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 Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz0.2 1 kHz 50 µs/km2 km Twisted pairs (multi-pair cables) 0 to 1 MHz0.7 1 kHz 5 µs/km2 km Coaxial cable0 to 500 MHz7 10 MHz 4 µs/km1 to 9 km Optical fiber186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km40 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 — — — — ——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 Bandwidth16 MHz100 MHz 200 MHz600 MHz Cable TypeUTPUTP/FTP SSTP Link Cost (Cat 5 =1)

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 to 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 typeApplication 820 to to 333 MultimodeLAN 1280 to to 222SSingle modeVarious 1528 to to 192CSingle modeWDM 1561 to to 192LSingle modeWDM

Attenuation in Guided Media

Adv. & Disadv. Of Optical Fiber Advantages 1.Higher Bandwidth 2.Less Signal Attenuation 3.Immunity to electromagnetic Interference 4.Resistance to corrosive materials 5.Light Weight Disadvantages 1.Require expertise in Installation and Maintenance 2.Unidirectional Light propagation 3.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 10 8 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

Free Space Loss

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 Receiver Sender Message Protocol Rules. 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. 51 PC Link

Number of node connected and share a single link. 52 Server PC 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. 54 Remote Control TV

Half-Duplex: two-way alternate. Walki-Talki Each side maybe sender or receiver but not a same time. 55 In different time

Duplex: two-way concurrent. Computer network Mobile Network Each side sender and receiver at same time. 56 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

Periodic Signals

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 Waves s(t) = A sin(2  ft +  )

Propagation Speed Speed of travelling Signal Depends on medium and frequency of signal In vacuum, light is propagated with a speed of 3*10 8 m/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*10 8 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

Frequency Domain 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. 67

1. Delivery: The system must deliver data to the correct destination. 2. Accuracy:  Data delivered accurately.  Altered data which left uncorrected are unusable. 3. Timelines: without delay (real-time). The system must deliver data in timely manner without delay (real-time). 4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. 68

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 Hz

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 log 10 P2/P1

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. 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 log 2 M

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) SNR db = 10 log 10 (signal/noise) Capacity C=B log 2 (1+SNR) – theoretical maximum capacity – get lower in practise