Unit 1

SRI KRISHNA COLLEGE OF TECHNOLOGY
DEPARTMENT OF CSE, 17CS313 - COMPUTER NETWORKS
NIVETHA N R P AP/CSE
UNIT - 1 DATA COMMUNICATIONS
Data communications and physical layer:
Introduction, history and development of computer networks, networks topologies,
ISO/OSI model and protocols. Different types of transmission media, errors in
transmission: attenuation, noise. Repeaters. Encoding (NRZ, NRZI, Manchester,
4B/5B).
DATA COMMUNICATIONS AND PHYSICAL LAYER
1. INTRODUCTION
 When we communicate, we are sharing information. This sharing can be local
or remote. Between individuals, local communication usually occurs face to
face, while remote communication takes place over distance.
 The term telecommunication, which includes telephony, telegraphy, and
television, means communication at a distance.
 The word data refers to information presented in whatever form is agreed upon
by the parties creating and using the data.
 Data communications are the exchange of data between two devices via some
form of transmission medium such as a wire cable. For data communications to
occur, the communicating devices must be part of a communication system
made up of a combination of hardware (physical equipment) and software
(programs).
 The effectiveness of a data communications system depends on four
fundamental characteristics: delivery, accuracy, timeliness, and jitter.
 Delivery. The system must deliver data to the correct destination. Data
must be received by the intended device or user and only by that device
or user.
 Accuracy. The system must deliver the data accurately. Data that have
been altered in transmission and left uncorrected are unusable.
 Timeliness. The system must deliver data in a timely manner. Data
delivered late are useless. In the case of video and audio, timely delivery
means delivering data as they are produced, in the same order that they
are produced, and without significant delay. This kind of delivery is
called real-time transmission.

 Jitter. Jitter refers to the variation in the packet arrival time. It is the
uneven delay in the delivery of audio or video packets. For example, let
us assume that video packets are sent every 30 ms. If some of the packets
arrive with 30 ms delay and others with 40 ms delay, an uneven quality in
the video is the result.
COMPONENTS
 A data communications system has five components.
Components of data communication
 Message. The message is the information (data) to be communicated.
Popular forms of information include text, numbers, pictures, audio, and
video.
 Sender. The sender is the device that sends the data message. It can be a
computer, workstation, telephone handset, video camera, and so on.
 Receiver. The receiver is the device that receives the message. It can be a
computer, workstation, telephone handset, television, and so on.
 Transmission medium. The transmission medium is the physical path by
which a message travels from sender to receiver. Some examples of
transmission media include twisted-pair wire, coaxial cable, fiber-optic
cable, and radio waves.
 Protocol. A protocol is a set of rules that govern data communications. It
represents an agreement between the communicating devices. Without a
protocol, two devices may be connected but not communicating, just as a
person speaking French cannot be understood by a person who speaks
only Japanese.
DATA REPRESENTATION
Information today comes in different forms such as text, numbers, images, audio, and
video.

TEXT
 In data communications, text is represented as a bit pattern, a sequence of
bits (0‟s or 1‟s).
 Different sets of bit patterns have been designed to represent text
symbols. Each set is called a code, and the process of representing
symbols is called coding.
 Today, the prevalent coding system is called Unicode, which uses 32 bits
to represent a symbol or character used in any language in the world.
 The American Standard Code for Information Interchange (ASCII),
developed some decades ago in the United States, now constitutes the first
127 characters in Unicode and is also referred to as Basic Latin.
NUMBERS
 Numbers are also represented by bit patterns. However, a code such as
ASCII is not used to represent numbers; the number is directly converted
to a binary number to simplify mathematical operations.
IMAGES
 Images are also represented by bit patterns. In its simplest form, an image
is composed of a matrix of pixels (picture elements), where each pixel is a
small dot.
 The size of the pixel depends on the resolution. For example, an image
can be divided into 1000 pixels or 10,000 pixels.
 In the second case, there is a better representation of the image (better
resolution), but more memory is needed to store the image.
 After an image is divided into pixels, each pixel is assigned a bit pattern.
 The size and the value of the pattern depend on the image. For an image
made of only black and- white dots (e.g., a chessboard), a 1-bit pattern is
enough to represent a pixel.
 If an image is not made of pure white and pure black pixels, you can
increase the size of the bit pattern to include gray scale.
 For example, to show four levels of gray scale, you can use 2-bit patterns.
A black pixel can be represented by 00, a dark gray pixel by 01, a light
gray pixel by 10, and a white pixel by 11.
 There are several methods to represent color images. One method is
called RGB, so called because each color is made of a combination of

