NETWORK BASICS

Exchanging of information between sender and receiver is called communication. This can be
achieved with the help of Networking. Few examples are as below.

 Samson sending an email to Margaret over internet (data)

 Various teams connecting over conference call (voice)
 Two users video-chatting over Skype application (video)

Network is required in every communication,

communication system.

Communication system is a collection of systems which are connected together to make

communication. A typical communication system consists of the following components.

 Sender - person or Device who sends the data. Example: Computer, Telephone, Radio
Broadcasting station.
 Receiver - person or Device who receives the data. Example: Computer, Telephone,
Radio sets.
 Transmission Media - physical medium through which the message travels from the
sender to the receiver. Example: Optical fibers, Coaxial Cables, Radio waves.
 Protocol - set of rules and regulations agreed upon by both the sender and receiver that
govern the message exchange.
Network is a collection of various devices and end user systems connected with each other to
achieve the purpose of communication. Networking has changed the way we do business and
day to day activities. With the help of Networking, we can get the information instantaneously
through email, telephone, fax etc.

Networking enables us to exchange data like text, audio and video across geographies.

NETWORK COMPONENTS

The major components of a Network are End user devices, Switch, Router and Interconnections.
A typical network looks as below.

Repeater, Hub, Bridge and Gateway are also components of Network. However components are
varied since every Network does not contain same type of devices.
Functions of components of Network

End User Devices

 These are the end points in the Network

 They help the end users to send and receive messages
 Examples: Computers, Servers, Smart Phones, Mobile Phones, Printers etc.

Hub

 It is typically the least expensive and it is a dumb device

 Performs very simple job - anything that comes in one port is sent out through all
the other ports
 It is used to form small networks

Note: port is an endpoint of communication in an operating system

Switch

 Switches contains multiple ports

 It connects various systems to form a simple local network
 Every switch maintains a database with source and destination information, it is called as
MAC Address table
 It also provide intelligent switching of the message with in the local network
Router

 Routers connect multiple networks

 They choose the best path between the networks
 Every router maintains a database for source and destination information, it is called as
Routing Table
 Routers usually connect different networks working in different locations. For example:
One network in a campus might have to communicate with a distant network connected
through fiber optics

Interconnections

 They help in providing a means for message to travel from one point to another.
The types of Interconnections are
 NIC (Network Interface Card) - it is a hardware part residing in all the machines which
efficiently translates the user data into a format that is fit to be transmitted in the network
 Media can be cables or wireless, they provide the channel to transmit the signals between
devices
 Connectors are the connection end points for the media
NETWORK TYPES

Following are the Types of network.

LAN (Local Area Network) WAN (Wide Area Network) MAN (Metropolitan Area Network)

LAN

LAN is used to connect networking devices that are in a very close geographic area, such as a
floor of a building, a building itself, or a campus environment. Switches and Hubs are the
primary devices to build a LAN. We can also build small network using only Hub.

WAN

WAN is used to connect Local Area Networks together. Typically, Wide Area Networks are
used to connect Local Area Networks which are separated by a large distance. Router is the
primary device to build a WAN.

MAN

MAN consists of a computer network across an entire city, college campus or small region. A
MAN is larger than a LAN, which is typically limited to a single building or site. Depending on
the configuration, this type of network can cover an area from several miles to tens of miles. A
MAN is often used to connect several LANs together to form a bigger network.

NETWORK COMMUNICATION TYPES

There are three types of communication which are used for different purposes:

 Unicast
 Multicast
 Broadcast

Unicast

In Unicast type of communication , there is only one source and one destination. In other words,
it is one to one communication. Example: Telephone call between two persons.

Multicast

In Multicast type of communication, source communicates with a group of destinations. In other

words, it is one to many. Example: A Manager sending a mail to his team.

Broadcast

In Broadcast communication, one to all. We have only one source and all the other hosts are the
destinations. Example: Radio Broadcasting service.
INTRODUCTION TO NETWORKING MODELS

The main aim of computer network is communication and information sharing. Suppose
computer A wants to communicate with computer B through a computer network. The two
systems might be few meters away or in different continents.

While communicating, data has to pass through many intermediate nodes which can be switch,
router etc.

