Computer NetworkComputer Network

Lucia Dwi Krisnawati, MALucia Dwi Krisnawati, MA

An OverviewAn Overview

The need of sharing databases Each desktop should be able to

communicate to each other. A computer Network:

A number of computers that are connected to each other

Communications BasicsCommunications Basics

Computers represent information in the form of electronic pulses traveling through a wire: 1: a voltage of certain level (high) 0: a voltage of lower level

Alternative: using waves to carry information

Spectrum: The variety of electromagnetic waves that

includes all forms of radiation from radio to light rays to gamma rays

Communications BasicsCommunications Basics

Type Frequency

Radio ~ 1 MHz-1 GHz

Microwaves (sometimes included in the radio band)

~ 1 GHz-300 GHz

Infrared ~ 300 GHz - 1,000 THz (1015 Hz)

Light (including ultraviolet) ~ 1015 Hz-1017Hz

X Rays/Gamma rays, etc ~ 1017 Hz-1022Hz

Figure 1: The electromagnetic Spectrum (a very rough cut)

Anatomy of a WaveAnatomy of a Wave

Frequency: The number of times in a second that a wave

complete a full cycle, from the midpoint of one wave to the midpoint of the next

measured in cycles per seconds (cps) Hertz (Gustav Hertz)

Amplitude: The height of the wave

Phase: Refers to pattern of the wave

1 complete cycle = 1 phase

Am

plitu

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WavesWaves

Frequencies are discrete: discreet Different frequencies do not interfere with each other

Colors Different frequencies that make up radio & TV channels

How do waves carry information? illustration on a white sheet of paper Carrier vs. modulation

Modulation: The process of modifying a wave so that it can carry

information. Three ways of modulating a wave:

Changing its amplitude Changing its frequency Changing its phase

Kinds of ModulationKinds of Modulation

Amplitude Modulation (AM): a carrier wave of a single frequency can be used to convey

information by varying its amplitude. Very efficient but vulnerable to interference Noise can easily change the height of the waves

Frequency Modulation (FM) the carrier represents one of many frequencies to be used Information is represented as changes in frequency Less vulnerable to interference but requires the use of more

frequency spectrum Phase Modulation (PM)

A receiver sense when the phase of wave differs from what would be expected

Then interpret the difference as information

ModulationsModulations

Am & FM

Phase Modulation

Bandwidth Bandwidth

Bandwidth: the amount of spectrum used to carry information Information carrying capacity, eg. “high bandwidth”

Broadcast Television Assignment

ChannelCarrier Frequency (example)

lowest frequency in channel

highest frequency in channel

Bandwidth

2 57 MHz 54 MHz 60 MHz 6 Mhz

3 63 MHz 60 MHz 66 MHz 6 Mhz

6 69 MHz 66 MHz 72 MHz 6 Mhz

Replacing Analog to Digital Replacing Analog to Digital SignalsSignals

Analog signal: Conveys information in a form that is analogues

to that of the source Steps in the analog-digital signal conversion:

Sampling: The original wave is checked at regular intervals Each sample is given a numerical (binary) value Result digital system sends a stream of bits

rather than an analog wave Digitalization:

Define a threshold amplitude The amplifier will note 1 if the signal is above the

threshold, and 0 if it is under the threshold

PicturePicture

Types of NetworkTypes of Network

LAN: Local Area Network Located within an organization All computers are situated within a radius of 1

KM Each computer has a unit called Network

Interface Unit (NIU) A place of data before being communicated to

another computer Has a job of delivering data with assigned address Some NIUs are connected to electronic circuit

called HUB Some HUBs are connected to a backbone HUB

NIU

CPU + Memory CPU + Memory

NIU

Network Connection

Computer p Computer q

HUB

NIUNIUNIU

PC1 PC2 PC3

Backbone HUB

HUB1 HUB2 HUB3

C1 C32C18C17C16C2 C34C33 C48

Client Server ComputingClient Server Computing

A server: A computer connected to a LAN wich is capable

of providing some special service The special service may be:

Access to a high cost peripheral A powerful CPU with good transaction

processing capability, hosting a large database A very powerful CPU with good number

crunching capability A client:

A computer which requires any of the resources provided by the server

Client-Server Architecture of Client-Server Architecture of LANLAN

HUB

ClientsPrint server E-mail Server Database server

Printer Disk

Peer-to-peer ComputingPeer-to-peer Computing

All computers in a network have equal standing

There is no servers Is good for a purpose of:

File sharing Sharing CPU resources

Technology used in LANTechnology used in LAN

Ethernet Connection Sending & receiving data using NIU, line & hub,

one message in a line

Preamble sender's address receiver's address data Error detection bits

Ethernet message format

Token-Ring ConnectionToken-Ring Connection

Computers are connected to form a ring A token:

Is a small bit string with specified unique format Is circulated in a ring Computer attaches messages in a token & puts

it back in the ring Each NIU examines whether the ring has a

message If the address in the token is address to comp 4,

the NIU 4 will detach the message & put the token back in the ring

Token-ring connectionToken-ring connection

Star ConnectionStar Connection

Clients are connected to a centralized server The message is sent directly to server Server will forward the message to receiver Advantages:

Each computer has an exclusive connection to a server

The bandwidth is not shared Disadvantages:

Traffic jam from server If the server fails, the entire network is down

Star ConnectionStar Connection

WANWAN

Wide area Network Area can include the whole country 2 types of WAN:

Public: using a number of different technologies such as microwave networks, communication satellites, fibre optic line

Private: using dedicated private communication infrastructure provided by the telephone companies to interconnect computers

WANWAN

WANWAN

Firewall: Is needed if LAN is connected to a public network Functions as a filter

Firewall actions are: Prevent unauthorized persons from accessing data

resource Block access to a certain undesirable sites Filter junk or advertisement e-mail Scan e-mails & e-mail attachment Prevent remote logging into computer in organization

Router: A device that connects to a public switched network Functions to direct the route