-1- Georgia State UniversitySensorweb Research Laboratory CSC4220/6220 Computer Networks Dr. WenZhan Song Associate Professor, Computer Science

-2- Georgia State UniversitySensorweb Research Laboratory Roadmap Physical Layer Theoretical basis Transmission medias Guided Wireless Multiplexing and Spectrum Reuse

-3- Georgia State UniversitySensorweb Research Laboratory The Physical Layer Lowest layer in Network Hierarchy Defines the mechanical, electrical, and timing interfaces to the network Physical transmission of data. Various flavors Copper wire, fiber optic, etc... Physical limits on transmission speed and bandwidth. Examples of how physical transmission is used in real networks.

-4- Georgia State UniversitySensorweb Research Laboratory Signals as functions of time We transmit signals by varying some physical property over time. e.g. vary voltage or current The faster we can (accurately) vary this property the more data we can send per unit of time. t 0v 5v dt

-5- Georgia State UniversitySensorweb Research Laboratory Attenuation As a signal propagates along a transmission line its amplitude decreases. This is known as signal 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 increases as a function of frequency. Therefore, amplifiers as designed to adjust for this. Equalizers equalize the attenuation across a band of frequencies. Fixes: Use short cables. Use amplifiers (repeaters) to boost the signal.

-6- Georgia State UniversitySensorweb Research Laboratory Attenuation of Digital Signals

-7- Georgia State UniversitySensorweb Research Laboratory Bandwidth Cutoff frequency Only transmit amplitudes from 0 to f c, because all frequencies above this attenuated. Bandwidth: range of frequencies transmitted without being strongly attenuated. In practice, the quoted bandwidth is from 0 to the frequency at which half the power gets through. Even more, filtered by providers. E.g., telephone line bandwidth 1MHz is filtered to just above 3000Hz Different transmission media has different bandwidth

-8- Georgia State UniversitySensorweb Research Laboratory Concerning physical data communication What are the signaling frequency limitations of various media? Is there an upper bound? What if noise is introduced into the system? What about attenuation and other factors? How does the speed of light limit us? How do we decide between various media for specific applications? We start with mathematical analysis. Convert functions of time to functions of frequency.

-9- Georgia State UniversitySensorweb Research Laboratory Fourier Analysis Fourier: We can construct any continuous periodic function by a linear combination of sinusoidal functions. g(t) is a continuous function over time t. Period of g is T. Fundamental frequency f = 1/T.

-10- Georgia State UniversitySensorweb Research Laboratory The Fourier transform Constant or “DC” term a n and b n are the sine and cosine amplitudes of the nth harmonics

-11- Georgia State UniversitySensorweb Research Laboratory Example Transmitting the ASCII character ‘b’ which is in binary. The magnitude of the amplitudes a n and b n are used to compute the “energy” of the signal at the corresponding frequency. root-mean-square

-12- Georgia State UniversitySensorweb Research Laboratory Approximation of original signals (a) A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

-13- Georgia State UniversitySensorweb Research Laboratory Approximation of original signals (d) – (e) Successive approximations to the original signal.

-14- Georgia State UniversitySensorweb Research Laboratory Understand distortion and attenuation Attenuation: the loss of energy as the signal propagates outward The loss is expressed in decibels per kilometer The amount of energy lost depends on the frequency, besides transmission media. Different Fourier component is attenuated by a different amount, which results in a different Fourier spectrum at the receiver Distortion: different Fourier components also propagate at different speeds in the wire. The speed difference leads to distortion of the signal received.

-15- Georgia State UniversitySensorweb Research Laboratory Harmonics vs. data rates Given: bit rate = b bits/sec The time required to send 8 bits with 1 bit a time(for example) is 8/b sec Frequency of 1st harmonic b/8 Hz. Number of highest harmonic passed through is 3000/(b/8) = 24,000/b, since voice grade line artificially-introduced cutoff frequency ~3000 Hz.

-16- Georgia State UniversitySensorweb Research Laboratory Harmonics vs. data rate table Relation between data rate and harmonics /2400 See Fig. (c)

-17- Georgia State UniversitySensorweb Research Laboratory Limited bandwidth Digital signal consists of many frequency components. Only those components that are within the bandwidth of the transmission medium are received. The larger the bandwidth of the medium… the more larger frequency components are passed; the more “faithful” the reproduction of the original signal at the receiving end.

-18- Georgia State UniversitySensorweb Research Laboratory 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

-19- Georgia State UniversitySensorweb Research Laboratory The Nyquist Theorem Given: Transmission medium with bandwidth H (a low pass filter may have been used to cancel out higher frequency terms). V discrete signal levels (V = 2 for binary signal). Noiseless channel. Wanted: Determine the maximum data rate of this transmission medium. Solution (Nyquist, 1924) maximum data rate = 2 H log 2 V bps Conclusion: We can reconstruct a binary signal with 2H samples per second. More samples are pointless

-20- Georgia State UniversitySensorweb Research Laboratory Nyquist example Example Problem A modem to be used with a PSTN uses a modulation scheme with 2 levels per signalling element. If the bandwidth of voice-graded phone line is 3000 Hz, deduce the Nyquist maximum data transfer rate: Data rate = 2 H log 2 V bps = (2)(3000)(1) = 6000 bps

-21- Georgia State UniversitySensorweb Research Laboratory Noise There is always random(thermal) noise present due to the motion of the molecules in the system In the presence of noise, the bit rate may be severely reduced. In the absence of a signal, a channel will ideally have zero electrical signal present. In practice, there are “random perturbations” on the line which are known as line noise. In the limit, an attenuated signal is reduced to line noise. How much “signal” is signal, and how much is noise?