To ensure that data from sender A reaches receiver B correctly and securely, we need to have a
set of rules which guides the communication between A and B. These set of rules are called as
Protocols. These protocols grouped together constitute a communication model. The agreed upon
set of rules are called as Standards.

Let us see a business scenario to understand host to host communication.

Boson, CEO of a British Company wants to send a letter to Lawrence, CEO of a French
company. Let us see how they communicate with each other. Boson and Lawrence have

employed secretaries to assist them.

The communication between Boson and Lawrence happens as given below.

The above scenario depicts a layered architecture wherein the person serves the person in the
above and gets the service from the person below. Similarly, in the communication system,
various services are categorized into layers. Each service in higher layer uses the service from
the layer below it. This is also termed as layered architecture for networking. Let us understand
the layered architecture in detail.

Andrew sends a text message from his iPhone to his friend Simon who is using Samsung Galaxy
Note. In this scenario, though both the users are using devices manufactured by different
companies there are able to communicate with each other. This is because of a set of rules agreed
upon by the vendors to exchange the information. These rules are defined in Networking Models.

 Two important Networking Models are OSI (Open System Interconnection) reference
model and TCP/IP (Transmission Control Protocol / Internet Protocol) Protocol suite
 The reference model is a conceptual framework for understanding communication
 Networking Models assist in exchanging the data between dissimilar hosts

E.g. you can communicate between a PC with Windows OS and a PC with Unix OS.
OSI stands for Open System Interconnection. OSI means that every system participating in this
model, is open for communicating with other systems. OSI reference model is developed by an
organization called ISO (International Organization for Standardization) which works on the
standardization of protocols. OSI Model is an ideal model that helps us in understanding how
data transfer happens between systems.

How the OSI Model look like?

 OSI model has been divided in seven separate layers

 Layer is a logical group of related functionalities
 Each layer has its own functionality and provides support to other layers
 All layers work together to move data through a network

Let us understand the properties and functions of each layer

Application Layer

 This layer serves as the window for users and application processes to access the network
services. An end user program that opens what was sent or creates what is to be sent
 Functionalities: Remote file access, resource sharing
 Protocols: HTTP/FTP/SMTP
 Data format (PDU - Protocol Data Unit) in this layer is called as user data

Presentation Layer

 This layer presents the data in the required format which may include:
encryption/decryption, compression/decompression, encoding/decoding
 It is a syntax layer which can be viewed as a “translator” for the network
 Functionalities: data conversion, data compression
 Protocols: ASCII/JPEG/GIF/EBCDIC/ASN.1
 Data format (PDU) in this layer is called as formatted data

Session Layer

 This layer establishes, manages, and terminates sessions between two communicating
hosts
 Functionalities: Perform security, Name recognition, Logging
 Protocols: RPC/SQL/NFS
 Data format (PDU) in this layer is called as formatted data
Transport Layer

 This layer breaks up the data from the sending host and then reassembles it in the
receiver. This Host to Host layer used for flow control
 It ensures that messages are delivered error-free, in sequence, and with no losses or
duplications
 Functionalities: Message segmentation, Message acknowledgement, Traffic control
 Data Format (PDU) in this layer is called as segment

Protocols: TCP/UDP/SPX

TCP (Transmission Control Protocol)

 It is a connection-oriented protocol
 It incurs an additional overhead to gain functions of order delivery, reliable delivery, and
flow control
 Applications that use TCP: Web Browsers, E-mail, File Transfers etc.

UDP (User Datagram Protocol)

 It is a simple, connection less protocol

 It provisions for low overhead data delivery
 The pieces of communication in UDP are called as datagrams. The datagrams are sent
using "best effort" service by this Transport layer protocol
 Applications that use UDP: Domain Name System (DNS),Video Streaming, Voice over
IP (VoIP) etc

Network Layer

 This layer makes “Best Path Determination” decisions based on logical addresses
(usually IP addresses)
 Functionalities: Routing, Network traffic control, Logical (ip)-Physical(mac) address
mapping
 Protocols: IP/IPX
 Associated device in this layer: Router
 Data format (PDU) in this layer is called as packet

