15 441 computer networking l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
15-441 Computer Networking PowerPoint Presentation
Download Presentation
15-441 Computer Networking

Loading in 2 Seconds...

play fullscreen
1 / 49

15-441 Computer Networking - PowerPoint PPT Presentation


  • 390 Views
  • Uploaded on

15-441 Computer Networking Lecture 2 – Physical Layer 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)

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about '15-441 Computer Networking' - andrew


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
15 441 computer networking

15-441 Computer Networking

Lecture 2 – Physical Layer

network protocols
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?

Lecture 2: Physical Layer

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

Network Protocol Stack

Lecture 2: Physical Layer

from signals to packets

Analog Signal

“Digital” Signal

0 0 1 0 1 1 1 0 0 0 1

Bit Stream

Packet

Transmission

0100010101011100101010101011101110000001111010101110101010101101011010111001

Packets

Sender

Receiver

Header/Body

Header/Body

Header/Body

From Signals to Packets

Lecture 2: Physical Layer

outline
Outline
  • RF introduction
  • Modulation
  • Antennas and signal propagation
  • Equalization, diversity, channel coding
  • Multiple access techniques
  • Wireless systems and standards

Lecture 2: Physical Layer

outline6
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

Lecture 2: Physical Layer

rf introduction
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.

Lecture 2: Physical Layer

wireless communication
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, …

Lecture 2: Physical Layer

spectrum allocation
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

Lecture 2: Physical Layer

what is an electromagnetic signal
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

Lecture 2: Physical Layer

decibels
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

Lecture 2: Physical Layer

analog and digital information
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)

Lecture 2: Physical Layer

outline13
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

Lecture 2: Physical Layer

the frequency domain
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.

Amplitude

Time

Frequency

Lecture 2: Physical Layer

signal sum of sine waves
Signal = Sum of Sine Waves

=

+ 1.3 X

+ 0.56 X

+ 1.15 X

Lecture 2: Physical Layer

modulation
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

Lecture 2: Physical Layer

amplitude and frequency modulation
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

Lecture 2: Physical Layer

baseband versus carrier modulation
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

Lecture 2: Physical Layer

amplitude carrier modulation
Amplitude Carrier Modulation

Signal

Carrier

Frequency

Modulated

Carrier

Lecture 2: Physical Layer

frequency division multiplexing multiple channels
Frequency Division Multiplexing:Multiple Channels

Determines Bandwidth of Link

Amplitude

Determines

Bandwidth

of Channel

Different Carrier

Frequencies

Lecture 2: Physical Layer

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.

Signal Bandwidth Considerations

Lecture 2: Physical Layer

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

Transmission Channel Considerations

Good

Bad

Frequency

Signal

Lecture 2: Physical Layer

the nyquist limit
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

Lecture 2: Physical Layer

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

Past the Nyquist Limit

psk

Psk+ AM

Lecture 2: Physical Layer

capacity of a noisy channel
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.

Lecture 2: Physical Layer

example modem rates
Example: Modem Rates

Lecture 2: Physical Layer

some examples
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

Lecture 2: Physical Layer

modulation vs ber
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.

Lecture 2: Physical Layer

outline29
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

Lecture 2: Physical Layer

what is an antenna
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

Lecture 2: Physical Layer

types of antennas
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

Lecture 2: Physical Layer

antennas and attenuation
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

Lecture 2: Physical Layer

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, …

Multi-element Antennas

Lecture 2: Physical Layer

directional antenna properties
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)

Lecture 2: Physical Layer

antennas
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.

Lecture 2: Physical Layer

propagation modes
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

Lecture 2: Physical Layer

limits to speed and distance
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

Lecture 2: Physical Layer

propagation degrades rf signals
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

Lecture 2: Physical Layer

refraction
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

Lecture 2: Physical Layer

free space loss
Free Space Loss

Loss = Pt / Pr = (4p d)2 / (Gr Gtl2)

  • 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

Lecture 2: Physical Layer

other los factors

Fairly

  • Predictable
  • Can be

planned for

or avoided

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

Lecture 2: Physical Layer

propagation mechanisms
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.

Lecture 2: Physical Layer

multipath effects
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).

Lecture 2: Physical Layer

example
Example

Lecture 2: Physical Layer

fading in the mobile environment
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

Lecture 2: Physical Layer

fading example
Fading - Example
  • Frequency of 910 MHz or wavelength of about 33 cm

Lecture 2: Physical Layer

fading channel models
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

Lecture 2: Physical Layer

wireless technologies
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

Lecture 2: Physical Layer

next lecture
Next Lecture
  • RF introduction
  • Modulation
  • Antennas and signal propagation
  • Equalization, diversity, channel coding
  • Multiple access techniques
  • Wireless systems and standards

Lecture 2: Physical Layer