Physical Layer Computer Networks Physical Layer Paolo Costa costa@cs.vu.nl http://www.cs.vu.nl/ costa Vrije Universiteit Amsterdam Provide the means to transmit bits from sender to receiver involves a lot on how to use (analog) signals for digital information Overview Theoretical background: signal transmission and Fourier analysis Transmission media wires and no wires Modulation techniques: the actual encoding (multiplexing, and switching) (Version April 23, 2008) Paolo Costa 02 - Physical Layer 1 / 94 Paolo Costa 02 - Physical Layer 2 / 94 Analog vs. Digital Transmitting Signals (1/2) We re living in a digital world, meaning that we d preferably want to send digital (i.e. two-valued) signals through wires. Wires are pretty much physical, meaning that Mother Nature will probably impose a few constraints here and there. Observation: Signals are not entirely transmitted through a wire as you would expect: The continuous signal might represent speech The discrete signal might represent binary 1s and 0s Paolo Costa 02 - Physical Layer Theoretical Background 3 / 94 Paolo Costa 02 - Physical Layer Theoretical Background 4 / 94
Transmitting Signals (2/2) Effect of frequency-dependent transmission delays: Sin Wave Definition s(t) = Asin(2πft + φ) Effect of frequency-dependent attenuation: Overall effect including noise: The sine wave is the fundamental periodic signal. A general sine wave can be represented by three parameters: peak amplitude A - the maximum value or strength of the signal over time; typically measured in volts. frequency f - the rate [in cycles per second, or Hertz (Hz)] at which the signal repeats. An equivalent parameter is the period (T) of a signal, so T = 1/f. phase φ - measure of relative position in time within a single period of a signal, illustrated subsequently Paolo Costa 02 - Physical Layer Theoretical Background 5 / 94 Paolo Costa 02 - Physical Layer Theoretical Background 6 / 94 Sin Wave Example Fourier Analysis Definition A periodic function with period T (and frequency f = 1/T ) g(t) can be written as: g(t) = 1 2 c + a n sin(2πnft) + b n cos(2πnft) n=1 n=1 Example: g(t) = n 1 k=1 2k 1 sin[(2k 1)t] (n is the number of harmonics we take into account) (a) f = 1Hz, A = 1, φ = 0 (b) f = 1Hz, A = 0.5, φ = 0 (c) f = 2Hz, A = 1, φ = 0 (d) f = 1Hz, A = 1, φ = π/4 Paolo Costa 02 - Physical Layer Theoretical Background 7 / 94 Paolo Costa 02 - Physical Layer Theoretical Background 8 / 94
Fourier Analysis Example Fourier Analysis Demo The transmission of the ASCII character b: 01100010 (a) A binary signal and its root-mean-square Fourier amplitudes a 2 n + b 2 n Let s play: http://www.phy.ntnu.edu.tw/ntnujava/index.php? topic=17 http://www.falstad.com/fourier/ (b)-(e) Successive approximations to the original signal Paolo Costa 02 - Physical Layer Theoretical Background 9 / 94 Paolo Costa 02 - Physical Layer Theoretical Background 10 / 94 Bandwidth Bandwidth Demo What does this all mean? Digital signal transmission can be thought of as being constructed as an infinite number of periodic analog signals. The quality of transmission is frequency dependent not all parts of the digital signal get through the wire as you would expect. Digital signal transmission is subject to attenuation, distortion, etc. This is partly caused by disallowing high-frequency components to pass through. The range of frequencies transmitted without being strongly attenuated is called bandwidth Let s play: http://www2.egr.uh.edu/~glover/applets/sampling/ Sampling.html Paolo Costa 02 - Physical Layer Theoretical Background 11 / 94 Paolo Costa 02 - Physical Layer Theoretical Background 12 / 94
Bandwidth Example The transmission of the ASCII character b: 01100010 (a) the original signal (b) the signal that results from a channel that allows only the first harmonic f 1 to pass through (c)-(e) reconstructed signals fro higher bandwidth channel Paolo Costa 02 - Physical Layer Theoretical Background 13 / 94 Bandwidth Example We are trying to transmit a single byte: With a bit rate of b bits/sec, it takes 8/b seconds to send a byte. i.e., 8/b seconds is the period T of the sin wave The frequency f 1 of the first harmonic is b/8 Hz; Assume maximum supported frequency is 3000 Hz. since all harmonics are multiple of the first harmonic, we have (roughly) 3, 000/(b/8) or 24, 000/b harmonics e.g., b = 300bps f 1 = 37.5Hz with 80 harmonics bps T (ms) f 1 # har. 300 26.67 37.5 80 600 13.33 75.0 40 1200 6.67 150.0 20 2400 3.33 300.0 10 4800 1.67 600.0 5 9600 0.83 1200.0 2 19200 0.42 2400.0 1 38400 0.21 4800.0 0 Paolo Costa 02 - Physical Layer Theoretical Background 14 / 94 Bandwidth Example (contd) bps T (ms) f 1 # har. 300 26.67 37.5 80 600 13.33 75.0 40 1200 6.67 150.0 20 2400 3.33 300.0 10 4800 1.67 600.0 5 9600 0.83 1200.0 2 19200 0.42 2400.0 1 38400 0.21 4800.0 0 Most telephone carriers cut off the highest frequency at 3000 Hz we can never transmit at 9600 bps (or at a higher speed) on the telephone line Question You gotta be kidding. How about my modem at 56 Kbps? We are assuming a simple encoding technique based on the fact that the line supports only two signal values. Paolo Costa 02 - Physical Layer Theoretical Background 15 / 94 Bandwidth Encoding Improvement: If there are four signal values available, we could encode 2 bits at a time: 00 0 volt 01 2 volt 10 4 volt 11 6 volt The number changes in a signal per second is called the baud. Example 1: A 2400 bauds line (modem) can make a bit rate of 9600 bps provided it uses 16 (2 4 ) signal values, i.e., 4 bits per signal: S bits S bits S bits S bits 0 0000 4 0100 8 1000 12 1100 1 0001 5 0101 9 1001 13 1101 2 0010 6 0110 10 1010 14 1110 3 0011 7 0111 11 1011 15 1111 Example 2: A 2400 bauds line (modem) can make a bit rate of 33,600 bps provided it uses 16,384 (2 14 ) signal values, i.e., 14 bits per signal. 56 kbps modems use a 8,000 baud line (4000 Hz) with 8 bits per sample (1 bit is reserved for control purpose) Paolo Costa 02 - Physical Layer Theoretical Background 16 / 94
