Download Physical layer

CS3505,
the Internet -physical layer
physical layer - purpose
 To
transmit bits, by encoding them onto signals;
and to receive the signals, interpreting them as bits
input: sequence of bit S, from DL or MAC layer
output: sequence of bit S’, to DL or MAC layer
ideally, S = S’
Physical layer definitions
 signal
1. mechanism used to carry information over
time OR distance
2. sign or gesture giving information
3. sequence of electrical or optical impulses or
waves
Signals
 examples:
 physical
gesture, wave, hand signal
 flashes of light (eg, Morse code)
 sound: vary tone, loudness or duration
 flags
 smoke
 mirrors
 electical voltages
transmission definition
1. the action of conveying electrical or optical signals from 1
point to 1 or more other points in space
2. the process of sending information from 1 point to another
 things necessary for transmission systems
 path for signal transfer (medium)
 transform signal to appropriate from (code)
 launch the signal (transmit)
 remove, receive or detect the signal (receive)
signal - can be modeled as
f(t)
t --> f(t) ... f(t) represents some physical quantity: voltage,
amplitude, frequency, etc.
digital/ analog signals
 digital
signal
1. assumed to take on finite number of
values, AND
2. has meaning only at discrete points in
time.
digital/analog signals
 analog
signal:
1. a signal that is an analog of the quantity being
represented; eg, signal voltage proportional to
volume of sound
2. continuous in range of values
3. also continuous in time; always valued.
digital/analog signals
 digital
data: text, bits; discrete valued.
 analog
data: sound, vision; music, etc. continuous
valued.
Note: digital (analog) signals can transport both
digital and analog data, so 4 combinations
(DD,DA,AD,AA) possible
transmission media

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
transmission medium: the physical element through
which signals must pass, from transmitter to receiver
examples: air, water, space, copper wires, optical fiber
two main categories: guided and unguided
propagation delays of signals in media
transmission terminology
 spectrum
signal
- range of frequencies making up a
 bandwidth
 data
rate
transmission terminology



bandwidth key factor in determining data rate;
however do not confuse bandwidth (hertz) and and data
rate (bps)
obstacles to transmission-
attenuation

interference

cross talk
modems, codecs
 modem
(modulator-demodulator)
 translates
a digital signal (bit) into an analog signal,
for transmission as an analog signal; receives the
corresponding analog signal, and translates back into
digital (bit)
 purpose:
use analog medium for digital data/signals
 example:
PC modem, phone lines; TV cable modems
modems, codecs, bauds, bits

codec (codec/decoder)
 converts analog data into digital form (bits), and the
reverse.
 main technique: PCM

PCM (pulse code modulation)
 absolute values, based on sampling theorem; (nearly)
total information
channel capacity
 channel
- a path in the transmission medium
through which signals/bits may pass
 channel
capacity - maximum number bits/sec the
channel can support
 factors
which determine channel capacity
 bandwidth
 signalling technique
 noise
transmission media
 Guided
Media
 twisted
pair (copper)
 coaxial cable (copper)
 optical fibers (silicon... plastic or glass)
 Unguided
 broadcast
Media
radio frequencies
 terrestrial microwave
 satellite microwave
 infrared, FSO
transmission media : twisted pair
copper a good conductor of electricity
 2 copper wires form circuit
 twisting gives better electrical properties
 backbone of the local telephone system
 also heavily used in data comm., LANs
 used for both digital, analog signals
 basic telephone wire vs. "Cat 5"

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, splicing 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
 suseptible to cross talk if lines close together
 susceptible to lightning strikes
 less bandwidth
transmission media : coaxial cable



thick cable, consisting of an inner copper core,
insulator, surrounded by another conductor (braided
shield), wrapped in a protective shield and outer cover
Properties (approx.)
 bandwidth: ~500Mhz, analog
 data rates: 500 Mbps or more
 repeater spacing: 1-10 Km
Two basic types:
 broadband (TV)
 baseband (ethernet)
transmission media : coaxial cable
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broadband: TV cable, analog signals
baseband: LANs, digital signals
Uses
 long distance telephone
 cable TV
 LANs
Note: higher capacity than t.p., but also 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
 less susceptible to interference
 easy to tap

disadvantages
 physically bulky
 limited bending radius
 heavier
transmission media : optical fiber



OF a major milestone in communications; first fibers
developed early1970s
since about 1988, majority of all U.S. long distance
traffic over 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; 100M to several G.

prop. speed approximately 2/3 c, as with tp and coax;
bits much smaller

repeater spacing: much higher... up to thousands of
miles
transmission media : optical fiber

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: total internal reflection

Two major types of fiber
1. multi-mode
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 100-300
Mhz 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

lower frequency ranges: roughly 30MHz-1GHz

omnidirectional

data rates not as high as microwave, so less useful for
data, but good for broadcast radio

better progagation characteristics; less attenuation, less
interference from rain, etc.
unguided media

lower frequency ranges: broadcast radio
30-300 kHz
MF AM radio
3-30 MHz
HF
shortwave radio, CB
30-300 MHz VHF FM radio, VHF TV

microwave frequency ranges: 2 to 40 GHz

infrared: just below visible light; frequency in hertz 1011
- 1014
unguided media :
terrestrial microwave

focused beam, 1-2 degrees

high frequencies 3-20 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 microwave

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
unguided media :
satellite microwave

need spacing of 4 deg. between satellites

significant prop delay ~ 250 ms

less difficulty with atmosphere

3 major differences with terr. microwave
unguided media : FSO,
or free space optics

uses optical signals in open air

limited distances only

very high transmission rates possible

much faster to set up and take down then conventional
wired networks

security less a problem than other wireless methods due
to focused beam and limited distance

very useful in some cases
unguided media : infrared

uses infrared light to transmit bits

similiar characteristics as free space optical

also very useful in some cases
voice channel & telephone system
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basic telephone network designed to deliver quality
voice service
voice emits analog signal - sound waves - from 30 to
10,000 Hz. Human ears detect up to 20K Hz.
most energy in 200-3500 Hz range; Standard analog
voice channel is 4000 Hz. This key number selected
many years ago by phone company.
standard PCM digital voice channel is 64 Kbps.
most local telephone loops still analog
all long distance in US is digital; majority is fiber.
voice channel & telephone system
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voice not very sensitive to most noise and distortion; for
this and other reasons, local telco loops not well suited
to modern data networks
However, the local telco networks are one of few
comm. links between homes, businesses and rest of the
world
Structure of U S Telephone networks /companies
 local loops “last mile” and telcos
 long distance networks and companies
 network equipment
video channels and cable TV system

TV cable system established recent decades

switching equipment designed for broadcast TV

standard TV - 6 MHz per channel

coaxial cables capable carry many TV channels.

these have capacity to carry thousands of voice channels
and/or high speed data -- but need appropriate
switching equipment at home office, and in homes

AT&T attempted, failed to use coax cable system for
Internet, voice -- probable a business error