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UNIT-1
INTRODUCTION

●DATA COMMUNICATIONS
Data communications refers to the transmission of the digital data between two or more
computers and a computer network or data network is a telecommunications network that
allows computers to exchange data. The physical connection between networked
computing devices is established using either cable media or wireless media. The best-
known computer network is the Internet.

Data communications incorporates several techniques and technologies with the primary
objective of enabling any form of electronic communication. These technologies include
telecommunications, computer networking and radio/satellite communication. Data
communication usually requires existence of a transportation or communication medium
between the nodes wanting to communicate with each other, such as copper wire, fiber
optic cables or wireless signals.

For example, a common example of data communications is a computer connected to the

Internet via a Wi-Fi connection, which uses a wireless medium to send and receive data
from one or more remote servers.

Some devices/technologies used in data communications are known as data communication

equipment (DCE) and data terminal equipment (DTE). DCE is used at the sending node, and
DTE is used at the receiving node.

●NETWORKS
A computer network is a set of connected computers. Computers on a network are called
nodes. The connection between computers can be done via cabling, most commonly the
Ethernet cable, or wirelessly through radio waves. Connected computers can share
resources, like access to the Internet, printers, file servers, and others. A network is a
multipurpose connection, which allows a single computer to do more.

Classification of computer networks

Computer networks are classified based on various factors. They includes:

• Geographical span
• Inter-connectivity
• Administration
• Architecture
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Geographical Span
Geographically a network can be seen in one of the following categories:

• It may be spanned across your table, among Bluetooth enabled devices,. Ranging not
more than few meters.
• It may be spanned across a whole building, including intermediate devices to
connect all floors.
• It may be spanned across a whole city.
• It may be spanned across multiple cities or provinces.
• It may be one network covering whole world.

Inter-Connectivity
Components of a network can be connected to each other differently in some fashion. By
connectedness we mean either logically , physically , or both ways.

• Every single device can be connected to every other device on network, making the
network mesh.
• All devices can be connected to a single medium but geographically disconnected,
created bus like structure.
• Each device is connected to its left and right peers only, creating linear structure.
• All devices connected together with a single device, creating star like structure.
• All devices connected arbitrarily using all previous ways to connect each other,
resulting in a hybrid structure.

Administration
From an administrator’s point of view, a network can be private network which belongs a
single autonomous system and cannot be accessed outside its physical or logical domain.A
network can be public which is accessed by all.

Network Architecture
Computer networks can be discriminated into various types such as Client-Server, peer-to-
peer or hybrid, depending upon its architecture.

• There can be one or more systems acting as Server. Other being Client, requests the
Server to serve requests. Server takes and processes request on behalf of Clients.
• Two systems can be connected Point-to-Point, or in back-to-back fashion. They both
reside at the same level and called peers.
• There can be hybrid network which involves network architecture of both the above
types.
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Applications of Communication & Computer Network

Computer systems and peripherals are connected to form a network.They provide numerous
advantages:

• Resource sharing such as printers and storage devices

• Exchange of information by means of e-Mails and FTP
• Information sharing by using Web or Internet
• Interaction with other users using dynamic web pages
• IP phones
• Video conferences
• Parallel computing
• Instant messaging

Transmission Modes in Computer Networks

Transmission mode means transferring of data between two devices. It is also known as
communication mode. Buses and networks are designed to allow communication to occur
between individual devices that are interconnected. There are three types of transmission
mode:-

Simplex Mode
Half-Duplex Mode
Full-Duplex Mode

Simplex Mode
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. The simplex mode can
use the entire capacity of the channel to send data in one direction.
Example: Keyboard and traditional monitors. The keyboard can only introduce input, the
monitor can only give the output.

Half-Duplex Mode
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 used in cases where there is no need for communication in both direction at the
same time. The entire capacity of the channel can be utilized for each direction.
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Example: Walkie- talkie in which message is sent one at a time and messages are sent in
both the directions.