three primary colors: red, green, and blue. The intensity of each color is
measured, and a bit pattern is assigned to it.
 Another method is called YCM, in which a color is made of a
combination of three other primary colors: yellow, cyan, and magenta.
AUDIO
 Audio refers to the recording or broadcasting of sound or music. Audio is
by nature different from text, numbers, or images.
 It is continuous, not discrete. Even when we use a microphone to change
voice or music to an electric signal, we create a continuous signal.
VIDEO
 Video refers to the recording or broadcasting of a picture or movie.
 Video can either be produced as a continuous entity (e.g., by a TV
camera), or it can be a combination of images, each a discrete entity,
arranged to convey the idea of motion.
 Again we can change video to a digital or an analog signal.
DATA FLOW
Communication between two devices can be simplex, half-duplex, or full-duplex
Data flow (simplex, half-duplex, and full-duplex)

SIMPLEX
 In simplex mode, the communication is unidirectional, as on a one-way
street. Only one of the two devices on a link can transmit; the other can
only receive.
 Keyboards and traditional monitors are examples of simplex devices. The
keyboard can only introduce input; the monitor can only accept output.
 The simplex mode can use the entire capacity of the channel to send data
in one direction.
HALF-DUPLEX
 In half-duplex mode, each station can both transmit and receive, but not at
the same time. When one device is sending, the other can only receive,
and vice versa.
 The half-duplex mode is like a one-lane road with traffic allowed in both
directions.
 When cars are traveling in one direction, cars going the other way must
wait. In a half-duplex transmission, the entire capacity of a channel is
taken over by whichever of the two devices is transmitting at the time.
 Walkie-talkies and CB (citizens band) radios are both half-duplex
systems.
 The half-duplex mode is used in cases where there is no need for
communication in both directions at the same time; the entire capacity of
the channel can be utilized for each direction.
FULL-DUPLEX
 In full-duplex mode (also called duplex), both stations can transmit and
receive simultaneously.
 The full-duplex mode is like a two-way street with traffic flowing in both
directions at the same time.
 In full-duplex mode, signals going in one direction share the capacity of
the link: with signals going in the other direction.
 This sharing can occur in two ways: Either the link must contain two
physically separate transmission paths, one for sending and the other for
receiving; or the capacity of the channel is divided between signals
traveling in both directions.

 One common example of full-duplex communication is the telephone
network.
 When two people are communicating by a telephone line, both can talk
and listen at the same time.
 The full-duplex mode is used when communication in both directions is
required all the time.
 The capacity of the channel, however, must be divided between the two
directions.
PHYSICAL STRUCTURES
Type of Connection
 A network is two or more devices connected through links.
 A link is a communications pathway that transfers data from one device to
another.
 For visualization purposes, it is simplest to imagine any link as a line
drawn between two points.
 For communication to occur, two devices must be connected in some way
to the same link at the same time.
 There are two possible types of connections: point-to-point and
multipoint.
Point-to-Point
 A point-to-point connection provides a dedicated link between two
devices.
 The entire capacity of the link is reserved for transmission between those
two devices.
 Most point-to-point connections use an actual length of wire or cable to
connect the two ends, but other options, such as microwave or satellite
links, are also possible.
 When you change television channels by infrared remote control, you are
establishing a point-to-point connection between the remote control and
the television's control system.
Multipoint
 A multipoint (also called multi drop) connection is one in which more
than two specific devices share a single link.

 In a multipoint environment, the capacity of the channel is shared, either
spatially or temporally.
 If several devices can use the link simultaneously, it is a spatially shared
connection. If users must take turns, it is a timeshared connection.
Network:
Definition:
A network is set of devices (nodes) connected by communication links (media)
A node can be a computer, printer or other device capable of sending and/or receiving
data Link connecting the devices are often called communication channels Most
network use distributed processing.
Distributed Processing
Networks use distributed processing in which a task divided among multiple
computers. Separate computers handle a subset instead of a single machine responsible
for all aspects of a process.
Performance
Performance can be measured in terms of transit time, response time, number of users,
type of transmission medium, and capabilities of the connected hardware and the
efficiency of the software.
Transit time
The time required for a message to travel from one device to another.
Response time
The time spent between an inquiry and a response
Reliability
It is measured by the frequency of failure and time required to recover from a failure.
Security
Network security is protecting data from unauthorized access.
Type of connection
Two types of connections
a. Point-to-point
b. Multipoint
In point-to-point connection the two devices are connected by a dedicated link. The
entire capacity of the link is reserved for transmission between those two devices.
A multipoint (also known as multidrop) connection is one in which more than two
specific devices share a single link. The capacity of the channel is shared either
spatially or temporally.

