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Transcript of Computer Networks(2)
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Computer Network
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Figure 1-1
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Data Communication System Components
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Data transmission
Simplex Half-Duplex Full-Duplex
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Figure 2-12
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Simplex
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Figure 2-13
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Half-Duplex
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Figure 2-14
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Full-Duplex
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Computer networkconnects two or moreautonomouscomputers.
The computers canbe geographicallylocated anywhere.
Computer Networks
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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
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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
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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
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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
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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
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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?
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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.
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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
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Complete topology
Basic costHigh O(n^2)
Communication costFast (1hop)
ReliabilityHigh
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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
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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.
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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
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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.
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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
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Linear bus
All sites share a common communication medium , only one sitecan communicate at a time.
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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
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Components Of Network
Physical Media
Interconnecting Devices
ComputersNetworking Software
Applications
Open Systems
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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.
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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.
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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
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OSI Reference Model: 7
Layers
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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
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Tasks involved in sendingletter
OSI Model
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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
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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
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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
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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
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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
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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
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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.
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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
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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.
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Routing devices
Repeaters
Bridges
Routers Gateway
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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.
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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.
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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
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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.