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UNIT1

Introduction of Networking

Advantages of Networking

Classification of Networking

Component Roles

Network Features

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Introduction

Data Communications

The term data refers to raw facts, letters orsymbols that are processed into meaningful

information.

Communication is the process of transferringmessages from one point to another.

Data communication is the transmission of

signals in a reliable and efficient manner.

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How do the computers send and receive the dataacross communication links?

By data communication softwares; instruct computersand devices as to how exactly data is to be transferredfrom one place to another.

The procedure of data transmission in the form ofsoftware is calledprotocol.

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Communication Model-

The main purpose of a communicationsystem is the exchange of data betweentwo entities. The model is as follows:

Source

Transmitter

Transmission

System

Receiver

Destination 4

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Data Communication

Components

Data Representation

Direction of Data Flow

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Figure 1.1 Components of data communication

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Figure 1.2 Simplex

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Figure 1.3 Half-duplex

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Figure 1.4 Full-duplex

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What is Network?

A Network means an interconnected system ofobjects, people or things.

For examples,

The cities connected by roads make a type of network

in which the cities are as nodes and roads are asconnected lines calledarcs,

Water-flow mechanism,

Type of graph,

Electric circuit,

Building structure, Computer Network

And so on..

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Computer Network

Definition A computer network is a connected set of

autonomous computers and able to exchange data.

It can be defined as a communications system thatlinks two or more computers and peripherals andenables transfer of data between the computers.

It is a hardware mechanism that computers use tocommunicate.

Basically, the computers in a network share

information, software, peripherals devices andprocessing mechanism.

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Advantages of Computer Networks

Resource Sharing-

Single printer or any other hardware can be shared bymany machines instead of requiring each machine tohave its own printer.

Information Sharing-Electronic mail, WWW, and news groups.

Reduced costs-

More processing power and storage capacity by buyingmany PCs and workstations than a single main frame

machine.

Improved Reliability-

Eliminate single points of failure through replication.

Effect on Society.

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Networks on Scale

Classification of interconnected processors byscale.

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Figure 1.12 Categories of networks

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Figure 1.13 LAN

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Figure 1.13 LAN (Continued)

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Introduction

Two computers linked together at home oroffice are the simplest form of a LAN.

In a typical LAN configuration, one

computer is designated as the file server. Computer are connected to the file server

are called workstations.

The server stores all of the software thatcontrols the entire network.

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Characteristics of LANs

Smaller scope as a single building orcampus, i.e., 10m to 1km.

Usually owned by same organization as

attached devices. Small size restriction binds the worst-case

transmission time and simplifies networkmanagement.

Distinguished from other networks by size

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Figure 1.14 MAN

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Figure 1.15 WAN

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WAN Characteristics

Communication Facility

Remote Data Entry

Centralized Information

Examples: Ethernet- developed by Xerox corporation

Arpanet- developed at Advanced Research ProjectsAgency of U.S. Department.

Connect more than 40 universities and institutes throughout USAand Europe.

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Home Network Categories

Computers (desktop PC, PDA, shared peripherals

Entertainment (TV, DVD, VCR, camera, stereo,MP3)

Telecomm (telephone, cell phone, intercom, fax)

Appliances (microwave, fridge, clock, furnace,airco)

Telemetry (utility meter, burglar alarm,babycam).

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Network Software

Protocol Hierarchies

Design Issues for the Layers

Connection-Oriented and Connectionless

Services Service Primitives

The Relationship of Services to Protocols

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Network SoftwareProtocol Hierarchies

Layers, protocols, and interfaces.27

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Design Issues for the Layers

Addressing

Error Control

Flow Control

Multiplexing

Routing

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Connection-Oriented and Connectionless

Services

Six different types of service.

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Service Primitives

Five service primitives for implementinga simple connection-oriented service.

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Service Primitives (2)

Packets sent in a simple client-server interactionon a connection-oriented network.

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Fig re 1 5 P i t t i t ti

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Figure 1.5 Point-to-point connection

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Figure 1 6 Multipoint connection

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Figure 1.6 Multipoint connection

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Figure 1 7 Categories of topology

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Figure 1.7 Categories of topology

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Mesh Topology

Mesh topology is an architecture and away to route data, voice and instructionsbetween nodes as shown in next slide.

It allows for continuous connections andreconfigurations around broken or blockedpaths by hopping from node to nodeuntil the destination is reached.

It is in two categories: Fully connected

Partially connected

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Mesh Topology Fully Connected

The type of network topology in which each ofthe nodes of the network is connected to eachof other nodes in the network with a point-to-point link.

This makes it possible for data to besimultaneously transmitted from any singlenode to all of the other nodes.

Partially connected

In which some of the nodes of the network areconnected to more than one other node in thenetwork with a point-to-point link.

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Figure 1.8 Mesh topology (for five devices)

Fully connected Partially connected

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Mesh Topology Advantages:-

The use of dedicated links guarantees thateach connection can carry its own data load,thus eliminating traffic problems.

