Basics of Computer Networks

5/27/2020Unit 1 Computer Networks Introduction - By M Indraja 1

Introduction to Network:
The “Advanced Research Projects Agency Network (ARPANET)” was an early packet-switching network
and the first network to implement the TCP/IP protocol suite. Both technologies became the technical
foundation of the Internet. The ARPANET was initially founded by the Advanced Research Projects Agency
(ARPA) of the United States Department of Defense.
The ARPANET originally used the Network Control Program. Later, the TCP/IP communications
protocols were developed by Robert Kahn and Vint Cerf.
As the network development progressed, protocols for were developed by which multiple separate
networks could be joined into a network of networks. Access to the ARPANE internetworking was expanded in
1981
ARPANET – Established in 1969.

OSI Layers:
International Standard Organization (ISO) established a committee in 1977 to develop an
architecture for systems communication.
Open System Interconnection (OSI) reference model is the result of this effort.
 This model allows any two different systems to communicate regardless of their underlying architecture.
 The OSI model describes how data flows from one computer, through a network to another computer.
 The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is
flexible and robust.
 The OSI model consists of seven separate but related layers, each of which defines a part of the process of
moving information across a network.

Physical Layer:
 Physical layer is the bottom(layer 1) of OSI model.
 It is responsible for the actual physical connection
between the devices.
 The physical layer is responsible for movements of
individual bits from one node to next.
 Some functions to physical Layer.
The Transmission medium can ensure the physical
characteristics of interface and medium.
It convert bits to signals.
It manage physical connection.
It can ensure the Bit rate control.

Data Link Layer:
The data link layer is responsible for moving frames from one node to
the next.
Functions to Data Link Layer:
Framing:-
Divides the data from N/W layer into frames.
Physical Addressing:-
Add a header to the frame to define the physical address of
the source and the destination machines.
Flow Control:-
It is the traffic regulatory mechanism implemented by Data
Link layer that prevents the fast sender from drowning the slow
receiver.
Error Control:-
It provides the mechanism of error control in which it detects
and retransmits damaged or lost frames.
Feedback:-
After transmitting the frames, the system waits for the
feedback.

Network Layer:
It is responsible for the source to destination
delivery of a
packets across multiple networks.
Routing:-
Provide mechanism to transmit data
over independent networks that are linked
together.
Logical addressing:-
Adds Logical addresses of sender
and Receiver.
The network layer also defines how to
fragment a packet into smaller packets to
accommodate different media.

Transport Layer:
It is responsible for source process to destination process
delivery of entire message.
Transport layer provides two types of services:
1) Connection Oriented Transmission:
In this type of transmission the receiving device
sends an acknowledgment back to the source after a packet
or group of packet is received.
2) Connectionless Transmission:
In this type of transmission the receiver does not
acknowledge receipt of a packet.

Functions of Transport Layer:
Segmentation and Reassembly:
Divide the message received from Session layer into Segments and number them to make a sequence
for reassembly at the receiving side.
Service point addressing:
Transport layer makes sure that the message is delivered to the correct process on destination
machine.
Error Control: Make sure that the entire message arrives without errors else retransmit.
Flow Control: Transport layer makes sure that the sender

Session Layer:
It is responsible for beginning, maintaining & ending the
communication between two devices, which is called session.
Establishment, maintaining and ending a session:
Sends SYN packet – establish request
Receives ACK & SYN- established
To end – Sender sends ACK
Dialog Control: The session layer allows two systems to enter into a
dialog.
Synchronization: Allows a process to add checkpoints to a stream of
data.

Presentation Layer:
This layer is concerned with the
syntax and semantics of the information exchanged
between two systems.
Data Translation:
Encoding and Decoding Sender to Common
format on Sending side Common to Receiving format on
Receiver side
Data Encryption:
For security and privacy purpose.
Data Compression:
Data compression reduces the number of bits
contained in the information.

Application Layer:
Provides User interfaces and support for Services,
like email, file transfer.
Network Virtual terminal:
It allows a user to log on to a remote host.
File Transfer Access, and Management:
This application allows a user to access files in a
remote host.
Mail Services:
This application provides various e-mail services.
Directory Services:
This application provides the distributed
database sources and access for global
information about various objects and services.

