Computer Network Lucia Dwi Krisnawati, MA

An 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 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 Basics TypeFrequency Radio~ 1 MHz-1 GHz Microwaves (sometimes included in the radio band) ~ 1 GHz-300 GHz Infrared~ 300 GHz - 1,000 THz (10 15 Hz) Light (including ultraviolet)~ Hz Hz X Rays/Gamma rays, etc~ Hz Hz Figure 1: The electromagnetic Spectrum (a very rough cut)

Anatomy 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 Amplitude

Waves 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 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

Modulations Am & FM Phase Modulation

Bandwidth Bandwidth:  the amount of spectrum used to carry information  Information carrying capacity, eg. “high bandwidth” Broadcast Television Assignment Channel Carrier Frequency (example) lowest frequency in channel highest frequency in channel Bandwidth 257 MHz54 MHz60 MHz6 Mhz 363 MHz60 MHz66 MHz6 Mhz 669 MHz66 MHz72 MHz6 Mhz

Replacing Analog to Digital Signals 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

Picture

Types 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

Client 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 LAN

Peer-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 LAN Ethernet Connection  Sending & receiving data using NIU, line & hub, one message in a line Preamblesender's addressreceiver's addressdataError detection bits Ethernet message format

Token-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 connection

Star 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 Connection

WAN 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

WAN

WAN 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  Scan s & attachment  Prevent remote logging into computer in organization Router:  A device that connects to a public switched network  Functions to direct the route