DATA COMMUNICATION- 5TH SEM-BSC(ITM)

UNIT-III
Sl.
UNIT QUESTIONS
No.
1 III What is transmission media and types of transmission media?
2 III What is switching and describe about circuit switching?
3 III What is multiplexing and types of multiplexing?
4 III What is spread spectrum modulation?
5 III What is virtual circuits networks?
6 III Describe Telephone networks?
7 III What are datagram networks?
8 III Describe cable TV networks.

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 1. What is transmission media and types of transmission media?
The Computer and other telecommunication devices use signals to represent data. These signals
are transmitted from one device to another in the form of electromagnetic energy.
Electromagnetic signals can travel through a vacuum, air or other transmission media.
Electromagnetic energy, a combination of electric and magnetic fields vibrating in relation to each
other.
It is of two types:
Guided Media
(b)Unguided Media
(a) Guided media: -
Guided Media, which are those that provide a conduit from one device to another. A signal
traveling along these media is directed and contained by physical limits of the medium.
Guided media is of three types
(i)Twisted pair cable
(ii)Coaxial cable
(iii)Fiber optics cable
(i) Twisted pair cable
Both UTP and STP uses a snap-in-plug connector like that used with telephone jacks. These
connectors are known as RJ (Redundant Jack) connector.
The various RJ connectors are –
RJ-11 (4 Pin)
RJ-13 (6 Pin)
RJ-45 (8Pin)
Twisted pair cable is of two types
(i)Unshielded Twisted pair cable (UTP)
(ii)Shielded Twisted pair cable (STP)
i)Unshielded Twisted pair cable
Unshielded Twisted Pair (UTP) cable is the most common type of telecommunication medium in
use today.
Its frequency range is suitable for transmitting both data and voice.
It supports a frequency band of 100Hz to 5 MHz. A UTP consist of two conductors (usually copper)
each with its own colored plastic insulation.
The plastic insulation is color banded for identification.
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(b)Shielded Twisted pair cable
STP Cable has a metal foil or mesh covering that encases each pair of insulating conductors.
The metal casing prevents the penetration of electromagnetic noise as a result STP is less
susceptible to noise. It supports a frequency band of 100Hz to 5 MHz.
ii) Coaxial Cable
Coaxial cable also known as coax carries signals of higher frequency ranges than twisted pair cable.
It supports frequency range from 100 MHz to 500 MHz Instead of having two wires, coax has a
central core conductor of solid copper wire enclosed in an insulating sheath, which is in turn,
encased in an outer conductor or metal foil. The outer metallic wrapping serves both as a shield
against noise and as a second conductor, which completes the circuit. This outer conductor is also
enclosed in an insulating sheath and the whole cable is protected by a plastic cover.

iii) Fiber optics Cable

Up until this point, we have discussed conductive (metal) cables that transmit signals in the form of
current. Optical fiber on the other hand, is made up of glass or plastic and transmits signals in the
form of light. To understand optical fiber, we first need to explore several aspects of the nature of
light.
Light is a form of energy that travels at its fastest in vacuum at speed of 300,000 kilometers/second
or 1,86,000 miles/second approx. The speed of light depends upon the density of medium through
which it is traveling (the higher the density, slower the speed).

Light travels in a straight line as long as it is moving though a single uniform substance.
If a ray of light traveling through one substance suddenly enters another (more or less dense)
substance, its speed changes abruptly, causing the ray to change direction. This change is called
refraction.
A beam of light moving from a less dense into a denser medium is bent toward the vertical axis. The
two angles made by the beam of light in relation to the vertical axis are called, “i” for incident and

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“r” for refracted. The beam travels from a denser medium to a less dense medium bents away from
the vertical axis.

