Post on 22-Dec-2015
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
de
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