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1 NET 221D:COMPUTER NETWORKS FUNDAMENTALS Networks and Communication Department Lecture 2: Physical Layer Introduction One of the major functions of the physical layer is to move data in the form of electromagnetic signals across a transmission medium. Thus, the data must be transformed electromagnetic signals to be transmitted. Data can be analog (continuous, with many levels) or digital (discrete, with a limited number of values). to Behrouz A. Forouzan” Data communications and Networking Introduction Both analog and digital signals can take one of two forms: periodic or nonperiodic. Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. Behrouz A. Forouzan” Data communications and Networking Sin Wave Signals A sine wave is represented by three parameters: Peak amplitude, Frequency, and Phase. Peak amplitude: it is the absolute value of the highest intensity. Frequency: it refers to the number of periods in 1 s. It is formally expressed in Hertz (Hz). Period is the amount of time, in seconds, a signal needs to complete one cycle (the completion of one full pattern). Behrouz A. Forouzan” Data communications and Networking Sin Wave Signals Behrouz A. Forouzan” Data communications and Networking Sin Wave Signals Phase: it describes the position of the waveform relative to time 0. It is measured in degree or radian. Behrouz A. Forouzan” Data communications and Networking Sin Wave Signals Wavelength binds the period or frequency of the simple sine wave to the propagation speed of the medium. Wavelength depends on both the frequency and the medium. Wavelength = propagation speed X period = propagation speed/ frequency Behrouz A. Forouzan” Data communications and Networking Sin Wave Signals In a vacuum, light is propagated with a speed of 3 X 108 m/s. (that speed is lower in air and cable.) . The frequency of red light is 4 X 1014 . Wavelength= c/f= (3 X 108 ) / (4 X 1014) = 0.75 X 10-6 m= 0.75 μm Behrouz A. Forouzan” Data communications and Networking Composite Signals According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. Behrouz A. Forouzan” Data communications and Networking Bandwidth The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Behrouz A. Forouzan” Data communications and Networking Fourier Analysis • Information can be transmitted on wires by varying some physical property such as voltage or current. • Any reasonably behaved periodic function, g(t) with period T, can be written as: Bandwidth-Limited Signals An example: the transmission of the ASCII character “b” encoded in an 8-bit byte. The bit pattern that is to be transmitted is 01100010. Bandwidth-Limited Signals root-mean-square amplitudes: Direct proportional with the transmitted signal energy at the corresponding freq. Any signal transmission occurs with power loss. Fourier coefficients are not affected proportionally by the power loss => signal amplitude is distorted Frequencies : 0-Fmax =>the amplitudes are undiminished – above they are attenuated. Bandwidth vs Data Rate • 1924 Henri Nyquist –relation between bandwidth and data rate in a noiseless channel (throughput): Nyquist Theorem: A data signal on a medium with H (Hz) bandwidth can be reconstructed by making 2H samples/sec. For a signal of V discrete levels: Maximum data rate=2H log2V bits/sec. 3 kHz channel (binary signals) => max_data_rate=6000 bps throughput =2*3000 log22 = 6000 bps. Throughput in a noisy channel S – the signal power; N – the noise power => S/N the signal to noise ratio. Signal to noise (decibels) 1 dB = 10 log10 S/N. Ex: S/N = 10 => 10 dB; S/N =100 => 20 dB Shannon’s Theorem The maximum throughput of a noisy channel of bandwidth H with a signal to noisy ratio of S/N is: Maximum throughput = H log2(1+S/N) bps. Ex: tel line Bandwidth=3kHz; S/N=30 dB => Max throughput = 3000 * log2(1+1000) =~ 30.000 bps = 28.8 kbps Twisted Pair • The purpose of the physical layer is to transport bits from one machine to another. • Media are grouped into : guided media, such as copper wire and fiber optics, and unguided media, such as terrestrial wireless, satellite, and lasers through the air. (a) Category 3 UTP. (b) Category 5 UTP. Twisted Pair • A twisted pair consists of two insulated copper wires, typically about 1 mm thick. • When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively. • The most common application of the twisted pair is the telephone system. • Twisted pairs can be used for transmitting either analog or digital information. • Twisted-pair cabling comes in several varieties. Twisted Pair • UTP (Unshielded Twisted Pair) consist simply of wires and insulators. • Links that can be used in both directions at the same time are called full-duplex links. Links that can be used in either direction, but only one way at a time are called half-duplex links. • • • A third category consists of links that allow traffic in only one direction. A category 5 twisted pair consists of two insulated wires gently twisted together. Coaxial Cable A coaxial cable. • • • It has better shielding and greater bandwidth than unshielded twisted pairs. 50-ohm cable: intended for digital transmission from the start. 75-ohm cable: analog transmission and cable television. Fiber Optics • Fiber optics are used for long-haul transmission in network backbones, high speed LANs, and high-speed Internet access. • An optical transmission system has three key components: light source, transmission medium, and the detector. • A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. 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. Fiber Optics • Multimode Fiber: any light ray incident on the boundary above the critical angle will be reflected internally, many different rays will be bouncing around at different angles. • Single-mode Fiber: light can propagate only in a straight line, without bouncing. Fiber Optics Three wavelength bands: they are centered at 0.85, 1.30, and 1.55 microns, respectively. All three bands are 25,000 to 30,000 GHz wide. • 0.85-micron band: has higher attenuation and so is used for shorter distances. • Last two bands have good attenuation properties (less than 5% loss per kilometer). Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. -Core: 50 microns for multi-mode, 8-10 microns for single mode -Cladding: glass with a lower refraction index, to keep the light in the core -Connection: -connectors (plug in) – about 20% attenuation -mechanical splicing, tuned by an operator – 10% attenuation -fused (melted together) – almost no attenuation Fiber Optic Networks A fiber optic ring with active repeaters. Fiber Optic Networks A passive star connection in a fiber optics network. Electromagnetic Spectrum • When electrons move, they create electromagnetic waves that can propagate through space (even in a vacuum). • The number of oscillations per second of a wave is called its frequency, f, and is measured in Hz. • The distance between two consecutive maxima (or minima) is called the wavelength. The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 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. Radio transmission • • • • • • Frequency ranges: 3 KHz to 1 GHz Omnidirectional Susceptible to interference by other antennas using same frequency or band Ideal for long-distance broadcasting May penetrate walls Apps: AM and FM radio, TV, maritime radio, cordless phones, paging Microwaves • • • • Frequencies between 1 and 300 GHz Unidirectional. Narrow focus requires sending and receiving antennas to be aligned. Issues: • • Line-of-sight (curvature of the Earth; obstacles) Cannot penetrate walls Politics of the Electromagnetic Spectrum The ISM bands in the United States. • • Bands for unlicensed usage. All devices in the ISM bands limit their transmit power (e.g., to 1 watt). Satellite Microwaves Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station. Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Satellite is relay station Television Long distance telephone Private business networks Satellite Microwaves Communication Satellites The principal satellite bands. Infrared • • • • • • Frequencies between 300 GHz and 400 THz. Short-range communication. High frequencies cannot penetrate walls. Requires line-of-sight propagation. Advantage: prevents interference between systems in adjacent rooms. Disadvantage: cannot use for long-range communication or outside a building due to sun’s rays. Baseband Transmission • • • • • Baseband transmission: the signal occupies frequencies from zero up to a maximum that depends on the signaling rate. It is common for wires. Passband transmission: the signal occupies a band of frequencies around the frequency of the carrier signal. It is common for wireless and optical channels. Multiplexing: Channels are often shared by multiple signals. Baseband Transmission • NRZ: a positive voltage to represent a 1 and a negative voltage to represent a 0. • The signal will be attenuated and distorted by the channel and noise at the receiver. NRZ needs a bandwidth of at least B/2 Hz when the bit rate is B bits/sec. By using four voltages, we can send 2 bits at once as a single symbol. • • Baseband Transmission Line codes: (a) Bits, (b) NRZ, (c) NRZI, (d) Manchester, (e) Bipolar or AMI. Clock Recovery • The receiver must know when one symbol ends and the next symbol begins to correctly decode the bits. • One strategy is to send a separate clock signal to the receiver. Manchester: mix the clock signal with the data signal by XORing them together. NRZI: coding a 1 as a transition and a 0 as no transition, or vice versa. • • • • 4B/5B: Every 4 bits is mapped into a5-bit pattern with a fixed translation table. The five bit patterns are chosen so that there will never be a run of more than three consecutive 0s. Clock Recovery 4B/5B mapping. Clock Recovery • Signals that have as much positive voltage as negative voltage even over short periods of time are called balanced signals. • Bipolar encoding: represent a logical 1, (say +1 V or −1 V) with 0 V representing a logical zero. To send a 1, the transmitter alternates between the +1 V and −1 V levels. Passband Transmission • we can take a baseband signal that occupies 0 to B Hz and shift it up to occupy a passband of S to S +B Hz. • Digital modulation is accomplished with passband transmission by regulating or modulating a carrier signal that sits in the passband. • In ASK (Amplitude Shift Keying), two different amplitudes are used to represent 0 and 1. • PSK (Phase Shift Keying): the carrier wave is systematically shifted 0 or 180 degrees at each symbol period. Passband Transmission (a) A binary signal. (b) Amplitude shift keying. (c) Frequency shift keying. (d) Phase shift keying. Multiplexing • FDM: divides the spectrum into frequency bands, with each user having exclusive possession of some band. • Guard band: It keeps the channels well separated. • When sending digital data, it is possible to divide the spectrum efficiently without using guard bands. • In OFDM (Orthogonal Frequency Division Multiplexing), the channel bandwidth is divided into many subcarriers that independently send data. • The frequency response of each subcarrier is designed so that it is zero at the center of the adjacent subcarriers. Frequency Division Multiplexing Frequency division multiplexing. (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. Frequency Division Multiplexing Orthogonal frequency division multiplexing (OFDM). Time Division Multiplexing • The users take turns (in a round-robin fashion), each one periodically getting the entire bandwidth for a little burst of time. • Bits from each input stream are taken in a fixed time slot and output to the aggregate stream. Code Division Multiplexing • CDM (Code Division Multiplexing) is a form of spread spectrum communication. • CDMA allows each station to transmit over the entire frequency spectrum all the time. • In CDMA, each bit time is subdivided into m short intervals called chips. • Typically, there are 64 or 128 chips per bit. • Each station is assigned a unique m-bit code called a chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the negation of its chip sequence. • Code Division Multiplexing (a) (b) Chip sequences for four stations. Signals the sequences represent