If a beam of light moves from high dense medium to less dense medium, the direction of beam
changes. It forms two angles with vertical axis, angle of incidence “i” and angle of refraction “r”. If
“i” “r” also increases, like this if we go on increasing the angle of incidence a point will come when r
= 90o, at this point angle of incidence is also known as Critical angle “c”. If we further increase “i”
than greater than “c” than a new phenomenon occurs called reflection. Light no longer passes into
the less dense medium at all. In this case i = r. Optical Fiber uses this reflection technique to guide
light through a channel.
Fiber optics cable composition
This cable consists of a fiber core made of glass or plastic surrounded by a cladding of less dense
glass or plastic. The difference in density of the two material must be such that a beam of light
moving through the core is reflected off the cladding instead of being refracted into it. Fiber core is
covered by a buffer layer that protects it from moisture. Finally, the entire cable is encased in an
outer jacket.
Advantages of Optical Fiber –
 Noise resistance
 Less signal attenuation
Disadvantages of Optical Fiber –
 High Cost
 Installation and Maintenance difficult
 Fragile
(b) Un-Guided Media: -

Infrared

Low frequency infrared waves are used for very short distance communication like TV remote,
wireless speakers, automatic doors, hand held devices etc. Infrared signals can propagate within a
room but cannot penetrate walls. However, due to such short range, it is considered to be one of
the most secure transmission modes.

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Radio Wave

Transmission of data using radio frequencies is called radio-wave transmission. We all are familiar
with radio channels that broadcast entertainment programs. Radio stations transmit radio waves
using transmitters, which are received by the receiver installed in our devices.
Both transmitters and receivers use antennas to radiate or capture radio signals. These radio
frequencies can also be used for direct voice communication within the allocated range. This
range is usually 10 miles.

Advantages of Radio Wave

These are some of the advantages of radio wave transmissions −

 Inexpensive mode of information exchange

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  No land needs to be acquired for laying cables
 Installation and maintenance of devices is cheap

Disadvantages of Radio Wave

These are some of the disadvantages of radio wave transmissions −

 Insecure communication medium

 Prone to weather changes like rain, thunderstorms, etc.

2. What is switching and describe about circuit switching?

Switching is a process to forward packets coming in from one port to a port leading towards the
destination.
Types of switching in categories
 Connection less
 Connection Oriented
WAN connection Types
 Message Switching
 Circuit Switching
 Packet switching
Circuit Switching
When two nodes (computer) communication with each other over a dedicated communication
path. It is called Circuit switching.
There is a need of prespecified rout from which data will trowels and no other data is permitted.
In circuit switching to transfer the data circuit must be established so that the data transfer can
take place.
Circuits can be permanent or temporary. Applications which use circuit switching may have to go
through three processes.
 Establish a circuit
 Transfer data
 Disconnect the circuit

3. What is multiplexing and types of multiplexing?

Multiplexing is a technique by which different Analog and digital streams of transmission can be
simultaneously processed over a shared link. Multiplexing divides the high capacity medium into low
capacity logical medium which is then shared by different streams.

Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical
fiber). All mediums are capable of multiplexing.

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When multiple senders try to send over a single medium, a device called Multiplexer divides the physical
channel and allocates one to each. On the other end of communication, a De-multiplexer receives data from
a single medium, identifies each, and sends to different receivers.

Frequency Division Multiplexing

When the carrier is frequency, FDM is used. FDM is an analog technology. FDM divides the
spectrum or carrier bandwidth in logical channels and allocates one user to each channel. Each user
can use the channel frequency independently and has exclusive access of it. All channels are divided
in such a way that they do not overlap with each other. Channels are separated by guard bands.
Guard band is a frequency which is not used by either channel.

Time Division Multiplexing

TDM is applied primarily on digital signals but can be applied on analog signals as well. In TDM the
shared channel is divided among its user by means of time slot. Each user can transmit data within
the provided time slot only. Digital signals are divided in frames, equivalent to time slot i.e. frame of
an optimal size which can be transmitted in given time slot.
TDM works in synchronized mode. Both ends, i.e. Multiplexer and De-multiplexer are timely
synchronized and both switch to next channel simultaneously.

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When channel A transmits its frame at one end, the De-multiplexer provides media to channel A on
the other end. As soon as the channel A’s time slot expires, this side switches to channel B. On the
other end, the De-multiplexer works in a synchronized manner and provides media to channel B.
Signals from different channels travel the path in interleaved manner.
Wavelength Division Multiplexing
Light has different wavelength (colors). In fiber optic mode, multiple optical carrier signals are
multiplexed into an optical fiber by using different wavelengths. This is an analog multiplexing
technique and is done conceptually in the same manner as FDM but uses light as signals.

