CS3502, Data and Computer Networks: the physical layer-2

channel capacity u channel - a path, contained in the transmission medium, through which signals/bits may pass u a part of the medium, not all u channel capacity - maximum number bits/sec the channel can support u factors which determine channel capacity u bandwidth u number signal levels u noise

channel capacity u basic channel capacity formulas u 2 cases:channel requirement, channel capacity. Case 1: The channel capacity required to digitize an analog signal which contains the highest frequency F max is given by the Nyquist formula R = 2 F max log 2 (V), where R : channel requirement in bps, F max :maximum frequency in hertz V : # signal levels

channel capacity u examples 1. F max 3100 Hz, 8 signal levels. What is R ? A: R = 2(3100) log(8) = 18,600 bps 2. R = 60 Kbps, F max is 6000 Hz. How many signal levels? A: ? 3. F max 10KHz, V is 16. What is R? A: ?

channel capacity u Case 2: Channel Capacity with noise present. Shannon formula. C = W log2 (1 + S/N) where: C = channel capacity in bps W= band width in hz S = signal strength in Watts N= noise strength in Watts Note 1 : upper bound, independent of signal levels. Note 2 : S/N often given in decibels; if so, must convert to absolute ratio using the formula: S/N dB = 10 log 10 (S/N)

channel capacity u example dB = 10 log 10 S/N ; --> S/N = 10**3 = S/N = 500, C = 1Mb/s. What bandwidth needed? A: 1 Mb/s = W log 2 (1+500), appr = W (9) W = Hz (approx) 3. S/N = 40dB, W = 6200 Hz. A: 81,840 (approx.)

channel capacity u note 1 : Shannon formulas is an upper bound; theoretical maximum. Actual data rates often much less. u note 2 : noise considered in Shannon is only thermal noise; no other type of noise. u note 3 : data compression not considered. This can raise the data limits considerably.

transmission media u Guided Media u twisted pair (copper) u coaxial cable (copper) u optical fibers (silicon... plastic or glass) u Unguided Media u broadcast radio frequencies u terrestrial microwave u satellite microwave Note: take the tables in Text on data rates, etc. as a general guide, NOT as absolute truth

transmission media : twisted pair u copper a good conductor of electricity u (side note: recent developments by IBM leading to use of copper on ICs - better chips) u 2 copper wires used to form a circuit between Xmitter, Rcvr u twisting gives better electrical properties u backbone of the local telephone system u also used for limited long distance telephones u also heavily used in data comm., LANs u used for both digital, analog signals

transmission media : twisted pair u various quality levels: voice grade, “Cat 5” u data rates: Mbps, depending on quality; voice grade at low end, Cat 5 top end. u higher quality are more tightly twisted u advantages u mature - well known technology u connections, splices easy u production, installation techniques well known u relatively cheap, easy to install

transmission media : twisted pair u disadvantages u cost of copper u signal attenuation increases with frequency, starting at low frequencies u often needs shield to reduce noise pickup u susceptible to cross talk if lines close together u susceptible to lightning strikes u less bandwidth than most other media u See text for further explanation

transmission media : coaxial cable u a thick cable, consisting of an inner copper core surrounded by an insulator, surrounded by another conductor (braided shield), wrapped in a protective shield and an outer cover. (see diagram in text) u Properties (approx.) u bandwidth: ~500Mhz, analog u data rates: 500 Mbps or more u repeater spacing: 1-10 Km u Two basic types: u broadband u baseband

transmission media : coaxial cable u broadband: TV cable, analog signals u baseband: LANs, digital signals u Uses u long distance telephone u cable TV u LANs u Note: higher capacity than t.p., but also much bulkier and difficult to work with in limited spaces

transmission media : coaxial cable u advantages u lower attenuation than t.p. at high frequencies u wider usable bandwidth u better isolation (less susceptible to interference) u easy to tap u disadvantages u physically larger, bulky u limited bending radius u heavier u fire code restrictions on materials

transmission media : optical fiber u development of OF a major milestone in communications; made feasible by invention of laser ~1960; first fibers developed ~1970 u twisted pair 19th century; coax ~ 1930; radio ~1900; integrated circuits ~ u since about 1988, majority of all U.S. long distance traffic over OF, though only about 5% of cable is OF. u due to OF, the networks have potential to be faster than the computer ---- a big flip flop

transmission media : optical fiber u A thin, flexible medium of extremely pure plastic/glass. Thickness about microns. Core often 62.5 microns. u much higher data rates; from 100M to several G. u note prop. speed approximately 2/3 c, as with tp and coax; bits much smaller u repeater spacing: much higher... u FDDI, DQDB, and SONET all optical fiber standards u principle: each bit is transported by a tiny ray of light(darkness), guided by the medium. u requires extremely accurate transmitters, receivers; much finer degree of synchronization

transmission media : optical fiber u principle: total internal reflection u Two major types of fiber 1. multi-mode u step index u graded index 2. single mode/monomode u limitations u modal dispersion (multimode) u material dispersion (single mode) u attenuation (single mode, at very high data rates)

transmission media : optical fiber u advantages u much higher bandwidth, real and potential u very low radiation, noise pickup; shielding not needed, crosstalk not a problem u very low attenuation, and little variation in.85,1.3,and 1.55 micro- meter range u not susceptible to lightning, etc. u small physical size and weight u cost will decrease u very difficult to tap

transmission media : optical fiber u disadvantages u cost u technology less mature u splicing difficult and critical u installation more difficult u Key note: fiber has literally made the network faster than the computer. We have far to go before we reach the potential data rates of fiber....

unguided media : broadcast radio TV u lower frequency ranges: roughly 30KHz-1GHz u omni-directional u data rates not as high as microwave, so less useful for data, but good for broadcast radio u better propagation characteristics; less attenuation, less interference from rain, etc.

unguided media u lower frequency ranges: broadcast radio kHzMFAM radio 3-30 MHzHFshortwave radio, CB MHzVHFFM radio, VHF TV... u microwave frequency ranges: ~1 to 40 GHz u infrared: just below visible light; frequency

unguided media : terrestrial microwave u focused beam, 1-2 degrees u high frequencies 3-40 GHz --> high data rates u paraboloid shaped antennas u better repeater spacing than cable u high data rates u more susceptible to rain, clouds, dust, etc. than others

unguided media : satellite microwave u high frequency; ( ~same as terr. uwave) u geosynchronous satellite --> repeater in sky u broadcast media u 22,300 miles --> 35,000 Km u receives, xmits on diff. frequencies to avoid interference u need spacing of 4 deg. between satellites u significant prop delay ~ 250 ms u less difficulty with atmosphere u 3 major differences with terr. microwave