A mesh topology is robust because the failure

of any computer does not bring the entirenetwork.

It provides security and privacy because everymessage sent travels along a dedicated line.

Point-to-point links make fault diagnose easy.

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Mesh Topology Disadvantages:-

Since every computer must be connected toevery other computer installation andreconfiguration is difficult.

The hardware require to connect each link

input/output and cable is expensive. Cabling cost is more.

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Star Topology

It consists of one central switch, hub orcomputer which acts as a conduit totransmit messages.

It does not amplify or regenerate thesignal.

An active star network has an activecentral node that usually has the means to

prevent echo-related problems.

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Figure 1.9 Star topology

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Figure 1.9 Star topology

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Star Topology

Advantages:- Good performance

Scalable, easy to setup and to expand

Any non-centralized failure will have very littleeffect on the network, whereas on a ringnetwork it would all fail with one fault.

Easy to detect fault

Data packets are sent quickly as they do nothave to travel through any unnecessary nodes.

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Bus Topology

In which a set of computers are connectedvia a shared communication line, calledbus.

Problem occurs when two clients want totransmit at the same time on the samebus.

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Bus Topology

Advantages:- Easy to implement and extend

Require less cable length than a star topology

Well suited for temporary or small networksnot requiring high speeds(quick setup)

Cheaper than other topologies.

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Figure 1.10 Bus topology

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g p gy

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Ring Topology In which each node connects to exactly

two other nodes, forming a circularpathway for signals: a ring

Data travels from node to node, with eachnode handling every message.

Because a ring topology provides only onepathway between any two nodes, ringnetworks may be disrupted by the failureof a single link.

A node failure or cable break might isolateevery node to attached to the ring.

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Figure 1.11 Ring topology

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g g p gy

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Networking Hardware

File Servers

Workstations

Network Interface Cards

Hubs

Repeaters

Bridges

Routers Switches

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File Servers

A file server is a very fast computer with alarge amount of RAM and storage space,along with a fast network interface card.

The network operating system softwareresides on this computer, along with anysoftware applications and data files thatneed to be shared.

The file server controls the communicationof information between the nodes on anetwork.

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Workstations

All the computers connected to the fileserver on a network are calledworkstations.

A workstation is a computer that isconfigured with a interface card,networking software, and the appropriatecables.

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Network Interface Cards (NIC)

NIC provides the physical connectionbetween the network and the computerworkstation.

NICs are a major factor in determining thespeed and performance of a network.

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Reference Models

The OSI Reference Model

The TCP/IP Reference Model

A Comparison of OSI and TCP/IP A Critique of the OSI Model and

Protocols

A Critique of the TCP/IP ReferenceModel

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R f M d l

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Reference Models

The OSI

referencemodel.

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R f M d l (2)

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Reference Models (2)

The TCP/IP reference model.

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R f r M d l (3)

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Reference Models (3) Protocols and networks in the TCP/IP

model initially.

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Functions of Protocols

Encapsulation

Segmentation

Connection Control

Ordered Delivery

Flow Control

Error Control

Addressing Multiplexing

Transmission Services59

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Encapsulation Encapsulation is the process of inserting the

information of upper layer into the data field of a

lower layer. Let's say you want to send email fromyour PC to another PC on the Internet. First you typea message that you want to send. This message isconverted into 1s and 0s by the application layer.Then, the presentation layer takes this message, and

adds it's own header and footer bytes to it. Yourmessage itself has not been changed, it is containedin the data field of the presentation layer.Then the session layer takes the resulting messageand adds it's own header and footer to it. Theprocess repeats until it gets to the physical layer.

It states that data as well as control information arecontained in each PDU (Protocol Data Unit). Thespecifics of the address are encapsulated into themessage.

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Segmentation It implies to segment the data stream into smaller

bounded size blocks / PDUs.

Connection Control

There are two types of data transfer that arefollows:

Connectionless data transfer- each PDU isindependent of other PDUs.

Connection-oriented data transfer.

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Connection-oriented data transfer

It is used if stations are to be connected for long timeor protocol details are to be worked out dynamically.

It is also known as virtual circuits with three phases:

Establish connection

Transfer data

Terminate connection

Ordered Delivery PDUs may not arrive in order in which they are sent.

The connection-oriented protocols are require thePDU order to be maintained. So the PDU numberedsequentially as they are generated.

There is problem if sequence numbers repeat afterover-flow.

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Flow control

Error control

Addressing Addressing level

At which an entity is named in the communicationsarchitecture

Addressing scope (Global address) Global non-ambiguity- one system to one address

Global applicability- any system can be identifiedfrom anywhere.

