Computer Networks(2)

8/9/2019 Computer Networks(2)

1/45

Computer Network


2/45

Figure 1-1

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Data Communication System Components


3/45

Data transmission

Simplex Half-Duplex Full-Duplex


4/45

Figure 2-12


Simplex


5/45

Figure 2-13


Half-Duplex


6/45

Figure 2-14


Full-Duplex


7/45

Computer networkconnects two or moreautonomouscomputers.

The computers canbe geographicallylocated anywhere.

Computer Networks


8/45

Introduction of Computer Network

A collection of computers interconnected to exchange

information

Need for computer networks

Intrinsically distributed informationResource sharing

Computational power (load sharing)

Reliability


9/45

Advantages of Computer Network

Intrinsically distributed informationInformation has to be kept at multi locations and has tobe networked togetherExample: Information in Airline and Train reservations

Useful to group the information and place it in a

hierarchical mannerExample: Branch Off., Zonal Off., Head Off.

Development work

Resource sharingWhen needed?

When Information needs to be shared among usersExample : Database, programs kept in a server

When it is not possible to afford expensive peripheralsExample : Fax and laser printers


10/45

Computational power

Distributed computing

Example : when an application requires GUI and heavy

number of crunching involving floating operationsLoad sharing

Example : Migrating processes

Reliability Graceful degradation

Entire system does not collapse when there is a collapse ofsome machines in the system

Alternative supply is available


11/45

Connecting computers via a communicationnetwork

Here is what a global picture of a computer network looks like.Here you see a wide variety of computers connected togetherranging from microprocessors to super computers


12/45

Real Computer Networks

Name Protocol Example

INTERNET TCP/IP ERNET,VSNL GIASBITNET IBM TIFR

USENET UUCP Access net(Dial upnodes on shakti)

INET X.25 DOT

NICNET SATELLITE District HQ

The above slide shows some examples of real networks the name ofthe network, the protocol and an example.

Acronyms:TCP\IP: Transmission Control Protocol and Internet ProtocolDOT: Dept of TelecommunicationUUCP: Unix to Unix Copy


13/45

Network topology

Topology

Refers to the way different sites are interconnected

Network Topology

Refers to the way different computers are connected to

each other

Motivation

Given, let us say 100 computers, how to connect them?


14/45

The network topologydefines the way in whichcomputers, printers, andother devices areconnected. A networktopology describes thelayout of the wire anddevices as well as thepaths used by data

transmissions.


15/45

Topology evaluation criteria

Basic cost

Cost of linking various sites

Communication cost Site A ----->(Time) Site B

Reliability costWhat happens if a site fails

How to judge the cost issue, say?All the following slides should be covered interactively!!Communication Cost: This depends on the communicationmedia such as copper link,

satellite connection etc. But what we are interested over here is inthe cost independent ofthe actual media. For e.g. how many no. of hops a message has tomake in order to reachthe destination from any given source.Reliability Cost: This is the cost which says how reliable is

the given topology when a site fails


16/45

Complete topology

Basic costHigh O(n^2)

Communication costFast (1hop)

ReliabilityHigh


17/45

Complete topology (Contd.)

It turns out that to connect n sites, we require n*(n-1)/2 links.Hence, the basic cost is high

and is in the order of O(n^2).

The communication is very fast as there is only one hop requiredto reach the destination

from any given source. However, the communication cost is highbecause every site is

connected to every other site.

Reliability is very good as there is an alternative path if one of thelink fails


18/45

Star topology

A central computer is used to connect all sites in the network. Thiscentral site acts as a switching element.

The star topology is the mostcommonly used architecture inEthernet LANs.

When installed, the star topologyresembles spokes in a bicycle wheel.

Larger networks use the extended startopology also called tree topology.

When used with network devices thatfilter frames or packets, like bridges,switches, and routers, this topologysignificantly reduces the traffic on thewires by sending packets only to thewires of the destination host.


19/45

Star topology

Basic cost

Linear : O(n)Communication cost

Low : 2 Hops Speed Central site : Bottle neck

Reliability Depends on central site

Basic cost to connect n sites is same as the number of sites i.e.O(n). Every node isconnected to the central node. Thus, if there are n nodes, then nlines are needed to ensureconnectivity. Hence, the basic cost of connecting the nodes is O(n).Communication cost is low (good) as it requires just two hops for asite to connect to anothersite.