2. HISTORY AND DEVELOPMENT OF COMPUTER NETWORKS
Evolution of Network
Need of a Network:
 An “network” additionally enables associated PCs to share documents and
information and also equipment assets, i.e., scanners, plotters, projectors and
storage devices, making it simpler to gather and administer data, and enabling
clients to work together.
 Advancement of systems administration began path back in 1969‟s by the
improvement of first system called APRANET, which prompted the
improvement of web.
 At that point after constantly everyday upgradation occur in the system
innovation. The system has gone through a few phases which are described
below:
ARPANET (Advanced Research Agency Network):
 ARPANET was the network that became the basis for the Internet. It was first
network that came into existence in 1969, which was designed and named by
the Advanced Research Projects Agency (ARPA) and US Department of
Defence (DoD)
 It was where a bunch of PCs were associated at various colleges and US DoD
for sharing of information and messages and playing long separation diversions
and associating with individuals to share their perspectives.
NSFNET (National Science Federation Network):
 In mid 80‟s another federal agency, NSFNET (National Science Federation
Network) created a new network which was more capable than ARPANET
and became the first backbone infrastructure for the commercial public
Internet.
 Its main aim was to use network only for academic research and not for any
kind of private business activity.
 Later, many privately owned businesses with their very own private systems
joined with ARPANET and NSFNET to make more capable and wide
network, the Internet.
ARPANET + NSFNET + PRIVATE NETWORKS = INTERNET

Internet:
 In the Internet, which is a network of networks, came into existence. The
internet has evolved from APANET.
 The internet is a globally connected network system that utilizes TCP/IP
to transmit information.
 It allows computers of different types to exchange information and is
known as internet. The Internet is the financially communications method
on the planet, in which the following services are instantly available:
 Email
 Web-enabled audio/video conferencing services
 Online movies and gaming
 Data transfer/file-sharing, often through File Transfer Protocol (FTP)
 Instant messaging
 Internet forums
 Social networking
 Online shopping
 Financial services
Interspace:
 Interspace is a software that allows multiple users in a client-server environment
to communicate with each other to send and receive data of various types such
as data files, video, audio and textual data.
 Interspace gives the most exceptional type form of communication available on
the Internet today.

3. NETWORK TOPOLOGY
Physical Topology
Physical Topology refers to the way in which network is laid out physically. The
topology of a network is the geometric representation of the relationship of all the
links and the linking devices. The physical or logical arrangement of a network is also
topology.
The basic topologies are
*) Mesh
*) Star
*) Bus
*) Ring
Mesh
Dedicated point-to-point links to every other device
Has n(n-1)/2 physical channel to link n devices
Devices have n-1 I/O
Advantages:
Dedicated link guarantees that each connection can carry its own data load and thus
eliminates the traffic problems that occur when links shared by multiple devices.
If one link becomes unusable, it does not incapacitate the entire system.
As every message travels along a dedicated line only the intended recipient, so it is
secure.
Disadvantages
*)More amount of cabling and the I/O ports required
*)Installation and reconnection are difficult
*) The hardware required to connect each link can be prohibitively expensive.
Star
*)Dedicated point-to-point links to central controller (hub)
*)Controller acts as exchange
Advantages
*) Less expensive than a mesh topology.
*)Each device needs only one link and I/O port to connect with Hub
*)Installation and reconfigure is easier.
*)If one link fails only that link is affected.
*)Requires less cable than a mesh.
Disadvantages
*)Yet requires more cable compared to bus and ring topologies.
Bus
*)Multipoint configuration
*)One long cable acts as a backbone to link all devices.