Addressing mode Individual or unicast address

Multicast or broadcast address

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Networking Connecting Devices

Repeaters

Bridges

Switches

Hubs

Routers

Gateways

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Networking Connecting Devices

APPLICATION LAYER APPLICATION GATEWAYS

TRANSPORT LAYER TRANSPORT GATEWAY

NETWORK LAYER ROUTER

DATA LINK LAYER BRIDGE, SWITCHPHYSICAL LAYER REPEATER, HUB

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Bridges:-

The device that can be used to interconnect

two separate LANs is known as a bridge. It is commonly used to connect two similar or

dissimilar LANs.

The interconnected individual LANs are called

as segments. The bridge filters or forwards the traffic

between two electrically independent cablesystems attached to it.

It operates in layer 2, i.e., data link layer andthat is why it is called level-2 relay withreference to the OSI model.

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Bridges (contd)-

The bridges are designed to store and forward

frames and it is protocol independent andtransparent to the end stations.

Bridges inspect incoming traffic and decidewhether to forward or discard it.

At each port of bridge it has physical layer andMAC sublayer. The physical layer and MACsublayer protocols at each port of bridgematch with the protocols of the respectiveLAN.

The MAC sublayer have relay and routingfunction between them.

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Function of Bridges:-

Frame filtering and forwarding

Learning the Address Routing- the process of deciding which frames

to forward and to where is called bridgerouting.

Types of bridges:- Transparent bridges

Source routing bridges

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Routers:-

A router is considered as a layer-3 relay that

operates in the network layer, which acts onnetwork layer frames.

It can be used to link two dissimilar LANs. Arouter isolates LANs into subnets to manageand control network traffic.

Schema diagram

Description of components of router is calledschematic diagram.

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Schematic diagram of a router

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Schematic diagram of a router

Port n

Port 1

Port 2

Port n

Port 1

Port 2

.

...

Input ports Output ports

RoutingProcessor

Switching Fabric

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R t

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Routers:- Schema diagram of the router has four basic

components-

Input ports Output ports The routing processor The switching fabric

Input port performs physical and data link layerfunctions of the router. The ports are also provided

with buffer to hold the packet before forwarding tothe switching fabric.

Output port performs the same functions as theinput ports, but in the reverse order.

The routing processor performs the function of the

network layer. The process involves table lookup. The switching fabric moves the packet from the

input queue to the output queue by usingspecialized mechanisms.

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Data link layerprocessor

Physical layerprocessor

Data link layerprocessor

Physical layerprocessor

Queue

Queue

Input Port

Output Port

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Gateways:- A gateway works above the network layer, such as

application layer. It is also called layer-7 relay.

The application layer gateways can look into the contentapplication layer packets such as email beforeforwarding it to the other side.

This property has made it suitable for use in firewalls.

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Communication through a gateway 74

Repeaters:

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Repeaters:- A repeater can be considered as two transreceivers joined

together and connected to two different segments of coaxial cable. The repeater passes the digital signal bit-by-bit in both directions

between the two segments. As the signal passes through a repeater, it is amplified and

regenerated at the other end. A repeater is considered as a level-1 relay.

Application

Presentation

Session

Transport

Network

Data-Link

Physical Physical Physical

Application

Presentation

Session

Transport

Network

Data-Link

Physical

Communication through a repeater 75

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Repeaters:-

Important features-

A repeater can connects different segments of a LAN. A repeater forwards every frame it receives.

A repeater is a regenerator, not an amplifier.

It can be used to create a single extended LAN.

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S it h

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Switches:-

Three possible forwarding approaches:

Cut-through A switch forwards a frame immediately after

receiving the destination address. As aconsequence, the switch forwards the framewithout collision and error detection.

Collision-free In this case, the switch forwards the frame after

receiving 64 bytes, which allows detection ofcollision.

Fully buffered

In this case, the switch forwards the frame onlyafter receiving the entire frame. So, the switchcan detect both collision and error free frames areforwarded.

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Functions of Layers of OSI reference model

1. Physical Layer The physical layer is responsible for two

functions: Communication with the Data Link layer

Transmission and receipt of data. It is responsible also for sending bits from

one computer to another.

The physical layer receive digital data fromdata link layer and process to convert as

digital signal. It defines transmission media, transmission

devices, physical topologies, data signaling.

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1 Physical layer (contd )

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1. Physical layer (contd..) The transmission media can be a copper cable,

a fiber optic cable, or a radio channel.

The transmission devices are responsible tosend and receive signals over each type ofphysical medium.

It can not detect errors in data transmission.

Four important characteristics Mechanical: physical properties of the interface to

transmission medium.

Electrical: representation of bits in terms of voltagelevels.

Functional: functions of individual circuits ofphysical interface between a system andtransmission medium.

Procedural: Sequence of events by which bitstreams are exchanged.

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2 Data Link Layer (contd )

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2. Data Link Layer (contd)

Bridges and switches are used at the data linklayer.

Most popular data link layer LAN protocol inuse today is Ethernet.

Data link layer protocol specifications includethe following three basic elements:

Frame format

Mechanism for controlling access to the networkmedium.

