Computer Networks (EC-321) Physical Layer

1. Computer Networks (EC-321)PhysicalLayer

2. Outline • Basis for Conventional Data Communication • Transmission Media • Wireless Transmission • Communication Satellites • Telephone System, Mobile Telephone System

3. Basis for Conventional Data Communication • Fourier Analysis • Bandwidth-Limited Signals • The Maximum Data Rate of a Channel

4. Fourier Analysis • Any periodic signal can be represented as a sum of sinusoids, known as a Fourier series: • f is known as the fundamental frequency • f = 1/T, where T is the period of the signal • multiples of f are referred to as harmonics • The values of the coefficients: • The root-mean-square amplitudes, , are proportional to the energy of the corresponding frequency.

5. Fourier Analysis • The spectrum of a signal is the range of frequencies that it contains • The absolute bandwidth is the width of the spectrum. • Most of the energy in a signal is contained in a relatively narrow band of frequencies. This band is referred to as the effective bandwidth, or just bandwidth. • Fourier analysis helps us estimate how well a signal produced by a transmitter will be carried by a particular channel.

6. Fourier Analysis (Example) 1 1 0 0 t ideal periodic signal t real composition (based on harmonics)

7. Fourier Analysis (Example)

8. Bandwidth-Limited Signals • All transmission media have a finite bandwidth, which is determined by physical properties of the medium and the distance traversed. • Thus, a given medium will filter out part of the signal (0 up to some frequency fc(cutoff)), namely, the higher frequency components. In addition, an artificial filter may be applied to the channel. • In real, cutoff is not sharp ------ Bandwidth is 0 to freq where received power fallen by half • Baseband and passband • Information carried not depend on starting and ending freqs

9. Effect of Limited Bandwidth Signals • channel: voice-grade line (3000 Hz)

10. The Maximum Data Rate of a Channel (Channel Capacity) • Impairments, such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • The maximum rate at which data can be transmitted over a given communication channel, under given conditions, is referred to as the channel capacity. • We would like to make as efficient use as possible of a given bandwidth, i.e., we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth.

11. The Maximum Data Rate of a Channel (Channel Capacity) • Two Formulas • Problem: given a bandwidth, what data rate can we achieve? • Nyquist Formula • Assume noise free • Shannon Capacity Formula • Assume white noise

12. NyquistFormula • Ifthe rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate. • Given bandwidth B, highest signal rate is 2B • If the signal consists of V discrete levels, Nyquist’s theorem states: Maximum Data Rate = 2B log2V bits /sec • Nyquist’s formula indicates that, • for a given bandwidth, the data rate can be increased by increasing the number of different signal elements/levels. • if all other things are equal, doubling the bandwidth doubles the data rate.

13. Shannon Capacity Formula Signal-to-Noise Ratio: • Ratio of the power in a signal to the power contained in the noise present at a particular point in the transmission. • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N) • A high SNR means a high-quality signal, low number of required intermediate repeaters

14. Shannon Capacity Formula • Shannon’s major result is that the maximum data rate or capacity of a noisy channel whose bandwidth is B Hz and whose signal-to-noise ratio is S/N, is given by: Maximum number of bits/sec = B log2 (1 + S/N) • Some remarks: • Given a level of noise, the data rate could be increased by increasing either signal strength or bandwidth. • As the signal strength increases, so do the effects of nonlinearities in the system which leads to an increase in intermodulation noise. • Because noise is assumed to be white, the wider the bandwidth, the more noise is admitted to the system. Thus, as B increases, SNR decreases.

15. Example • Consider an example that relates the Nyquist and Shannon formulations. Suppose the spectrum of a channel is between 3 MHz and 4 MHz, and SNRdB = 24dB. So, B = 4 MHz – 3 MHz = 1 MHz SNRdB = 24 dB = 10 log10(SNR)  SNR = 251 • Using Shannon’s formula, the capacity limit C is: C = 106 x 1og2(1+251) ≈ 8 Mbps. • If we want to achieve this limit, how many signaling levels are required at least? By Nyquist’s formula: C = 2Blog2M We have 8 x 106 = 2 x 106 x log2M  M = 16.

16. Transmission Media • Media are roughly grouped into • guided media • unguided media • There are many options for sending data from point A to point B. • Guided media • Magnetic • Twisted pair • Coaxial cable • Power lines • Fiber optics • Unguided media • Wireless

17. Guided media • Magnetic Media • One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or removable media • High Bandwidth and less cost • Delay characteristics are poor Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.

18. Guided media • Twisted pair • Consists of pair of twisted, insulated copper wires, about 1mm thick. To gain bandwidth, sets of pairs are grouped into a single cable. • twisting the wires reduces EMI • used for both analog and digital transmission • various grades of cable are used • data rate up to 10Gbps • used for many decades in the phone system, still dominate local loops • Very commonly used in LANs today

19. Guided media • Twisted pair (EMI)

20. Guided media • Twisted pair (Terminology) • The cables are marked as 'XbaseY'. For example, 10base10, where X means the data rate in Mbps, base means 'baseband'. Baseband means a transmission medium of signals and Y means the category of cabling.