*)Stations are connected through tap and drop lines.
Advantages
*)Ease of installation.
*)Uses less cabling than mesh or star topologies.
Disadvantages
*)Difficult reconnection and isolation.
*)Signal reflection at the taps can cause degradation in quality.
*)A fault or break in the bus cable stops all transmission.
Ring
*)Dedicated point-to-point configuration to neighbors
*)Signal passes from device to device until it reaches destination
*)Each device functions as a repeater
Advantages
*)Easy to install and reconfigure.
*)Only two connections are to be changed to add or delete a device.
*)If one device does not receive the signal within a specified period, it issue an alarm
that alerts the network operator to the problem and its location
Disadvantages
*)A break in the ring breaks the entire network.
Categories of Network
Three primary categories of network
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN)
The category into which a network fall is determined by its size, ownership, the
distance it covers and
its physical architecture.
LAN
*)Usually privately owned and links the devices in a single office, building, or campus
*)LAN size is limited to a few kilometers.
*)LANs are designed to allow resources to be shared (hardware , software and data )
*)Today LANs to have data rates of 100 Mbps to 10Gbps
*)Backbone Networks (BN), have a scale of a few hundred meters to a few kilometers.
Include a high speed backbone linking the LANs at various locations.
MAN
*)A MAN is designed to cover an entire city.
*)May be a single network such as cable TV network
*)May be a means of connecting a number of LANs into a larger network
*)MANs have data rates of 1 Mbps to 100 Mbps

*)Resources may be shared LAN to LAN as well as device to device
*)A company can use a MAN to connect the LANs in all its offices throughout a city.
*)A MAN can be owned by a private company or it may be a service provided by a
public company ,such as local telephone company
*)Telephone companies provide a popular MAN service called (SMDS) Switched
Multi-megabit Data Services.
WAN
*)WAN provides long distance transmission of data, voice, image, and video
information over large geographical areas
*) Comprise a country, a continent, or even the whole world (Interlink age of many
LANs and MANs)
*) Low data transmission rate (below 1 Mbps)
*)Unlimited num ber of miles example: Internet Network
4.a) ISO/OSI MODEL
OSI stands for Open Systems Interconnection. It has been developed by ISO –
„International Organization of Standardization„, in the year 1974. It is a 7 layer
architecture with each layer having specific functionality to perform. All these 7 layers
work collaboratively to transmit the data from one person to another across the globe.

1. Physical Layer (Layer 1) :
The lowest layer of the OSI reference model is the physical layer. It is responsible for
the actual physical connection between the devices. The physical layer contains
information in the form of bits. It is responsible for the actual physical connection
between the devices. When receiving data, this layer will get the signal received and
convert it into 0s and 1s and send them to the Data Link layer, which will put the
frame back together.
The functions of the physical layer are :
1. Bit synchronization: The physical layer provides the synchronization of the
bits by providing a clock. This clock controls both sender and receiver thus
providing synchronization at bit level.
2. Bit rate control: The Physical layer also defines the transmission rate i.e. the
number of bits sent per second.
3. Physical topologies: Physical layer specifies the way in which the different,
devices/nodes are arranged in a network i.e. bus, star or mesh topolgy.
4. Transmission mode: Physical layer also defines the way in which the data
flows between the two connected devices. The various transmission modes
possible are: Simplex, half-duplex and full-duplex.
* Hub, Repeater, Modem, Cables are Physical Layer devices.
** Network Layer, Data Link Layer and Physical Layer are also known as Lower
Layers or Hardware Layers.
2. Data Link Layer (DLL) (Layer 2) :
The data link layer is responsible for the node to node delivery of the message. The
main function of this layer is to make sure data transfer is error free from one node to
another, over the physical layer. When a packet arrives in a network, it is the
responsibility of DLL to transmit it to the Host using its MAC address.
Data Link Layer is divided into two sub layers :
1. Logical Link Control (LLC)
2. Media Access Control (MAC)
The packet received from Network layer is further divided into frames depending on
the frame size of NIC(Network Interface Card). DLL also encapsulates Sender and
Receiver‟s MAC address in the header.
The Receiver‟s MAC address is obtained by placing an ARP(Address Resolution
Protocol) request onto the wire asking “Who has that IP address?” and the destination
host will reply with its MAC address.

