per bit time and may have as many as two transitions.
Thus, the maximum modulation rate is twice that for NRZ; this means that
the bandwidth required is correspondingly greater. Because a transition between
two voltage levels is required to represent each bit, twice as many voltage pulses are
needed to transmit a given number of bits in comparison to NRZ. However, biphase
schemes have two major advantages:
• Synchronization: Because there is a predictable transition during each bit
time, the receiver can synchronize on that transition. For this reason, the
biphase codes are known as self-clocking codes.
• Error detection: The absence of an expected transition can be used to detect
errors. Noise on the line would have to invert both the signal before and after
the expected transition to cause an undetected error.

29. Manchester Code Transition in the middle of each bit period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0
Used in Ethernet and other LANs
In the Manchester code (Figure 5.7c), there is a transition at the middle of
each bit period. The midbit transition serves as a clocking mechanism and also
represents data: A high-to-low transition represents a 0, and a low-to-high transition
represents a 1. Manchester coding is used in 10 Mbps Ethernet and a number
of other local area networks.

30. Differential Manchester
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential encoding
Used in token-ring
In differential Manchester (Figure 5.7d), the midbit
transition is used only to provide clocking. The encoding of a 0 is represented
by the presence of a transition at the beginning of a bit period, and a 1 is represented
by the absence of a transition at the beginning of a bit period. Differential
Manchester is used in token ring LANs and in some types of magnetic and optical
storage systems.

31. Table 5.5 4B/5B Digital Encoding
While Manchester encoding schemes help ensure synchronization between
transmitters and receivers, they are relatively inefficient because the baud rate is
twice the data rate. If these were used in a Gigabit Ethernet LAN, a baud rate of
two billion would be needed. Other digital encoding schemes have been developed to
maintain synchronization without having to have a baud rate which is twice the data
(bits per second) rate. One of these is the 4B/5B encoding scheme. In 4B/5B encoding,
the transmitter takes each 4-bit string of data and converts it into a 5-bit sequence
that ensures frequent signal transitions (for synchronization). This is illustrated in
Table B/5B uses NRZI signals. Receivers take the 5-bit codes and convert them
back into the original 4-bit data strings that they represent.
In a 100 Mbps Ethernet LAN running at full capacity, each 100 million transmitted
bits “carries” 80 million bits of data. The other 20 million bits are those
added by 4B/5B encoding to ensure synchronization. This makes 4B/5B only 80%
efficient. Higher speed Ethernet networks use digital encoding schemes that operate
in a manner that is similar to 4B/5B. Gigabit Ethernet, for example, uses 8B/10B
encoding. 8B/10B encoding is also used in Firewire (IEEE 1394), Fibre Channel,
InfiniBand, and USB B/66B encoding is used in 10 Gigabit Ethernet,
10 Gigabit Fibre Channel, and InfiniBand.

32. Analog Encoding of Analog Information
Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components and transmitted on a voice-grade telephone line
Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band
A higher frequency may be needed for effective transmission
Analog-to-analog modulation permits frequency-division multiplexing
In some instances, analog information can be converted directly into an analog signal
that occupies the same bandwidth. The best example of this is voice. Voice-generated
sound wave in the range of 300–3400 Hz can be represented by an electromagnetic
signal with the same frequency components. This signal can then be directly transmitted
on a voice-grade telephone line.
It is also possible to use an analog signal to modulate a carrier to produce
a new analog signal that conveys the same information but occupies a different
frequency band. There are two principal reasons for doing this:
• A higher frequency may be needed for effective transmission. For unguided
media, it is virtually impossible to transmit low-frequency signals; the required
antennas would be many kilometers in diameter. Guided media also have
constraints on frequency range. Optical fiber, for example, requires that the
frequency be on the order of Hz.
• Analog-to-analog modulation permits frequency division multiplexing, an
important technique explored in Chapter 6.
As with digital-to-analog modulation, analog-to-analog modulation involves
an information source that is used to modulate one of the three principal characteristics
of a carrier signal: amplitude, frequency, or phase.

33. Figure 5.8 Sine-Wave Signals
Figure 5.8 illustrates the three possibilities. With amplitude modulation
(AM), the amplitude of the carrier varies with the pattern of the modulating signal.
Similarly, frequency modulation (FM) and phase modulation (PM) modulate the
frequency and phase of a carrier, respectively.

