UNIT - II

COMPUTER NETWORKS

• Guided - wire• Unguided - wireless• Characteristics and quality determined by

medium and signal• For guided, the medium is more important• For unguided, the bandwidth produced by the

antenna is more important• Key concerns are data rate and distance

Transmission Media

Design Factors

• Bandwidth– Higher bandwidth gives higher data rate

• Transmission impairments– Attenuation

• Interference• Number of receivers– In guided media– More receivers (multi-point) introduce more

attenuation

Guided Transmission Media

• Twisted Pair• Coaxial cable• Optical fiber

Transmission Characteristics of Guided Media

 

  Frequency Range

Typical Attenuatio

n

Typical Delay

Repeater Spacing

Twisted pair (with loading)

0 to 3.5 kHz 0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 to 1 MHz 0.7 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable

0 to 500 MHz

7 dB/km @ 10 MHz

4 µs/km 1 to 9 km

Optical fiber 186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

Twisted Pair

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

Twisted Pair - Pros and Cons

• Cheap• Easy to work with• Low data rate• Short range

Twisted Pair - Transmission Characteristics

• Analog – 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

Near End Crosstalk

• Coupling of signal from one pair to another• Coupling takes place when transmit signal

entering the link couples back to receiving pair• i.e. near transmitted signal is picked up by

near receiving pair

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)

UTP Categories• Cat 3

– up to 16MHz– Voice grade found in most offices– Twist length of 7.5 cm to 10 cm

• Cat 4– up to 20 MHz

• Cat 5– up to 100MHz– Commonly pre-installed in new office buildings– Twist length 0.6 cm to 0.85 cm

• Cat 5E (Enhanced) –see tables• Cat 6• Cat 7

Comparison of Shielded and Unshielded Twisted Pair  Attenuation (dB per 100 m) Near-end Crosstalk (dB)

Frequency (MHz)

Category 3 UTP

Category 5 UTP

150-ohm STP

Category 3 UTP

Category 5 UTP

150-ohm STP

1 2.6 2.0 1.1 41 62 58

4 5.6 4.1 2.2 32 53 58

16 13.1 8.2 4.4 23 44 50.4

25 — 10.4 6.2 — 41 47.5

100 — 22.0 12.3 — 32 38.5

300 — — 21.4 — — 31.3

Twisted Pair Categories and Classes

  Category 3 Class C

Category 5 Class D

Category 5E

Category 6 Class E

Category 7 Class F

Bandwidth

16 MHz 100 MHz 100 MHz 200 MHz 600 MHz

Cable Type

UTP UTP/FTP UTP/FTP UTP/FTP SSTP

Link Cost (Cat 5 =1)

0.7 1 1.2 1.5 2.2

Coaxial Cable

Coaxial Cable Applications

• Most versatile medium• Television distribution– Ariel 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

Coaxial Cable - Transmission Characteristics

• Analog– Amplifiers every few km– Closer if higher frequency– Up to 500MHz

• Digital– Repeater every 1km– Closer for higher data rates

Optical Fiber

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

Optical Fiber - Applications

• Long-haul trunks• Metropolitan trunks• Rural exchange trunks• Subscriber loops• LANs

Optical Fiber - Transmission Characteristics

• Act 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

Optical Fiber Transmission Modes

Frequency Utilization for Fiber Applications

Wavelength (in vacuum) range (nm)

Frequency range (THz)

Band label

Fiber type Application

820 to 900 366 to 333   Multimode LAN

1280 to 1350 234 to 222 S Single mode

Various

1528 to 1561 196 to 192 C Single mode

WDM

1561 to 1620 185 to 192 L Single mode

WDM

Attenuation in Guided Media

Adv. & Disadv. Of Optical Fiber

• Advantages1. Higher Bandwidth2. Less Signal Attenuation3. Immunity to electromagnetic Interference4. Resistance to corrosive materials5. Light Weight• Disadvantages1. Require expertise in Installation and Maintenance 2. Unidirectional Light propagation3. Expensive

Wireless Transmission Frequencies• 3KHz to 1GHz

– Omnidirectional– Radio Waves– Broadcast radio

• 1GHz to 300GHz– Microwave– Highly directional– Point to point– Satellite

• 300GHz to 400THz – Infrared– Local

Electromagnetic Spectrum

Antennas• Electrical conductor (or system of..) used to radiate

electromagnetic energy or collect electromagnetic energy• Transmission

– Radio frequency energy from transmitter– Converted to electromagnetic energy– By antenna– Radiated into surrounding environment

• Reception– Electromagnetic energy impinging on antenna– Converted to radio frequency electrical energy– Fed to receiver

• Same antenna often used for both

Radiation Pattern

• Power radiated in all directions• Not same performance in all directions• Isotropic antenna is (theoretical) point in

space– Radiates in all directions equally– Gives spherical radiation pattern

Parabolic Reflective Antenna• Used for terrestrial and satellite microwave• Source placed at focus will produce waves reflected from

parabola in parallel to axis– Creates (theoretical) parallel beam of light/sound/radio