The functions of the data Link layer are :
1. Framing: Framing is a function of the data link layer. It provides a way for a
sender to transmit a set of bits that are meaningful to the receiver. This can be
accomplished by attaching special bit patterns to the beginning and end of the
frame.
2. Physical addressing: After creating frames, Data link layer adds physical
addresses (MAC address) of sender and/or receiver in the header of each frame.
3. Error control: Data link layer provides the mechanism of error control in
which it detects and retransmits damaged or lost frames.
4. Flow Control: The data rate must be constant on both sides else the data may
get corrupted thus , flow control coordinates that amount of data that can be sent
before receiving acknowledgement.
5. Access control: When a single communication channel is shared by multiple
devices, MAC sub-layer of data link layer helps to determine which device has
control over the channel at a given time.
* Packet in Data Link layer is referred as Frame.
** Data Link layer is handled by the NIC (Network Interface Card) and device
drivers of host machines.
*** Switch & Bridge are Data Link Layer devices.
3. Network Layer (Layer 3) :
Network layer works for the transmission of data from one host to the other located in
different networks. It also takes care of packet routing i.e. selection of the shortest path
to transmit the packet, from the number of routes available. The sender & receiver‟s IP
address are placed in the header by network layer.
The functions of the Network layer are :
1. Routing: The network layer protocols determine which route is suitable from
source to destination. This function of network layer is known as routing.
2. Logical Addressing: In order to identify each device on internetwork uniquely,
network layer defines an addressing scheme. The sender & receiver‟s IP address
are placed in the header by network layer. Such an address distinguishes each
device uniquely and universally.
* Segment in Network layer is referred as Packet.
** Network layer is implemented by networking devices such as routers.
4. Transport Layer (Layer 4) :

Transport layer provides services to application layer and takes services from network
layer. The data in the transport layer is referred to as Segments. It is responsible for
the End to End delivery of the complete message. Transport layer also provides the
acknowledgment of the successful data transmission and re-transmits the data if an
error is found.
• At sender’s side:
Transport layer receives the formatted data from the upper layers,
performs Segmentation and also implements Flow & Error control to ensure proper
data transmission. It also adds Source and Destination port number in its header and
forwards the segmented data to the Network Layer.
Note: The sender need to know the port number associated with the receiver‟s
application.
Generally, this destination port number is configured, either by default or manually.
For example, when a web application makes a request to a web server, it typically uses
port number 80, because this is the default port assigned to web applications. Many
applications have default port assigned.
• At receiver’s side:
Transport Layer reads the port number from its header and forwards the Data which it
has received to the respective application. It also performs sequencing and
reassembling of the segmented data.
The functions of the transport layer are :
1. Segmentation and Reassembly: This layer accepts the message from the
(session) layer , breaks the message into smaller units . Each of the segment
produced has a header associated with it. The transport layer at the destination
station reassembles the message.
2. Service Point Addressing: In order to deliver the message to correct process,
transport layer header includes a type of address called service point address or
port address. Thus by specifying this address, transport layer makes sure that the
message is delivered to the correct process.
The services provided by transport layer :
1. Connection Oriented Service: It is a three-phase process which include
– Connection Establishment
– Data Transfer
– Termination / disconnection
In this type of transmission, the receiving device sends an acknowledgment, back
to the source after a packet or group of packet is received. This type of
transmission is reliable and secure.
2. Connection less service: It is a one phase process and includes Data Transfer.
In this type of transmission, the receiver does not acknowledge receipt of a
packet. This approach allows for much faster communication between devices.
Connection oriented Service is more reliable than connection less Service.

* Data in the Transport Layer is called as Segments.
** Transport layer is operated by the Operating System. It is a part of the OS and
communicates with the Application Layer by making system calls.
Transport Layer is called as Heart of OSI model.
5. Session Layer (Layer 5) :
This layer is responsible for establishment of connection, maintenance of sessions,
authentication and also ensures security.
The functions of the session layer are :
1. Session establishment, maintenance and termination: The layer allows the
two processes to establish, use and terminate a connection.
2. Synchronization : This layer allows a process to add checkpoints which are
considered as synchronization points into the data. These synchronization point
help to identify the error so that the data is re-synchronized properly, and ends of
the messages are not cut prematurely and data loss is avoided.
3. Dialog Controller : The session layer allows two systems to start
communication with each other in half-duplex or full-duplex.
**All the below 3 layers(including Session Layer) are integrated as a single layer in
TCP/IP model as “Application Layer”.
**Implementation of these 3 layers is done by the network application itself. These
are also known as Upper Layers or Software Layers.
6. Presentation Layer (Layer 6) :
Presentation layer is also called the Translation layer.The data from the application
layer is extracted here and manipulated as per the required format to transmit over the
network.
The functions of the presentation layer are :
1. Translation : For example, ASCII to EBCDIC.
2. Encryption/ Decryption : Data encryption translates the data into another form
or code. The encrypted data is known as the cipher text and the decrypted data is
known as plain text. A key value is used for encrypting as well as decrypting data.
3. Compression: Reduces the number of bits that need to be transmitted on the
network.
7. Application Layer (Layer 7) :
At the very top of the OSI Reference Model stack of layers, we find Application layer
which is implemented by the network applications. These applications produce the
data, which has to be transferred over the network. This layer also serves as a window