34. Figure 5.9 Asynchronous Transmission
Two approaches are common for achieving the desired synchronization. The first
is called, oddly enough, asynchronous transmission . The strategy with this scheme
is to avoid the timing problem by not sending long, uninterrupted streams of bits.
Instead, data are transmitted one character at a time, where each character is
5 to 8 bits in length. Timing or synchronization must only be maintained within
each character; the receiver has the opportunity to resynchronize at the beginning
of each new character.
Figure 5.9 illustrates this technique. When no character is being transmitted,
the line between transmitter and receiver is in an idle state. The definition of
idle is equivalent to the signaling element for binary 1. Thus, for NRZ-L signaling
(see Figure 5.7), which is common for asynchronous transmission, idle would be the
presence of a negative voltage on the line. The beginning of a character is signaled
by a start bit with a value of binary 0. This is followed by the 5 to 8 bits that actually
make up the character. The bits of the character are transmitted beginning with
the least significant bit. For example, for IRA characters, the data bits are usually
followed by a parity bit, which therefore is in the most significant bit position. The
parity bit is set by the transmitter such that the total number of ones in the character,
including the parity bit, is even (even parity) or odd (odd parity), depending
on the convention being used. This bit is used by the receiver for error detection,
as discussed in Section 5.4. The final element is a stop element , which is a binary 1.
A minimum length for the stop element is specified, and this is usually 1, 1.5, or
2 times the duration of an ordinary bit. No maximum value is specified. Because the
stop element is the same as the idle state, the transmitter continues to transmit the
stop element until it is ready to send the next character.
The timing requirements for this scheme are modest. For example, IRA characters
are typically sent as 8-bit units, including the parity bit. If the receiver is 5%
slower or faster than the transmitter, the sampling of the eighth character bit is
displaced by 45% but is still correctly sampled. Figure 5.9c shows the effects of a
timing error of sufficient magnitude to cause an error in reception. In this example,
we assume a data rate of 10,000 bits per second (10 Kbps); therefore, each bit is of
0.1 millisecond (ms), or 100 μs, duration. Assume that the receiver is fast by 6%,
or 6 μs per bit time. Thus, the receiver samples the incoming character every 94 μs
(based on the transmitter’s clock). As can be seen, the last sample is erroneous.
Asynchronous transmission is simple and cheap but requires an overhead
of 2 to 3 bits per character. For example, for an 8-bit character with no parity bit,
using a 1-bit-long stop element, 2 out of every 10 bits convey no information but
are there merely for synchronization; thus the overhead is 20%. Of course, the
percentage overhead could be reduced by sending larger blocks of bits between
the start bit and stop element. However, as Figure 5.9c suggests, the larger the
block of bits, the greater the cumulative timing error. To achieve greater efficiency,
a different form of synchronization, known as synchronous transmission,
is used.

35. Synchronous Transmission
Block of bits transmitted in a steady stream without start and stop codes
Clocks of transmitter and receiver must somehow be synchronized
Provide a separate clock line between transmitter and receiver works well over short distances
Embed the clocking information in the data signal
Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern
The data plus preamble, postamble, and control information are called a frame
Involves the use of a data link control procedure which automatically detects transmission error and causes a frame in error to be retransmitted
With synchronous transmission , a block of bits is transmitted in a steady stream
without start and stop codes. The block may be many characters in length. To prevent
timing drift between transmitter and receiver, their clocks must somehow be
synchronized. One possibility is to provide a separate clock line between transmitter
and receiver. One side (transmitter or receiver) pulses the line regularly with
one short pulse per bit time. The other side uses these regular pulses as a clock.
This technique works well over short distances, but over longer distances the clock
pulses are subject to the same impairments as the data signal, and timing errors can
occur. The other alternative is to embed the clocking information in the data signal.
For digital signals, this can be accomplished with Manchester, differential
Manchester encoding, or 4B/5B encoding, as explained in Section 5.2. For analog
signals, a number of techniques can be used; for example, the carrier frequency
itself can be used to synchronize the receiver based on the phase of the carrier.
With synchronous transmission, there is another level of synchronization
required to allow the receiver to determine the beginning and end of a block of
data. To achieve this, each block begins with a preamble bit pattern and generally
ends with a postamble bit pattern. In addition, other bits added to the block
convey control information used in the data link control procedures, discussed
in Chapter 6. The data plus preamble, postamble, and control information are
called a frame . The exact format of the frame depends on which data link control
procedure is being used.
For large systems and computer networks, the efficiency of synchronous transmission
is needed, even though it introduces the technical problem of synchronizing
the clocks of transmitter and receiver.
In addition to the requirement for efficiency, large data transfers introduce a
requirement for error checking. While the interactive application user often checks
his or her own input and output for errors by looking at the screen and rekeying
portions that contain errors, such a procedure is clearly impractical for long file
transfers that occur at fast rates and often without an operator present. As we shall
see, synchronous transmission involves the use of a data link control procedure,
which automatically detects transmission errors and causes a frame in error to be
retransmitted.

36. Error Control Process
All transmission media have potential for introduction of errors
All data link layer protocols must provide a method for controlling errors
Error control process has two components
Error detection
Redundancy introduced so that the occurrence of an error will be detected
Error correction
Receiver and transmitter cooperate to retransmit frames that were in error
As discussed in Chapter 4, noise and other signal impairments have the
potential to cause data communication errors. The ability to detect and recover
from errors has always been an important aspect of computer networks and its
importance has grown over time. This is partly because data integrity is increasingly
important to business operations. No business needs a network that captures,
transmits, and stores data that is deficient in quality. It is important to detect data
that has been corrupted in transit before it is stored or used for database updates.
Certain data simply cannot be wrong; for example, consider the effect of an
undetected data error on an electronic funds transfer. Businesses have become
less tolerant of errors in communication and mass storage systems as bandwidths
and volumes of data have increased. In general, uncorrected and undetected
errors can degrade performance and response times in any system that handles
large volumes of data.
The process of error control involves two elements:
• Error detection : Redundancy is introduced into the data stream so that the
occurrence of an error is detected.
• Error correction : Once an error is detected by the receiver, the receiver and
the transmitter cooperate to cause the frames in error to be retransmitted.

