2. Data & Information: Data refers to the raw facts that are collected while
information refers to processed data that enables us to take decisions.
Ex. When the result of a particular test is declared it contains data of all
students, when you find the marks you have scored you have the
information that lets you know whether you have passed or failed.
The word data refers to any information which is presented in a form
that is agreed and accepted upon by is creators and users.
BASICS OF DATA COMMUNICATION
3. Data Communication: Data Communication is a process of exchanging
data or information In case of computer networks this exchange is done
between two devices over a transmission medium.
This process involves a communication system that is made up of
hardware and software. The hardware part involves the sender and
receiver devices and the intermediate devices through which the data
passes. The software part involves certain rules which specify what is to
be communicated, how it is to be.
BASICS OF DATA COMMUNICATION
4. CHARACTERISTICS OF DATA
COMMUNICATION
The effectiveness of any data communications system depends upon the
following four fundamental characteristics:
Delivery: The data should be delivered to the correct destination and correct
user.
Accuracy: The communication system should deliver the data accurately,
without introducing any errors. The data may get corrupted during
transmission affecting the accuracy of the delivered data.
Timeliness: Audio and Video data has to be delivered in a timely manner
without any delay; such a data delivery is called real time transmission of
data.
Jitter: It is the variation in the packet arrival time. Uneven Jitter may affect
the timeliness of data being transmitted.
6. Message: Message is the information to be communicated by the sender to
the receiver.
Sender: The sender is any device that is capable of sending the data
(message).
Receiver: The receiver is a device that the sender wants to communicate the
data (message).
Transmission Medium: It is the path by which the message travels from
sender to receiver. It can be wired or wireless and many subtypes in both
COMPONENTS OF DATA COMMUNICATION
7. Protocol: It is an agreed-upon set or rules used by the sender and
receiver to communicate data. A protocol is a set of rules that governs
data communication. A Protocol is a necessity in data communications
without which the communicating entities are like two persons trying to
talk to each other in a different language without knowing the other
language.
COMPONENTS OF DATA COMMUNICATION
8. DATA REPRESENTATION
Data is a collection of raw facts which is processed to deduce
information.
Text: Text includes a combination of alphabets in small case as well
as upper case. It is stored as a pattern of bits. Prevalent encoding
system: ASCII, Unicode.
Numbers: Numbers include a combination of digits from 0 to 9. It is
stored as a pattern of bits. Prevalent encoding system: ASCII,
Unicode.
9. Images: A Pixel is the smallest element of an image. To put it in
simple terms, a picture or image is a matrix of pixel elements.
Audio: Data can also be in the form of sound which can be recorded
and broadcast. Example: What we hear on the radio is a source of
data or information. Audio data is continuous, not discrete.
Video: Video refers to the broadcasting of data in the form of
picture or movies.
DATA REPRESENTATION
10. DATA FLOW
The data can flow between the two devices in the
following ways.
Simplex
Half Duplex
Full Duplex
14. COMPUTER NETWORK
Computer Networks are used for data communications
A computer network can be defined as a collection of nodes. A
node can be any device capable of transmitting or receiving data.
The communicating nodes have to be connected by
communication links.
18. CATEGORIES OF TOPOLOGY
The arrangement of a network that comprises nodes and connecting lines via
sender and receiver is referred to as Network Topology. The various network
topologies are:
•Point to Point Topology
•Mesh Topology
•Star Topology
•Bus Topology
•Ring Topology
•Tree Topology
•Hybrid Topology
19. POINT TO POINT TOPOLOGY
Point-to-Point Topology is a type of
topology that works on the
functionality of the sender and
receiver. It is the simplest
communication between two nodes,
in which one is the sender and the
other one is the receiver. Point-to-
Point provides high bandwidth.
20. In a mesh topology, every device is
connected to another device via a
particular channel. In Mesh Topology,
the protocols used are AHCP (Ad Hoc
Configuration Protocols), DHCP
(Dynamic Host Configuration
Protocol), etc.