Full-Duplex Mode
In full-duplex mode, both stations can transmit and receive simultaneously. In full_duplex
mode, signals going in one direction share the capacity of the link with signals going in other
direction, this sharing can occur in two ways: Either the link must contain two physically
separate transmission paths, one for sending and other for receiving. Or the capacity is
divided between signals travelling in both directions.

Full-duplex mode is used when communication in both direction is required all the time. The
capacity of the channel, however must be divided between the two directions.
Example: Telephone Network in which there is communication between two persons by a
telephone line, through which both can talk and listen at the same time.

●Types of Computer Networks

The Network allows computers to connect and communicate with different computers via
any medium. LAN, MAN and WAN are the three major types of the network designed to
operate over the area they cover. There are some similarities and dissimilarities between
them. One of the major differences is the geographical area they cover, i.e. LAN covers the
smallest area; MAN covers an area larger than LAN and WAN comprises the largest of all.

Personal Area Network

• Personal Area Network is a network arranged within an individual person, typically
within a range of 10 meters.
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• Personal Area Network is used for connecting the computer devices of personal use
is known as Personal Area Network.
• Personal Area Network covers an area
of 30 feet.

LAN (Local Area Network)

• A Local Area Network (LAN) is a group of computer and peripheral devices which are
connected in a limited area such as school, laboratory, home, and office building.
• LAN is used for connecting two or more personal computers through a communication
medium such as twisted pair, coaxial cable, etc.
• It is less costly as it is built with inexpensive hardware such as hubs, network adapters, and
ethernet cables.
• The data is transferred at an extremely faster rate in Local Area Network.
• Local Area Network provides higher security.

Advantages of LAN

• You can use the same software over the network instead of purchasing the licensed
software for each client in the network.
• Data of all network users can be stored on a single hard disk of the server computer.
• You can easily transfer data and messages over networked computers.
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• Local Area Network offers the facility to share a single internet connection among all
the LAN users.

Disadvantages of LAN

• LAN will indeed save cost because of shared computer resources, but the initial cost
of installing Local Area Networks is quite high.
• The LAN admin can check personal data files of every LAN user, so it does not offer
good privacy.
• Unauthorized users can access critical data of an organization in case LAN admin is
not able to secure centralized data repository.

MAN (Metropolitan Area Network)

• A metropolitan area network is a network that covers a larger geographic area by
interconnecting a different LAN to form a larger network.
• Government agencies use MAN to connect to the citizens and private industries.
• In MAN, various LANs are
connected to each other
through a telephone exchange
line.
• The most widely used
protocols in MAN are RS-232,
Frame Relay, ATM, ISDN, OC-
3, ADSL, etc.
• It has a higher range than
Local Area Network (LAN).

Advantages of MAN

• It offers fast communication using high-speed carriers, like fiber optic cables.
• It provides excellent support for an extensive size network and greater access to
WANs.
• The dual bus in MAN network provides support to transmit data in both directions
concurrently.
• A MAN network mostly includes some areas of a city or an entire city.

Disadvantages of MAN

• You need more cable to establish MAN connection from one place to another.
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• In MAN network it is tough to make the system secure from hackers

WAN (Wide Area Network)

• A Wide Area Network is a network that extends over a large geographical area such as
states or countries.
• A Wide Area Network is quite bigger network than the LAN.
• A Wide Area Network is not limited to a single location, but it spans over a large
geographical area through a telephone line, fibre optic cable or satellite links.
• The internet is one of the biggest WAN in the world.
• A Wide Area Network is widely used in the field of Business, government, and education.

Advantages of WAN

• WAN helps you to cover a larger geographical area. Therefore business offices
situated at longer distances can easily communicate.
• Contains devices like mobile phones, laptop, tablet, computers, gaming consoles,
etc.
• WLAN connections work using radio transmitters and receivers built into client
devices.

