Computer Network Introduction full

Slides Prepared by Aneeb.A Assiatant Professor HM College of Science and Technology Manjeri

 FILE SHARING
–
Networks offer a quick and
easy way to share files directly.
 RESOURCE SHARING – All computers in the
as
and
network
printers,
modems.
can
fax
share resources such
machines, scanners,
 COMMUNICATION –Those on the network can
communicate with each other via e-mail,
instant messages, etc.

 Flexible Access - Networks allow their users
to access files from computers throughout
the network.
-
share
 Sharing of Information Computer
networks enable us
the
to data
that
and
areinformation with computers
located geographically large distance apart.

P2P
Stands for "Peer to Peer." In a P2P network, the "peers" are computer systems
which are connected to each other via the Internet. Files can be shared directly
between systems on the network without the need of a central server. In other
words, each computer on a P2P network becomes a file server as well as a
client.

DifferentTypes of Networks
• Depending upon the geographical
by a network, it is classified as:
area covered
– Local Area Network (LAN)
– Metropolitan Area Network (MAN)
– Wide Area Network (WAN)
– Personal Area Network (PAN)

Local Area Network (LAN)
A LAN is a network that is used for communicating among
computer devices, usually within an office building or home.
•
• LAN’s enable the sharing of resources such as files or
hardware devices that may be needed by multiple users
•
• Is limited in size, typically spanning a few hundred meters,
and no more than a mile
• Is fast, with speeds from 10 Mbps to 10 Gbps
• Requires little wiring, typically a single cable connecting to
each device
• Has lower cost compared to MAN’s or WAN’s

Local Area Network (LAN)
LAN’s can be either wired or wireless.Twisted pair,
coax or fibre optic cable can be used in wired LAN’s.
•
• Every LAN uses a protocol – a set of rules that governs
how packets are configured and transmitted.
• Nodes in a LAN are linked together with a certain
topology.These topologies include:
–
–
–
Bus
Ring
Star
• LANs are capable of very high transmission rates (100s
Mb/s to G b/s).

Advantages of LAN
• Speed
• Cost
• Security
• E-mail
• Resource Sharing

Disadvantages
ExpensiveTo Install
of LAN
•
• Requires Administrative Time
• File Server May Fail
• Cables May Break

Metropolitan Area Network (MAN)
A metropolitan area network (MAN) is a large
computer network that usually spans a city or a
large campus.
•
• A MAN is optimized for a larger geographical
area than a LAN, ranging from several blocks
buildings to entire cities.
of
• A MAN might be owned and operated by a
single organization, but it usually will be used
by many individuals and organizations.

A MAN often acts as a high speed network to
allow sharing of regional resources.
•
• A MAN typically covers an area of between
and 50 km diameter.
5
• Examples of MAN:Telephone company
network that provides a high speed DSL to
customers and cable TV
network,Networks of Banks like
institution which cover a district

Wide Area Network (WAN)
WAN covers a large geographic area such as
country, continent or even whole of the world.
•
• AWAN is two or more LANs connected
together.The LANs can be many miles apart.
• To cover great distances,WANs may transmit
data over leased high-speed phone lines or
wireless links such as satellites.

Wide Area Network (WAN)
Multiple LANs can be connected together
using devices such as bridges, routers, or
gateways, which enable them to share data.
•
• The world's most popularWAN is the Internet.

Personal Area Network (PAN)
A PAN is a network that is used for
communicating among computers and computer
devices (including telephones) in close
proximity of around a few meters within a room
•
•
It can be used for communicating between the
devices themselves, or for connecting to a larger
network such as the internet.
• PAN’s can be wired or wireless

Personal Area Network (PAN)
A personal area network (PAN) is a computer
network used for communication among
computer devices, including telephones and
personal digital assistants, in proximity to an
individual's body.
•
• The devices may or may not belong to the
person in question.The reach of a PAN is
typically a few meters.

 It is the physical way in which computers
are interconnected.
 Five basic network structures are :

 Devices are connected to a central computer
called HUB.
 A Star network is particularly appropriate for
organizations that require a centralized data
base or a centralized processing facility.
 For example, a star network may be used in
banking for centralized record keeping in an
on-line branch office environment.