for the application services to access the network and for displaying the received
information to the user.
Ex: Application – Browsers, Skype Messenger etc.
**Application Layer is also called as Desktop Layer.
The functions of the Application layer are :
1. Network Virtual Terminal
2. FTAM-File transfer access and management
3. Mail Services
4. Directory Services
OSI model acts as a reference model and is not implemented in Internet because of its
late invention. Current model being used is the TCP/IP model.
4.B PROTOCOLS
*)A protocol is a set of rules that governs data communication; the key elements of a
protocol are
*)Syntax – data formats and Signal levels
*)Semantics – control information and error handling
*)Timing – speed matching and sequencing Standards are necessary to ensure that
products from different manufacturers can work together as expected.
Standards
Why do we need standards?
To create and maintain an open and competitive market for equipment manufacturers
To guarantee national and international interoperability of data, telecommunication
technology and process
To give a fixed quality and product to the customer
To allow the same product to be re used again elsewhere
To aid the design and implementation of ideas
To provide guidelines to manufacturers, vendors, government agencies and other
service providers to ensure kind of interconnectivity.
Data communication standards are divided into two types
De facto (from the fact):
Standards that have not been approved by an organized body.
It has been adopted as standards through widespread use.
This is often established originally by manufacturers to define the functionality of a
new product or technology.
De jure (by law):
Those that have been legislated by an officially recognized body.
Standards organizations

Standards are developed through the cooperation of standards creation committees,
forums, and government regulatory agencies.
Standards Creation Committees
ITU, International Telecommunications Union formerly the (CCITT):
It a standard for telecommunication in general and data systems in particular.
ISO, International Standards Organization:
It is active in developing cooperation in the realms of scientific, technological and
economic activity.
ANSI, American National Standards Institute:
It is a private nonprofit corporation and affiliated with the U.S federal government.
IEEE, Institute of Electrical and Electronics Engineers:
It aims to advance theory, creativity, and product quality in the fields of electrical
engineering, electronics radio and in all related branches of Engineering.
It oversees the development and adoption of international standards for computing and
communications.
EIA, Electronic Industries Association:
It is a nonprofit organization devoted to the promotion of electronics manufacturing
concerns. Its activities include public awareness education and lobbying efforts in
addition to standards development.
It also made significant contributions by defining physical connection interfaces and
electronic signaling specifications for data communication.
Forums
It works with universities and users to test, evaluate and standardize new technologies.
The forums are able to speed acceptance and use of those technologies in the
telecommunications community.
It presents their conclusions to standard bodies.
Regulatory Agencies:
Its purpose is to protect the public interest by regulating radio, television and wire
cable communications.
It has authority over interstate and international commerce as it relates to
communication.
Internet Standards
It is a thoroughly tested specification that is useful to and adhered to by those who
work with the internet.
It is a formalized regulation that must be followed.
A specification begins as an internet draft and attains Internet standard status.
An Internet draft is a working document and it may be published as Request for
Comment
(RFC).RFC is edited, assigned a number, and made available to all interested
parties.

5. TRANSMISSION MEDIA
In data communication terminology, a transmission medium is a physical path
between the transmitter and the receiver i.e it is the channel through which data is sent
from one place to another. Transmission Media is broadly classified into the following
types:
1. Guided Media:
It is also referred to as Wired or Bounded transmission media. Signals being
transmitted are directed and confined in a narrow pathway by using physical links.
Features:
 High Speed
 Secure
 Used for comparatively shorter distances
There are 3 major types of Guided Media:
(i) Twisted Pair Cable –
It consists of 2 separately insulated conductor wires wound about each other.
Generally, several such pairs are bundled together in a protective sheath. They are the
most widely used Transmission Media. Twisted Pair is of two types:
1. Unshielded Twisted Pair (UTP):
This type of cable has the ability to block interference and does not depend on a
physical shield for this purpose. It is used for telephonic applications.
Advantages:
 Least expensive
 Easy to install
 High speed capacity
Disadvantages:

