15-441: Computer Networking Lecture 3: Application Layer and Socket Programming.
1 15-441 Computer Networking Lecture 2 – Physical Layer.
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Transcript of 1 15-441 Computer Networking Lecture 2 – Physical Layer.
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15-441 Computer Networking
Lecture 2 – Physical Layer
1-18-06 Lecture 2: Physical Layer 2
Network Protocols
• Protocol• A set of rules and formats that govern the
communication between communicating peers
• Protocol layering• Decompose a complex problem into smaller
manageable pieces (e.g., Web server)• Abstraction of implementation details• Reuse functionality• Ease maintenance• Cons?
1-18-06 Lecture 2: Physical Layer 3
Network Protocol Stack
• Application: supporting network applications• FTP, SMTP, HTTP
• Transport: host-host data transfer• TCP, UDP
• Network: routing of datagrams from source to destination
• IP, routing protocols
• Link: data transfer between neighboring network elements
• WiFi, Ethernet
• Physical: bits “on the wire”• Radios, coaxial cable, optical fibers
application
transport
network
link
physical
1-18-06 Lecture 2: Physical Layer 4
From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream 0 0 1 0 1 1 1 0 0 0 1
Packets0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Body Header/Body Header/Body
ReceiverSenderPacket
Transmission
1-18-06 Lecture 2: Physical Layer 5
Outline
• RF introduction
• Modulation
• Antennas and signal propagation
• Equalization, diversity, channel coding
• Multiple access techniques
• Wireless systems and standards
1-18-06 Lecture 2: Physical Layer 6
Outline
• RF introduction• What is “RF”• Digital versus analog contents
• Modulation
• Antennas and signal propagation
• Equalization, diversity, channel coding
• Multiple access techniques
• Wireless systems and standards
1-18-06 Lecture 2: Physical Layer 7
RF Introduction
• RF = Radio Frequency.• Electromagnetic signal that propagates through “ether”• Ranges 3 KHz .. 300 GHz• Or 10 km .. 0.1 cm (wavelength)
• Has been used for communication for a long time, but improvements in technology have made it possible to use higher frequencies.
1-18-06 Lecture 2: Physical Layer 8
Wireless Communication
• 300 GHz is huge amount of spectrum!• Spectrum can also be reused in space
• Not quite that easy:• Most of it is hard or expensive to use!• Noise and interference limits efficiency• Most of the spectrum is allocated by FCC
• FCC controls who can use the spectrum and how it can be used.
• Need a license for most of the spectrum• Limits on power, placement of transmitters, coding, ..• Need rules to optimize benefit: guarantee emergency services,
simplify communication, return on capital investment, …
1-18-06 Lecture 2: Physical Layer 9
Spectrum Allocation
See: http://www.ntia.doc.gov/osmhome/allochrt.html
Most bands are allocated.• Industrial, Scientific, and Medical (ISM) bands are
“unlicensed”.• But still subject to various constraints on the operator, e.g. 1 W
output• 433-868 MHz (Europe)• 902-928 MHz (US)• 2.4000-2.4835 GHz• Unlicensed National Information Infrastructure (UNII) band is
5.725-5.875 GHz
1-18-06 Lecture 2: Physical Layer 10
What Is an Electromagnetic Signal
• We will be vague about this and we will use two “cartoon” views:
• Think of it as energy that radiates from an antenna and is picked up by another antenna.
• Can easily explain properties such as attenuation
• Can also view it as a “wave” that propagates between two points.
• Can easily explain properties
Space and Time
1-18-06 Lecture 2: Physical Layer 11
Decibels
• A ratio between signal powers is expressed in decibels
decibels (db) = 10log10(P1 / P2)• Is used in many contexts:
• The loss of a wireless channel• The gain of an amplifier
• Note that dB is a relative value.• Can be made absolute by picking a reference
point.• Decibel-Watt – power relative to 1W• Decibel-milliwatt – power relative to 1 milliwatt
• 4.5 mW = (10*log10 4.5) dBm
1-18-06 Lecture 2: Physical Layer 12
Analog and Digital Information
• Initial RF use was for analog information.• Radio and TV stations• The information that is sent is of a continuous nature
• In digital transmission, the signal consists of discrete units (e.g. bits).