Data Link Layer

 This layer provides error free transfer of data frames from one node to another over the
physical layer
 It makes decisions based on physical addresses (usually MAC addresses)
 Functionalities: Frame traffic control, Frame sequencing, Frame error checking
 Protocols: Ethernet, PPP/HDLC
 Associated device in this layer: Switch/Bridge
 Data format (PDU) in this layer is called as frame

Physical Layer

 This layer is concerned with the transmission and reception of the unstructured raw bit
stream over the physical medium (cables, hubs, etc.)
 Functionalities: Data encoding, Physical Medium attachments, Transmission technique,
physical medium transmission bits & volts
 Associated device in this layer: Hub/Repeater
 Data format (PDU) in this layer is called as bit
The application (upper) layers

 Layer 7: Application layer

 Layer 6: Presentation layer
 Layer 5: Session layer

The data-flow (lower) layers

 Layer 4: Transport layer

 Layer 3: Network layer
 Layer 2: Data link layer
 Layer 1: Physical layer

The communication between a same layer on source and destination is known as peer layer communication.

The following image illustrates various Protocol Data Unit represented at each layer of OSI model. The same
terminologies are also applicable for TCP-IP model.

Encapsulation and De-capsulation of data at different layers of OSI model as follows.

Let us understand the data flow in OSI model.

System A wants to communicate with system B over the network. They are the end systems
connected by a network.

The intermediate nodes implement only the bottom 3 layers of communication. Since their task
is to pass on the data along the navigation path.
The top 4 layers are implemented by end systems and bottom 3 layers are implemented by all
nodes in the path.

Every layer has some protocol using which it communicates with the corresponding layer in the
other system. Application layer protocol, Presentation layer protocol, Session layer protocol etc.
are such protocols. Every layer communicates with the layer above and below it. Also it provides
some service to the layer above it. Protocols works between same layers of different machines,
where services works with different layers within same machine.
Now we will learn how data get transmitted in a network. To illustrate this we’ll study how
browser request for a page.

Step 1: User types a URL in the browser. Here URL is the user data.
Step 2: Application layer in the browser adds metadata(header) to user data. This results in a
HTTP GET request. This is also called a message.

Step 3: Presentation layer formats message from a format used by application layer into a
common format at the sender system. This layer adds metadata(header) to Message and creates
Formatted message.
Step 4: Session layer adds metadata(header) to formatted message. This Header contains the
detail of session establishment between both the host.

Step 5: Transport layer adds metadata to massage and creates segment.

 Destination port : used to identify process on destination device

 Source port : will be used by the receiver as the destination port for sending reply
 Sequence no : used for reliable data delivery
Step 6: Network layer adds metadata to segment and creates a packet.

 Destination IP : used to identify destination device

 Source IP : will be used by the receiver as the destination IP for sending reply

Step 7: Data link layer adds metadata to packet and creates a frame.

 Destination mac : used to identify the next router or destination device

 Source mac : will be used as the destination mac for sending reply
Step 8: Physical layer converts frame to signal (0,1) and transmits via the media.

Step 9: Frame (bits) reaches the switch of the sender’s network.

Step 10: Data link layer of switch read the header in the frame to find destination MAC. It then
uses MAC table to identify the port for forwarding the data.

Step 11: It forwards frame to physical link (E.g. ethernet cable) through the identical port. then
frame (bits) reaches the router.
Step 12: The frame has reached the data link layer of the router. The layer will remove the
metadata from frame and deliver the packet to the network layer.

Step 13: The network layer of router reads the destination ip from the packet, and determines the
destination path using routing table.
Step 14: The data link layer of the router adds metadata to packet based on destination path. It
changes the source MAC and the destination MAC.

Step 15: Physical layer converts frame to signals and transmits via the media. Frame (bits)
reaches the switch of the receiving network.
Step 16: Data link layer of switch read the header in the frame to find destination MAC. It then
uses MAC table to identify the port for forwarding the data.

Step 17: It forwards frame to physical link (E.g. ethernet cable) through the identical port. Frame
(bits) reaches the Receiver machine.
Step 18: The frame has reached the data link layer of receiver. The layer will remove the
metadata from frame and deliver the packet to network layer.