 It is easy to add new and remove nodes.
 A node failure does not bring let down
the entire network.
 It is easier to diagnose network
through a central hub.
problems

 If the central hub fails, the whole network
ceases to function.
 It costs more to cable a star configuration
than other topologies because more cable is
required than other topologies.

 In Bus topology a single network cable runs in
the building or campus and all nodes are
line
or
linked along with this communication
with two endpoints called the bus
backbone.
 This structure is very popular for local area
networks

 Reliable in very small networks as well as
easy to use and understand.
 Requires the least amount of cable to connect
the computers together and therefore is less
expensive than other cabling arrangements.

 Heavy network traffic can slow a bus
considerably.
 Each connection between cables weakens the
electrical signal.
If the common cable fails, then the whole
system will crash down

 In Ring topology the network cable passes
from one node to another until all nodes are
connected in the form of a “loop or ring”.
 Transmits in only one direction.
 Used in LAN’S and WAN’S.

1.One station is known as monitor station which takes all the
responsibility to perform the operations.
2.To transmit the data, station has to hold the token. After the
transmission is done, the token is to be released for other stations to
use.
3.When no station is transmitting the data, then the token will circulate
in the ring.

 Ring networks can span longer distances
than other types of networks.
 Ring networks are easily extendable.

 Relatively expensive and difficult to install.
 Failure of one computer on the network
can affect the whole network.
 Adding or removing computers can disrupt
the network.

 In mesh network, there is random connection
of nodes using communication links.
 Mesh topology is the general topology for
wide area network.
 In mesh topology, every device is connected
to another device via particular channel.
In mesh topology, every device is connected to another
device via particular channel.

Figure 1 : Every device is connected with another via dedicated channels. These channels are known
as links.
•If suppose, N number of devices are connected with each other in mesh topology, then total number of
ports that is required by each device is ? N-1. In the Figure 1, there are 5 devices connected to each
other, hence total number of ports required is 4.
•If suppose, N number of devices are connected with each other in mesh topology, then total number of
dedicated links required to connect them is NC2 i.e. N(N-1)/2. In the Figure 1, there are 5 devices
connected to each other, hence total number of links required is 5*4/2 = 10

 The reliability is very high
alternate paths available if
as there are always
direct link between
two nodes is down.
 Yields the greatest amount
of the
of redundancy in
the event that one
can
nodes fails where
network
node.
traffic be redirected to another

 The cost of installation and maintenance is
high ( more cable is required than any other
configuration).

 A tree topology connects one star network
the other star network.
to
 It is an extension of star topology.
 Here, we divided the whole network into
segment which can be easily managed and
maintained.

In this the various secondary hubs are connected to the central hub which contains the repeater.

 Each segment is provided with dedicated
point-to-point wiring to central hub.
 Error detection and correction is easy.
 If one segment is damaged, other segment
are not affected.
 Expansion of network is possible and easy.

 As multiple segments are connected to a
central hub, the networks depend heavily on
the hub. Its failure affects the entire network.
 Maintenance is not easy and cost are high.
 With increase in size beyond a point, the
management becomes difficult.

Hybrid Topology :
This topology is a collection of two or more topologies which are described above. This is a scalable topology
which can be expanded easily. It is reliable one but at the same it is a costly topology

INTERNETWORK/Internet
Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are connected to
one another. When two or more networks are connected, they become an internetwork,
or internet.
As an example, assume that an organization has two offices, one on the east coast and the
other on the west coast. The established office on the west coast has a bus topology
LAN; the newly opened office on the east coast has a star topology LAN. The president of
the company lives somewhere in the middle and needs to have control over the company
from her horne. We can make communicate them using internet via switches and routers
like network devices

NETWORK MODELS
Computer Network Models : Introduction
 For data communication to take place and two or more users can transmit data from one to other, a systematic approach
is required. This approach enables users to communicate and transmit data through efficient and ordered path. It is
implemented using models in computer networks and are known as computer network models.
 Computer network models are responsible for establishing a connection among the sender and receiver and transmitting
the data in a smooth manner respectively.
 There are two computer network models i.e. OSI Model and TCP/IP Model on which the whole data communication
process relies.

