cse 447 lecture 1.ppt

8/13/2019 CSE 447 Lecture 1.ppt

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Introduction

Course Code: CSE 447

Course Name: Computer NetworksCourse Teacher: Shebuti Rayana


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What is a Computer Network?

It is basically a collection of autonomous computers

interconnected by a single technology.

Computers can be geographically located anywhere.

Two computers are said to be interconnected if they

are able to exchange information.

Connection via copper wire, fiber optics,

microwaves, infrared or communication satellite.

Internet is not a single network it is a network of

networks.


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Uses of Computer Network

Business Applications Resource sharing

Information Sharing

Communication medium

Client-server model E-commerce

Home Applications Access to remote information

Person-to-person communication

Interactive Entertainment

Electronic commerce

Peer-to-peer communication


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Uses of Computer Network (contd)

Mobile Users

Wireless network

Communication

Email Message broadcast

M-commerce

Social Issues

Security

Privacy


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Network Hardware

Two important dimensions:

Transmission Technology

Scale

Two types of transmission technology are in

widespread use:

Broadcast Link

Point-to-point Link


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Broadcast Link

A single communication channel shared by allmachines.

It allows the possibility of addressing a packet

to all destinations by a special code(Broadcasting).

It also supports transmission to a subset of

machines (multicasting). Reserve 1 bit to indicate multicasting

Remaining bits hold a group number


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Point-to-point Link

It consists of many connections between

individual pairs of machines.

On the way from source to destination the packet

may visit one or more intermediate machines.

When there are multiple paths, finding the best

path is the main challenge.

Point-to-point transmission with one sender andone receiver is sometimes called unicasting.


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Example of P2P


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Classifying network using Scale

Classifying the network using their physical

size.

Personal Area Network (PAN)

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area Network (WAN)

The Internetwork (Internet)


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PAN

Networks that are meant for one person.

1 meter range

For example: A wireless network connecting acomputer with its mouse, keyboard and

printer.


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LAN

Privately owned networks within a singlebuilding or campus of up to a few kilometersin size.

Resource sharing Information exchange etc.

Three characteristics:

Size Transmission Technology

Topology


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LAN (contd)

Speed

10Mbps to 100Mbps for traditional LANs

10Gbps for new ones

Various Topology: Star

Bus etc.

IEEE 802.3 standard, popularly called Ethernetis a bus based broadcast network withdecentralized control(10Mbps-10Gbps).


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LAN (contd)

IEEE 802.5 is a ring based LAN (4 16 Mbps).

Depending on channel allocation broadcast

networks are of two types:

Static

Dynamic


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Static Allocation

Divide the time into discrete intervals and use

a round robin algorithm, allowing each

machine to broadcast only when its time slot

comes up.

It waste channel capacity.

So, most system attempt to allocate the

channel dynamically.


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Dynamic Allocation

Either centralized or decentralized.

Centralized Allocation

A single entity determines who goes next

It may do this by accepting request or by internal

algorithm.

Decentralized Allocation

Each machine decide by itself whether to

transmit.


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Ethernet bus Ethernet hub

transceivers

Figure 1.17Leon-Garcia & Widjaja: Communication

Networks

Copyright 2000 The McGraw Hill

Companies

Wired LANs


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Wireless LANs

Figure (a) Wireless networking with a base station. (b)Ad hoc networking.


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MAN

It covers a city.

Example: Cable

television

network, high-speed wireless

internet.

A metropolitan area network based on cable TV.


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WAN

Spans a large geographic area, a country or a

continent.

Hosts are connected by communication

subnet.

Customer owns the host

Telephone company or ISP owns the subnet

Subnet carries the message from host to host.


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WAN (contd)

In most WANs, subnet consists of twocomponents:

Transmission Lines

Move bits between machines

Made of copper wire, optical fiber or radio link

Switching Elements

Specialized devices connect three or more transmission

lines. When data arrives at incoming line it forward them by

outgoing line.

Example: Router


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WAN (contd)

Packet Switched Subnet(store-and-forward)

Sending host cuts the message into packets.

Each packet has a sequence number. Packets are injected into the network one at a

time.

At the receiver packets are reassembled. Routing decision of the packets are made

locally (Routing Algorithm needed).


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Packet-Switched Subnet

Figure: A stream of packets from sender to receiver.


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WAN (contd)

Satellite system

Each router has an antenna through which it can

send and receive.

Routers hear the output of the satellite and

upward transmission of fellow routers to the

satellite.

Sometimes routers are connected point-to-point subnet, with only some of them having

satellite antenna.


