15 441 Computer Networking Lecture 2 Physical Layer

15 -441 Computer Networking Lecture 2 – Physical Layer 1

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 2

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 • Wi. Fi, Ethernet • Physical: bits “on the wire” • 1 -18 -06 Radios, coaxial cable, optical fibers Lecture 2: Physical Layer application transport network link physical 3

From Signals to Packets Analog Signal “Digital” Signal Bit Stream Packets Packet Transmission 1 -18 -06 0 0 1 1 1 0 0 0 1 01000101110010101110111000000111101010101011010111001 Header/Body Sender Lecture 2: Physical Layer Header/Body Receiver 4

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 5

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 6

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 7

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 8

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 9

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 10

Decibels • A ratio between signal powers is expressed in decibels (db) = 10 log 10(P 1 / P 2) • Is used in many contexts: • • The loss of a wireless channel The gain of an amplifier • Note that d. B is a relative value. • Can be made absolute by picking a reference point. • • Decibel-Watt – power relative to 1 W Decibel-milliwatt – power relative to 1 milliwatt • 1 -18 -06 4. 5 m. W = (10*log 10 4. 5) d. Bm Lecture 2: Physical Layer 11

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 12

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 13

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) Amplitude • Again: Similar to radio and TV signals. Time 1 -18 -06 Lecture 2: Physical Layer Frequency 14

Signal = Sum of Sine Waves = + 1. 3 X + 0. 56 X + 1. 15 X 1 -18 -06 Lecture 2: Physical Layer 15

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 16

Amplitude and Frequency Modulation 00110001110001110 0 1 -18 -06 1 1 0 Lecture 2: Physical Layer 0 0 1 17

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 18

Amplitude Carrier Modulation Signal 1 -18 -06 Carrier Frequency Lecture 2: Physical Layer Modulated Carrier 19

Frequency Division Multiplexing: Multiple Channels Amplitude Determines Bandwidth of Link Determines Bandwidth of Channel 1 -18 -06 Different Carrier Frequencies Lecture 2: Physical Layer 20

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 21

Transmission Channel Considerations • Every medium supports transmission in a certain frequency range. • Good Bad 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. • Frequency Thanks to our EE friends 1 -18 -06 Lecture 2: Physical Layer Signal 22

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 23

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 psk 1 -18 -06 Psk + AM Lecture 2: Physical Layer 24

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 (30 db) What is the upper limit on capacity? • 1 -18 -06 Modems: Teleco internally converts to 56 kbit/s digital signal, which sets a limit on B and the S/N. Lecture 2: Physical Layer 25

Example: Modem Rates 1 -18 -06 Lecture 2: Physical Layer 26

Some Examples • Differential quadrature phase shift keying • • Four different phases representing a pair of bits Used in 802. 11 b 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 28

Modulation vs. BER • More symbols = • • Higher data rate: More information per baud Higher bit error rate: Harder to distinguish symbols • Why useful? • • 802. 11 b uses DBPSK (differential binary phase shift keying) for 1 Mbps, and DQPSK (quadriture) for 2, 5. 5, and 11. 802. 11 a uses four schemes - BPSK, 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 29

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 30

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 31

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 32

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 33

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 34

Directional Antenna Properties • d. Bi: antenna gain in d. B relative to an isotropic antenna with the same power. • Example: an 8 d. Bi Yagi antenna has a gain of a factor of 6. 3 (8 db = 10 log 6. 3) 1 -18 -06 Lecture 2: Physical Layer 35

Antennas • Spatial reuse: • Directional antennas allow more communication in same 3 D 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 8 d. B gain antenna b/c of spatial characteristics, but not always an 8 d. B amplifier. 1 -18 -06 Lecture 2: Physical Layer 36

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 37

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 38

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 39

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. • • denser E. g. sky wave propagation But also local, small scale differences in the air 1 -18 -06 Lecture 2: Physical Layer 40

Free Space Loss = Pt / Pr = (4 p d)2 / (Gr Gt l 2) • Loss increases quickly with distance (d 2). • 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 41

Other LOS Factors • There are many noise sources. • • Thermal noise: caused by agitation of the electrons Intermodulation noise: result of mixing signals; appears at f 1 + f 2 and f 1 – f 2 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 Fairly Predictable ØCan be planned for or avoided • 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 1 -18 -06 Lecture 2: Physical Layer 42

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 43

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 44

Example 1 -18 -06 Lecture 2: Physical Layer 45

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 47

Fading - Example • Frequency of 910 MHz or wavelength of about 33 cm 1 -18 -06 Lecture 2: Physical Layer 48

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 49

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 50

Next Lecture • • • 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 51