Computer Network Models : The OSI Reference Model
 The OSI Model is one of the general purpose networking or communication model among computer network models,
which is responsible for establishing connection in an open manner between all the communicable devices present
across the globe.
 OSI stands for “Open System Interconnection” and the name of this reference model was given by an organization
known as “International Organization for Standardization”. The ISO is responsible for generating and promoting
industrial and commercial standards applicable for all the users or universally.
 Apart from OSI Model, another computer network models which is widely used is TCP/IP Model.
 OSI model having a layered architecture, allows easy data communication as each layer has
predefined structured and functionalities.
The functionalities are different for each layer and thus when combined together forms the OSI Model. There
are in total seven layers in general purpose OSI model

Physical Layer
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 transmitting individual bits from one node to the next. 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 rate control: The Physical layer also defines the transmission rate i.e. the
number of bits sent per second.
2. 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.
3. 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-One direction only communication Eg:Radio
half-duplex -At a time one direction only-Eg:Walkie Talkie
and full-duplex.-Two way communication Eg:Telephone
* 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.

Data Link Layer
 Data Link Layer provides or acts as an intermediate, ensuring delivery of data message to
its respective destination in the network by using the physical address of the device known
as MAC (Media Access Control) Address.
 This layer translates the data received in the form of bits from physical layer into respective
format and forwards it to network layer.
 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.
 Data Link Layer is divided into two sub layers :
o Logical Link Control (LLC)
o 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 functions of the data Link layer are :
Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits(frame) that
are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of
the frame.
Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of sender and/or
receiver in the header of each frame.
Error control: Data link layer provides the mechanism of error control in which it detects and retransmits damaged or
lost frames.
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.
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

Network Layer
 The network layer is responsible for facilitating data transfer between two different networks. If the two devices
communicating are on the same network, then the network layer is unnecessary.
 Ie Network layer works for the transmission of data from one host to the other located in different networks
 Ie it enable source to destination transfer of packets
 The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender ’s device,
and reassembling these packets on the receiving device.
 The network layer also finds the best physical path for the data to reach its destination; this is known as routing.

Functions
 Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem
locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source
and destination systems Called IP Address. The network layer adds a header to the packet coming from the upper
layer that, among other things, includes the logical addresses of the sender and receiver.
 Routing. When independent networks or links are connected to create intemetworks (network of networks) or a
large network, the connecting devices (called routers or switches) route or switch the packets to their final
destination.
Segment in Network layer is referred as Packet.
Network layer is implemented by networking devices such as routers.

Transport Layer
 The transport layer is responsible for process-to-process delivery of the entire message. A process is an application
program running on a host..
 Layer 4 is responsible for end-to-end communication between the two devices. This includes taking data from the
session layer and breaking it up into chunks called segments before sending it to layer 3. The transport layer on the
receiving device is responsible for reassembling the segments into data the session layer can consume.
 The transport layer also provides the acknowledgement of the successful data transmission and re-transmits the data
if an error is found
 Functions of Transport Layers
1. Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the message
into smaller units(Segments) . 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 the transport layer :
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 acknowledgement, back to the source after a packet or group of
packet is received. This type of transmission is reliable and secure.
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 connectionless 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.

Session Layer
The session layer (layer 5) is responsible for establishing, managing, synchronizing and terminating sessions between
end-user application processes.
The main functions of the session layer are as follows −
 It works as a dialog controller. It allows the systems to communicate in either half-duplex or full-duplex mode of
communication.
 It is responsible for token management. Through this, it prevents the two users to simultaneously attempt the same
critical operation.
 It synchronizes communication. It adds synchronization points or checkpoints in data streams for long
communications. This ensures that data streams up to the checkpoints are successfully received and acknowledged. In
case of any failures, only the streams after the checkpoints have to be re-transmitted.

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.

Application Layer
The application layer enables the user, whether human or software, to access the network. It provides user
interfaces and support for services such as electronic mail, remote file access and transfer, shared database
management, and other types of distributed information services.
It is the user interface layer.
Ex: Application – Browsers, Skype Messenger etc.

