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• Sharing of Hardware– Computing Resources– Disks– Printer

• Sharing of Software– Multiple Single User Licenses are more

expensive than a single multiple user license– Maintaining of Systems becomes easier

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Why Networking

• Sharing of Information- easy accessibility from anywhere- search capability

• Communication– e-mail– internet telephony– audio conferencing– video conferencing

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Types of Networks

• Local Area Network (LAN):

– Network within a floor/building/campus

– At most a few kilometers distance

• Wide Area Network (WAN):

– Country-wide or global network

– Distance: 100s or 1000s of kilometers

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Distributed System

– Based on a computer network.– Network is transparent to the user.– Users don't have to remember computer names, their

architectures, addresses, etc.– The application may be running on multiple

computers.

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Layered Architecture

• Networks are organized as

layers or levels.• Reduces design complexity.• Each network architecture has

different:– number of layers– names of layers

– functions of each layer• Between adjacent layers there is an interface.• Reference models exist to architect a network.

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Protocols

• Rules and conventions used by two communicating entities are collectively known as protocol.

• Each layer of network architecture

has its own protocol.• Two communicating entities at the

same layer are called peers.

• A list of protocols used by a system is called protocol stack.

• A set of layers and protocols is referred to as network architecture.

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OSI Reference Model • Open Systems Interconnection (OSI) model has

been developed by ISO.

• It defines seven layers for communication.

• It is called a Reference Model since different networks may use less or more layers, have somewhat different division of services amongst the layers.

• Functions for each layer are defined.

• Peer layers communicate as per defined protocol.

• Each layer provides services to the upper layer.

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OSI Layers

Application

Presentation

Session

TransportNetwork

Data link

Physical

Application

Presentation

Session

TransportNetwork

Data link

Physical

Application

Presentation

Session

TransportNetwork

Data link

Physical

App YApp X

Data

Data

Data

Data

Data

Data

Data

H

H

H

H

H

H

Outgoing Packets Incoming Packets

Physical Path

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Physical Layer • Concerned with transmitting raw bits

over a channel.• Makes sure that when one side sends a

1 bit,the other side receives it as a 1.

• Covers all aspects for communication,

including,mechanical, electrical, functional and procedural.• Mechanical interface defines the connectors used, number

of pins, their placement, size, material used.• Electrical interface defines the voltage/current levels of

signals.• Functional interface attaches meaning to each signal.

• Procedural interface specifies the sequence of events.

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Data Link Layer

• Data link layer attempts to make physical link reliable.• May divide upper layer packet into multiple frames.• Ensure that peer entity will recognize frame boundaries.• May introduce sequence numbers and acknowledgments.• Some mechanism for error control and flow control.• How to share the link, if it is half-duplex or multipoint.• Can be connection-oriented or connectionless.

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Network Layer

• Network layer is designed to facilitate communication between systems across a communication network.

• Implements network routing and message delivery through networks to the correct destination.

• Congestion control can also be done at network layer.

• Internetworking.• Internet protocol or IP is one example.

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Transport Layer• Implements end-to-end protocol.• Implements required quality-of-service.• May detect erroneous packets.• Reorders packets which arrive out-of-sequence.• Ensures that there is no loss or duplication of

packets.• May provide connection-less or connection-

oriented type of service.

• Provides for the connection management.• Multiplexing and demultiplexing.• Packetization, flow control, etc.• Examples: TCP, and UDP 

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Session Layer• Allows users on different computers to establish a session.• This layer requests for a logical connection to be established based on an end-user's request.• Any necessary log-on or password validation is also handled by this layer.• This layer provides services like dialogue discipline which can be full duplex or half duplex.• Session layer can also provide check pointing (or synchronization) mechanism. • If a failure occurs between checkpoints, all data can be retransmitted from the last checkpoint.

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Presentation and Application Layers

• Presentation layer defines the data format to be exchanged between the programs.• Manages abstract data structures, and converts them host-representation to network- representation and vice versa.• Application layer is the highest level of OSI model.• May define common standards like Network Virtual Terminal.• Examples at this layer are file transfer, electronic mail, remote login, etc.•

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TCP/IP Reference Model

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

APPLICATION APPLICATION

TRANSPORT

INTERNET

HOST-TO-NETWORK

(Data Link/ Physical)

OSI TCP/IP

Not presentIn the model

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Transmission Media

• Carries information in its raw form.• Electrical, optical or radio signals.• Guided Media Twisted pair (shielded, unshielded) Coaxial cable Optical fiber • Unguided Media (wireless) Radio link Terrestrial microwave Satellite microwave

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Data Transmission • Point-to-point link

- Two nodes are connected to each other. • Multi-point link

- Shared media

- Any two nodes can communicate with each other. - Broadcasting:

* Information is sent to all nodes at the same

time. - Multicasting:

* Information is sent to a group of nodes.

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Types of Links• Simplex:

• Information can be sent only in one direction.

• Half-duplex:

• Information can be sent in both directions.

but only in one direction at any given time.

• Full-duplex:

• Information can be sent in both directions simultaneously.

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Multiplexing of Several Communications

• Frequency Division Multiplexing (FDM)• Frequency spectrum is divided among the logical

channels.• Each user has exclusive control of of one

frequency band.• Time Division Multiplexing (TDM)• Users take turn in round-robin fashion.• Code Division Multiplexing (CDM)• Asynchronous TDM or Statistical Multiplexing

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Data Switching• If two nodes are not directly connected, the information

may need to travel through intermediate nodes, called, switches.

• Circuit-switched:• A channel for data transfer is setup before communication

starts. All resources are reserved.• Message-switched:• Message is sent to next-hop, where it is stored, and

forwarded to next node.• Also known as ``store-and-forward'' network• Packet-switched:• Message is divided into smaller packets.• Packets are sent independently through ``store-and-forward'' mechanism.

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