• Data networks, cell phones• Focus of this course
• We can also send analog information as digital data.• Sample the signal, i.e. analog digital analog
• E.g., Cell phones, …• Also digital analog digital (e.g. modem)
1-18-06 Lecture 2: Physical Layer 13
Outline
• RF introduction• Modulation
• Baseband versus carrier modulation• Forms of modulation• Channel capacity
• Antennas and signal propagation• Equalization, diversity, channel coding• Multiple access techniques• Wireless systems and standards
1-18-06 Lecture 2: Physical Layer 14
The Frequency Domain
• A (periodic) signal can be viewed as a sum of sine waves of different strengths.
• Corresponds to energy at a certain frequency• Every signal has an equivalent representation in the
frequency domain.• What frequencies are present and what is their strength (energy)
• Again: Similar to radio and TV signals.
TimeFrequency
Am
plit
ude
1-18-06 Lecture 2: Physical Layer 15
Signal = Sum of Sine Waves
=
+ 1.3 X
+ 0.56 X
+ 1.15 X
1-18-06 Lecture 2: Physical Layer 16
Modulation
• Sender changes the nature of the signal in a way that the receiver can recognize.
• Assume a continuous information signal for now
• Amplitude modulation (AM): change the strength of the carrier according to the information.
• High values stronger signal
• Frequency (FM) and phase modulation (PM): change the frequency or phase of the signal.
• Frequency or Phase shift keying
• Digital versions are sometimes called “shift keying”.• Amplitude (ASK), Frequency (FSK) and Phase (PSK) Shift Keying
1-18-06 Lecture 2: Physical Layer 17
Amplitude and FrequencyModulation
0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 0
0 1 1 0 1 1 0 0 0 1
1-18-06 Lecture 2: Physical Layer 18
Baseband versus Carrier Modulation
• Baseband modulation: send the “bare” signal.• Use the lower part of the spectrum• Everybody competes – not attractive for wireless
• Carrier modulation: use the (information) signal to modulate a higher frequency (carrier) signal.
• Can be viewed as the product of the two signals• Corresponds to a shift in the frequency domain
1-18-06 Lecture 2: Physical Layer 19
Amplitude Carrier Modulation
Signal CarrierFrequency
ModulatedCarrier
1-18-06 Lecture 2: Physical Layer 20
Frequency Division Multiplexing:Multiple Channels
Am
plitu
de
Different CarrierFrequencies
DeterminesBandwidthof Channel
Determines Bandwidth of Link
1-18-06 Lecture 2: Physical Layer 21
Signal Bandwidth Considerations
• The more frequencies are present in a signal, the more detail can be represented in the signal.
• The signal can look “cleaner”
• Energy is distributed over a larger part of the spectrum, i.e. it consumes more (spectrum) bandwidth
• Signals with more detail can represent more bits, so in general, higher (spectrum) bandwidth translates into a higher (information) bandwidth.
1-18-06 Lecture 2: Physical Layer 22
Transmission Channel Considerations• Every medium supports
transmission in a certain frequency range.
• Outside this range, effects such as attenuation, .. degrade the signal too much
• Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band.
• Tradeoffs between cost, distance, bit rate
• As technology improves, these parameters change, even for the same wire.
• Thanks to our EE friends
Frequency
Good Bad
Signal
1-18-06 Lecture 2: Physical Layer 23
The Nyquist Limit
• A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H.
• E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second
• Assumes binary amplitude encoding
1-18-06 Lecture 2: Physical Layer 24
Past the Nyquist Limit
• More aggressive encoding can increase the channel bandwidth.
• Example: modems• Same frequency - number of symbols per second• Symbols have more possible values
pskPsk+ AM
1-18-06 Lecture 2: Physical Layer 25
Capacity of a Noisy Channel
• Can’t add infinite symbols - you have to be able to tell them apart. This is where noise comes in.
• Shannon’s theorem:• C = B x log(1 + S/N)• C: maximum capacity (bps)• B: channel bandwidth (Hz)• S/N: signal to noise ratio of the channel
• Often expressed in decibels (db). 10 log(S/N).
• Example:• Local loop bandwidth: 3200 Hz• Typical S/N: 1000 (30db)• What is the upper limit on capacity?
• Modems: Teleco internally converts to 56kbit/s digital signal, which sets a limit on B and the S/N.