TCP/IP PROTOCOL MODEL
The OSI Model we just looked at is just a reference/logical model. It was designed to describe the functions of the
communication system by dividing the communication procedure into smaller and simpler components. But when we
talk about the TCP/IP model, it was designed and developed by Department of Defense (DoD) in 1960s and is based on
standard protocols. It stands for Transmission Control Protocol/Internet Protocol. The TCP/IP model is a concise
version of the OSI model. It contains four layers, unlike seven layers in the OSI model.
Network Access Layer/Link layer
internet Layer
host-to-host or Transport Layer,
and application Layer

1. Network Access Layer –
This layer corresponds to the combination of Data Link Layer and Physical Layer of the OSI model. It looks out
for hardware addressing and the protocols present in this layer allows for the physical transmission of data.
2. Internet Layer –
This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for
logical transmission of data over the entire network. The main protocols residing at this layer are :
1. IP – stands for Internet Protocol and it is responsible for delivering packets from the source host to the destination host
by looking at the IP addresses in the packet headers. IP has 2 versions:
IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is growing as the number of IPv4
addresses are limited in number when compared to the number of users.
2. ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for
providing hosts with information about network problems.
3. ARP – stands for Address Resolution Protocol. Its job is to find the hardware address of a host(MAC Addres) from a
known IP address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or
station address called MAC 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.
ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP and Inverse ARP

3. Host-to-Host Layer –
This layer is analogous to the transport layer of the OSI model. It is responsible for end-to-
end communication and error-free delivery of data. It shields the upper-layer applications
from the complexities of data. The two main protocols present in this layer are :
1. Transmission Control Protocol (TCP) – It is known to provide reliable and error-free
communication between end systems. It performs sequencing and segmentation of data. It
also has acknowledgment feature and controls the flow of the data through flow control
mechanism. It is a very effective protocol but has a lot of overhead due to such features.
Increased overhead leads to increased cost.
User Datagram Protocol (UDP) – Is also provide unrealiable communication between end
systems.On the other hand does not provide any such features error free,flow control..
Unlike TCP, which is connection-oriented protocol, UDP is connectionless

Application Layer –
This layer performs the functions of top three layers of the OSI model: Application,
Presentation and Session Layer. It is responsible for node-to-node communication and
controls user-interface specifications. Some of the protocols present in this layer are:
HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP, SNMP, NTP, DNS, DHCP, NFS, X
Window, LPD. Have a look at Protocols in Application Layer for some information
about these protocols. Protocols other than those present in the linked article are :
1.HTTP and HTTPS – HTTP stands for Hypertext transfer protocol. It is used
by the World Wide Web to manage communications between web browsers and
servers. HTTPS stands for HTTP-Secure. It is a combination of HTTP with
SSL(Secure Socket Layer). It is efficient in cases where the browser need to fill
out forms, sign in, authenticate and carry out bank transactions.
2.Telnet-Enable remote logging

PHYSICAL LAYER
Physical layer in the OSI model plays the role of interacting with actual hardware and signaling mechanism. Physical layer is
the only layer of OSI network model which actually deals with the physical connectivity of two different stations. This layer
defines the hardware equipment, cabling, wiring, frequencies, pulses used to represent binary signals etc.
Physical layer provides its services to Data-link layer. Data-link layer hands over frames to physical layer. Physical layer
converts them to electrical pulses, which represent binary data.The binary data is then sent over the wired or wireless media.
DATA TERMINAL EQUIPMENT (DTE)
Data Terminal Equipment is equipment which acts as source or destinations in digital communication and
which is capable of converting information to signals and also reconverting received signals.The data terminal
equipment may be a single piece of equipment or an interconnected subsystem of multiple pieces of equipment that
perform all the required functions necessary to permit users to communicate. A user interacts with the DTE (e.g.
through a human-machine interface), or the DTE may be the user. Usually, the DTE device is the terminal (or a
computer emulating a terminal), and the DCE is a modem

DATA CIRCUIT-TERMINATING EQUIPMENT (DCE)
Data Communications Equipment, or DCE, is any device that supports data transmission over a serial
telecommunications link. Typically, data communications equipment (DCE) refers to modems, Channel
Service Unit/Data Service Units (CSU/DSUs), multiplexers, and similar devices. The purpose of a DCE is to
provide termination for the telecommunications link and an interface for connecting data terminal
equipment (DTE) to the link.
A data circuit-terminating equipment (DCE) is a device that sits between the data terminal equipment (DTE)
and a data transmission circuit. It is also called data communication(s) equipment and data carrier
equipment. Usually, the DTE device is the terminal (or computer), and the DCE is a modem

Slides Prepared by
Aneeb.A
Assiatant Professor
HM College of Science and Technology Manjeri

Computer Network Introduction full

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