Further, on each wavelength time division multiplexing can be incorporated to accommodate more
data signals.
Code Division Multiplexing
Multiple data signals can be transmitted over a single frequency by using Code Division
Multiplexing. FDM divides the frequency in smaller channels but CDM allows its users to full
bandwidth and transmit signals all the time using a unique code. CDM uses orthogonal codes to
spread signals.
Each station is assigned with a unique code, called chip. Signals travel with these codes
independently, inside the whole bandwidth. The receiver knows in advance the chip code signal it
has to receive.

4. What is spread spectrum modulation?

Collective classes of signaling techniques are employed before transmitting a signal to provide a secure
communication, known as the Spread Spectrum Modulation. The main advantage of spread spectrum
communication technique is to prevent “interference” whether it is intentional or unintentional.

The signals modulated with these techniques are hard to interfere and cannot be jammed. An intruder with
no official access is never allowed to crack them. Hence, these techniques are used for military purposes.
These spread spectrum signals transmit at low power density and has a wide spread of signals.

Pseudo-Noise Sequence:

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A coded sequence of 1s and 0s with certain auto-correlation properties, called as Pseudo-Noise coding
sequence is used in spread spectrum techniques. It is a maximum-length sequence, which is a type of cyclic
code.

Narrow-band and Spread-spectrum Signals:

Both the Narrow band and Spread spectrum signals can be understood easily by observing their frequency
spectrum as shown in the following figures.

Narrow-band Signals
The Narrow-band signals have the signal strength concentrated as shown in the following frequency
spectrum figure.

Following are some of its features −

Band of signals occupy a narrow range of frequencies.

Power density is high.
Spread of energy is low and concentrated.
Though the features are good, these signals are prone to interference.

Spread Spectrum Signals

The spread spectrum signals have the signal strength distributed as shown in the following frequency
spectrum figure.

Following are some of its features −

Band of signals occupy a wide range of frequencies.

Power density is very low.
Energy is wide spread.
With these features, the spread spectrum signals are highly resistant to interference or jamming. Since
multiple users can share the same spread spectrum bandwidth without interfering with one another, these
can be called as multiple access techniques.

Advantages of Spread Spectrum

Following are the advantages of spread spectrum −
Cross-talk elimination
Better output with data integrity
Reduced effect of multipath fading
Better security
Reduction in noise
Co-existence with other systems
Longer operative distances

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Hard to detect
Not easy to demodulate/decode
Although spread spectrum techniques were originally designed for military uses, they are now
being used widely for commercial purpose.

5. What is a virtual circuit’s network?

A virtual circuit (VC) is a means of transporting data over a packet switched computer network in
such a way that it appears as though there is a dedicated physical layer link between the source and
destination end systems of this data. The term virtual circuit is synonymous with virtual
connection and virtual channel. Before a connection or virtual circuit may be used, it has to be
established, between two or more nodes or software applications, by configuring the relevant parts
of the interconnecting network. After that, a bit stream or byte stream may be delivered between
the nodes; hence, a virtual circuit protocol allows higher level protocols to avoid dealing with the
division of data into segments, packets, or frames.
Virtual circuit communication resembles circuit switching, since both are connection oriented,
meaning that in both cases data is delivered in correct order, and signaling overhead is required
during a connection establishment phase. However, circuit switching provides a constant bit rate
and latency, while these may vary in a virtual circuit service due to factors such as:
varying packet queue lengths in the network nodes, varying bit rate generated by the application,
varying load from other users sharing the same network resources by means of statistical
multiplexing, etc.
Many virtual circuit protocols, but not all, provide reliable communication service through the use
of data retransmissions because of error detection and automatic repeat request (ARQ).
Examples of protocols that provide virtual circuits
Examples of transport layer protocols that provide a virtual circuit:
Transmission Control Protocol (TCP), where a reliable virtual circuit is established on top of the
underlying unreliable and connectionless IP protocol. The virtual circuit is identified by the source
and destination network socket address pair, i.e. the sender and receiver IP address and port
number. Guaranteed QoS is not provided.
Stream Control Transmission Protocol (SCTP), where a virtual circuit is established on top of the IP
protocol.
Examples of network layer and datalink layer virtual circuit protocols, where data always is
delivered over the same path:
X.25, where the VC is identified by a virtual channel identifier (VCI). X.25 provides reliable node-to-
node communication and guaranteed QoS.
Frame relay, where the VC is identified by a DLCI. Frame relay is unreliable, but may provide
guaranteed QoS.