One or more physical layer specifications for use

with the protocol.

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2 Data Link Layer

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2. Data Link Layer

L3 data

1010110100

L3 data

From Network layer To Network layer

Frame

From Physical layer

1010110100T2 T2H2 H2

10101011010010 10101011010010

To Physical layer

Data link

layerData link

layer

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3. Network Layer

Network layer provides connectivity and pathselection between two end systems.

Network layer is the layer at which routingoccurs.

The network layer is concerned with thefollowing primary functions:

Communication with the transport layer above.

Management of connectivity and routing betweenhosts or networks.

Communication with the Data link layer below.

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The network layer decides the following:

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The network layer decides the following:

Logical addressing-software addresses tohardware addresses are resolved.

Routing of message (packets) between hostsand networks.

Determining the best route (makes routingdecisions and forwards packets/datagrams)for devices that could be farther away than asingle link.

Moves information to the correct address.

Sends messages and reports errors regardingpacket delivery.

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3. Network Layer

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3. Network Layer

L4 data L4 data

From Transport layer To Transport layer

From Data link layer

H3H3

L3 data L3 data

To Data link layer

Network

layerNetwork

layer

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4 Transport Layer

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4. Transport Layer

This layer is responsible for reliable networkcommunication between nodes.

The primary functions of transport layer are:

Communication with the Session layer above.

Detect errors and lost data, retransmit data,reassemble datagrams into data streams.

Communicate with the Network layer below.

The aim of transport layer is to isolate theupper three layers from the network, so thatany changes to the network equipment

technology will be confined to the lower threelayers.

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The transport layer decides the following:

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The transport layer decides the following:

Responsible for packet handling. Ensures error free

delivery.

Ensures proper sequencing and without loss andduplication.

Takes action to correct faulty transmissions.

Controls flow of data.

Acknowledges successful receipt of data.

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The transport layer protocols are concerned with

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The transport layer protocols are concerned withthe following issues:

Establishment and termination of host-to-host

connections. Efficient and cost-effective delivery of data across the

network from one host to another.

Multiplexing of data, if necessary, to improve use ofnetwork bandwidth, and de-multiplexing at the other

end. Flow control between hosts.

Addressing of messages to their correspondingconnections. The address information appears as a partof the message header.

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4. Transport Layer

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. a spo t aye

L5 data

H4

From Session layer

L4 data

To Network layer

H4 H4

L4 data

L4 data

L5 data

H4

To Session layer

L4 data

From Network layer

H4 H4

L4 data

L4 data

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6 Presentation Layer

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6. Presentation Layer

The presentation layer performs the followingfunctions:

Communication with the Application layer above.

Translation of standard data formats to formatsunderstood by the local machine.

Communication with session layer below.

The presentation layer deals with the formator representation, of computer information.

It provides security by encrypting anddecrypting data.

It also compress the data before transmittingit.

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Design layer issues:

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Design layer issues:

Abstract representation of application data

Binary representation of application data Conversion between the binary representation

of application data and a common format fortransmission between peer applications

Data compression to better utilize networkbandwidth

Data encryption as a security measure

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7. Application Layer

The application layer provides services toapplication processes such as electronic mail,file transfer, etc., that are outside of the OSImodel.

It identifies and establishes the availability ofintended communication partners,

synchronizes cooperating applications, andestablishes agreement on procedures forerror recovery and control of data integrity.

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7. Application Layer (contd)

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7. Application Layer (contd)

The application layer is the entrance pointthat programs use to access the OSI modeland utilize network resources.

Most common application layer protocolsprovide services that programs use to accessthe network, such as SMTP (Simple Mail

Transfer Protocol), which most e-mailprograms use to send e-mail messages.

In some cases, such as the FTP (File TransferProtocol), the application layer protocol is a

program in itself.

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TCP/IP Reference Model

TCP/IP reference model is the networkmodel used in the current Internetarchitecture.

The name TCP/IP refers to a suite of data

communication protocols. Generally, TCP/IP applications use four

layers: Application layer

Transport layer Network layer

Host to Network layer

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TCP/IP Features

The popularity of the TCP/IP protocols did not growrapidly just because the protocols were there, orbecause connecting to the Internet mandated theiruse. They had several important features as

Open protocol standards, freely available anddeveloped independently from any specificcomputer hardware or operating system.

Because it is so widely supported TCP/IP is ideal

for uniting different hardware and software,even if you dont communicate over the internet.

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TCP/IP Features

Independence from specific physical networkhardware. This allows TCP/IP integrate manydifferent kinds of networks.

A common addressing scheme that allows any

TCP/IP device to uniquely address any otherdevice in the entire network, even if the networkis as large as the worldwide Internet.

Standardized high-level protocols for consistent,

widely available user services.

