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Komunikasi Data danJaringan Komputer
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Referensi
W. Stallings, Data and ComputerCommunications, 4ed, Macmillan, 1994.
F. Halsall, Data Communications,Computer Networks and OpenSystems, Addison Wesley, 1996.
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A Communications Model
Source
generates data to be transmitted
Transmitter
Converts data into transmittable signalsTransmission System
Carries data
Receiver
Converts received signal into data
Destination
Takes incoming data
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Simplified Communications
Model - Diagram
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Simplified Data
Communications Model
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Key Communications Tasks
Transmission System Utilization
Interfacing
Signal Generation
SynchronizationExchange Management
Error detection and correction
Addressing and routing
Recovery
Message formatting
Security
Network Management
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Communications Standard
Many types of connection media :
telephone lines, optical fibers,
cables, radios, etc.
Many different types of machines and
operating systems
Many different network applications
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What Standard means?
How many volts pulse is a 0 and 1 ?
How to determine the end of a message ?
How to handle lost messages ?
How many bits for different data types ?
Integers/Strings, etc.; are ASCII chars ?
How machines are identified ?
How to find the way to reach a machine ?
How applications speaks together through the
network ?
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Standard Bodies
International Telecommunications Union
Telecommunications Sector (ITU-T)
Institute of Electrical and Electronics
Engineers (IEEE)
International Standards Organization (ISO)
Electronic Industries Alliance (EIA)dll
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The ISO/OSI Model
ISO (the International Standards Organization) has
developed a reference model for communications,
called the
OSI(Open Systems Interconnection)
OPEN SYSTEM means that it can communicate with
any other system that follows the specified standards,formats and semantics.
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OSI Networking Model
Application ApplicationDataAH
Presentation PresentationDataunitPH
Session SessionData unitSH
Transport TransportData unitTH
Network NetworkData unitNH
Data link Data linkData unitLH LT
Physical PhysicalBits
Physical transmission medium
DataProgram X Program Y
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OSI Layers (1)
Physical
Physical interface between devices
Mechanical
ElectricalFunctional
Procedural
Data Link
Means of activating, maintaining and deactivating areliable link
Error detection and control
Higher layers may assume error free transmission
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OSI Layers (2)
Network
Transport of information
Higher layers do not need to know about underlying technology
Not needed on direct links
Transport
Exchange of data between end systems
Error free
In sequence
No losses No duplicates
Quality of service
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OSI Layers (3)
Session
Control of dialogues between applications
Dialogue discipline
Grouping Recovery
Presentation
Data formats and coding
Data compression
Encryption
Application
Means for applications to access OSI environment
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Transmission Medium
Guided - wire
Unguided - wireless
Characteristics and quality determined bymedium and signal
For guided, the medium is more important
For unguided, the bandwidth produced bythe antenna is more important
Key concerns are data rate and distance
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Guided Transmission Media
Twisted Pair
Coaxial cable
Optical fiber
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Twisted pair - INEXPENSIVE
Two wires twisted together.
Makes them less susceptible to acting likean antenna and picking up radio frequency
information or appliance noise.
Telephone company uses twisted-pair
copper wires to link telephones.
Twisted Pair
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Twisted Pair
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Twisted Pair - Applications
Most common medium
Telephone network
Between house and local exchange
(subscriber loop)
Within buildings
To private branch exchange (PBX)
For local area networks (LAN)
10Mbps or 100Mbps
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Twisted Pair - Pros and Cons
Cheap
Easy to work with
Low data rateShort range
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Twisted Pair - Transmission
CharacteristicsAnalog
Amplifiers every 5km to 6km
Digital
Use either analog or digital signals repeater every 2km or 3km
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100MHz)
Susceptible to interference and noise
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Unshielded and Shielded TP
Unshielded Twisted Pair (UTP)
Ordinary telephone wire
Cheapest
Easiest to install Suffers from external EM interference
Shielded Twisted Pair (STP)
Metal braid or sheathing that reduces interference
More expensive
Harder to handle (thick, heavy)
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UTP Categories
Cat 3
up to 16MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cmCat 4
up to 20 MHz
Cat 5 or Cat 6
up to 100MHz
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
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Coaxial Cable (1)
Coaxial cable
Also two wires:
One of the wires is woven of fine strands ofcopper forming a tube.
The wire mesh surrounds a solid copper
wire that runs down the center.
Space between has a non-conductingmaterial.
Makes them more impervious to outside
noise.
Use this when1. Long distances2. Lots of interference
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Coaxial Cable (2)
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Coaxial Cable (3)
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Coaxial Cable Applications
Most versatile medium
Television distribution
Aerial to TV
Cable TV
Long distance telephone transmission
Can carry 10,000 voice calls simultaneously
Being replaced by fiber optic
Short distance computer systems links
Local area networks
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Coaxial Cable - Transmission
CharacteristicsAnalog
Amplifiers every few km
Closer if higher frequency
Up to 500MHz
Digital
Repeater every 1km
Closer for higher data rates
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Optical Fiber (1)
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Optical Fiber (2)
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Optical Fiber (3)
Fiber-optic cable
(BIG JOBS + EXPENSIVE)
Light is electromagnetic. Can transmit more information down a single
strand.
It can send a wider set of frequencies.
Each cable can send several thousand phone
conversations or computer communications.
