phys_layer-2

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Transcript phys_layer-2

CS3502,
Data and Computer Networks:
the physical layer-2
channel 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 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),
where
R : channel requirement in bps,
Fmax :maximum frequency in hertz
V : # signal levels
channel capacity
 examples
1. 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: ?
channel capacity
 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 log10 (S/N)
channel capacity
 example
1. 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.)
channel capacity
 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 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 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

transmission media : twisted pair
 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
transmission media : twisted pair
 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 cable
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 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 cable
 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 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 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
transmission media : optical fiber
 principle:
 Two
total internal reflection
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)
transmission media : optical fiber
 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
transmission media : optical fiber
 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 radio 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
 lower
frequency ranges: broadcast radio
300-3000 kHz
MF AM radio
3-30 MHz
HF shortwave radio, CB
30-300 MHz
VHF FM radio, VHF TV
...
 microwave frequency ranges: ~1 to 40 GHz
 infrared:
1014
just below visible light; frequency 1011 -
unguided media : terrestrial microwave
 focused
 high
beam, 1-2 degrees
frequencies 3-40 GHz --> high data rates
 paraboloid
 better
 high
shaped antennas
repeater spacing than cable
data rates
 more
susceptible to rain, clouds, dust, etc. than
others
unguided media : satellite microwave
 high
frequency; ( ~same as terr. uwave)
 geosynchronous
 broadcast
 22,300
satellite --> repeater in sky
media
miles --> 35,000 Km
 receives,
xmits on diff. frequencies to avoid
interference
 need
spacing of 4 deg. between satellites
 significant
 less
3
prop delay ~ 250 ms
difficulty with atmosphere
major differences with terr. microwave