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F i f l

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Functions of layers

1. Application layer The following TCP/IP application layer

protocols are essential:

File Transfer Protocol (FTP)

TELNET

Simple Mail Transfer Protocol (SMTP)

Hyper Text Transfer Protocol (HTTP)

Domain Name System (DNS)

Routing Information Protocol (RIP) Network File System (NFS)

Simple Network Management Protocol (SNMP)

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Application Layer Protocols

1. File Transfer Protocol (FTP) It performs basic interactive file transfers

between hosts.

The FTP allows a user on any computer to get

files from another computer, or to send filesto another computer.

Security is handled by requiring the user tospecify a user name and password for the

other computer. FTP is a utility that you run any time you

want to access a file on another computer.

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2. TELNET It enables users to execute terminal sessions

with remote hosts.

The network terminal protocol (TELNET)

allows a user to log in on any other computeron the network.

You start a remote session by specifying acomputer to connect to. From that time until

you finish the session, anything you type isdirectly sent to the other computer.

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3. Simple Mail Transfer Protocol (SMTP) It supports basic message delivery services.

This facility allows user to send messages toother users on other computers.

The SMTP protocol is used for thetransmission of e-mails. It takes care ofsending your email to another computer.

Normally, the e-mail is sent to an email

server (SMTP server), then to anotherserver(s), and finally to its destination.

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4. Hyper Text Transfer Protocol (HTTP) It supports the low-overhead transport of files

consisting of a mixture of text and graphics.

It uses a stateless, connection and object

oriented protocol with simple commands thatsupport selection and transport of objectsbetween the client and the server.

5. Domain Name System (DNS)

It is also called name service.

This application maps IP addresses to thenames assigned to network devices.

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p y

It is designed to allow peer entities on

the source and destination hosts to carryon a conversation, just as in the OSItransport layer.

Two end-to-end transport protocols havebeen classified:-

TCP (Transmission Control Protocol)

UDP (User Datagram Protocol)

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TCP (Transmission Control Protocol)

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( )

TCP is a reliable connection-oriented protocolthat allows a byte stream originating on one

machine to be delivered without error on anyother machine in the internet.

It fragments the incoming byte stream intodiscrete messages and passes each one on to

the internet layer. At the destination, the receiving TCP process

reassembles the received messages into theoutput stream.

TCP also handles flow control to make sure afast sender cannot swamp a slow receiverwith more messages than it can handle.

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UDP (User Datagram Protocol)

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g

UDP is an unreliable, connectionless protocolfor applications that do not want TCPs

sequencing or flow control and wish toprovide their own.

It is also widely used as client-server-typerequest-reply queries and applications in

which prompt delivery is more important thanaccurate delivery, such as transmittingspeech or video.

The relation of IP, TCP, and UDP is shown in

next slide:

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TELNET FTP SMTP DNS

TCP UDP

IP

ARPANET SATNET PKT RADIO LAN

Protocols

Networks

Layer (OSI names)

Application

Transport

Network

Physical +data link

Figure: Protocols and networks in the TCP/IP model initially

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Comparison of the OSI and TCP/IP

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Reference Model

S.No OSI Reference Model TCP/IP Reference Model

1. The protocols in the OSI model arehidden.

The protocols in the TCP/IP model arenot hidden.

2. Change can be made based on thetechnology changes.

Change can not be made based on thetechnology changes

3. It supports both connectionless and

connection-oriented communication inthe network layer, but only connection-oriented communication in transportlayer, where it counts (because thetransport service is visible to theusers).

It has only one mode in the network

layer (connectionless) but supports bothmodes in the transport layer, giving theusers a choice.

4. OSI model is based on three concepts

i.e. services, interfaces, protocols.Each concept is defined separately.

TCP/IP model did not originally clearly

distinguish between service, interfaceand protocol.

5. OSI model has specific protocols forbottom two layers that correspond tophysical layer and data link layer.

TCP/IP model does not provide specificprotocols for bottom two layers thatcorrespond to physical layer and datalink layer.

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TransmissionMedia

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Digital data can be transmitted over many

different types of media.

As we know that selecting a transmissionmedium is guided by comparing

transmission requirements against themedium characteristics as shown in thenext slide:

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TransmissionMedia

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Bandwidth:-

It is the maximum frequency range that can bepractically supported by a medium. Thegreater bandwidth of the signal, the higher thedata rate can be achieved.

Cost: two types of cost are relevant as

The cost of installing the medium, includingthe medium specific equipment that may beneeded.

The cost of running and maintaining themedium and its equipment.

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TransmissionMedia

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Reliability

Coverage:-

The physical characteristics of a mediumdictate how long a signal can travel in it beforeit is distorted beyond recognition.

To cover larger areas, repeaters are needed torestore the signal, and this increases the costs.

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Figure 7.2 Classes of transmission media

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Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

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Figure 7.3 Twisted-pair cable

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Twisted pair cable

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In twisted pair technology, two copper

wires are strung between two points: The two wires are typicallytwistedtogether

in a helix to reduce interference between thetwo conductors as shown in the previous

slide. Twisting decreases the cross-talkinterference between adjacent pairs in acable.