1-18-06 Lecture 2: Physical Layer 26
Example: Modem Rates
100
1000
10000
100000
1975 1980 1985 1990 1995 2000
Year
Mod
em r
ate
1-18-06 Lecture 2: Physical Layer 28
Some Examples
• Differential quadrature phase shift keying• Four different phases representing a pair of bits• Used in 802.11b networks
• Quadrature Amplitude Modulation• Combines amplitude and phase modulation• Uses two amplitudes and 4 phases to represent
the value of a 3 bit sequence
1-18-06 Lecture 2: Physical Layer 29
Modulation vs. BER
• More symbols =• Higher data rate: More information per baud• Higher bit error rate: Harder to distinguish symbols
• Why useful?• 802.11b uses DBPSK (differential binary phase shift keying) for
1Mbps, and DQPSK (quadriture) for 2, 5.5, and 11. • 802.11a uses four schemes - BPSK, PSK, 16-QAM, and 64-AM,
as its rates go higher.
• Effect: If your BER / packet loss rate is too high, drop down the speed: more noise resistance.
• We’ll see in some papers later in the semester that this means noise resistance isn’t always linear with speed.
1-18-06 Lecture 2: Physical Layer 30
Outline
• RF introduction
• Modulation
• Antennas and signal propagation• How do antennas work• Propagation properties of RF signals
• Equalization, diversity, channel coding
• Multiple access techniques
• Wireless systems and standards
1-18-06 Lecture 2: Physical Layer 31
What is an Antenna?
• Conductor that carries an electrical signal and radiates an RF signal.
• The RF signal “is a copy of” the electrical signal in the conductor
• Also the inverse process: RF signals are “captured” by the antenna and create an electrical signal in the conductor.
• This signal can be interpreted (i.e. decoded)
• Efficiency of the antenna depends on its size, relative to the wavelength of the signal.
• E.g. half a wavelength
1-18-06 Lecture 2: Physical Layer 32
Types of Antennas
• Abstract view: antenna is a point source that radiates with the same power level in all directions – omni-directional or isotropic.
• Not common – shape of the conductor tends to create a specific radiation pattern
• Note that isotropic antennas are not very efficient!!• Unless you have a very large number of receivers
• Shaped antennas can be used to direct the energy in a certain direction.
• Well-known case: a parabolic antenna• Pringles boxes are cheaper
1-18-06 Lecture 2: Physical Layer 33
Antennas and Attenuation
• Isotropic Radiator: A theoretical antenna• Perfectly spherical radiation.• Used for reference and FCC regulations.
• Dipole antenna (vertical wire)• Radiation pattern like a doughnut
• Parabolic antenna• Radiation pattern like a long balloon
• Yagi antenna (common in 802.11)• Looks like |--|--|--|--|--|--|• Directional, pretty much like a parabolic reflector
1-18-06 Lecture 2: Physical Layer 34
Multi-element Antennas
• Multi-element antennas have multiple, independently controlled conductors.
• Signal is the sum of the individual signals transmitted (or received) by each element
• Can electronically direct the RF signal by sending different versions of the signal to each element.
• For example, change the phase in two-element array.
• Covers a lot of different types of antennas.
• Number of elements, relative position of the elements, control over the signals, …
1-18-06 Lecture 2: Physical Layer 35
Directional Antenna Properties
• dBi: antenna gain in dB relative to an isotropic antenna with the same power.
• Example: an 8 dBi Yagi antenna has a gain of a factor of 6.3 (8 db = 10 log 6.3)
1-18-06 Lecture 2: Physical Layer 36
Antennas
• Spatial reuse:• Directional antennas allow more communication in same 3D space
• Gain:• Focus RF energy in a certain direction• Works for both transmission and reception
• Frequency specific• Frequency range dependant on length / design of antenna, relative
to wavelength.
• FCC bit: Effective Isotropic Radiated Power. (EIRP).• Favors directionality. E.g., you can use an 8dB gain
antenna b/c of spatial characteristics, but not always an 8dB amplifier.
1-18-06 Lecture 2: Physical Layer 37
Propagation Modes
• Line-of-sight (LOS) propagation.• Most common form of propagation• Happens above ~ 30 MHz• Subject to many forms of degradation (next set of
slides)• Ground-wave propagation.
• More or less follows the contour of the earth• For frequencies up to about 2 MHz, e.g. AM radio
• Sky wave propagation.• Signal “bounces” off the ionosphere back to earth – can
go multiple hops• Used for amateur radio and international broadcasts
1-18-06 Lecture 2: Physical Layer 38
Limits to Speed and Distance
• Noise: “random” energy is added to the signal
• Attenuation: some of the energy in the signal leaks away
• Dispersion: attenuation and propagation speed are frequency dependent.
