CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them...

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CS3505, CS3505, the Internet -- the Internet -- physical layer physical layer

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Transcript of CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them...

Page 1: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

CS3505,CS3505,

the Internet --the Internet -- physical layer physical layer

Page 2: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

physical layer - purposephysical 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 layerideally, S = S’

Page 3: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

Physical layer definitionsPhysical 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

Page 4: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

SignalsSignals

examples: physical gesture, wave, hand signal flashes of light (eg, Morse code) sound: vary tone, loudness or duration flags smoke mirrors electical voltages

Page 5: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission definitiontransmission definition1. the action of conveying electrical or optical signals

from 1 point to 1 or more other points in space2. 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)

Page 6: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

signal - can be modeled assignal - can be modeled as f(t)f(t)

t --> f(t) ... f(t) represents some physical quantity: voltage, amplitude, frequency, etc.

Page 7: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

digital/ analog signalsdigital/ analog signals

digital signal1. assumed to take on finite number of values, AND

2. has meaning only at discrete points in time.

Page 8: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

digital/analog signalsdigital/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.

Page 9: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

digital/analog signalsdigital/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

Page 10: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission mediatransmission media

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

Page 11: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission terminologytransmission terminology spectrum - range of frequencies making up

a signal

bandwidth

data rate

Page 12: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission terminologytransmission 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

Page 13: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

modems, codecsmodems, 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

Page 14: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

modems, codecs, bauds, bitsmodems, 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

Page 15: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

channel capacitychannel 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

Page 16: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

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 infrared, FSO

Page 17: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : twisted pairtransmission 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"

Page 18: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : twisted pairtransmission 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

Page 19: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : twisted pairtransmission 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

Page 20: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : coaxial cabletransmission 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)

Page 21: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : coaxial cable 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 bulkier and difficult to work with in limited spaces

Page 22: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : coaxial cabletransmission 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

Page 23: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : optical fibertransmission 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

Page 24: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

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; 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

Page 25: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : optical fibertransmission 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

Page 26: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : optical fibertransmission 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)

Page 27: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : optical fibertransmission 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

Page 28: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

transmission media : optical fibertransmission 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....

Page 29: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : broadcast radiounguided 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.

Page 30: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media unguided media

lower frequency ranges: broadcast radio30-300 kHz MF AM radio3-30 MHz HF shortwave radio,

CB30-300 MHzVHF FM radio, VHF TV

microwave frequency ranges: 2 to 40 GHz

infrared: just below visible light; frequency in hertz 1011 - 1014

Page 31: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : unguided media : terrestrial microwaveterrestrial 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

Page 32: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : unguided media : satellite microwavesatellite 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

Page 33: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : unguided media : satellite microwavesatellite microwave

need spacing of 4 deg. between satellites

significant prop delay ~ 250 ms

less difficulty with atmosphere

3 major differences with terr. microwave

Page 34: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : FSO, unguided media : FSO, or free space opticsor 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

Page 35: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

unguided media : infrared unguided media : infrared

uses infrared light to transmit bits

similiar characteristics as free space optical

also very useful in some cases

Page 36: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

voice channel & telephone systemvoice channel & telephone system

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.

Page 37: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

voice channel & telephone systemvoice channel & telephone system

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

Page 38: CS3505, the Internet -- physical layer. physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting.

video channels and cable TV systemvideo 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


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