2.12
Summary of layers

TCP/IP Protocol suite:
The TCP/IP protocol suite was developed prior to
the OSI model. Therefore, the layers in the TCP/IP
protocol suite do not exactly match those in the
OSI model.
TCP/IP protocol suite was defined as
having four layers: host-to-network, internet,
transport, and application.
The first four layers provide physical
standards, network interfaces, internetworking,
and transport functions that correspond to the
first four layers of the OSI model. The three
topmost layers in the OSI model, however, are
represented in TCP/IP by a single layer called the
application layer

Layers and Protocols:
Physical and Data Link Layers:
The physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and
proprietary protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network.
Network Layer:
At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking
Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP.
Internetworking:
To connect multiple networks of 1 or more organizations into a large, uniform communication
system.

The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address. On a typical
physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted
on the network interface card (NIC). ARP is used to find the physical address of the node when its Internet address is
known.
The Reverse Address Resolution Protocol (RARP) allows a host to discover its Internet address when it knows
only its physical address. It is used when a computer is connected to a network for the first time or when a
diskless computer is booted.
The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of
datagram problems back to the sender. ICMP sends query and error reporting messages.
The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous transmission of a
message to a group of recipients.

The User Datagram Protocol (UDP) is the simpler of the two standard TCP/IP transport protocols. It is a process-to-
process protocol that adds only port addresses, checksum error control, and length information to the data from
the upper layer.
The Stream Control Transmission Protocol (SCTP) provides support for newer applications such as voice over the
Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

TYPES OF COMPUTER NETWORKS:
1. LAN (Local Area Network)
2. WAN (Wired Area Network)
3. MAN ( Metropolitan Area Network)
4. PAN (Personal Area Network)
5. VPN( Virtual Private Network)
LOCAL AREA NETWORK( LAN ):
Local area networks (LANs) are computer networks ranging in size from a
few computers in a single office to hundreds or even thousands of devices spread across several buildings.
 LANs in turn may be plugged into larger networks, such as larger LANs or wide area networks (WANs),
connecting many computers within an organization to each other and/or to the Internet.
 Typically, connected devices share the resources of a single processor or server within a small
geographic area (for example, within an office building).
 Usually, the server has applications and data storage that are shared in common by multiple computer
users.

WIDE AREA NETWORK(WAN):
A wide area network (WAN) is a computer network that covers a broad area (i.e. any network whose
communications links cross metropolitan, regional, or national boundaries).
 WAN is a network that uses routers and public communications links.
 The largest and most well-known example of a WAN is the Internet. A WAN is a data communications network
that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and
that often uses transmission facilities provided by common carriers, such as telephone companies.
 WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer,
the data link layer, and the network layer.

METROPOLITAN AREA NETWORK (MAN):
A metropolitan area network (MAN) is a network that connects two or more local area networks or campus area
networks together but does not extend beyond the boundaries of the immediate town/city.
 Routers, switches and hubs are connected to create a metropolitan area network.
 The term is applied to the interconnection of networks in a city into a single larger network (which may then
also offer efficient connection to a wide area network).
 It is also used to mean the interconnection of several local area networks by bridging them with backbone lines.

PERSONALAREA NETWORK (PAN):
A personal area network (PAN) is a computer network used for communication among computer devices close to one
person.
 Examples of devices that are used in a PAN are :printers, fax machines, telephones, PDAs and scanners.
 The reach of a PAN is typically about 20-30 feet approximately 6-9 meters.

VIRTUAL PRIVATE NETWORK (VPN):
A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open
connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires.
 A VPN may have best-effort performance, or may have a defined service level agreement (SLA) between the VPN
customer and the VPN service provider.
Generally, a VPN has a topology more complex than point-to-point.
 A VPN allows computer users to appear to be editing from an IP address location other than the one which connects
the actual computer to the Internet.

Thank you…

Basics of Computer Networks

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