Disadvantage of WAN

• The initial setup cost of investment is very high.

• It is difficult to maintain the WAN network. You need skilled technicians and network
administrators.
• There are more errors and issues because of the wide coverage and the use of
different technologies.
• Offers lower security compared to other types of networks.
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Internetwork
• A network of networks is called an internetwork, or simply the internet. It is the
largest network in existence on this planet.The internet hugely connects all WANs
and it can have connection to LANs and Home networks. Internet uses TCP/IP
protocol suite and uses IP as its addressing protocol. Present day, Internet is widely
implemented using IPv4. Because of shortage of address spaces, it is gradually
migrating from IPv4 to IPv6.
• Internet enables its users to share and access enormous amount of information
worldwide. It uses WWW, FTP, email services, audio and video streaming etc. At
huge level, internet works on Client-Server model.
• Internet uses very high speed backbone of fiber optics. To inter-connect various
continents, fibers are laid under sea known to us as submarine communication
cable.

Internet is serving many proposes and is involved in many aspects of life. Some of them are:

Web sites, E-mail, Instant Messaging, Blogging, Social Media, Marketing, Networking
Resource Sharing Audio and Video Streaming.

●NETWORK TOPOLOGIES
A Network Topology is the arrangement with which computer systems or network devices are
connected to each other. Topologies may define both physical and logical aspect of the network.
Both logical and physical topologies could be same or different in a same network.

BUS Topology
Bus topology is a network type in which every computer and network device is connected to
single cable. When it has exactly two endpoints, then it is called Linear Bus topology.
It is one of the simple forms of networking where a failure of a device does not affect the
other devices. But failure of the shared communication line can make all other devices stop
functioning. Both ends of the shared channel have line terminator.
The data is sent in only one direction and as soon as it reaches the extreme end, the
terminator removes the data from the line.
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STAR TOPOLOGY
All hosts in Star topology are connected to a central device, known as hub device, using a point-to-
point connection. That is, there exists a point to point connection between hosts and hub.

As in Bus topology, hub acts as single point of failure. If hub fails, connectivity of all hosts to all other
hosts fails. Every communication between hosts takes place through only the hub. Star topology is
not expensive as to connect one more host, only one cable is required and configuration is simple

RING Topology
In ring topology, each host machine connects to exactly two other machines, creating a circular
network structure. When one host tries to communicate or send message to a host which is not
adjacent to it, the data travels through all intermediate hosts. To connect one more host in the
existing structure, the administrator may need only one more extra cable.

Failure of any host results in failure of the whole ring. Thus, every connection in the ring is a point of
failure. There are methods which employ one more backup ring.
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MESH Topology
In this type of topology, a host is connected to one or multiple hosts. This topology has hosts in
point-to-point connection with every other host or may also have hosts which are in point-to-point
connection with few hosts only.

Hosts in Mesh topology also work as relay for other hosts which do not have direct point-to-
point links. Mesh technology comes into two types:

Full Mesh: All hosts have a point-to-point connection to every other host in the network.
Thus for every new host n(n-1)/2 connections are required. It provides the most reliable
network structure among all network topologies.
Partially Mesh: Not all hosts have point-to-point connection to every other host. Hosts
connect to each other in some arbitrarily fashion. This topology exists where we need to
provide reliability to some hosts out of all.

TREE Topology
Also known as Hierarchical Topology, this is the most common form of network topology in
use presently. This topology imitates as extended Star topology and inherits properties of
Bus topology.
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This topology divides the network into multiple levels/layers of network. Mainly in LANs, a
network is bifurcated into three types of network devices. The lowermost is access-layer
where computers are attached. The middle layer is known as distribution layer, which works
as mediator between upper layer and lower layer. The highest layer is known as core layer,
and is central point of the network, i.e. root of the tree from which all nodes fork.

