The physical layer
description
Transcript of The physical layer
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The physical layer
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Physical Layer
Sending raw bits across “the wire”. Issues:
– What’s being transmitted.
– Transmission medium.
– How it’s being transmitted.
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Signal
Signal: electro-magnetic wave carrying information. Time domain: signal as a function of time.
– Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities.
– Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).
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Time Domain
Periodic signals:–Same signal pattern repeats over time.–Example: sine wave
Amplitude (A) Period (or frequency) (T = 1/f) Phase
)()(
)2sin()(
tsTts
ftAts
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Frequency Domain
Signal consists of components of different frequencies. Spectrum of signal: range of frequencies signal contains. Absolute bandwidth: width of signal’s spectrum.
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Example:
))3(2sin(3/1)2sin()( 11 tftfts
Spectrum of S(f) extends from f1 to 3f1.
Bandwidth is 2f1.
S(f)
f1 2 3
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Analog Technology
• Analog devices maintain exact physical analog of information– E.g., microphone: the voltage at the output of
the mic is proportional to the sound pressure• Early telephones were all analog• Problems with analog signals:
– Difficult to store (e.g.: audio tapes, videotapes)– Must be processed by analog systems which
often add distortion– Noise always adds to the signal
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Digital Technology
• It use numbers to record and process information– Inside a computer, all information is
represented by numbers– Analog-to-digital conversion: ADC– Digital-to-analog conversion: DAC
• All signals (including multimedia) can be encoded in digital form
• Digital information does not get distorted while being stored, copied or communicated
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Digital Communication Technology
• Example: The telegraph (Morse code)– Uses dots and dashes to transmit letters– It is digital even though uses electrical signals
• The telephone has become digital• CDs and DVDs• Digital communication networks form the
Internet• The user is unaware that the signal is
encoded in digital form
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2 Levels Are Sufficient
• Computers encode numbers using only two levels: 0 and 1
• A bit is a digit that can only assume the values 0 and 1 (it is a binary digit)
• A word is a number formed by several bits– Example: ASCII standard for encoding text
• A = 1000001; B = 1000010; …
• A byte is a word with 8 bits
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Definitions• 1 byte = 8 bits• 1 KB = 1 kilobyte = 1,024 bytes = 8*1,024 bits
• 210 = 1,024 is powr of 2 closest to 1,000.• [also 1,000 bytes]
• 1 MB = 1 megabyte = 1,000 KB• 1 GB = 1 gigabyte = 1,000 MB• 1 TB = 1 terabyte = 1,000 GB
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Definitions (cont’d)
1 Kb = 1 kilobit = 1,024 bits
[also, 1,000 bits] 1 Mb = 1 megabit = 1,000 Kb 1 Gb = 1 gigabit = 1,000 Mb 1 Tb = 1 terabit = 1,000 Gb
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Digitization
• Digitization is the process that allows us to convert analog to digital (implemented by ADC)
• Analog signals: x(t)– Defined on continuum (e.g. time)– Can take on any real value
• Digital signals: q(n)– Sequence of numbers (samples) defined in a
discrete set (e.g., integers)
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Digitization - Example
1.35 1.355 1.36 1.365 1.37 1.375-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
x(t)
1.35 1.355 1.36 1.365 1.37 1.375-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
q(n)
Analog signal x(t) Digitized signal q(n)
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Some Definitions
• Interval of time between two samples:– Sampling Interval (T)
• Sampling frequency F=1/T• E.g.: if the sampling interval is 0.1 seconds,
then the sampling frequency is 1/0.1=10– Measured in samples/second or Hertz
• Each sample is defined using a word of B bits– E.g.: we may use 8 bits (1 byte) per sample.
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Bit-rate
• Bit-rate = numbers of bits per second we need to transmit– For each second we transmit F=1/T samples – Each sample is defined with a word of B bits– Bit-rate = F*B
• Example: if F is 10 samples/s and B=8, then the bit rate is 80 bits/s
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Example of Digitization
Time (seconds)0 1 2
F=4 samples/second
10101110010100110011010000110100
B=4 bits/sample
Bit-rate=BF=16 bits/second
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Bit-rate - Example 1
• What is the bit-rate of digitized audio?– Sampling rate: F= 44.1 KHz– Quantization with B=16 bits– Bit-rate = BF= 705.6 Kb/s– Example: 1 minute of uncompressed
stereo music takes more than 10 MB!