-22- Georgia State UniversitySensorweb Research Laboratory Signal to Noise ratio An important parameter associated with a transmission medium is the signal-to-noise ratio (typically measured in decibels dB). SNR = 10 log 10 (S/N) dB S = signal power N = noise power High SNR is good. Low SNR is bad.

-23- Georgia State UniversitySensorweb Research Laboratory Shannon-Hartley Law Maximum data rate of a noisy channel. Max Data rate = H log 2 (1 + S/N) bps No matter how many or how few signal levels are used No matter how often or how infrequently samples are taken Example: Assuming that a PSTN has a bandwidth of 3000 Hz and a typical SNR of 30 dB. Determine the max theoretical data rate that can be achieved. 30 dB = 10 log 10 (S/N), S/N = 1000 Date rate = H log 2 (1 + S/N) bps = 3000 log 2 ( ) = about 35kbps

-24- Georgia State UniversitySensorweb Research Laboratory Roadmap Physical Layer Theoretical basis Transmission medias Guided Wireless Multiplexing and Spectrum Reuse

-25- Georgia State UniversitySensorweb Research Laboratory Guided Transmission Data Twisted Pair Coaxial Cable Fiber Optics

-26- Georgia State UniversitySensorweb Research Laboratory Twisted Pair

-27- Georgia State UniversitySensorweb Research Laboratory UTP Categories Category 3 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Category 5 up to 100MHz Commonly pre-installed in new office buildings for high-speed computer communications Twist length 0.6 cm to 0.85 cm

-28- Georgia State UniversitySensorweb Research Laboratory Twisted Pair - Applications Most common medium Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) Local area networks (LAN) 10Mbps or 100Mbps

-29- Georgia State UniversitySensorweb Research Laboratory Coaxial Cable Coaxial Cable. Higher bandwidth(e.g., 1 GHz) and better noise immunization

-30- Georgia State UniversitySensorweb Research Laboratory Coaxial Cable Applications Most versatile medium Television distribution 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

-31- Georgia State UniversitySensorweb Research Laboratory Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

-32- Georgia State UniversitySensorweb Research Laboratory Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources.

-33- Georgia State UniversitySensorweb Research Laboratory Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection.

-34- Georgia State UniversitySensorweb Research Laboratory Transmission of Light through Fiber Attenuation of light through fiber in the infrared region.

-35- Georgia State UniversitySensorweb Research Laboratory Fiber Optic Networks A fiber optic ring with active repeaters.

-36- Georgia State UniversitySensorweb Research Laboratory Optical Fiber Applications Long-haul trunks (1500km) Metropolitan trunks (15km) Rural exchange trunks (40-160km) Subscriber loops LANs

-37- Georgia State UniversitySensorweb Research Laboratory

-38- Georgia State UniversitySensorweb Research Laboratory Roadmap Physical Layer Theoretical basis Transmission medias Guided Wireless Multiplexing and Spectrum Reuse

-39- Georgia State UniversitySensorweb Research Laboratory Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere Tend to be absorbed by earth

-40- Georgia State UniversitySensorweb Research Laboratory Politics of the Electromagnetic Spectrum The ISM bands in the United States.

-41- Georgia State UniversitySensorweb Research Laboratory Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

-42- Georgia State UniversitySensorweb Research Laboratory Roadmap Physical Layer Theoretical basis Transmission medias Guided Wireless Multiplexing and Spectrum Reuse

-43- Georgia State UniversitySensorweb Research Laboratory Multiplexing in physical trunk Multiplexing many low-bandwidth communications over a high-bandwidth physical trunk FDM (Frequency Division Multiplexing) WDM(Wavelength Division Multiplexing) for fiber optical channels TDM (Time Division Multiplexing)

-44- Georgia State UniversitySensorweb Research Laboratory Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.

-45- Georgia State UniversitySensorweb Research Laboratory Wavelength Division Multiplexing Wavelength division multiplexing.

-46- Georgia State UniversitySensorweb Research Laboratory Time Division Multiplexing The T1 carrier (1.544 Mbps).

-47- Georgia State UniversitySensorweb Research Laboratory Time Division Multiplexing Multiplexing T1 streams into higher carriers. Far more widespread, but only used for digital data

-48- Georgia State UniversitySensorweb Research Laboratory Cellular Networks Concept, 1968 by AT&T Cellular Telephone System, 1983 by AT&T

-49- Georgia State UniversitySensorweb Research Laboratory Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. Handoff: Soft vs Hard Neither 1 st or 2 nd generation device can do

-50- Georgia State UniversitySensorweb Research Laboratory Channel Categories The 832 full-duplex channels are divided into four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data

-51- Georgia State UniversitySensorweb Research Laboratory D-AMPS: Digital Advanced Mobile Phone System (a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users kbps

-52- Georgia State UniversitySensorweb Research Laboratory GSM: Global System for Mobile Communications GSM uses 124 frequency channels, each of which uses an eight-slot TDM system kbps

-53- Georgia State UniversitySensorweb Research Laboratory GSM (2) A portion of the GSM framing structure.

-54- Georgia State UniversitySensorweb Research Laboratory CDMA – Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal Sprint PCS: CDMA AT&T: D-AMPS

-55- Georgia State UniversitySensorweb Research Laboratory CDMA Encode/Decode slot 1 slot 0 d 1 = Z i,m = d i. c m d 0 = slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

-56- Georgia State UniversitySensorweb Research Laboratory CDMA: two-sender interference

-57- Georgia State UniversitySensorweb Research Laboratory Simplex/Duplex 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, Modem, ADSL, cable different frequency for different direction