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06/13/22 1 Computer Networks Introduction to Computer Networks What is a Network? Why do we need Network? Network Classification Types of Network Basic Hardware Components

934 Ch1 Networks

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Computer Networks

Introduction to Computer Networks

What is a Network?Why do we need Network?Network ClassificationTypes of NetworkBasic Hardware Components

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What is a Network?• A network is a collection of computers and devices connected to each other. The

network allows computers to communicate with each other and share resources and information. The Advance Research Projects Agency (ARPA) designed "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of Defence. It was the first computer network in the world in late 1960's and early1970's.

Why a Network? • Distribute pieces of computation among computers (nodes) • Resource Sharing • Data Sharing• Remote Data/File Access , Remote I/O Devices • Personal communications (like e-mail, chat, audio/video conferencing) • World Wide Web

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Network ClassificationOn the basis of

ScaleConnection MethodFunctional RelationshipNetwork Topology

Based on their scale, networks can be classified as Local Area Network (LAN), Wide Area Network (WAN), Metropolitan Area Network (MAN), Personal Area Network (PAN), Virtual Private Network (VPN), Campus Area Network (CAN), Storage Area Network (SAN), etc. Connection Method such as Optical Fiber, Ethernet, Wireless LAN etc.Network Architecture such as Client-Server or Peer to PeerTopology- Bus, Star, Ring , Mesh, Tree, Hybrid

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• Types of Network– Personal Area Network (PAN)

– Local Area Network (LAN)

– Campus Area Network (CAN)

– Metropolitan Area Network (MAN)

– Wide Area Network (WAN)

– Global Area Network (GAN)

– Virtual Private Network (VPN)

– Inter network• Intranet

• Extranet

• Internet

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• Closing Quiza) Without Computer networks, Computers will be half the use.Comment.

b) What are the various topologies? Which one is the most secure?

c) Difference between Internet and Intranet.

d) Difference between Client Server and Work-group.

e) Wifi stands for?

f) What are the various types of networks? Discuss all of them

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• Uses of Computer Network– Business Application

• Client Server Architecture

• Email

• E Commerce

– Home Applications• Access to Remote Information

• Person to Person Communication

• Interactive Entertainment

• E-Commerce

– Mobile User

– Social Issues

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Network Hardware

There are two types of transmission technology that are widespread in use-

Broadcast Link

Point to Point Link

What is Broadcasting, Multicasting & Unicasting?

What is Simplex, Half Duplex and Full Duplex mode?


Wireless Network

Inter network

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Network Software

Protocol Hierarchies – To reduce the design complexity, most networks are organized as a stack of layers or levels, built one over the other.

They name of each layer, content and functions of each layer are different.

The purpose of each layer is to offer services to higher layer, shielding those layers from the details of how the offered services are actually implemented.

Protocol is an agreement between the communicating parties on how communication is to proceed.

Figure 1.13 from the text book

No data is directly transferred from layer n of 1 machine to the layer n of the other , instead they have to use the Physical Layer.

A Set of Layers and Protocols together is called network architecture.

Communication Example

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Design Issues for the Layers

Every layer needs a mechanism for identifying senders and receivers

We need to identify with whom a machine wants to talk, i.e. some form of addressing mechanism is required.

Rules for Data transfer need to be established. In some cases data only travels one direction, in others both ways. Also need to find out how many logical channels?

Error Control :- Various error-detecting and error correcting codes are there that have to be used, also at times, the sequence in which data is sent is not the way in which it is received, hence reassembly also needs to be done.

Flow Control

Inability to receive long messages, we disassemble, transmit and then reassemble data

At times sending data packets separately is not efficient, in that case we use multiplexing and demultiplexing.


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Connection Oriented /Less Services

Connection-oriented service : Just like a telephone system, we establish a connection, use and then release the connection.

Firstly, negotiations take place about the parameters used, such as maximum message size, quality of service required and other issues.

Connectionless Service : Just like the postal system

A reliable service is ensured by having the receiver acknowledge the receipt of each message so that sender is sure that it arrived. This introduces overheads and delays.

Example of connection oriented and connection less service

Remote login- connection oriented

Request Reply – Database Query etc

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Service Primitives

A service is formally specified by a set of primitives ( operations) available to a user process to access the service.

Primitive Meaning

LISTEN Block waiting for an incoming connection

CONNECT Establish a connection with a waiting peer

RECEIVE Block waiting for an incoming message

SEND Send a message to the peer

DISCONNECT Terminate a Connection

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Client Machine Server Machine

1)Connection Request


3)Request for Data




How communication takes place?

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Services & Protocols

A service is a set of primitives (operations) that a layer provides to the layer above it.

A service defines what operation the layer is prepared to perform but cannot tell how they will be performed.

A Protocol is a set of rules governing the format and meaning of the packers or messages that are exchanged by the peer entities within a layer.

Closing Quiz1)What is broadcasting, multicasting and unicasting?

2)Justify the purpose behind the layered architecture, define service & protocol

3)What are the various design issues pertaining to Layers?

4) Differentiate Connection Less & Connection Oriented Service.

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OSI Reference Model

Open System Interconnection model has seven layers.

The principles that were applied to arrive at the seven layers can be summarized as follows :

A layer should be created where a different abstraction is needed.

Each Layer should perform a well-defined function.