Data rates of twisted pair cable has several

Mbps. It can be used for several kilometers.

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Twisted pair cable

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It can carry both analog and digital signals.

Data rate determined by wire thickness andlength.

Characteristics:--

The data rate that can be supported over a

twisted-pair is inversely proportional to thesquare of the line length.

For analog voice signals, amplifiers arerequired about every 6 km and for digital

signals, repeaters are needed for about 2 km.

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Figure 7.4 UTP and STP

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Table 7.1 Categories of unshielded twisted-pair cables

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Category Bandwidth Data Rate Digital/Analog Use

1 very low < 100 kbps Analog Telephone

2 < 2 MHz 2 Mbps Analog/digital T-1 lines

3 16 MHz 10 Mbps Digital LANs



6 (draft) 200 MHz 200 Mbps Digital LANs

7 (draft) 600 MHz 600 Mbps Digital LANs

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Coaxial cable

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It is made up of a copper core

surrounded by insulating material and abraided outer conductor.

Physical connection consists of metal pintouching the copper core.

Two kinds of coaxial cable are widelyused:

One kind, 50-ohm cable for digital

transmission from the start. The other kind, 75-ohm cable for analog

transmission and cable television.

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Figure 7.7 Coaxial cable

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Fiber Optics

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An optical transmission system has three

key components: The light source

A pulse of light indicates a 1 bit and the absence oflight indicates a 0 bit.

The transmission medium It is an ultra-thin fiber of glass.

The detector

The detector generates an electrical pulse when

light falls on it.

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Fiber Optics

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By attaching a light source to one end of

an optical fiber and a detector to theother;

a unidirectional data transmission systemthat accepts an electrical signal, convertsand transmits it by light pulses, and thenreconverts the output to an electricalsignal at the receiving end.

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Fiber Optics

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A light ray is said to have a different

mode, so a fiber having this property iscalled a multimode fiber.

The light can propagate only in a straightline without bouncing, yielding a single-mode fiber.

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Figure 7.13 Modes

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Fiber Cables

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(a) Side view of a single fiber.

(b) End view of a sheath with three fibers.

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Fiber Cables

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Two kinds of light sources are used to do thesignaling, LEDs (Light emitting Diodes) andsemiconductor lasers.

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TEST SERIES # 1

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Note: Attempt all the following questions:

What do you mean by OSI?

What are the seven layers of ISOs OSI model?

What are the key functions of data link layer?

What do you mean by Protocol?

Two networks each provide reliable connection-orientedservice. One of them offers a reliable byte stream and

the other offers a reliable message stream. Are theseidentical? If so, why is the distinction made? If not, give

an example of how they differ ?

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Unguided transmission is used when running a

physical cable (either fiber or copper) between twoend points is not possible.

Infrared signals typically used for short distances;

Microwave signals commonly used for longer

distances.

Difficulties

Weather interferes with signals, may adversely affect

communication. Signals bouncing off of structures may lead to out-of-phase

signals that the receiver must filter out.

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Radio Transmission

Radio signals have been used for long distancecommunication. It may operate at a variety of frequency

bands, ranging from hundreds of Hz to hundreds of GHz.

Microwave signals are used for radio transmission. It

operates in the GHz range with data rates in order of 100sof Mbps per channel.

Example, a satellite system, which is essentially a

microwave system has a large repeater in the sky. The

signals transmitted by earth stations are received,amplified, and retransmitted to other earth stations by the

satellite.

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Radio Transmission

Because of their high bandwidths, satellites are capable ofsupporting an enormous number and variety of channels,

including TV, telephone, and data etc.

Cellular radio(another popular form of radio)

It is recently being used by carriers for providing mobiletelephone networks. These operate in the VHF band and

subdivide their coverage area into conceptual cells, where

each cell represents a limited area which served by a lower-

power transmitter and receiver station. As the mobile user moves from one cell area to another, its

communication is handed over from one station to another .

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Table 7.4 Bands

Band Range Propagation Application

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VLF 330 KHz Ground Long-range radio navigation

LF 30300 KHz GroundRadio beacons andnavigational locators

MF 300 KHz3 MHz Sky AM radio

HF 330 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF 30300 MHzSky and

line-of-sight

VHF TV,

FM radio

UHF 300 MHz3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF 3

30 GHz Line-of-sight Satellite communication

EHF 30300 GHz Line-of-sight Long-range radio navigation

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It is a technology which allows the various users to

share the channel simultaneously. It reduces the cost of transmission media and modem.