 Susceptible to external interference
 Lower capacity and performance in comparison to STP
 Short distance transmission due to attenuation
2. Shielded Twisted Pair (STP):
This type of cable consists of a special jacket to block external interference. It is
used in fast-data-rate Ethernet and in voice and data channels of telephone lines.
Advantages:
 Better performance at a higher data rate in comparison to UTP
 Eliminates crosstalk
 Comparitively faster
Disadvantages:
 Comparitively difficult to install and manufacture
 More expensive
 Bulky
(ii) Coaxial Cable –
It has an outer plastic covering containing 2 parallel conductors each having a separate
insulated protection cover. Coaxial cable transmits information in two modes:
Baseband mode(dedicated cable bandwidth) and Broadband mode(cable bandwidth is
split into separate ranges). Cable TVs and analog television networks widely use
Coaxial cables.
Advantages:
 High Bandwidth
 Better noise Immunity
 Easy to install and expand
 Inexpensive
Disadvantages:
 Single cable failure can disrupt the entire network
(iii) Optical Fibre Cable –
It uses the concept of reflection of light through a core made up of glass or plastic. The
core is surrounded by a less dense glass or plastic covering called the cladding. It is
used for transmission of large volumes of data.
Advantages:
 Increased capacity and bandwidth
 Light weight
 Less signal attenuation
 Immunity to electromagnetic interference
 Resistance to corrosive materials
Disadvantages:
 Difficult to install and maintain

 High cost
 Fragile
 unidirectional, ie, will need another fibre, if we need bidirectional
communication
2. Unguided Media:
It is also referred to as Wireless or Unbounded transmission media.No physical
medium is required for the transmission of electromagnetic signals.
Features:
 Signal is broadcasted through air
 Less Secure
 Used for larger distances
There are 3 major types of Unguided Media:
(i) Radiowaves –
These are easy to generate and can penetrate through buildings. The sending and
receiving antennas need not be aligned. Frequency Range:3KHz – 1GHz. AM and FM
radios and cordless phones use Radiowaves for transmission.
Further Categorized as (i) Terrestrial and (ii) Satellite.
(ii) Microwaves –
It is a line of sight transmission i.e. the sending and receiving antennas need to be
properly aligned with each other. The distance covered by the signal is directly
proportional to the height of the antenna. Frequency Range:1GHz – 300GHz. These
are majorly used for mobile phone communication and television distribution.
(iii) Infrared –
Infrared waves are used for very short distance communication. They cannot penetrate
through obstacles. This prevents interference between systems. Frequency
Range:300GHz – 400THz. It is used in TV remotes, wireless mouse, keyboard,
printer, etc.
6. ERRORS IN TRANSMISSION
TRANSMISSION IMPAIRMENT
*)Signals travel through transmission media, which are not perfect.
*)The imperfection causes signal impairment.
*)This means that the signal at the beginning of the medium is not the same as the
signal at the end of the medium.
*)What is sent is not what is received.
*)Three causes of impairment are attenuation, distortion, and noise.

Attenuation: Loss of energy i.e. Loss in signal strength. It is measured in decibel (dB)
Suppose a signal travels through a transmission medium and its power is reduced to
one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power)
can be calculated as
Distortion: The change in shape or form of a signal. This occurs mainly in composite
signals.
Noise: Unwanted signal mixed with the original signal. The noise can be of different
types like Thermal Noise, Induced Noise, Cross Talk and Impulse Noise.
Thermal Noise: Produced due to random movement of free electrons in a wire creating
extra unwanted signal.
Induced Noise: Produced if the source is an electrical motor or appliance.
Cross talk: Effect of one wire over another.
Impulse Noise: Is a spike produced during earth quake, thunder and lightning etc.
Example:
The power of a signal is 10 mW and the power of the noise is 1 μW; what are the
values of SNR
and SNRdB ?
Solution
The values of SNR and SNRdB can be calculated as follows:
More about Signals:
In data communication four other measurements are used. They are Throughput,
Propagation speed, Propagation time and wave length.
Throughput: It is the measurement of how fast data can pass through an entity.
Propagation Speed: It measures the distance that a signal or bit can pass through the
medium in one second.
Propagation time: The time required by a signal to travel from one point of
transmission to another. Propagation time= Distance/Propagation speed
Wave length: Wave length= Propagation speed X period
7. REPEATERS
*) Repeater is a device used to regenerate the signal over the same network
*) A Repeaters connects segments of LAN.
*) A Repeater is a device that operates only at the physical layer of OSI model.