Step 19: The packet has reached the network layer of receiver. This layer will remove metadata
from packet and deliver the segment to the transport layer.
Step 20: The segment has reached the transport layer of receiver. This layer will remove
metadata from segment to exact message. Next it will use the port number to deliver the message
to the application layer of the correct application (In this case the Web Server).

Step 21: The Formatted message has reached the session layer of receiver. This layer will
remove metadata from Formatted message.
Step 22: The Formatted message has reached the presentation layer of receiver. This layer will
remove metadata from Formatted message to extract message. It will translate the message into
the format needed by allocation layer.

Step 23: The message has reached the application layer of receiver. This layer will remove
metadata from message. It will use the removed metadata to pass user data to appropriate
component in web server.
Step 24: The web server receives the user data (http request).
TCP/IP PROTOCOL SUITE
Before OSI Reference Model was developed, the most widely used reference model was TCP/IP
Protocol Suite. This was developed as a result of a researched program sponsored by United
States Department of Defense. Unlike OSI reference model, TCP/IP has only 4 layers. This
model is as given below.

 Although the OSI reference model is universally recognized, the historical and technical
open standard of the internet is TCP/IP (Transmission Control Protocol / Internet
Protocol)
 The TCP/IP reference model and the TCP/IP protocol stack make data communication
possible between any two computers, anywhere in the world, at nearly the speed of light
 The U.S. Department of Defense (DoD) created the TCP/IP reference model

let us understand the functions of each layer in TCP/IP model.

Application Layer

 This layer contains all the higher level protocols including file transfer, email, web
services etc.
 Application, Presentation and Session layers in OSI model together, correspond to
Application layer of TCP/IP model

Transport Layer

 This layer takes care of the conversation between source and destination. The
conversation can be connection oriented or connection less. Protocols in this layer are
TCP and UDP. TCP functionalities are fragmenting the original message into discrete
pieces and reassembling the same when they reach the destination. On the other hand,
UDP is unreliable protocol.
 Transport layer in both the models are same
Internet Layer

 This layer was the result of packet switching network. The main functionality of this
layer is to transmit packets to any network through the best path from the source. The
major protocol in this layer is IP (Internet Protocol)
 Network layer of OSI model corresponds to Internet layer in TCP/IP model

Network Access Layer

 TCP/IP protocol suite does not specify much below the Internet layer. This layer is quite
undefined
 The data link layer and physical layer of OSI form the Network Access layer in TCP/IP
Protocol suite
TCP/IP model data

Let's say that you are browsing Sparsh. By the time the Sparsh page loads in your desktop, the
data has gone through so many software and hardware.This is sort of like how the postal system
works. Many people in the postal department, do their respective jobs to make sure your parcel
reaches to your friend.

Like the postal department, in computer networks, many layers of software and hardware must
do their respective jobs to ensure smooth communication. It is called as the TCP/IP model. It
defiles how networks should work. TCP/IP is a set of four layers of protocols. Each layer focuses
on how to package the data so that the corresponding layer on the receving device can
understand it. It does not bother how the data is handled by subsequent layers.

Application Layer

This layer is responsible for converting the data in a form understandable by the destination
application. Layer is just a concept. It is implemented in a software or a hardware. This
application and application layer are not the same.

Transport Layer

This layer ensures that the data has reached the destination. It does this by looking for an
acknowledgement from the receiver. It sends a unique sequence number for each data and
expects it back in the acknowledgement. If it does not receive an acknowledgement within a
certain time, it resends the data.
Internet Layer

This layer is responsible for finding the best possible route for the data to reach the destination.
Router is the network device works in this layer and it actively use this layer to find the best
route.

Link Layer

Here the Network Access layer referred as link layer. The link layer is responsible for ensuring
the data delivery to the correct host with error-free using CRC (Cyclic Redundancy Check)
checks.

The sender uses CRC algorithm to generate a unique number for the data. The CRC and data are
transmitted to the destination. The receiver uses the same CRC algorithm and checks if the
generated CRC matches with the transmitted CRC. If both are matched, then the data has arrived
uncorrected. If not, the received data is discarded.

Physical Layer

This layer is outside of the TCP/IP model. It coverts data to appropriate signals for transmission
through wired and wireless media.
Protocol Stack

There can be many combination of protocols to make up a stack.

Following is the comparison between OSI model and TCP/IP along with protocols.