1-Introduction to Computer Networks

8/7/2019 1-Introduction to Computer Networks

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Introduction to Computer

NetworksS.S. Satapathy



Introduction- Computer Network

- Collection of Computer Systems interconnected bycommunication channels

HH

H

H

H

H

HH H

H Subnet

H - Host

- Switch



Network Types• Based on the scope and size the networks

are classified as:– WAN – Satellite, Long distance optical

fiber, leased line based networks spreadacross cities, states or countries.

– MAN – Spread over an area with a radiusof up to several tens of km.• Wi Max, Cable based networks.

– LAN – Confined within a hall, building or a campus.• Ethernet, Wi Fi.

– PAN/ BAN – Confined within a room.•

Bluetooth



Topology



Network Topology• A network's topology is the layout of the

cables and devices that connect the nodes.• The four most common network topologies

are:

– Bus. Each node is connected in series along asingle conduit.

– Star. All nodes are connected to a central hub.

– Ring. Nodes are connected in a circular chain,

with the conduit beginning and ending at thesame computer.

– Mesh. Each node has a separate connection toevery other node.





Switching Techniques



Switching Techniques

• Circuit Switching

• Packet Switching

– Virtual Circuit– Datagram



Simple Switched Network



Circuit Switching• Dedicated communication path between two stations

• Three phases– Establish

– Transfer

– Disconnect

• Must have switching capacity and channel capacity to establish

connection• Must have intelligence to work out routing

• Inefficient– Channel capacity dedicated for duration of connection

– If no data, capacity wasted

• Set up (connection) takes time• Once connected, transfer is transparent

• Developed for voice traffic (phone)



Packet Switching

• Data transmitted in small packets

containing

– user data &

– some control info

• Routing (addressing) info

• Packets are received, stored briefly

(buffered) and past on to the next node– Store and forward



Use of Packets



Advantages• Line efficiency

– Single node to node link can be shared by many

packets over time

– Packets queued and transmitted as fast as possible

• Data rate conversion– Each station connects to the local node at its own

speed

– Nodes buffer data if required to equalize rates

• Packets are accepted even when network is busy– Delivery may slow down

• Priorities can be used



Datagram• Each packet treated

independently• Packets can take any

practical route

• Packets may arrive

out of order • Packets may go

missing

• Up to receiver to re-order packets andrecover from missingpackets



VirtualCircuit• Preplanned route

established before any

packets sent• Call request and call

accept packets establishconnection (handshake)

• Each packet contains avirtual circuit identifier

instead of destinationaddress

• No routing decisionsrequired for each packet

• Clear request to dropcircuit

• Not a dedicated path



Virtual Circuits v Datagram• Virtual circuits

– Network can provide sequencing and error control

– Packets are forwarded more quickly• No routing decisions to make

– Less reliable• Loss of a node looses all circuits through that node

• Datagram

– No call setup phase• Better if few packets

– More flexible• Routing can be used to avoid congested parts of the

network



Packet Size



Multiple Access Methods



Multiple Access Methods

Three major types:

– Frequency Division Multiple Access (FDMA)

– Time Division Multiple Access (TDMA)

– Code Division Multiple Access (CDMA)

• Frequency hopping (FH-CDMA)

• Direct sequence (DS-CDMA)



Frequency-Time Plane

Time

Frequency

Partition of signalspace into time slots and frequency bands



FDMA

Time

Frequency

Different userstransmit at differentfrequency bands simultaneously.



Frequency Division Multiple Access

(FDMA)

• The spectrum of each link (forward or reverse) is

further divided into frequency bands

• Each station assigned fixed frequency band

f r e q u e n c

y b a n d s

idle

idle

idle



TDMA

Time

Frequency

Different userstransmit at differenttime slots.

Each user occupy thewhole freq. spectrum.



Time Division Multiple Access

(TDMA)

• The mobile users access the channel in round-robin fashion.

• Each station gets one slot in each round.

Slots 2, 5 and 6 are idle



FDMA/TDMA, example GSM

1 2 3 7 8

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

Each freq. carrier is divided into 8 time slots.



CDMA

Received

signal

Use B’s

Chip

User A’s

Chip

• Spread spectrum modulation– originally developed for the

military

– resists jamming and many kindsof interference

• All users share same (large)block of spectrum

• Almost all accepted 3G radiostandards are based on

CDMA– CDMA2000, W-CDMA and TD-

SCDMA



Frequency Hopping CDMA

Frequency

Time

At each successive time

slot, the frequencyband assignments arereordered.

Each user employs acode that dictates the

frequency hoppingpattern.

Benefits include improved privacy,decreased narrowband

interference



Direct Sequence CDMA

Time

Frequency

All users occupy the wholebandwidth all the time.

Signals of different usersoverlap with one other.

How can it be done?

N t k A hit t



Network Architecture– Has to deal with very wide range of issues

– Cable, connector, signal form, voltage, current . . .