• On reception, signal is concentrated at focus, where detector is placed

Parabolic Reflective Antenna

Antenna Gain

• Measure of directionality of antenna• Power output in particular direction compared

with that produced by isotropic antenna• Measured in decibels (dB)• Results in loss in power in another direction• Effective area relates to size and shape– Related to gain

Terrestrial Microwave

• Parabolic dish• Focused beam• Line of sight• Long haul telecommunications• Higher frequencies give higher data rates

Satellite Microwave

• Satellite is relay station• Satellite receives on one frequency, amplifies or

repeats signal and transmits on another frequency

• Requires geo-stationary orbit– Height of 35,784km

• Television• Long distance telephone• Private business networks

Satellite Point to Point Link

Satellite Broadcast Link

Broadcast Radio

• Omnidirectional- signals sent in all diections.• FM radio• UHF and VHF television• Line of sight• Suffers from multipath interference– Reflections

Infrared

• Modulate noncoherent infrared light• Line of sight (or reflection)• Blocked by walls• e.g. TV remote control

Wireless Propagation• Signal travels along three routes– Ground wave

• Follows contour of earth• Up to 2MHz• AM radio

– Sky wave• Amateur radio, BBC world service, Voice of America• Signal reflected from ionosphere layer of upper atmosphere• (Actually refracted)

– Line of sight• Above 30Mhz• May be further than optical line of sight due to refraction

Ground Wave Propagation

Sky Wave Propagation

Line of Sight Propagation

Refraction• Velocity of electromagnetic wave is a function of density of material

– ~3 x 108 m/s in vacuum, less in anything else• As wave moves from one medium to another, its speed changes

– Causes bending of direction of wave at boundary– Towards more dense medium

• Index of refraction (refractive index) is– Sin(angle of incidence)/sin(angle of refraction)– Varies with wavelength

• May cause sudden change of direction at transition between media• May cause gradual bending if medium density is varying

– Density of atmosphere decreases with height– Results in bending towards earth of radio waves

Optical and Radio Horizons

Line of Sight Transmission• Free space loss

– Signal disperses with distance– Greater for lower frequencies (longer wavelengths)

• Atmospheric Absorption– Water vapour and oxygen absorb radio signals– Water greatest at 22GHz, less below 15GHz– Oxygen greater at 60GHz, less below 30GHz– Rain and fog scatter radio waves

• Multipath– Better to get line of sight if possible– Signal can be reflected causing multiple copies to be received– May be no direct signal at all– May reinforce or cancel direct signal

• Refraction– May result in partial or total loss of signal at receiver

FreeSpaceLoss

Multipath Interference

Transmission Terminology

• data transmission occurs between a transmitter & receiver via some medium

• guided medium– eg. twisted pair, coaxial cable, optical fiber

• unguided / wireless medium– eg. air, water, vacuum

Data Communication and Computer Networks 1303330 49

ReceiverSender

Message

ProtocolRules...

Rules...

Protocol

Transmission Medium

Transmission Terminology

• direct link– no intermediate devices

• point-to-point– direct link – only 2 devices share link

• multi-point– more than two devices share the link

• A pair of nodes connected together via dedicated link.

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PC PCLink

• Number of node connected and share a single link.

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Server

PCPCPC

Link

Transmission Terminology

• simplex– one direction• eg. television

• half duplex– either direction, but only one way at a time• eg. police radio

• full duplex– both directions at the same time• eg. telephone

• Simplex: one direction only.

• Always one side sender and another side receiver.

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Remote Control

TV

• Half-Duplex: two-way alternate.

Walki-Talki

• Each side maybe sender or receiver but not a same time.

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In different time

• Duplex: two-way concurrent. Computer network

Mobile Network

• Each side sender and receiver at same time.

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At same time

Frequency, Spectrum and Bandwidth

• time domain concepts– analog signal• various in a smooth way over time

– digital signal• maintains a constant level then changes to another

constant level– periodic signal• pattern repeated over time

– aperiodic signal• pattern not repeated over time

Analogue & Digital Signals

PeriodicSignals

Sine Wave

• peak amplitude (A)– maximum strength of signal– volts

• frequency (f)– rate of change of signal– Hertz (Hz) or cycles per second– period = time for one repetition (T)– T = 1/f

• phase ()– relative position in time

Varying Sine Wavess(t) = A sin(2ft +)

Propagation Speed

• Speed of travelling Signal• Depends on medium and frequency of signal• In vacuum, light is propagated with a speed of

3*108m/s.• That speed is lower in air and even lower in

cable.

Wavelength ()• is distance a simple signal can travel in one period.• Dependent on frequency and medium.• between two points of corresponding phase in two

consecutive cycles• assuming signal velocity v have = vT• Wavelength=Propagation Speed * period• or equivalently f = v especially when v=c

c = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts

• A single frequency signal is not useful in Data communication.