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Optical Fiber - Spectrum
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Optical Fiber - Benefits
Greater capacity
Data rates of hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing 10s of km at least
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Optical Fiber - Applications
Long-haul trunks
Metropolitan trunks
Rural exchange trunksSubscriber loops
LANs
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Optical Fiber - Transmission
CharacteristicsAct as wave guide for 1014 to 1015 Hz
Portions of infrared and visible spectrum
Light Emitting Diode (LED)
Cheaper Wider operating temp range
Last longer
Injection Laser Diode (ILD)
More efficient Greater data rate
Wavelength Division Multiplexing (WDM)
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Optical Fiber Transmission
Modes
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Wireless Transmission
Unguided media
Transmission and reception via antenna
Directional Focused beam
Careful alignment required
Omni-directional Signal spreads in all directions
Can be received by many antenna
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Frequencies
2GHz to 40GHz
Microwave
Highly
directional
Point to point Satellite
30MHz to 1GHz
Omni-
directional
Broadcast radio
3 x 1011 to 2 x 1014
Infrared
Local
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Terrestrial Microwave
Parabolic dish
Focused beam
Line of sightLong haul telecommunications
Higher frequencies give higher data rates
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Satellite Microwave
Satellite is relay station
Satellite receives on one frequency,
amplifies or repeats signal and transmitson another frequency
Requires geo-stationary orbit
Height of35,784kmTelevision
Long distance telephone
Private business networks
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Broadcast Radio
Omni-directional
FM radio
UHF and VHF televisionLine of sight
Suffers from multi-path interference
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Infrared
Modulate non-coherent infrared light
Line of sight (or reflection)
Blocked by wallse.g. TV remote control, IRD port
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Terminology (1)
Transmitter
Receiver
Medium Guided medium
e.g. twisted pair, optical fiber
Unguided medium
e.g. air, water, vacuum
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Terminology (2)
Direct link
No intermediate devices
Point-to-point
Direct link
Only 2 devices share link
Multi-point
More than two devices share the link
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Terminology (3)
Simplex
One direction
e.g. Television
Half duplex
Either direction, but only one way at a time
e.g. police radio
Full duplex Both directions at the same time
e.g. telephone
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Terminology (4)
Bits per second (bps).
The number of bits (0s and 1s) that travel
down the channel per second.
Baud rate
The number of bits that travel down the
channel in a given interval.
The number is given in signal changes persecond, not necessarily bits per second.
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Terminology (5)
Asynchronous transmission Information is sent byte by byte.
Cheaper and more commonly used.
Synchronous transmission Data is sent in large blocks rather than in
small pieces.
Preceded by special information, concerningerror detection and block size.
These modems are expensive but very fast.
A l d Di it l D t
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Analog and Digital Data
TransmissionData
Entities that convey meaning
Signals
Electric or electromagnetic representations of
data
Transmission
Communication of data by propagation and
processing of signals
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Data
Analog
Continuous values within some interval
e.g. sound, video
Digital
Discrete values
e.g. text, integers
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Data and Signals
Usually use digital signals for digital data
and analog signals for analog data
Can use analog signal to carry digital data
Modem
Can use digital signal to carry analog data
Compact Disc audio
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Signals
Type of signal communicated (analog or digital).
Analog: Those signals that vary with smooth continuouschanges.
A continuously changing signal similar to that found on
the speaker wires of a high-fidelity stereo system. Digital: Those signals that vary in steps or jumps from
value to value. They are usually in the form of pulses of
electrical energy (represent 0s or 1s).
Analog Signals Carr ing Analog
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Analog Signals Carrying Analog
and Digital Data
Digital Signals Carrying Analog
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Digital Signals Carrying Analog
and Digital Data
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Analog Transmission
Analog signal transmitted without regard to
content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Also amplifies noise
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Digital Transmission
Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater receives signal
Extracts bit pattern
Retransmits
Attenuation is overcomeNoise is not amplified
Advantages of Digital
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Advantages of Digital
TransmissionDigital technology
Low cost LSI/VLSI technology
Data integrity
Longer distances over lower quality lines
Capacity utilization
High bandwidth links economical
High degree of multiplexing easier with digital techniques
Security & Privacy
Encryption
Integration
Can treat analog and digital data similarly
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Transmission Impairments
Signal received may differ from signaltransmitted
Analog - degradation of signal quality
Digital - bit errorsCaused by
Attenuation and attenuation distortion
Propagation delay Noise
Interference
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Attenuation
Signal strength falls off with distance
Depends on medium
Received signal strength:
must be enough to be detected
must be sufficiently higher than noise to be
received without error
Attenuation is an increasing function of
frequency
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Propagation Delay
The time required for a signal to travel
from one point to another.
Propagation velocity varies with frequency.
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Noise (1)
Additional signals inserted between
transmitter and receiver
Thermal
Due to thermal agitation of electrons
White noise
Inter-modulation
Signals that are the sum and difference of
original frequencies sharing a medium
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Noise (2)
Crosstalk
A signal from one line is picked up by another
Impulse
Irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude
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Channel Capacity
Data rate
In bits per second
Rate at which data can be communicated
Bandwidth
In cycles per second of Hertz
Constrained by transmitter and medium
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Modulation Techniques
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Adaptive Modulation
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Data Rate and Bandwidth
Any transmission system has a limited
band of frequencies
This limits the data rate that can be carried
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Multiplexing
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Time Division Multiplexing
Data rate of medium exceeds data rate of
digital signal to be transmitted
Multiple digital signals interleaved in time
May be at bit level of blocks
Time slots preassigned to sources and
fixed
Time slots allocated even if no data
Time slots do not have to be evenly
distributed amongst sources
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Time Division Multiplexing
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TDM System
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Frequency Division Multiplexing
FDM
Useful bandwidth of medium exceeds
required bandwidth of channel
Each signal is modulated to a different
carrier frequency
Carrier frequencies separated so signals
do not overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
Frequency Division Multiplexing
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Frequency Division Multiplexing
Diagram
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FDM System