MESH TOPOLOGY
21. STAR TOPOLOGY
In Star Topology, all the devices are
connected to a single hub through
a cable. This hub is the central
node and all other nodes are
connected to the central node.
22. BUS TOPOLOGY
Bus Topology is a network type in which every computer and network device is
connected to a single cable. It is bi-directional. It is a multi-point connection and
a non-robust topology because if the backbone fails the topology crashes. In
Bus Topology, various MAC (Media Access Control) protocols are followed by
LAN ethernet connections like TDMA, Pure Aloha, CDMA, Slotted Aloha, etc.
23. RING TOPOLOGY
In a Ring Topology, it forms a ring connecting devices with exactly two
neighboring devices. A number of repeaters are used for Ring topology with a
large number of nodes, because if someone wants to send some data to the last
node in the ring topology with 100 nodes, then the data will have to pass
through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters
are used in the network.
The data flows in one direction, i.e. it is unidirectional, but it can be made
bidirectional by having 2 connections between each Network Node, it is
called Dual Ring Topology. In-Ring Topology, the Token Ring Passing protocol is
used by the workstations to transmit the data.
25. TREE TOPOLOGY
This topology is the variation of the Star topology. This topology has a
hierarchical flow of data. In Tree Topology, protocols like DHCP and SAC
(Standard Automatic Configuration ) are used.
26. HYBRID TOPOLOGY
This topological technology is the combination of all the various
types of topologies we have studied above. Hybrid Topology is
used when the nodes are free to take any form. It means these
can be individuals such as Ring or Star topology or can be a
combination of various types of topologies seen above. Each
individual topology uses the protocol that has been discussed
earlier.
28. NETWORK MODELS AND
ADDRESSING
•LAYERED TASKS
We use the concept of layers in our daily life. As an example,
let us consider two friends who communicate through postal
mail The process of sending a letter to a friend would be
complex if there were no services available from the post
office.
The below Figure shows the steps in this task. In the Figure,
we have a sender, a receiver, and a carrier that transports the
letter. There is a hierarchy of tasks.
30. NETWORK MODELS AND
ADDRESSING
At the Sender Site
The activities that take place at the sender site :
» Higher layer: The sender writes the letter, inserts the letter in an
envelope, writes the sender and receiver addresses, and drops the
letter in a mailbox.
» Middle layer: The letter is picked up by a letter carrier and
delivered to the post office.
» Lower layer: The letter is sorted at the post office; a carrier
transports the letter.
31. NETWORK MODELS AND
ADDRESSING
0n the Way
The letter is then on its way to the recipient. On the way to the recipient's
local post office, the letter may actually go through a central office. In
addition, it may be transported by truck, train, airplane, boat, or a
combination of these.
At the Receiver Site
» Lower layer: The carrier transports the letter to the post office.
» Middle layer: The letter is sorted and delivered to the recipient's mailbox.
» Higher layer: The receiver picks up the letter, opens the envelope and
reads it.
32. NETWORK MODELS AND
ADDRESSING
0n the Way
The letter is then on its way to the recipient. On the way to the recipient's
local post office, the letter may actually go through a central office. In
addition, it may be transported by truck, train, airplane, boat, or a
combination of these.
At the Receiver Site
» Lower layer: The carrier transports the letter to the post office.
» Middle layer: The letter is sorted and delivered to the recipient's mailbox.
» Higher layer: The receiver picks up the letter, opens the envelope and
reads it.
33. THE OSI MODEL
OSI stands for Open Systems Interconnection. It has 7 layers
Physical layer, Data Link layer, Network layer, Transport layer,
Session layer, Presentation layer, and Application layer. Each
layer performs its task independently. It was developed in 1984
by the International Organization for Standardization (ISO).
43. LAYERS IN OSI
Session Layer: This layer maintains sessions between remote
hosts. For example, once user/password authentication is done, the
remote host maintains this session for a while and does not ask for
authentication again in that time span.