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Bit-rate - Example 2
• What is the bit-rate of digitized speech?– Sampling rate: F = 8 KHz – Quantization with B = 16 bits– Bit-rate = BF = 128 Kb/s
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Bandwidth and Bit Rate
Bit rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal).– Example: 2Mbits/sec, or 2Mbps.
If data rate of signal is W bps, good representation achieved with 2*W Hz bandwidth.
Nyquist-Shannon sampling theorem:
If a function x(t) contains no frequencies higher than B hertz, it is completely determined by giving its ordinates at a series of points spaced 1/(2B) seconds apart.
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Data Transmission
Analog and digital transmission.– Example of analog data: voice and video.
– Example of digital data: character strings Use of codes to represent characters as sequence of bits (e.g., ASCII).
Historically, communication infrastructure for analog transmission.– Digital data needed to be converted: modems (modulator-
demodulator).
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Digital Transmission
Current trend: digital transmission.– Cost efficient: advances in digital circuitry (VLSI).
Advantages:– Data integrity: better noise immunity.
– Security: easier to integrate encryption algorithms.
– Channel utilization: higher degree of multiplexing (time-division mux’ing).
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The Theoretical Basis for Data Communication
• Fourier Analysis• Any periodical signal can be decomposed as a
sum of sinusoidal signals at frequencies which are multiple of the original frequency
• We call those the “harmonics”
• Bandwidth-Limited Signals• Not all harmonics pass through a channel• The result is a distortion in the shape of the
signal
• Maximum Data Rate of a Channel
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Bandwidth-Limited Signals
A binary signal and its root-mean-square Fourier amplitudes.
(b) – (c) Successive approximations to the original signal.
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Bandwidth-Limited Signals (2)
(d) – (e) Successive approximations to the original signal.
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Bandwidth-Limited Signals (3)
Relation between data rate and harmonics.
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Guided Transmission Data
•Magnetic Media• Write the data on a storage system (eg. tapes or hard drive), carry them over physically
•Twisted Pair•Coaxial Cable•Fiber Optics
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Twisted Pair
Category 3 UTP (unshielded twisted pair)– Possible bandwidth 16MHz, telephony systems, 10BASE-T Ethernet
Category 5 UTP – standard for Fast Ethernet– Up to 100MHz– since about 1988 – more twists, less crosstalk, better signal over
longer distances
• Category 6 UTP – standard for Gigabit Ethernet• Up to 250MHz (500MHz for 6a)
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Coaxial Cable
• More expensive than twisted pair• High bandwidth and excellent noise immunity• Impedance is an important metric (50-75ohms)• It must be manufactured to exact specifications, not only an inner
conductor wrapped in a shielding (as audio cables are)• Can transmit bandwidths way into the GHz.
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Fiber Optics
(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
-Not a mirror! That would lead to losses at every reflection!
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Single mode vs multi-mode
• Multi-mode fiber: light reflected on various angles inside the fiber.
• If the fiber is so narrow that it is only several wavelengths, the light can travel only in a single way, in a straight line, without bouncing. • The fiber acts like a wave guide
• Called a single mode fiber
• Smaller loss, more suitable for long distance transmission
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Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region.
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Fiber Cables
-Core: 50 microns for multi-mode, 8-10 microns for single mode-Cladding: glass with a lower refraction index, to keep the light in the
core-Connection:
-connectors (plug in) – about 20% attenuation-mechanical splicing, tuned by an operator – 10% attenuation-fused (melted together) – almost no attenuation
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Fiber Cables (2)
A comparison of semiconductor diodes and LEDs as light sources.
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Fiber Optic Networks
A fiber optic ring with active repeaters.
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Fiber Optic Networks (2)
A passive star connection in a fiber optics network.
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Wireless Transmission
• The Electromagnetic Spectrum• Radio Transmission• Microwave Transmission• Infrared and Millimeter Waves• Lightwave Transmission
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Narrow-band vs spread spectrum
Spectrum– About 8 bits / Hz (using all the tricks in the book)
Narrowband: – Δf / f << 1
Spread spectrum– Frequency hopping spread spectrum
Several times / sec, military communications, good resistance to multipath fading
– Direct sequence spread spectrum DSSS: 802.11b, CDMA telephony, GPS, Galileo, ZigBee
– Ultra-wide band any radio technology having bandwidth exceeding the lesser of 500 MHz
or 20% of the arithmetic center frequency
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The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
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Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
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Politics of the Electromagnetic Spectrum
The ISM bands in the United States (Industrial, Scientifical, Medical: also known as unlicenced bands)
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Lightwave Transmission
Convection currents can interfere with laser communication systems.