*) As the distance of the two segments increases, the signal strength
automatically keeps decreasing due to attenuation. To overcome this, devices
like repeaters that span long distances are used

Functions of Repeaters
*) Repeaters receive signals from one network segment and it regenerates and
retransmits those signals to another network segment
*) When a signal becomes weak,they copy the signal bit by bit and regenerate it
at the original strength.
*) A Repeater can extend only the physical length of network
*) A Repeater forwards every frame, it has no filtering capacity.
Advantages
*) Extend length of network
*) It strengthen the weak signals
*) Remove the unwanted noise in an incoming signal.
Disadvantages:
*) It has no filtering capacity
*) It cant connect two different architectures
*) Number of repeaters used must be limited otherwise it causes propagation
delay
8. ENCODING
DIGITAL TRANSMISSION
DIGITAL-TO-DIGITAL CONVERSION:
We can represent digital data by using digital signals.

The conversion involves three techniques: line coding, block coding, and scrambling.
Line coding is always needed.
Block coding and scrambling may or may not be needed.
Line Coding & Decoding:
Line Coding is the process of converting Binary data into digital signals.
Signal element versus data element
Effect of lack of synchronization
Self-synchronization To correctly interpret the signals received from the sender, the
receiver's bit intervals must correspond exactly to the sender's bit intervals. If the
receiver clock is faster or slower, the bit intervals are not matched and the receiver
might misinterpret the signals.
A self-synchronizing digital signal includes timing information in the data being
transmitted. This can be achieved if there are transitions in the signal that alert the

receiver to the beginning, middle, or end of the pulse. If the receiver‟s clock is out of
synchronization, these points can reset the clock
LINE CODING SCHEMES
The line coding schemes are divided into five broad categories.
Line coding schemes
Line coding schemes
Unipolar NRZ scheme:
Unipolar encoding uses only one polarity.0 is represented by zero voltage and 1 is
represented by positive voltage. It is inexpensive to implement. Unipolar encoding has
two problems :
*)Lack of synchronization
*)A dc component

Polar NRZ-L and NRZ-I schemes:
NRZ
The level of the signal is always either positive or negative.
NRZ-L
The level of the signal depends on the type of bit it represents.
The bit 0 is represented by positive voltage
The bit 1 is represented by negative voltage.
NRZ-I
The 1 bit is represented by an inversion (transition between a positive and a negative
voltage) of the voltage level.
The existence of 1‟s in the data stream allows the receiver to resynchronize its timer to
the actual arrival of the transmission.
A string of 0‟s can still cause problems.
In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the
inversion or the lack of inversion determines the value of the bit.
Polar RZ scheme:
RZ
It uses three values
Positive
Negative
Zero
In RZ the signal changes during each bit. A 1 bit is actually represented by positive-to-
zero and A 0 bit is actually represented by negative-to-zero.
Demerits

It requires two signal changes to encode one bit.
It occupies more bandwidth.
Polar biphase: Manchester and differential Manchester schemes:
Biphase
The signal changes at the middle of the bit interval and does not return to zero.
There are two types of biphase encoding
Manchester
Differential Manchester
Manchester
It uses the inversion at the middle of each bit interval for both synchronization and bit
representation.
The bit 1 is represented by negative -to-positive transition.
The bit 0 is represented by positive-to-negative transition
Transition at the middle is used for synchronization
The minimum bandwidth is 2 times that of NRZ
Differential Manchester
Inversion at the middle of the bit interval is used for synchronization.
Presence or absence of additional transition at the beginning of the interval is used to
identify the bit.
A bit 0 is represented by a transition. A bit 1 means no transition.
It requires two signal changes to represent binary 0,but only one to represent binary 1.

Manchester and differential Manchester schemes
The minimum bandwidth of Manchester and differential Manchester is 2 times that of
NRZ.
Block coding concept
Block coding is normally referred to as mB/nB coding; it replaces each m-bit group
with an n-bit group.
Block coding normally involves three steps:
Division : In the division step, a sequence of bits is divided into groups of m bits. For
example, in 4B/5B encoding, the original bit sequence is divided into 4-bit groups.
Substitution: In substitution step, we substitute an m-bit group for an n-bit group. For
example, in 4B/5B encoding we substitute a 4-bit code for a 5-bit group.
Combination: The n-bit groups are combined together to form a stream. The new
stream has more bits than the original bits.

Using block coding 4B/5B with NRZ-I line coding scheme
4B/5B
4B/5B Mapping Codes:
Substitution in 4B/5B block coding

Substitution in 4B/5B block coding

Unit 1

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Unit 1