44. LAYERS IN OSI
Presentation Layer: This layer defines how data in the native
format of the remote host should be presented in the native
format of the host.
45. LAYERS IN OSI
Application Layer: This layer is responsible for providing an
interface to the application user. This layer encompasses
protocols that directly interact with the user.
47. TCP/IP MODEL
The layers in the TCP/IP protocol suite do not exactly match
those in the OSI model. The original TCP/IP protocol suite
was defined as having four layers: host-to-network, internet,
transport, and application. However, when TCP/IP is
compared to OSI, we can say that the TCP/IP protocol suite
is made of five layers: physical, data link, network, transport,
and application.
48. TCP/IP MODEL
The main work of TCP/IP is to transfer the data of a computer
from one device to another. The main condition of this process is
to make data reliable and accurate so that the receiver will receive
the same information that is sent by the sender. To ensure that,
each message reaches its final destination accurately, the TCP/IP
model divides its data into packets and combines them at the
other end, which helps in maintaining the accuracy of the data
while transferring from one end to another end.
51. ADDRESSING
Four levels of addresses are used in an internet employing the
TCP/IP protocols: physical, logical, port, and specific
Topics discussed in this section:
Physical Address
Logical Address
Port Address
Specific Address
2.51
54. 2.54
In Figure a node with physical address 10 sends a frame
to a node with physical address 87. The two nodes are
connected by a link (bus topology LAN). As the figure
shows, the computer with physical address 10 is the
sender, and the computer with physical address 87 is the
receiver.
Example 1
56. 2.56
Most local-area networks use a 48-bit (6-byte) physical
address written as 12 hexadecimal digits; every byte (2
hexadecimal digits) is separated by a colon, as shown
below:
Example 2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
57. 2.57
Figure 2.20 shows a part of an internet with two routers
connecting three LANs. Each device (computer or router)
has a pair of addresses (logical and physical) for each
connection. In this case, each computer is connected to
only one link and therefore has only one pair of
addresses. Each router, however, is connected to three
networks (only two are shown in the figure). So each
router has three pairs of addresses, one for each
connection.
Example 3
59. 2.59
Figure 2.21 shows two computers communicating via the
Internet. The sending computer is running three processes
at this time with port addresses a, b, and c. The receiving
computer is running two processes at this time with port
addresses j and k. Process a in the sending computer
needs to communicate with process j in the receiving
computer. Note that although physical addresses change
from hop to hop, logical and port addresses remain the
same from the source to destination.
Example 4
61. 2.61
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Note
62. 2.62
Example 5
A port address is a 16-bit address represented by one
decimal number as shown.
753
A 16-bit port address represented
as one single number.
63. DATA & SIGNALS
An electrical or electromagnetic quantity (current, voltage, radio wave,
microwave, etc.) that carries data or information from one system (or
network) to another is called a signal.
Two basic types of signals are used for carrying data, viz. analog signal,
and digital signal.
Analog and digital signals are different from each other in many aspects.
One major difference between the two signals is that an analog signal is
a continuous function of time, whereas a digital signal is a discrete
function of time.
65. SIGNALS CAN BE OF TWO
TYPES
Analog Signal: They have infinite values in a range.
What is an Analog Signal?
A signal which is a continuous function of time and used to carry the
information is known as an analog signal. An analog signal represents a
quantity analogous to another quantity, for example, in the case of an analog
audio signal, the instantaneous value of signal voltage represents the pressure
of the sound wave.
Analog signals utilize the properties of medium to convey the information. All
the natural signals are the examples of analog signals. However, the analog
signals are more susceptible to the electronic noise and distortion which can
degrade the quality of the signal.
66. SIGNALS CAN BE OF TWO
TYPES
What is a Digital Signal?
A signal that is discrete function of time, i.e. which is not a
continuous signal, is known as a digital signal. The digital
signals are represented in the binary form and consist of
different values of voltage at discrete instants of time.