• Composite signal are made up of many frequencies components are sine waves

• Relationship between amplitude and frequency, is called frequency domain plot.

• Fourier analysis can shown that any signal is made up of component sine waves

• can plot frequency domain functions

Addition of Frequency

Components(T=1/f)

• c is sum of f & 3f

FrequencyDomain

Representations

• freq domain func of single square pulse

1. Message: data. 2. Sender: The device that send the message.

3. Receiver: The device that receive the message.

4. Transmission Medium: The physical path between sender and receiver, the message travel.

5. Protocol: Is a set of rules that governs data communication. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

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1. Delivery: The system must deliver data to the correct destination.

2. Accuracy: Data delivered accurately.Data delivered accurately. Altered data which left uncorrected are unusable.Altered data which left uncorrected are unusable.

3. Timelines: The system must deliver data in timely manner without delay without delay

(real-time).(real-time).4. Jitter: Jitter refers to the variation in the packet arrival time. It Jitter refers to the variation in the packet arrival time. It

is the uneven delay in the delivery of audio or video is the uneven delay in the delivery of audio or video packets.packets.

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Simplified Communications Model - Diagram

Analog and Digital Data Transmission

• data – entities that convey meaning

• signals & signalling– electric or electromagnetic representations of

data, physically propagates along medium

• transmission– communication of data by propagation and

processing of signals

Simplified Data Communications Model

Acoustic Spectrum (Analog)

Audio Signals

• freq range 20Hz-20kHz (speech 100Hz-7kHz)• easily converted into electromagnetic signals• varying volume converted to varying voltage• can limit frequency range for voice channel to 300-

3400Hz

Data• Analog:

Continuous values within some interval.e.g. sound, video.

• Digital:Discrete values.e.g. text, integers.

Analog Signals

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.

Digital Signals

Digital Transmission• Concerned with content.• Integrity endangered by noise,

attenuation etc..• Repeaters used.• Repeater receives signal.• Extracts bit pattern.• Retransmits.• Attenuation is overcome.• Noise is not amplified.

Advantages & Disadvantages of Digital Signals

• cheaper• less susceptible to noise• but greater attenuation• digital now preferred choice

Advantages of Digital Transmission

• Digital 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.

Transmission Impairments

• signal received may differ from signal transmitted causing:– analog - degradation of signal quality– digital - bit errors

• most significant impairments are– attenuation and attenuation distortion– delay distortion– noise

Attenuation• Means loss of energy in propagation due to the

resistance of medium.• Energy is measured in decibel(dB).• Decibel measues the relative strength of two

signals or one signal at two different points.• dB=10 log10 P2/P1

AttenuationSignal 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.• increase strength using amplifiers/ repeaters

Distortion• Means that Signal changes its Form and shape. • only occurs in guided media• composite signal made up by different frequency

Signals and various frequency components arrive at different times

• propagation velocity varies with frequency• Difference in delay may create a difference in phase,

Therefore, a Shape of Signal is changed.• particularly critical for digital data• since parts of one bit spill over into others• causing intersymbol interference

Noise • Additional signals inserted between transmitter

and receiver.• Induced: comes from different sources like

motors an other appliances• Thermal: The random motion of electrons in wire

which creates an extra signal, not originally sent by Transmitter

• Intermodulation: Signals that are the sum and difference of original frequencies sharing a medium.

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.

Channel Capacity

• Data rate: In bits per second.Rate at which data can be communicated.Depends on three factors:• Bandwidth available• Level of Signals• Quality of the Channel

• Bandwidth: In cycles per second of Hertz.Constrained by transmitter and medium.

Nyquist Bandwidth( Noise Less Channel)

• consider noise free channels• if rate of signal transmission is 2B then can

carry signal with frequencies no greater than B (Bandwidth)– ie. given bandwidth B, highest signal rate is 2B

• for binary signals, 2B bps needs bandwidth B Hz

• can increase rate by using M signal levels• Nyquist Formula is: C = 2B log2M

Nyquist Bandwidth( Noise Less Channel)

• In no. of Signal Level (M) is 2. They can easily distinguish by receiver (either 0 or 1)

• But if no. of signal level is 64. then it’s very difficult to distinguish by receiver.

• So Increasing the levels of a signal may reduce the reliability of the system.

• so increase rate by increasing signals– at cost of receiver complexity– limited by noise & other impairments

Shannon Capacity Formula (Noisy Channel)

• consider relation of data rate, noise & error rate– faster data rate shortens each bit so bursts of noise affects

more bits– given noise level, higher rates means higher errors

• Shannon developed formula relating these to signal to noise ratio (in decibels)

• SNRdb=10 log10 (signal/noise)

• Capacity C=B log2(1+SNR)– theoretical maximum capacity– get lower in practise