21. Guided media • Coaxial cable • It has better shielding and greater bandwidth than unshielded twisted pairs • so it can span longer distances at higher speeds • Two kinds of coaxial cable are widely used • 50-ohm • 75-ohm

22. Guided media • Coaxial cable (50-ohm) • 50-ohm, also called baseband • up to 1-2 Gbps over 1 km • previously widely used in long-haul • telephone lines (now largely replaced by fiber) • May still be used in some LANs

23. Guided media • Coaxial cable (75-ohm) • 75-ohm cable, also called broadband • Originally, analog transmission and cable TV • about 450 MHz for 100 km • usually, the cable is divided into 6 MHz channels • amplifiers are required to boost signals

24. Guided media • Power lines • Used at low data rates by electricity companies for years • Use in the home: data signal is superimposed on a low-frequency power signal. • Difficulties with household wiring: • It doesn’t carry high-frequency signals well • Its electrical properties vary • It can act as an antenna

25. Guided media • Optical Fibre • Composition • ultra-thin (2 to 125 µm) fiber of glass that can transmit light pulses in one direction • three concentric sections • core: innermost; one or more very thin strands of glass or plastic • cladding: surrounds each fiber, glass or plastic coating with optical properties much different from core (specifically, lower index of refraction) • jacket: plastic surrounding one or more cladded fibers, to protect against moisture, abrasion, crushing, etc.

26. Guided media • Optical Fibre (Optical Transmission and Reception) • Internal reflection occurs with indices of refraction of core and cladding and angle of incident • Refractive index n= C/V • Core has high ‘n’ compare to cladding

27. Guided media • Optical Fibre (Optical Transmission and Reception) • Multiple angles (modes) may propagate • multimode fiber - multiple modes propagate • single-mode fiber - only one mode propagates • which is more expensive? • Light sources • Light Emitting Diode (LED) • Injection Laser Diode (ILD) • Reception - photodiode

28. Guided media • Advantages of Optical Fibre • Higher bandwidth • data rates of multiple Gbps over 10s of kilometersdemonstrated • Small size and light weight compared to copper • Much lower attenuation (0.1dB/km), implying greater spacing of repeaters • Immunity to electromagnetic interference • do not radiate energy • cause no interference • very difficult to tap

29. Guided media Attenuation of light through fiber in the infrared region

30. Unguided media Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared Transmission • Light Transmission

31. The Electromagnetic Spectrum • Electromagnetic Waves • Moving electrons create electromagnetic waves that can propagate through free space. • Wave characteristics • Frequency • Wavelength • Wireless communication is based on broadcast/receipt of waves using electronic circuits attached to antennas. • In a vacuum, all waves propagate at the same speed • Fundamental relation among frequency, wavelength, and speed:

32. The Electromagnetic Spectrum • Radio, microwave, infrared, and visible light are good for transmission • UV, X-rays, gamma rays would be good due to their higher frequencies, but.. • hard to produce and modulate • do not propagate well through buildings • interfere with our ability to live

33. The Electromagnetic Spectrum • Frequency Bands and Bandwidth • Carrying capacity is proportional to the bandwidth of the media • Computing a frequency band: • c = fλ • df/dλ= − c/λ2 • Then ∆f = c∆λ/λ2 • Given the width of a wavelength band ∆λ, you can compute corresponding frequency band ∆f.

34. The Electromagnetic Spectrum • Radio Transmission • Description • frequencies in 30 MHz to 1 GHz • radio is omnidirectional, whereas microwaves are focused • line of sight is not required • Tight licensing needed due to interference • Uses • packet radio (often a defence application) • ALOHA-type networks • cellular telephone • personal communication network (PCN) • wireless LANs (growing rapidly)

35. The Electromagnetic Spectrum • Use of Frequencies • Most transmissions use a narrow frequency band: ∆f/f << 1 • • Wide band variations: • Frequency hopping spread spectrum • Hard to detect, impossible to jam • the receiver will not be stuck on an impaired frequency for long enough to shut down communication • Good resistance to multipath fading and narrowband interference • Good for ISM band (crowded part of spectrum) • Direct sequenced spread spectrum • Ultra-WideBand(UWB)

36. The Electromagnetic Spectrum Use of Frequencies

37. The Electromagnetic Spectrum Use of Frequencies

38. The Electromagnetic Spectrum Radio Transmission Characteristics • VLF, LF, and MF bands follow the ground • HF and VHF bands bounce off ionosphere

39. The Electromagnetic Spectrum • Terrestrial Microwave • • Description • parabolic dishes mounted on towers • “line of sight” distance from one another • Ex: 100 ft towers can be 80 km apart • for long distance transmission, use relay • towers in a point-to-point fashion • • Uses • long-haul telecommunications service, as alternative to coax cable for television and voice • communication between buildings (closed-circuit TV) • digital data in small regions (radius < 10 km)