All neighboring hosts have point-to-point connection between them. Similar to the Bus
topology, if the root goes down, then the entire network suffers even though it is not the
single point of failure. Every connection serves as point of failure, failing of which divides the
network into unreachable segment.

DAISY CHAIN
This topology connects all the hosts in a linear fashion. Similar to Ring topology, all hosts are
connected to two hosts only, except the end hosts. Means, if the end hosts in daisy chain
are connected then it represents Ring topology.

Each link in daisy chain topology represents single point of failure. Every link failure splits the
network into two segments. Every intermediate host works as relay for its immediate hosts.

Hybrid Topology
A network structure whose design contains more than one topology is said to be hybrid
topology. Hybrid topology inherits merits and demerits of all the incorporating topologies.
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The above picture represents an arbitrarily hybrid topology. The combining topologies may
contain attributes of Star, Ring, Bus, and Daisy-chain topologies. Most WANs are connected
by means of Dual-Ring topology and networks connected to them are mostly Star topology
networks. Internet is the best example of largest Hybrid topology.

●PROTOCOLS
Network Protocols are a set of rules governing exchange of information in an easy, reliable
and secure way. Before we discuss the most common protocols used to transmit and
receive data over a network, we need to understand how a network is logically organized or
designed. The most popular model used to establish open communication between two
systems is the Open Systems Interface (OSI) model proposed by ISO.

OSI Model
Open System Interconnect is an open standard for all communication systems. OSI model is
established by International Standard Organization (ISO).
→ The OSI model is a layered framework for the design of network systems that allows
communication between all types of computer systems.
→ The purpose of the OSI model is to show how to facilitate communication between
different systems without requiring changes to the logic of the underlying hardware and
software.
→The OSI model is not a protocol; it is a model for understanding and designing a network
architecture that is flexible, robust, and interoperable.
→It consists of seven separate but related layers, each of which defines a part of the
process of moving information across a network. An understanding of the fundamentals of
the OSI model provides a solid basis for exploring data communications
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Application Layer: This layer is responsible for providing interface to the application user.
This layer encompasses protocols which directly interact with the user.
Presentation Layer: This layer defines how data in the native format of remote host should
be presented in the native format of host.
Session Layer: This layer maintains sessions between remote hosts. The session layer is the
network dialog controller. It establishes, maintains, and synchronizes the interaction among
communicating systems. 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.
Transport Layer: This layer is responsible for end-to-end delivery
between hosts. The transport layer ensures that the whole
message arrives intact and in order, overseeing both error control
and flow control at the source-to-destination level.
Network Layer: The network layer is responsible for the source-to-
destination delivery of a packet, possibly across multiple networks
(links). This layer is responsible for address assignment and
uniquely addressing hosts in a network.
Data Link Layer: This layer is responsible for reading and writing
data from and onto the line. Link errors are detected at this layer.
Physical Layer: This layer defines the hardware, cabling, wiring,
power output, pulse rate etc.

TCP/IP PROTOCOL
TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a
specific functionality; however, the modules are not necessarily interdependent. Whereas
the OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP
protocol suite contain relatively independent protocols that can be mixed and matched
depending on the needs of the system. The term hierarchical means that each upper-level
protocol is supported by one or more lower-level protocols.
→The TCP/IP model consists of five layers: the application layer, transport layer, network
layer, data link layer and physical layer.
→The first four layers provide physical standards, network interface, internetworking, and
transport functions that correspond to the first four layers of the OSI model and these four
layers are represented in TCP/IP model by a single layer called the application layer.
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Application Layer: This layer defines the protocol which enables user to interact with the
network. For example, FTP, HTTP etc.
Transport Layer: This layer defines how data should flow between hosts. Major protocol at
this layer is Transmission Control Protocol (TCP). UDP and TCP are transport level protocols
responsible for delivery of a message from a process (running program) to another process.
This layer ensures data delivered between hosts is in-order and is responsible for end-to-
end delivery.
Internet Layer: Internet Protocol (IP) works on this layer. It is an unreliable and
connectionless protocol-a best-effort delivery service. The term best effort means that IP
provides no error checking or tracking.This layer facilitates host addressing and recognition.
This layer defines routing.