37. Parity Checks
Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)
Good for detecting single-bit errors only
Noise impulses are often long enough to destroy more than one bit
The ability of parity checking to detect errors is dependent on the total number of bits corrupted by noise impulses and the parity convention that is used
The simplest approach to error detection is to append a parity bit to the end of a
block of data. A typical example is asynchronous transmission of IRA characters,
in which a parity bit is attached to each 7-bit character. The value of this bit is
selected so that the character has an even number of 1s (even parity) or an odd
number of 1 s (odd parity). So, for example, if the transmitter is transmitting the
character G ( ) and using odd parity, it appends a 1 and transmits
The receiver examines the received character and, if the total number of 1s is odd,
assumes that no error has occurred. If one bit (or an odd number of bits) is erroneously
inverted during transmission (e.g., ), then the receiver detects an
error. Note, however, that if two (or any even number of) bits are inverted due to
error, an undetected error occurs. Hence, the use of the parity bit is not a foolproof
data detection scheme.
In practice, noise impulses are often long enough in duration to destroy
more than one bit, particularly at high data rates. Impulse noises can corrupt
parity bits in addition to data bits. The ability of parity checking to detect errors
is therefore dependent on the total number of bits corrupted by noise impulses
(odd or even) and the parity convention that is used (odd or even). Typically,
even parity is used for synchronous transmission and odd parity for asynchronous
transmission.

38. Figure 5.10 Error Detection Process
When synchronous transmission is used, it is possible to employ an error detection
technique that is both more efficient (lower percentage of overhead bits) and more
powerful (more errors detected) than the simple parity bit. This technique requires
the addition of a frame check sequence (FCS) , or error-detecting code , to each synchronous
frame. The use of an FCS is illustrated in Figure 5.10, using the cyclical
redundancy check (CRC) code described in this subsection. On transmission,
a calculation is performed on the bits of the frame to be transmitted; the result is
inserted as an additional field in the frame. On reception, the same calculation is
performed on the received bits and the calculated result is compared to the value
stored in the incoming frame. If there is a discrepancy, the receiver assumes that an
error has occurred.

39. Cyclic Redundancy Check (CRC)
One of the most common and powerful error-detecting codes
Data in frame is treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame
17-bit divisor leaves bit remainder, 33-bit divisor leaves bit remainder
For a CRC of length N, errors undetected are 2-N
Overhead is low (1-3%)
One of the most common and powerful error-detecting codes is the cyclic
redundancy check (CRC) . For this technique, the message to be transmitted is
treated as one long binary number. This number is divided by a unique prime
binary number (a number divisible only by itself and 1), and the remainder is
attached to the frame to be transmitted. When the frame is received, the receiver
performs the same division, using the same divisor, and compares the calculated
remainder with the remainder received in the frame. The most commonly used divisors
are a 17-bit divisor, which produces a 16-bit remainder, and a 33-bit divisor,
which produces a 32-bit remainder. Respectively, these are called CRC-16 and
CRC-32; the numbers 16 and 32 represent the size, in bits, of the FCS appended
to each transmitted frame.
The measure of effectiveness of any error-detecting code is what percentage
of errors it detects. For a CRC of length N , the rate of undetected errors is on the
order of 2–N (see [STAL04] for details). This means that CRC-16 detects % of
communication errors while CRC-32 detects nearly %. In summary, the
CRC is a very powerful means of error detection and requires very little overhead.
As an example, if a 16-bit FCS is used with frames of 1000 bits, then the overhead is
only 1.6%. With a 32-bit FCS, the overhead is 3.2%.

40. Table 5.6 Data and Signal Combinations
Both analog and digital information can be encoded as either analog or digital signals.
The particular encoding that is chosen depends on the specific requirements
to be met and the media and communications facilities available. For example, to
transmit digital information over an analog telephone line, a modem is used to convert
the digital data into analog form. Similarly, there is an increasing use of digital
facilities, and voice information must be encoded in digital form to be transmitted
on these digital facilities. The various combinations of data and signals found in
today’s networks are summarized in Table 5.6.

41. Summary Asynchronous transmission
Error detection
The need for error control
Parity checks
Cyclic redundancy check
Analog and digital data communications
Data encoding techniques
Analog encoding of digital information
Digital encoding of analog information
Digital encoding of digital data
Analog encoding of analog information
Chapter 5 summary.
Chapter 5: Data Communication Fundamentals