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Asynchronous Transfer Mode (ATM), where the circuit is identified by a virtual path identifier (VPI)
and virtual channel identifier (VCI) pair. The ATM layer provides unreliable virtual circuits, but the
ATM protocol provides for reliability through the ATM adaptation layer (AAL) Service Specific
Convergence Sub layer (SSCS) (though it uses the terms "assured" and "non-assured" rather than
"reliable" and "unreliable").
6. Describe Telephone networks?
The earliest electronic network is the telephone system. This telephone network commonly uses
analog technology that was quite different from digital technology used in the computer-based
networks. The advantages of digital technology over the analog technology in terms of economics
and services forced the telephone industry to move rapidly to install fiber and digital networks.
The telephone network transmits Analog signals and hence a modem is required whenever a computer or
terminal is connected to the telephone line. The modem then converts digital data from a computer to an
Analog signal that can be transmitted via a telecommunication line and converts the Analog signal received
to computer data.

Dial-up Telephone Networks

The telephone network consists of the subscriber's line, switchboards, and trunk lines. Each subscriber line
has an address, i.e. telephone number. When a caller transmits a dial signal to the switchboard, the
switchboard connects the caller's subscriber line to that of the receiver, enabling communication. The trunk
line between the caller and the receiver is occupied until either discontinues the communication.

Advantages of Telephone Networks

It is circuit-switching network; therefore, any receiver can be selected and' there is virtually no
transmission delay.
As it is widely spread therefore it is available at a low price.
Disadvantages of Telephone Networks: -
It requires a long time for connection. A dial-up operation is necessary before the line can be
connected to the receiver. This dial-up time is too long to use in data communication systems.
It has low transmission speed.
The line quality is not sufficient for data transmission, and is therefore, not appropriate for high
speed data transmission because telephone lines were originally developed for audio
communication,

7. What are datagram networks?

A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are
typically structured in header and payload sections. Datagrams provide a connectionless
communication service across a packet-switched network. The delivery, arrival time, and order of
arrival of datagrams need not be guaranteed by the network.

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A datagram needs to be self-contained without reliance on earlier exchanges because there is no
connection of fixed duration between the two communicating points as there is, for example, in
most voice telephone conversations.[3]
Datagram service is often compared to a mail delivery service; the user only provides the
destination address, but receives no guarantee of delivery, and no confirmation upon successful
delivery. Datagram service is therefore considered unreliable. Datagram service routes datagrams
without first creating a predetermined path. Datagram service is therefore
considered connectionless. There is also no consideration given to the order in which it and other
datagrams are sent or received. In fact, many datagrams in the same group can travel along
different paths before reaching the same destination.
Each datagram has two components, a header and a data payload. The header contains all the
information sufficient for routing from the originating equipment to the destination without relying
on prior exchanges between the equipment and the network. Headers may include source and
destination addresses as well as a type field. The payload is the data to be transported. This process
of nesting data payloads in a tagged header is called encapsulation.

8. Describe cable TV networks.

Cable television is a popular television system that delivers television programming services
through cables. This is different from terrestrial television (where radio waves are transmitted over
air and received by antennas) and satellite television (where signals are sent by communication
satellites and received by satellite dish).
Types of cables used in cable TV
Coaxial cables through which radio-frequency signals are transmitted
Fiber optic cables through which light pulses are sent
Services offered by Cable TV
Originally used for broadcasting television services, the functionalities of cable TV has now been
extended for providing different services of computer networks as well.
Some of the most predominant services of cable TV are −
 FM programming
 Cable Internet
 Telephone services

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