• Changes the shape of the signal
1-18-06 Lecture 2: Physical Layer 39
Propagation Degrades RF Signals
• Attenuation in free space: signal gets weaker as it travels over longer distances.
• Radio signal spreads out – free space loss• Absorption
• Obstacles can weaken signal through absorption or reflection.
• Part of the signal is redirected• Multi-path effects: multiple copies of the signal interfere
with each other.• Similar to an unplanned directional antenna
• Mobility: moving receiver causes another form of self interference.
• Receiver moves ½ wavelength -> big change in wavelength
1-18-06 Lecture 2: Physical Layer 40
Refraction
• Speed of EM signals depends on the density of the material.
• Vacuum: 3 x 108 m/sec• Denser: slower
• Density is captured by refractive index.
• Explains “bending” of signals in some environments.
• E.g. sky wave propagation• But also local, small scale
differences in the air
denser
1-18-06 Lecture 2: Physical Layer 41
Free Space Loss
Loss = Pt / Pr = (4 d)2 / (Gr Gt 2)
• Loss increases quickly with distance (d2).• Need to consider the gain of the antennas at
transmitter and receiver.• Loss depends on frequency: higher loss with
higher frequency.• But careful: antenna gain depends on frequency too
• For fixed antenna area, loss decreases with frequency• Can cause distortion of signal for wide-band signals
1-18-06 Lecture 2: Physical Layer 42
Other LOS Factors
• There are many noise sources.• Thermal noise: caused by agitation of the electrons• Intermodulation noise: result of mixing signals;
appears at f1 + f2 and f1 – f2
• Cross talk: picking up other signals (i.e. from other source-destination pairs)
• Impulse noise: irregular pulses of high amplitude and short duration – harder to deal with
• Absorption of energy in the atmosphere.• Very serious at specific frequencies, e.g. water vapor
(22 GHz) and oxygen (60 GHz)• Obviously objects also absorb
FairlyPredictableCan be planned foror avoided
1-18-06 Lecture 2: Physical Layer 43
Propagation Mechanisms
• Besides line of sight, signal can reach receiver in three other “indirect” ways.
• Reflection: signal is reflected from a large object.
• Diffraction: signal is scattered by the edge of a large object – “bends”.
• Scattering: signal is scattered by an object that is small relative to the wavelength.
1-18-06 Lecture 2: Physical Layer 44
Multipath Effects
• Receiver receives multiple copies of the signal, each following a different path
• Copies can either strengthen or weaken each other.
• Depends on whether they are in our out of phase
• Small changes in location can result in big changes in signal strength.
• Short wavelengths, e.g. 2.4 GHz 12 cm
• Difference in path length can cause inter-symbol interference (ISI).
1-18-06 Lecture 2: Physical Layer 45
Example
1-18-06 Lecture 2: Physical Layer 47
Fading in the Mobile Environment
• Fading: time variation of the received signal strength caused by changes in the transmission medium or paths.
• Rain, moving objects, moving sender/receiver, …
• Fast versus slow fading.• Fast: changes in distance of about half a wavelength – result in big
fluctuations in the instantaneous power• Slow: changes in larger distances affects the paths – result in a
change in the average power levels around which the fast fading takes place
• Selective versus non-selective (flat) fading.• Does the fading affect all frequency components equally• Region of interest is the spectrum used by the channel
1-18-06 Lecture 2: Physical Layer 48
Fading - Example
• Frequency of 910 MHz or wavelength of about 33 cm
1-18-06 Lecture 2: Physical Layer 49
Fading Channel Models
• Statistical distribution that captures the properties of classes of fading channels.
• Raleigh distribution: multiple indirect paths but no dominating, direct LOS path.
• E.g. urban environment with large cells, in buildings
• Ricean distribution: LOS path plus indirect paths.
• Open space or small cells
1-18-06 Lecture 2: Physical Layer 50
Wireless Technologies
• Great technology: no wires to install, convenient mobility, ..• High attenuation limits distances.
• Wave propagates out as a sphere• Signal strength reduces quickly (1/distance)3
• High noise due to interference from other transmitters.• Use MAC and other rules to limit interference• Aggressive encoding techniques to make signal less sensitive to
noise• Other effects: multipath fading, security, ..• Ether has limited bandwidth.
• Try to maximize its use• Government oversight to control use
1-18-06 Lecture 2: Physical Layer 51
Next Lecture
• RF introduction
• Modulation
• Antennas and signal propagation
• Equalization, diversity, channel coding
• Multiple access techniques
• Wireless systems and standards