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

CS3502,CS3502,Data and Computer Networks:Data and Computer Networks:

the physical layer-2the physical layer-2

channel capacitychannel capacity

channel - a path, contained in the transmission medium, through which signals/bits may pass a part of the medium, not all

channel capacity - maximum number bits/sec the channel can support

factors which determine channel capacity bandwidth number signal levels noise

channel capacitychannel capacity basic channel capacity formulas 2 cases:channel requirement, channel capacity.Case 1: The channel capacity required to digitize

an analog signal which contains the highest frequency Fmax

is given by the Nyquist formula R = 2 Fmax log2 (V),

whereR : channel requirement in bps,

Fmax :maximum frequency in hertz

V : # signal levels


examples1. Fmax 3100 Hz, 8 signal levels. What is R?

A: R = 2(3100) log(8) = 18,600 bps

2. R = 60 Kbps, Fmax is 6000 Hz. How many signal levels? A: ?

3. Fmax 10KHz, V is 16. What is R?

A: ?


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 toabsolute ratio using the formula:

S/N dB = 10 log10 (S/N)


example1. 30 dB = 10 log10 S/N ; --> S/N = 10**3 =

1000.

2. S/N = 500, C = 1Mb/s. What bandwidth needed? A: 1 Mb/s = W log2 (1+500), appr. 1000000 = W (9)

W = 111111 Hz (approx)

3. S/N = 40dB, W = 6200 Hz. A: 81,840 (approx.)


note 1: Shannon formulas is an upper bound; theoretical maximum. Actual data rates often much less.

note 2: noise considered in Shannon is only thermal noise; no other type of noise.

note 3: data compression not considered. This can raise the data limits considerably.

transmission mediatransmission media

Guided Media twisted pair (copper) coaxial cable (copper) optical fibers (silicon... plastic or glass)

Unguided Media broadcast radio frequencies terrestrial microwave satellite microwave

Note: take the tables in Text on data rates, etc. as a general guide, NOT as absolute truth

transmission media : twisted pairtransmission media : twisted pair

copper a good conductor of electricity (side note: recent developments by IBM

leading to use of copper on ICs - better chips) 2 copper wires used to form a circuit between

Xmitter, Rcvr twisting gives better electrical properties backbone of the local telephone system also used for limited long distance telephones also heavily used in data comm., LANs used for both digital, analog signals


various quality levels: voice grade, “Cat 5”

data rates: 1-100 Mbps, depending on quality; voice grade at low end, Cat 5 top end.

higher quality are more tightly twisted

advantages mature - well known technology connections, splices easy production, installation techniques well known relatively cheap, easy to install


disadvantages cost of copper signal attenuation increases with frequency,

starting at low frequencies often needs shield to reduce noise pickup susceptible to cross talk if lines close together susceptible to lightning strikes less bandwidth than most other media

See text for further explanation

transmission media : coaxial transmission media : coaxial cablecable

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)

Properties (approx.) bandwidth: ~500Mhz, analog data rates: 500 Mbps or more repeater spacing: 1-10 Km

Two basic types: broadband baseband

transmission media : coaxial transmission media : coaxial cable cable

broadband: TV cable, analog signals baseband: LANs, digital signals Uses

long distance telephone cable TV LANs

Note: higher capacity than t.p., but also much bulkier and difficult to work with in limited spaces

transmission media : coaxial transmission media : coaxial cablecable

advantages lower attenuation than t.p. at high frequencies wider usable bandwidth better isolation (less susceptible to interference) easy to tap

disadvantages physically larger, bulky limited bending radius heavier fire code restrictions on materials

transmission media : optical fibertransmission media : optical fiber

development of OF a major milestone in communications; made feasible by invention of laser ~1960; first fibers developed ~1970

twisted pair 19th century; coax ~ 1930; radio ~1900; integrated circuits ~1950...

since about 1988, majority of all U.S. long distance traffic over OF, though only about 5% of cable is OF.

due to OF, the networks have potential to be faster than the computer ---- a big flip flop

transmission media : optical fibertransmission media : optical fiber A thin, flexible medium of extremely pure plastic/glass.

Thickness about 2-125 microns. Core often 62.5 microns.

much higher data rates; from 100M to several G.

note prop. speed approximately 2/3 c, as with tp and coax; bits much smaller

repeater spacing: much higher...

FDDI, DQDB, and SONET all optical fiber standards

principle: each bit is transported by a tiny ray of light(darkness), guided by the medium.

requires extremely accurate transmitters, receivers; much finer degree of synchronization


principle: total internal reflection

Two major types of fiber

1. multi-mode step index

graded index

2. single mode/monomode

limitations modal dispersion (multimode)

material dispersion (single mode)

attenuation (single mode, at very high data rates)


advantages much higher bandwidth, real and potential very low radiation, noise pickup; shielding not

needed, crosstalk not a problem very low attenuation, and little variation

in .85,1.3,and 1.55 micro- meter range not susceptible to lightning, etc. small physical size and weight cost will decrease very difficult to tap


disadvantages cost technology less mature splicing difficult and critical installation more difficult

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 unguided media : broadcast radio TVradio TV

lower frequency ranges: roughly 30KHz-1GHz

omni-directional

data rates not as high as microwave, so less useful for data, but good for broadcast radio

better propagation characteristics; less attenuation, less interference from rain, etc.

unguided media unguided media

lower frequency ranges: broadcast radio300-3000 kHz MF AM radio3-30 MHz HF shortwave radio,

CB30-300 MHz VHF FM radio, VHF TV...

microwave frequency ranges: ~1 to 40 GHz

infrared: just below visible light; frequency 1011 - 1014

unguided media : terrestrial unguided media : terrestrial microwavemicrowave

focused beam, 1-2 degrees

high frequencies 3-40 GHz --> high data rates

paraboloid shaped antennas

better repeater spacing than cable

high data rates

more susceptible to rain, clouds, dust, etc. than others

unguided media : satellite unguided media : satellite microwavemicrowave

high frequency; ( ~same as terr. uwave)

geosynchronous satellite --> repeater in sky

broadcast media

22,300 miles --> 35,000 Km

receives, xmits on diff. frequencies to avoid interference

need spacing of 4 deg. between satellites

significant prop delay ~ 250 ms

less difficulty with atmosphere

3 major differences with terr. microwave

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