Basically, a digital signal represents the data and information
as a sequence of separate values at any given time. The
digital signal can only take on one of a finite number of
values.
67. ANALOG AND DIGITAL
Analog-to-analog conversion is the representation of
analog information by an analog signal. One may ask why
we need to modulate an analog signal; it is already analog.
Modulation is needed if the medium is bandpass in nature
or if only a bandpass channel is available to us.
Topics discussed in this section:
Amplitude Modulation
Frequency Modulation
Phase Modulation
69. Amplitude Modulation
A carrier signal is modulated only in amplitude value
The modulating signal is the envelope of the carrier
The required bandwidth is 2B, where B is the
bandwidth of the modulating signal
Since on both sides of the carrier freq. fc, the spectrum
is identical, we can discard one half, thus requiring a
smaller bandwidth for transmission.
73. Frequency Modulation
The modulating signal changes the freq. fc of the carrier signal
The bandwidth for FM is high
It is approx. 10x the signal frequency
74. The total bandwidth required for FM can
be determined from the bandwidth
of the audio signal: BFM = 2(1 + β)B.
Where is usually 4.
Note
77. Phase Modulation (PM)
The modulating signal only changes the phase of the carrier signal.
The phase change manifests itself as a frequency change but the
instantaneous frequency change is proportional to the derivative of the
amplitude.
The bandwidth is higher than for AM.
79. The total bandwidth required for PM can
be determined from the bandwidth
and maximum amplitude of the
modulating signal:
BPM = 2(1 + β)B.
Where = 2 most often.
Note
80. ANALOG-TO-DIGITAL
CONVERSION
A digital signal is superior to an analog signal because it is more
robust to noise and can quickly be recovered, corrected, and
amplified. For this reason, today’s tendency 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)
81. PCM
PCM consists of three steps to digitize an analog signal:
1. Sampling
2. Quantization
3. Binary encoding
Before we sample, we have to filter the signal to limit
the maximum frequency of the movement as it affects
the sampling rate.
Filtering should ensure that we do not distort the signal,
i.e. remove high-frequency components that affect the
signal shape.
83. Sampling
Analog signal is sampled every TS secs.
Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
There are 3 sampling methods:
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flattop - sample and hold, like natural but with single amplitude
value
The process is referred to as pulse amplitude modulation PAM and
the outcome is a signal with analog (non integer) values
87. Noiseless Channel
Noiseless Channel: Nyquist Bit Rate: For a noiseless channel, the
Nyquist bit rate formula defines the theoretical maximum bit rate
Nyquist proved that if an arbitrary signal has been run through a
low-pass filter of bandwidth, the filtered signal can be completely
reconstructed by making only 2*Bandwidth (exact) samples per
second. Sampling the line faster than 2*Bandwidth times per second
is pointless because the higher-frequency components that such
sampling could recover have already been filtered out. If the signal
consists of L discrete levels, Nyquist’s theorem states:
88. Noiseless Channel
Noiseless Channel: Nyquist Bit Rate: For a noiseless channel, the
Nyquist bit rate formula defines the theoretical maximum bit rate
Nyquist proved that if an arbitrary signal has been run through a low-
pass filter of bandwidth, the filtered signal can be completely
reconstructed by making only 2*Bandwidth (exact) samples per second.
Sampling the line faster than 2*Bandwidth times per second is pointless
because the higher-frequency components that such sampling could
recover have already been filtered out. If the signal consists of L discrete
levels, Nyquist’s theorem states:
BitRate = 2 * Bandwidth * log2(L) bits/sec
89. Nyquist Bit Rate
BitRate = 2 * Bandwidth * log2(L) bits/sec
In the above equation, bandwidth is the bandwidth of the channel, L
is the number of signal levels used to represent data, and BitRate is
the bit rate in bits per second.
Bandwidth is a fixed quantity, so it cannot be changed. Hence, the
data rate is directly proportional to the number of signal levels.
