Sharif University of Technology The Physical Layer defines the mechanical, electrical, and timing interfaces to the network. Topics in this chapter:  theoretical analysis of data transmission  three kinds of transmission media : guided : copper wire and fiber optics Wireless: terrestrial radio satellite  This material will provide background information on the key transmission technologies used in modern networks

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication Information can be transmitted on wires by varying some physical property such as voltage or current. we can model the behavior of the signal and analyze it mathematically (f(t)) Fourier Analysis :  periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines :

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication A data signal that has a finite duration (which all of them do) can be handled by just imagining that it repeats the entire pattern over and over forever. Computation of series coefficients :

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication Bandwidth-Limited Signals  example: the transmission of the ASCII character ''b'' encoded in an 8-bit byte. bit pattern : Root-mean-square:

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication  all transmission facilities diminish different Fourier components by different amounts, thus introducing distortion.  Usually, the amplitudes are transmitted undiminished from 0 up to some frequency fc [measured in cycles/sec or Hertz (Hz)] with all frequencies above this cutoff frequency attenuated.  The range of frequencies transmitted without being strongly attenuated is called the bandwidth (0 - fc )  The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication  In some cases a filter is introduced into the circuit to limit the amount of bandwidth available to each customer. Example:  bandwidth of a telephone wire for short distance: 1 MHz  Restricted bandwidth by telephone company : 3100 Hz  Given a bit rate of b bits/sec  The time required to send 8 bits is 8/b sec  The frequency of the first harmonic is b/8 Hz  An ordinary telephone line, often called a voice-grade line, has an artificially-introduced cutoff frequency just above 3000 Hz

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication  The number of the highest harmonic passed through is roughly 3000/(b/8) or 24,000/b  Relation between data rate and harmonics :

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication The Maximum Data Rate of a pure Channel  Nyquist’s theorem : an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. If the signal consists of V discrete levels, Nyquist's theorem states:  Example: A noiseless 3-kHz channel cannot transmit binary (i.e., two- level) signals at a rate exceeding 6000 bps

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication The Maximum Data Rate of a noisy Channel  there is always random (thermal) noise present due to the motion of the molecules in the system.  The amount of thermal noise present is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio. If we denote the signal power by S and the noise power by N, the signal-to-noise ratio is S/N. Usually, the ratio itself is not quoted; instead, the quantity 10 log10 S/N is given (decibels (dB) ).

Sharif University of Technology Physical layer: The Theoretical Basis for Data Communication  Shannon's theorem:  Example: A channel of 3000-Hz bandwidth with a signal to thermal noise ratio of 30 dB (typical parameters of the analog part of the telephone system) can never transmit much more than 30,000 bps.  No matter how many or how few signal levels are used and no matter how often or how infrequently samples are taken.  This is only an upper bound and real systems rarely achieve it.

Sharif University of Technology Guided Transmission Media Magnetic Media  Bandwidth characteristics is excellent  Delay characteristics is poor Twisted Pair  A twisted pair consists of two insulated copper wires, typically about 1 mm thick.  Twisting is done because two parallel wires constitute a fine antenna.  Twisted pair categories: Category 3 UTP (Unshielded Twisted Pair) (a) Category 5 UTP (Unshielded Twisted Pair) (b)  They are similar to category 3 pairs, but with more twists per centimeter, which results in less crosstalk and a better-quality signal over longer distances, making them more suitable for high-speed computer communication.

Sharif University of Technology Guided Transmission Media Coaxial Cable  It has better shielding than twisted pairs, so it can span longer distances at higher speeds.  Two kinds of coaxial cable 50-ohm cable : is commonly used when it is intended for digital transmission. 75-ohm cable: is commonly used for analog transmission and cable television The bandwidth possible depends on the cable quality, length, and signal-to-noise ratio of the data signal.

Sharif University of Technology Guided Transmission Media (Fiber Optics ) Fiber Optics  An optical transmission system has three key components: Light source: Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. Transmission medium: is an ultra-thin fiber of glass Detector: generates an electrical pulse when light falls on it.

Sharif University of Technology Guided Transmission Media (Fiber Optics )  When a light ray passes from one medium to another, for example, from fused silica to air, the ray is refracted (bent) at the silica/air boundary  Fiber types multimode fiber single-mode fiber: diameter is reduced to a few wavelengths of light

Sharif University of Technology Guided Transmission Media (Fiber Optics )  Transmission of Light through Fiber The attenuation of light through glass depends on the wavelength of the light Attenuation of light through fiber in the infrared region

Sharif University of Technology Guided Transmission Media (Fiber Optics ) Fiber Cables  multimode fibers : The core is typically 50 microns in diameter, about the thickness of a human hair  single-mode fibers : T he core is 8 to 10 microns.

Sharif University of Technology Guided Transmission Media (Fiber Optics )  Fibers can be connected in three different ways Connectors Mechanical Fusion  Two kinds of light sources are typically used to do the signaling LEDs (Light Emitting Diodes) Semiconductor lasers  receiving end of an optical fiber consists of a photodiode A comparison of semiconductor diodes and LEDs as light sources.

Sharif University of Technology Guided Transmission Media (Fiber Optics ) Fiber Optic Networks  Active repeaters  Passive star A passive star connection in a fiber optics network A fiber optic ring with active repeaters

Sharif University of Technology Guided Transmission Media (Fiber Optics ) Comparison of Fiber Optics and Copper Wire  Fiber can handle much higher bandwidths than copper  Fiber not being affected by power surges, electromagnetic interference, or power failures  Fiber is thin and lightweight  Security in fiber is high  Fiber is a less familiar technology requiring skills not all engineers have  Fibers can be damaged easily by being bent too much  Optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber  fiber interfaces cost more than electrical interfaces