Reliability of the star topology depends on the central elementwhich can be the bottleneck


20/45

Ring topology

Each site is connected to its neighbors as shown in the diagram in the

slide above

A frame travels around the ring,

stopping at each node. If a nodewants to transmit data, it addsthe data as well as thedestination address to the frame.

The frame then continues around

the ring until it finds thedestination node, which takes thedata out of the frame.

Single ring All the devices onthe network share a single

cableDual ring The dual ringtopology allows data to be sentin both directions.


21/45

Ring topology (contd..)

Basic cost Linear : O(n)Communication cost (Worst case)

Unidirectional : n - 1 Bidirectional : n/2Reliability (Partition) Unidirectional : 1 site

Bidirectional : 2 site

The basic cost to connect n sites is O(n).

Communication cost depends on whether the site is uni-directional or bi-directional. The communication cost is high for auni-directional ring topology , say for e.g. the site that is themost far away from any given site is (n-1) site


22/45

Linear bus

All sites share a common communication medium , only one sitecan communicate at a time.


23/45

Linear bus (contd..)

Basic costLinear : O(n)

Communication cost

Link is the bottleneck Reliability Link

Commonly referred to as a linear bus,

all the devices on a bus topology are

connected by one single cable.

node

Transmission MEdium


24/45

Components Of Network

Physical Media

Interconnecting Devices

ComputersNetworking Software

Applications

Open Systems


25/45

Open SystemsInterconnection (OSI)

ModelInternational standard organization (ISO)established a committee in 1977 to develop anarchitecture for computer communication.

Open Systems Interconnection (OSI) referencemodel is the result of this effort.

In 1984, the Open Systems Interconnection (OSI)reference model was approved as an internationalstandard for communications architecture.

Term open denotes the ability to connect any

two systems which conform to the reference modeland associated standards.


26/45

OSI Reference ModelThe OSI model is now considered the primaryArchitectural model for inter-computercommunications.

The OSI model describes how information or datamakes its way from application programmes

(such as spreadsheets) through a networkmedium (such as wire) to another applicationprogramme located on another network.

The OSI reference model divides the problem ofmoving information between computers over a

network medium into SEVEN smaller and moremanageable problems .

This separation into smaller more manageablefunctions is known as layering.


27/45

OSI Layer model

Deals with the connection and communication among open systems

Has a layered approach which helps in

Reducing the design complexity of each layer

Helps in shielding the implementation details of the services offeredby each layer

Each layer provides services to the layer above

Different protocols are followed between corresponding layers in Senderand Receiver

The OSI model is based on a proposal developed by the InternationalStandard Organization.(ISO) as a first step towards standardizing the protocols used in thedifferent layers. Thismodel is called ISO OSI (Open System Interconnection) Reference

Model because it dealswith the connection of systems that are open for communication withother systemsotherwise called as open systems.The OSI defined seven layers for a computer network architecture.Aprotocol is a mutually acceptable language between two systems

which may be defined as a set of rules governing the exchange of data.A rotocol states ver clearl what is


28/45

OSI Reference Model: 7

Layers


29/45

7 Layers

7. Application Layer

6. Presentation Layer

5. Session Layer4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer

All

People

SeemTo

Need

DataProcessing

Please

Do

NotTrust

Sales

PersonsApproach


30/45

Tasks involved in sendingletter

OSI Model


31/45

Physical LayerProvides physical interface for transmission ofinformation.

Defines rules by which bits are passed from one

system to another on a physical communicationmedium.

Covers all - mechanical, electrical, functional andprocedural - aspects for physical communication.

Such characteristics as voltage levels, timing of

voltage changes, physical data rates, maximumtransmission distances, physical connectors, andother similar attributes are defined by physicallayer specifications.

OSI Model

OSI Model


32/45

Data Link LayerData link layer attempts to provide reliablecommunication over the physical layer interface.

Breaks the outgoing data into frames andreassemble the received frames.

Create and detect frame boundaries.Handle errors by implementing anacknowledgement and retransmission scheme.Implement flow control.Supports points-to-point as well as broadcast

communication.Supports simplex, half-duplex or full-duplexcommunication.