90. Examples
Input1: Consider a noiseless channel with a bandwidth of 3000 Hz
transmitting a signal with two signal levels. What can be the
maximum bit rate?
Output1 : BitRate = 2 * 3000 * log2(2) = 6000bps
Input2: We need to send 265 kbps over a noiseless channel with a
bandwidth of 20 kHz. How many signal levels do we need?
Output2 : 265000 = 2 * 20000 * log2(L) log2(L) = 6.625 L = 26.625 =
98.7 levels
91. Quantization
Quantization is the process of mapping continuous infinite
values to a smaller set of discrete finite values. In the context
of simulation and embedded computing, it is about
approximating real-world values with a digital
representation that introduces limits on the precision and
range of a value.
94. Sampling results in a series of pulses of varying
amplitude values ranging between two limits: a min
and a max.
The amplitude values are infinite between the two
limits.
We need to map the infinite amplitude values onto a
finite set of known values.
This is achieved by dividing the distance between min
and max into L zones, each of height
= (max - min)/L
Quantization
96. Quantization Error
When a signal is quantized, we introduce an error -
the coded signal is an approximation of the actual
amplitude value.
The difference between actual and coded value
(midpoint) is referred to as the quantization error.
The more zones, the smaller which results in
smaller errors.
BUT, the more zones the more bits required to
encode the samples -> higher bit rate
97. Bit rate and bandwidth
requirements of PCM
The bit rate of a PCM signal can be calculated form the
number of bits per sample x the sampling rate
Bit rate = nb x fs
The bandwidth required to transmit this signal depends on
the type of line encoding used. Refer to previous section for
discussion and formulas.
A digitized signal will always need more bandwidth than the
original analog signal. Price we pay for robustness and other
features of digital transmission.
98. 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:
Example
99. PCM Decoder
To recover an analog signal from a digitized signal
we follow the following steps:
◦ We use a hold circuit that holds the amplitude value of a
pulse till the next pulse arrives.
◦ We pass this signal through a low pass filter with a cutoff
frequency that is equal to the highest frequency in the
pre-sampled signal.
The higher the value of L, the less distorted a signal
is recovered.
101. We have a low-pass analog signal of 4 kHz. If we send
the analog signal, we need a channel with a minimum
bandwidth of 4 kHz. If we digitize the signal and send 8
bits per sample, we need a channel with a minimum
bandwidth of 8 × 4 kHz = 32 kHz.
Example
102. Delta Modulation
This scheme sends only the difference between pulses, if the pulse at
time tn+1 is higher in amplitude value than the pulse at time tn, then a
single bit, say a “1”, is used to indicate the positive value.
If the pulse is lower in value, resulting in a negative value, a “0” is used.
This scheme works well for small changes in signal values between
samples.
If changes in amplitude are large, this will result in large errors.
106. Delta PCM (DPCM)
Instead of using one bit to indicate positive and
negative differences, we can use more bits ->
quantization of the difference.
Each bit code is used to represent the value of the
difference.
The more bits the more levels -> the higher the
accuracy.
107. 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
111. 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.
Note
114. 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. The bits
are usually sent as bytes and many
bytes are grouped in a frame. A frame is
identified with a start and an end byte.
Note
117. Multiplexing
Multiplexing is the sharing of a medium or
bandwidth. It is the process in which multiple signals
coming from multiple sources are combined and
transmitted over a single communication/physical line.
119. FDM
Frequency Division Multiplexing :
Frequency division multiplexing is defined as a type of multiplexing
where the bandwidth of a single physical medium is divided into a
number of smaller, independent frequency channels.
120. FDM
Frequency Division Multiplexing is used in radio and television
transmission.
In FDM, we can observe a lot of inter-channel cross-talk, due to the
fact that in this type of multiplexing the bandwidth is divided into
frequency channels. In order to prevent inter-channel cross-talk,
unused strips of bandwidth must be placed between each channel.
These unused strips between each channel are known as guard
bands.