A bidirectional system with two lasers is pictured here.
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Communication Satellites
• Geostationary Satellites• Medium-Earth Orbit Satellites• Low-Earth Orbit Satellites• Satellites versus Fiber
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Communication Satellites
Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and
number of satellites needed for global coverage.
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Communication Satellites (2)
The principal satellite bands.
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Communication Satellites (3)
VSATs using a hub.
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Low-Earth Orbit SatellitesIridium
(a) The Iridium satellites from six necklaces around the earth.
(b) 1628 moving cells cover the earth.
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Globalstar
(a) Relaying in space.(b) Relaying on the ground.
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Traditional telephony
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Public Switched Telephone System
• Structure of the Telephone System• The Politics of Telephones• The Local Loop: Modems, ADSL and Wireless
• Trunks and Multiplexing• Switching
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Structure of the Telephone System
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
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Structure of the Telephone System (2)
A typical circuit route for a medium-distance call.
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Major Components of the Telephone System
• Local loops Analog twisted pairs going to houses and businesses
• Trunks Digital fiber optics connecting the switching offices
• Switching offices Where calls are moved from one trunk to another
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The Politics of Telephones
The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it.
LATA: local access and transport areasLEC: local exchange carrierIXC: interexchange carrierThis is the result of the 1984 breakup of the AT&T monopoly.
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The Local Loop: Modems, ADSL, and Wireless
The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the
modems and codecs.
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Modems
(a) A binary signal
(b) Amplitude modulation(c) Frequency modulation
(d) Phase modulation
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Modems (2)
(a) QPSK.
(b) QAM-16.
(c) QAM-64.
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Modems (3)
(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.
(a) (b)
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Digital Subscriber Lines
Bandwidth versus distance over category 3 UTP for DSL.
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Digital Subscriber Lines (2)
Operation of ADSL using discrete multitone modulation.
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Digital Subscriber Lines (3)A typical ADSL equipment configuration.
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Multiplexing
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What is multiplexing?
Sending multiple flows of data through the same physical channel.
Examples:– Frequency division multiplexing (FDM) – used everywhere
– Time division multiplexing (TDM)
– Wavelength division multiplexing (FDM in the optical domain)
– Code Division Multiplexing (CDMA – wireless telephony)
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Frequency Division Multiplexing
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
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Wavelength Division Multiplexing
Wavelength division multiplexing.
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Time Division Multiplexing
The T1 carrier (1.544 Mbps).
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Time Division Multiplexing (2)
Delta modulation.
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Time Division Multiplexing (3)
Multiplexing T1 streams into higher carriers.
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Time Division Multiplexing (4)
Two back-to-back SONET frames.
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Time Division Multiplexing (5)
SONET and SDH multiplex rates.
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Code division multiple access
Different from FDMA and CDMA Heavily promoted by Qualcomm (large amount of
intellectual property) – Do not confuse CDMA (the technology idea) with cdma2000 etc,
the various standards currently used by the telco’s
Spread spectrum technology– Obviously (why?)
Key ideas:– Mutually orthogonal code words assigned to senders
– Encoding / decoding using the code words
– Signals are summed up in the air Make sure you understand “interference”
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Example of synchronous CDMA
(a) Binary chip sequences for four stations(b) Bipolar chip sequences (c) Six examples of transmissions(d) Recovery of station C’s signal
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Circuit switching, packet switching, message switching
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Circuit Switching
(a) Circuit switching.
(b) Packet switching.
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Message Switching
(a) Circuit switching (b) Message switching (c) Packet switching
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Packet Switching
A comparison of circuit switched and packet-switched networks.
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Cable television internet access
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Cable Television
• Community Antenna Television• Internet over Cable• Spectrum Allocation• Cable Modems• ADSL versus Cable
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Community Antenna Television
An early cable television system.
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Internet over Cable
Cable television
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Internet over Cable (2)
The fixed telephone system.
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Spectrum Allocation
Frequency allocation in a typical cable TV system used for Internet access
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Cable Modems
Typical details of the upstream and downstream channels in North America.