– Procedural steps in sending signals.– Reliably exchanging data between adjacent switches.– Dealing with interference, errors, loss of data.– Specifying the destination host.– Finding the path to the destination.

– Security issues.– Issues specific to kind of use- Application– . . .

– Designed in layered modules- each module/

layer taking care of a set of issues– Each layer has a set of rules & mechanisms (protocol)

for the exchange of information.

– The set of layers, also called the protocol stack,define the architecture of the network.



ISO/OSI Reference Model

• To address the growing tangle of incompatibleproprietary network protocols, in 1984 the ISO formed a

committee to devise a unified protocol standard.

• The result of this effort is the ISO Open Systems

Interconnection Reference Model (ISO/OSI RM).

• The ISO’s work is called a reference model because

virtually no commercial system uses all of the features

precisely as specified in the model.

• The ISO/OSI model does, however, lend itself to

understanding the concept of a unified communications

architecture.



ISO/OSI Reference Model• The OSI RM contains seven protocol

layers, starting with physical media

interconnections at Layer 1, throughapplications at Layer 7.• OSI model defines only the functions of

each of the seven layers and theinterfaces between them.

• Implementation details are not part of themodel.

• Interface: It defines which primitiveoperations and services the lower layer offers to the upper layer.

• Peer: The similar layer on a differentmachine.

• Protocol: It is a set of rules andconventions used by the layer to

communicate with similar peer layer inanother (remote) system.

• The peer processes communicate with eachother using a protocol .

• A set of layers and protocols is called anetwork architecture.



• The Physical layer receives a stream of bits from the Data Link layer above it,

encodes them and places them on thecommunications medium.• The Physical layer conveys transmission

frames, called Physical Protocol DataUnits, or Physical PDUs. Each physical

PDU carries an address and hasdelimiter signal patterns that surroundthe payload , or contents, of the PDU.

• It concerns with transmitting raw bitsover a communication channel.

• It deals with:– Voltage Levels for 0 and 1.– Connectors: Number of bins and purpose of

each bin.– Transmission Media.

– Attenuation and Distortion.

Physical layer



• The Data Link layer negotiates framesizes and the speed at which they aresent with the Data Link layer at theother end.

– The timing of frame transmission iscalled flow control .

• Data Link layers at both endsacknowledge packets as they areexchanged. The sender retransmitsthe packet if no acknowledgement isreceived within a given time interval.

Data Link layer



• At the originating computers, the

Network layer adds addressinginformation to the Transport layer PDUs.

• The Network layer establishes the routeand ensures that the PDU size iscompatible with all of the equipment

between the source and the destination.• Its most important job is in moving PDUs

across intermediate nodes.• It deals with:

– Routing: It determines how packets are

routed from source to destination.– Congestion Control: Many packets in the

subnet trying to use the same route.– Internetworking: It allows heterogeneous

networks to be interconnected.– Accounting Function.

Network layer



• the OSI Transport layer provides end-to-end acknowledgement and error correctionthrough its handshaking with the Transportlayer at the other end of the conversation.

• Transport layer assures the Session layer that there are no network-induced errors inthe PDU.

• Disassembling and Reassembling: Itaccepts data from a session layer, split it upto smaller units if needed, pass these to thenetwork layer, and ensure that the pieces allarrive correctly at the other end.

• End-to-end error control.• End-to-end flow control.• It defines Quality of Service (QOS).• It is an end-to-end layer. Lower layers

communicate with intermediate nodes.

Transport layer



• It allows users on different machines to establishsessions between them.

• Interaction Management: The Session layer arbitrates the dialogue between twocommunicating nodes, opening and closing thatdialogue as necessary. The data exchangeassociated with a dialog may be:– Duplex: Two-way simultaneous.

– Half-Duplex: Two-way alternate.– Simplex: One-way.

• Synchronization: For lengthy transaction, theuser may choose to establish synchronizationpoints associated with the transfer. If a faultdevelops during a transaction, the dialog may be

restarted at an agreed synchronization point• It also supplies recovery checkpoints during file

transfers.• Checkpoints are issued each time a block of data

is acknowledged as being received in goodcondition.

Session layer



• The Presentation layer provideshigh-level data interpretationservices for the Application layer above it, such as EBCDIC-to-ASCII translation.

• Presentation layer services arealso called into play if we useencryption or certain types of data compression.

Presentation layer



• The Application layer suppliesmeaningful information andservices to users at one end of the communication andinterfaces with system resources(programs and data files) at theother end of the communication.

• All that applications need to do isto send messages to the

Presentation layer, and the lower layers take care of the hard part.

Application layer

D t t i i i OSI R f



Data transmission in OSI Reference

Model

Data

DataPH

DataAH

DataSH

DataTH

DataNH

DataDH DT

Application

Data Link

Presentation

Network

Physical

Session

Transport

Receiving

Process

Application

Data Link

Presentation

Network

Physical

Session

Transport

Sending

Process

Bits

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1-Introduction to Computer Networks