Physical and Data Link Layers

At the physical and data link layers, TCPIIP does not define any specific protocol. It
supports all the standard and proprietary protocols. A network in a TCPIIP internetwork
can be a local-area network or a wide-area network.

Difference between TCP/IP and OSI Model:

TCP/IP OSI
TCP refers to Transmission Control
OSI refers to Open Systems Interconnection.
Protocol.
TCP/IP has 4 layers. OSI has 7 layers.
TCP/IP is more reliable OSI is less reliable
TCP/IP does not have very strict
OSI has strict boundaries
boundaries.

TCP/IP follow a horizontal approach. OSI follows a vertical approach.

TCP/IP uses both session and

presentation layer in the application OSI uses different session and presentation layers.
layer itself.
TCP/IP developed protocols then
OSI developed model then protocol.
model.

Transport layer in TCP/IP does not In OSI model, transport layer provides assurance
provide assurance delivery of packets. delivery of packets.

Connection less and connection oriented both

TCP/IP model network layer only
services are provided by network layer in OSI
provides connection less services.
model.
While in OSI model, Protocols are better covered
Protocols cannot be replaced easily in
and is easy to replace with the change in
TCP/IP model.
technology.
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●ADDRESSING
A network address is any logical or physical address that uniquely distinguishes a network
node or device over a computer or telecommunications network. It is a numeric/symbolic
number or address that is assigned to any device that seeks access to or is part of a network.
Four levels of addresses are used in an internet employing the TCP/IP protocols:

1. Physical Addresses(link)
2. Logical Addresses(IP)
3. Port Addresses
4. Specific Addesses

Physical Addresses
The physical address, also known as the link address, is the address of a node as defined by
its LAN or WAN. It is included in the frame used by the data link layer. It is the lowest-level
address.
The physical addresses have authority over the network (LAN or WAN). The size
and format of these addresses vary depending on the network. For example, Ethernet
uses a 6-byte (48-bit) physical address that is imprinted on the network interface card
(NIC). LocalTalk (Apple), however, has a I-byte dynamic address that changes each
time the station comes up.

Logical Addresses
Logical addresses are necessary for universal communications that are independent of
underlying physical networks. Physical addresses are not adequate in an internetwork
environment where different networks can have different address formats. A universal
addressing system is needed in which each host can be identified uniquely, regardless
of the underlying physical network.
The logical addresses are designed for this purpose. A logical address in the Internet
is currently a 32-bit address that can uniquely define a host connected to the Internet. No
two publicly addressed and visible hosts on the Internet can have the same IP address.

Port Addresses
The IP address and the physical address are necessary for a quantity of data to travel
from a source to the destination host. However, arrival at the destination host is not the
final objective of data communications on the Internet. A system that sends nothing but
data from one computer to another is not complete. Today, computers are devices that
can run multiple processes at the same time. The end objective of Internet communication
is a process communicating with another process. For example, computer A can
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communicate with computer C by using TELNET. At the same time, computer A

communicates with computer B by using the File Transfer Protocol (FTP).
For these processes to receive data simultaneously, we need a method to label the different
processes. In other words, they need addresses. In the TCPIIP architecture, the label
assigned to a process is called a port address. A port address in TCPIIP is 16 bits in length.

Specific Addresses
Some applications have user-friendly addresses that are designed for that specific address.
Examples include the e-mail address (for example, forouzan@fhda.edu) and the Universal
Resource Locator (URL) (for example, www.mhhe.com). The first defines the recipient of
an e-mail the second is used to find a document on the World Wide Web.These addresses,
however, get changed to the corresponding port and logical addresses by the sending
computer.