The function of each layer should be chosen with an eye toward defining internationally standardized protocols.

The layer boundaries should be chosen to minimize the information flow across the interfaces.

The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

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Layers in OSI

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Layers in OSI

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The Application Layer

It is the spot where the User communicates with the Computer.

Example of IE.

It is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist.

Application Layer acts as an interface between the actual application programs. This means that Microsoft Word, for e.g. does not reside at the Application Layer but instead interfaces with the Application Layer Protocols.

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The Presentation Layer

It presents Data to the Application Layer and is responsible for Data translation and Code Formatting.

This layer is essentially a translator and provides coding and conversion functions.

By providing translation services, the Presentation Lauer ensures that data transferred from the Application Layer of one system can be read by the Application Layer of another one.

Tasks like Data Compression, Decompression, Encryption and Decryption are associated with this layer.

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The Session Layer

Responsible for setting up, managing and then tearing down sessions between Presentation Layer entities.

It also provides Dialogue control between devices and nodes.

Simplex, Half Duplex or Full Duplex mode? The choice is made here

It keeps different application’s data separate from other application data.

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The Transport Layer

Data is in the form of segments here.

The Transport Layer segments and reassembles data into data stream.

They provide end-to-end data transport services and can establish a logical connection between sending host and destination host on an inter network.

Both TCP and UDP work at this Layer, TCP is reliable service and UDP is not. (reliable means acknowledgements, sequencing and flow control be used.)


Flow Control

Connection Oriented Communication



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The Transport Layer ( Cont…)

Connection Oriented Communication is reliable, a device that wants to transmit sets up a connection-oriented communication with a remote device by creating a session.

A session is established after a 3-way handshake.

While information is being transferred between hosts, the two machines periodically check in with each other, communicating through their protocol software to ensure that all is going well.

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What happens when a machine receives a flood of datagrams too quickly to process?

It stores them in a memory section called buffer, if the buffer even gets full, transport layer sends a “Not Ready” indicator to the sender.

A service is considered to be connection-oriented if it has following characteristics –

A virtual circuit is set up.It uses sequencing.It uses acknowledgements.It uses flow control.

The Transport Layer ( Cont…)

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The quantity of data segments (in bytes) that the transmitting machine is allowed to send without receiving an acknowledgement for them is called window.


Non-duplication of data is achieved through positive acknowledgement with re transmission.

The sender documents each segment it sends and waits for this acknowledgement before sending the next segment. It starts a timer and retransmits if it expires before an acknowledgement is returned from receiving end.

The Transport Layer ( Cont…)

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It manages device addressing, tracks the location of devices on the network and determines the best way to move data, which means that the NL must support transport traffic between devices that aren’t locally attached.

Routers are specified at the NL and provide the routing services

How does it work?

Two type of packets are used : Data Packets (Routed Packets) & Route Update Packets.

The Routing Table consist of the following information : Network AddressInterfaceMetric

The Network Layer

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The Network Layer ( Cont…)

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The Data Link LayerThe DLL provides Physical Transmission of the Data and handles error notification, network topology and flow control. That means that DLL will make sure that the messages are delivered to proper device on a LAN using hardware address and will translate messages from the Network Layer into bits for the Physical Layer to transmit.

Data is in the form of Frames here.DLL adds a custom header containing the Hardware Destination and Source Address.

The DLL consists of two sublayers :-MAC (Media Access Control)LLC ( Logical Link Control)

Bridges and Switches work at DLL

Collision Domain / Broadcast Domain

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The Physical LayerThe Physical Layer does 2 things –send bits and receive bits.

The Physical Layer communicates directly with the various types of actual communication media.

Different type of media can be used to represent bit value. Some use audio tones, while others use State Transitions- Change in voltage from high to low and vice versa.

The Physical Layer specifies the electrical, mechanical, procedural and functional requirements for activating, maintaining & deactivating a physical link between end systems.

Interface between DCE and DTE is identified here. DCE is usually located at the service provider and DTE is attached device.

Hubs and Repeaters work at Physical Layer.

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The TCP/IP Model

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The TCP/IP ModelTCP/IP model is also called DOD Model

It has the following LayersProcess / Application LayerHost-to-Host LayerInternet LayerNetwork Access Layer

TCP/IP Protocol Suite

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The Process/Application Layer

It has the following ProtocolsTelnetFTPTFTPNFSSMTPLPDX WindowSNMPDNSDHCP/BootP

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The Host-to-Host Layer

The main purpose of this layer is to shield the upper-layer applications from the complexities of the network

It consists of 2 protocolsTCPUDP

TCP takes large blocks of information from an application and breaks them into segments.

It numbers and sequences each segment so that the destination’s TCP can put the segments back into the order.

Before sending, it establishes a connection, a virtual circuit, provides reliable service and works in a full-duplex mode.

It also waits for an acknowledgement from the receiver.

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The Host to Host Layer

TCP/IP Segment Format

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The Host-to-Host Layer UDP on the other hand, is a scale-down model.

It does not sequence the segments and does not care in which order the segments arrive at the destination.

UDP does not create a virtual circuit either.

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The Host-to-Host Layer

The Internet Layer

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The Network Layer The Physical Connectivity & actual Transmission of data takes place here.

Comparison Of OSI & TCP/IP

Revolves around the concept of ServicesInterfacesProtocols