In functioning, the MUX is connected to the DEMUX

by a single data link. The MUX combines data from

these n input lines and transmits them through the

single high capacity data link, which is being demulti-

plexed at the other end and is delivered to the suitable

output lines.Thus, multiplexing can also be defined as a technique

that allows simultaneous transmission of multiple

signals across a single data link. 139

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Multiplexing techniques can be categorized into the

following types: Frequency-division multiplexing (FDM)

It is used extensively in radio and TV transmission. The frequency

spectrum is divided into several logic channels, giving each user

exclusive possession of a particular frequency band.

Time-division multiplexing (TDM)

It is also called synchronous TDM, which is commonly used for

multiplexing digitized voice stream.

Statistical TDM

This is also called asynchronous TDM. This scheme simplyimproves on the efficiency of synchronous TDM.

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Frequency Division Multiplexing

A i t f l i f

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142

Assignment of non-overlapping frequencyranges to each useror signal on a medium.Thus, all signals are transmitted at the sametime, each using different frequencies.

A multiplexor accepts inputs and assignsfrequencies to each device.

The multiplexor is attached to a high-speedcommunications line.

A corresponding multiplexor, or demultiplexor,

is on the end of the high-speed line andseparates the multiplexed signals.

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143

Frequency Division Multiplexing

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144

Analog signaling is used to transmits the

signals. Broadcast radio and television, cable

television, and cellular phone systems usefrequency division multiplexing.

This technique is the oldest multiplexingtechnique.

Since it involves analog signaling, it is moresusceptible to noise.

Time Division Multiplexing

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Sharing of the signal is accomplished by

dividing available transmission time on amedium among users.

Digital signaling is used exclusively.

Time division multiplexing comes in two basic

forms:

1. Synchronous time division multiplexing, and

2. Statistical, or asynchronous time division

multiplexing.

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147

Synchronous Time Division Multiplexing

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148

If one device generates data at a faster rate

than other devices, then the multiplexor musteither sample the incoming data stream fromthat device more often than it samples theother devices, or buffer the faster incoming

stream. If a device has nothing to transmit, the

multiplexor must still insert a piece of datafrom that device into the multiplexed stream.

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150

Synchronous time division multiplexing

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So that the receiver may stay synchronized with the incoming

data stream, the transmitting multiplexor can insert alternating1s and 0s into the data stream.

Synchronous Time Division Multiplexing

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152

Three types popular today:T-1 multiplexing (the classic)

ISDN multiplexing

SONET (Synchronous Optical NETwork)

The T1 (1.54 Mbps) multiplexor stream is a continuous series offrames of both digitized data and voice channels

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frames of both digitized data and voice channels.

24 separate 64Kbps channels

Data Communications and Computer NetworksChapter 5

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154

The ISDN multiplexor stream is also a continuous stream of

frames. Each frame contains various control and sync info.

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Synchronous TDM

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Very popular Line will require as much bandwidth as all

the bandwidths of the sources

Statistical Time Division Multiplexing

l l l l h

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157

A statistical multiplexor transmits only the

data from active workstations (or why workwhen you donthave to).

If a workstation is not active, no space iswasted on the multiplexed stream.

A statistical multiplexor accepts the incomingdata streams and creates a frame containingonly the data to be transmitted.

Statistical Time Division Multiplexing

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Wavelength Division Multiplexing (WDM)

Gi e e h me ge diffe ent elength (f eq en )

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Give each message a different wavelength (frequency)

Easy to do with fiber optics and optical sources

Dense Wavelength Division Multiplexing (DWDM)

Dense wavelength division multiplexing is often called justwavelength division multiplexing

Dense wavelength division multiplexing multiplexes multiple

data streams onto a single fiber optic line.

Different wavelength lasers (called lambdas) transmit themultiple signals.

Each signal carried on the fiber can be transmitted at adifferent rate from the other signals.

Dense wavelength division multiplexing combines many (30,40, 50, 60, more?) onto one fiber.


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161


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163

SWITCHING

As e kno that the telephone s stem

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164

As we know that the telephone system

consists of three major components: Local loops

analog twisted pairs going into houses andbusinesses

Trunks

digital fiber optics connecting the switchingoffices

Switching offices where calls are moved from one trunk to

another

Local loops

Modems

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Modems

When a computer wishes to send digitaldata over an analog dial-up line, the datamust first be converted to analog form fortransmission over the local loop.

This conversion is done by a device calleda modem.

The modem is inserted between the(digital) computer and the (analog)telephone system.

Local loops

ADSL (Asymmetric Digital Subscriber Line)

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ADSL (Asymmetric Digital Subscriber Line)

ADSL modem is actually a digital signalprocessor that has been set up to act as250 QAM modems operating in parallel atdifferent frequencies.

Wireless Local Loops (WLL)

In a certain sense, a fixed telephone usinga wireless local loop is a bit like a mobilephone.

Switching

The phone system is divided into two main

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The phone system is divided into two main

parts: Outside plant (the local loops and trunks, since they

are physically outside the switching offices)

Inside plant (the switches)

Two different switching techniques are usednow-a-days:

Circuit switching

Packet switching

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Circuit Switching

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(a) Circuit switching. (b) Packet switching.