OSI Model

OSI Model


33/45

Network LayerImplements routing of frames (packets)through the network.

Defines the most optimum path the packet

should take from the source to the destinationDefines logical addressing so that any endpointcan be identified.

Handles congestion in the network.

Facilitates interconnection between

heterogeneous networks (Internetworking).The network layer also defines how tofragment a packet into smaller packets toaccommodate different media.

OSI Model

OSI Model


34/45

Transport LayerPurpose of this layer is to provide a reliablemechanism for the exchange of data between twoprocesses in different computers.

Ensures that the data units are delivered error free.

Ensures that data units are delivered in sequence.

Ensures that there is no loss or duplication of dataunits.

Provides connectionless or connection oriented

service.Provides for the connection management.

Multiplex multiple connection over a singlechannel.

OSI Model

OSI Model


35/45

Session LayerSession layer provides mechanism for controllingthe dialogue between the two end systems. Itdefines how to start, control and end conversations(called sessions) between applications.

This layer requests for a logical connection to beestablished on an end-users request.Any necessary log-on or password validation is alsohandled by this layer.Session layer is also responsible for terminating theconnection.

This layer provides services like dialogue disciplinewhich can be full duplex or half duplex.Session layer can also provide check-pointingmechanism such that if a failure of some sort occursbetween checkpoints, all data can be retransmitted

from the last checkpoint.

OSI Model

OSI Model


36/45

Presentation LayerPresentation layer defines the format inwhich the data is to be exchanged betweenthe two communicating entities.

Also handles data compression and data

encryption (cryptography).

OSI Model


37/45

Application LayerApplication layer interacts with applicationprograms and is the highest level of OSImodel.

Application layer contains management

functions to support distributedapplications.

Examples of application layer areapplications such as file transfer, electronicmail, remote login etc.


38/45

Remember

A convenient aid for remembering the OSI

layer names is to use the first letter of each

word in the phrase:

All People Seem ToNeed Data Processing

Please Do Not Trust Sales Person

Approach


39/45


40/45

OSI in ActionA message begins at the topapplication layer and movesdown the OSI layers to thebottom physical layer.

As the message descends,each successive OSI model

layer adds a header to it.

A header is layer-specificinformation that basicallyexplains what functions thelayer carried out.

Conversely, at the receivingend, headers are striped fromthe message as it travels upthe corresponding layers.


41/45

Routing devices

Repeaters

Bridges

Routers Gateway


42/45

Repeater A repeater receives a signal, regenerates it, andpasses it on.

It can regenerate and retime network signals at thebit level to allow them to travel a longer distanceon the media.

It operates at Physical Layer of OSI The Four Repeater Rule for 10-Mbps Ethernet

should be used as a standard when extending LANsegments.

This rule states that no more than four repeaters

can be used between hosts on a LAN.

This rule is used to limit latency added to frametravel by each repeater.


43/45

Hub Hubs are used to connect

multiple nodes to a singlephysical device, whichconnects to the network.

Hubs are actually multiport

repeaters. Using a hub changes the

network topology from alinear bus, to a star.

With hubs, data arrivingover the cables to a hub port

is electrically repeated on allthe other ports connected tothe same network segment,except for the port on whichthe data was sent.


44/45

Bridge Bridges are used to logically separate

network segments within the samenetwork.

They operate at the OSI data link layer(Layer 2) and are independent ofhigher-layer protocols.

The function of the bridge is to makeintelligent decisions about whether ornot to pass signals on to the nextsegment of a network.

When a bridge receives a frame on the

network, the destination MAC address islooked up in the bridge table todetermine whether to filter, flood, orcopy the frame onto another segment

Broadcast Packets are forwarded


45/45

Switch Switches are Multiport Bridges.

Switches provide a unique network segment on each port,thereby separating collision domains.

Today, network designers are replacing hubs in their wiringclosets with switches to increase their network performance

and bandwidth while protecting their existing wiringinvestments.

Like bridges, switches learn certain information about the datapackets that are received from various computers on thenetwork.

Switches use this information to build forwarding tables todetermine the destination of data being sent by one computerto another computer on the network.

Documents

Computer Networks(2)