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Network Topology

The network topologydefines the way in whichcomputers, printers, andother devices are

connected.

It describes the layout ofthe wire and devices aswell as the paths used bydata transmissions.


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Bus Topology

Commonly referred to

as a linear bus

All the devices on a bus

topology are connectedby one single cable.


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Star and Tree Topology

The star topology is the most

commonly used architecture

in Ethernet LANs.

When installed, the star

topology resembles spokes

in a bicycle wheel.

Larger networks use the

extended star topology also

called tree topology.


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Star and Tree Topology (contd)

When used with network devices that filter

frames or packets, like bridges, switches, and

routers, this topology significantly reduces the

traffic on the wires by sending packets only tothe wires of the destination host.


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Ring Topology

A frame travels around the

ring, stopping at each node.

If a node wants to transmit

data, it adds the data as well

as the destination address tothe frame.

The frame then continues

around the ring until it

finds the destination node,which takes the data out of

the frame.

Single ringAll the devices

on the network are

organized in a single loop


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Ring Topology

A frame travels around the

ring, stopping at each node.

If a node wants to transmit

data, it adds the data as well

as the destination address tothe frame.

The frame then continues

around the ring until it

finds the destination node,which takes the data out of

the frame.

Dual Ring

The dual

ring topology allows

data to be sent in both

direction.

The primary disadvantage of ring topology is

the failure of one machine will cause the entire

network to fail.


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Mesh Topology

The mesh topology connectsall devices (nodes) to eachother for redundancy and faulttolerance.

It is used in WANs tointerconnect LANs and formission critical networks likethose used by banks andfinancial institutions.

Implementing the meshtopology is expensive anddifficult.


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Network Software

Protocol Hierarchies

Design Issues for the Layers

Connection-Oriented and ConnectionlessServices


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Protocol Hierarchies Networks are organized as a stack of layers or levels, each

one build upon the one below it.

The number of layers, the name of each layer, the contentsof each layer and the function of each layer varies from

network to network.

Each layer is a kind of virtual machine offering certainservice to the layer above it.

Layer n of one machine communicate with layer n of anothermachine using some protocol called layer n protocol(agreement between the two parties on howcommunication is to proceed).


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Layered Tasks

An example from the everyday life

Hierarchy?

Services


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Layers, Protocols, Interfaces

Layer 5

Layer 4

Layer 1

Layer 2

Layer 3

Layer 5

Layer 4

Layer 1

Layer 2

Layer 3

Physical Medium

Host 1 Host 2Layer 5 Protocol

Layer 4 Protocol

Layer 3 Protocol

Layer 2 Protocol

Layer 1 Protocol

Layer 4/5 interface

Layer 3/4 interface

Layer 2/3 interface

Layer 1/2 interface

Five Layer Network

Entities from the same Layer called- Peers


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Layers

In reality, no data are directly transferred from layer

n on one machine to layer n on another machine.

Each layer passes data and control information tothe layer immediately below it, until the lowest

layer is reached.

Below layer 1 is the physical medium throughwhich actual communication occurs.

Between each pair of adjacent layers is an interfacewhich offer service to the upper layer.


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Layers and Protocols

A set of layers and protocols is called network

architecture.

Networks are designed following the network

architecture.

A list of protocols used by a certain system,

one protocol per layer, is called a protocol

stack.


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Example: Protocol Stack

Peer protocol


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Addressing To identify source and destination

Rules for data transfer Unidirectional or bidirectional

At least two logical channel for each connection Normal data

Urgent data

Error Control Error-detecting and error-correcting codes are used

The receiver tells the sender which messages arecorrectly received and which are not.


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Flow Control

To synchronize the sender and receiver

Multiplexing and Demultiplexing

Routing


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Layer Services

Layers can offer two different types of

services:

Connection-Oriented Services

Connectionless Services


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Connection-Oriented Services

Three Step Service:

User first establish a connection

Use the connection

Release the connection In most cases the order of transmission is

preserved.

After connection is established a negotiation is

conducted between sender, receiver and subnet.

We can compare it with a Telephone service.


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Connectionless Services

It can be compared with a postal service.

Each packet carries a full destination address.

Routed through the network independently.

Ensure Quality of service. Message sequence

byte streams

Unreliable connectionless service is often calleddatagram service like telegram with noacknowledgement.


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Why unreliable connection is preferred

over reliable connection?

Reliable connection may not be available

Ethernet does not provide reliable connection

Packets can occasionally be damaged in transit.

It is up to higher protocol to deal with the

problem.

The delay in the reliable service is

unacceptable.


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Thank You

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