Message Switching

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(a) Circuit switching (b) Message switching (c) Packet switching

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Functions of the Data Link Layer

P id i i t f t th

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Provide service interface to the

network layer

Dealing with transmission errors

Regulating data flow

Slow receivers not swamped by fastsenders

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Functions of the Data Link Layer (2)

Relationship between packets and frames

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Relationship between packets and frames.

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Services Provided to Network Layer

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(a) Virtual communication. (b) Actual communication.

175

Framing

A h

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A character stream:

(a) Without errors. (b) With one error.

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Framing (2)

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(a) A frame delimited by flag bytes.

(b) Four examples of byte sequences before and after stuffing.

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Framing (3)

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Bit stuffing:--

(a) The original data.

(b) The data as they appear on the line.

(c)The data as they are stored in receivers memory afterdestuffing.

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Error Detection and Correction

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Error-Correcting Codes Error-Detecting Codes

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Error-Correcting Codes

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Use of a Hamming code to correct burst errors.

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Error-Detecting Codes

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Calculation of the polynomial code checksum.

181

Elementary Data Link Protocols

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An Unrestricted Simplex Protocol A Simplex Stop-and-Wait Protocol

A Simplex Protocol for a Noisy

Channel

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Sliding Window Protocols

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A One-Bit Sliding Window Protocol

A Protocol Using Go Back N

A Protocol Using Selective Repeat

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Sliding Window Protocols (2)

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A sliding window of size 1, with a 3-bit sequence number.

(a) Initially.

(b) After the first frame has been sent.(c) After the first frame has been received.

(d) After the first acknowledgement has been received.

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A One-Bit Sliding Window Protocol (2)

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Two scenarios for protocol 4. (a) Normal case. (b) Abnormalcase. The notation is (seq, ack, packet number). An asteriskindicates where a network layer accepts a packet.

185

A Protocol Using Go Back N

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Pipelining and error recovery. Effect on an error when

(a)Receivers window size is 1.

(b)Receivers window size is large.186

Sliding Window Protocol Using Go Back N (2)

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Simulation of multiple timers in software.

187

Example Data Link Protocols

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HDLC High-Level Data Link Control

The Data Link Layer in the Internet

188

High-Level Data Link Control

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Frame format for bit-oriented protocols.

189

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The Data Link Layer in the Internet

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A home personal computer acting as aninternet host.

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PPPPoint to Point Protocol

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The PPP full frame format for unnumberedmode operation.

192

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PPPPoint to Point Protocol (3)

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The LCP frame types.

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Domain Name System

It is a naming scheme that uses a hierarchical

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It is a naming scheme that uses a hierarchical,

domain-based naming scheme on a distributeddatabase system.

The domain names are case insensitive, so comand COM have same meaning.

Every node has a label which is 63 characterslong. The top-level domains are divided into twocategories: genericand countries.

For example,

Domain Name System

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Each domain name is named by the path upward from it to theroot. The components are separated by dots.

Whenever a new system is installed in a zone, the DNSadministrator for the zone allocates name and IP address forthe new servers database.

A zone is a subtree of the DNS tree that is administratedseparately.

Domain Name System

S No Domain Name Description

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S.No. Domain Name Description

1 com Commercial applications

2 edu Educational institutions

3 gov USA Governmental organizations

4 mil USA military

5 net Network

6 org Other organizations

7 int International organization

8 ac Academic institutions

9 in,uk,fr,it

Symbols for countries like india, united

kingdom, france, italy etc.

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ELECTRONIC MAIL (Email)

Each user, who intends to participate in e-mailcommunication is assigned a mailbox where

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communication, is assigned a mailbox, where

outgoing and incoming messages are buffered,allowing the transfer to take place in thebackground.

The message contains the header that specifiesthe sender, recipients, and subject, followed by abody that contains message.

The TCP/IP protocol that supports e-mail on theinternet is called Simple Mail Transfer Protocol(SMTP).

ELECTRONIC MAIL (Email)

The SMTP supports the following features:Sending a message to one o man ecipients

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Sending a message to one or many recipients.

Sending messages that include text, voice, video, or graphics.

A software package, known asUserAgent, is used tocompose, read, reply or forward e-mails and handle

mailboxes.

The e-mail address consists of two parts divided by a@ character. The first part is the local name thatidentifies mailbox and the second part is a domainname.

E-mail operation

The following steps are:

User agent

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User agent

SMTP sender

SMTP receiver

Mail Access Protocol

As, we know that the e-mail delivery takes place inthree stages The first and second stage use SMTP but

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three stages. The first and second stage use SMTP but

the third stage does not use SMTP because it is pushprotocol, so the third stage uses a mail accessprotocol.

There are two mail access protocols being used: POP3 (Post office protocol version 3)

IMAP4 (Internet Mail Access protocol version 4)

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