Nyquist & Shannon Why cannot we increase the number of samples per seconds? Nyquist showed that if the cut-off frequency is H Hz, the filtered signal can be reconstructed by making 2H samples. No more, no less. maximum transmission rate = 2H log 2 V bps (where V is the number of signal values) http://www2.egr.uh.edu/~glover/applets/sampling/ Sampling.html e.g., H = 3, 000Hz and V = 2 (binary transmission over a telephone line) data rate cannot exceed 6000 bps Group Quorum System Properties Shannon showed that a noisy channel with a signal-to-noise ration S/N, has a limit with respect to the bit rate: maximum transmission rate = H log 2 (1 + S/N) bps e.g., A telephone line with H = 3000 and 10 log 10 (S/N) = 30 db, can do no better than 30 kbps, no matter how you do your encoding (excluding compression). 56 kbps is still possible because with H = 4000 and little noise but no more than 70 kbps Paolo Costa 02 - Physical Layer Theoretical Background 17 / 94 Nyquist & Shannon Shannon showed that a noisy channel with a signal-to-noise ration S/N, has a limit with respect to the bit rate: Question maximum transmission rate = H log 2 (1 + S/N) bps e.g., A telephone line with H = 3000 and 10 log 10 (S/N) = 30 db, can do no better than 30 kbps, no matter how you do your encoding (excluding compression). 56 kbps is still possible because with H = 4000 and little noise but no more than 70 kbps No way. My brand new ADSL goes at 4 Mbps! ADSL uses up to 224 4-kHz channels. With 15 bits/baud and 4000 baud, the downstream bandwidth would be 13.4 Mbps (more details later on) Paolo Costa 02 - Physical Layer Theoretical Background 18 / 94 Transmission Media Magnetic Tape Throughput vs. Latency Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway Take a standard videotape that can carry about 7 gigabytes of data. A box of 50 50 50 cm can hold about 1000 tapes, which corresponds to 7000 gigabytes. Sending such a box can be done within 24 hours, worldwide. Throughput is the rate at which information can be moved Latency is the amount of time between request and response We ve got a transmission rate of 648 Mbps! Question What is overlooked in this reasoning? We re overlooking latency. Paolo Costa 02 - Physical Layer Transmission Media 19 / 94 Paolo Costa 02 - Physical Layer Transmission Media 20 / 94
Twisted Pair (TP) Twisted pair: Two insulated copper wires, twisted like a DNA string (reduces electrical inference). Often, twisted pairs go by the bundle. Comparable to telephone wiring at home. (a) category 3: traditional phone wires, 10 Mbps Ethernet (b) category 5: 100Mbps Ethernet Twisted Pair Characteristics limited distance needs a repeater every 2-3km limited bandwidth 1MHz limited data rate For long-distance, data rates of up to a few Mbps are possible; for very short distances, data rates of up to 10 Gbps have been achieved in commercially available products. susceptible to interference and noise Paolo Costa 02 - Physical Layer Transmission Media 21 / 94 Paolo Costa 02 - Physical Layer Transmission Media 22 / 94 Coaxial Cable Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies Coaxial Cable Characteristics superior frequency characteristics to TP performance limited by attenuation and noise Coax is better than twisted pair when you need more bandwidth, but is now rapidly being replaced with fiber. amplifiers every few km bandwidth: up to 500MHz Like the one you use for your TV Set Paolo Costa 02 - Physical Layer Transmission Media 23 / 94 Paolo Costa 02 - Physical Layer Transmission Media 24 / 94
Fiber Optics Principle Fiber Optics Medium Rather than using electrical signals, we use optical ones that are passed through optical fiber. Principal working is based on the refraction property of light: When a light ray passes from one medium to another (e.g., silica to air) the ray is refracted at the boundary. e.g., the ray incident at an angle α 1 emerges at an angle β For angle of incidence above a certain critical value, the light is refracted back into the silica Paolo Costa 02 - Physical Layer Transmission Media 25 / 94 An optical fiber is a thin (2 to 125 micron), flexible medium capable of guiding an optical ray Various glasses and plastics can be used to make optical fibers. Paolo Costa 02 - Physical Layer Transmission Media 26 / 94 Fiber Optics Attenuation Fiber Optics Bandwidth As it turns out, attenuation is extremely well in optical fiber. This means that they can be used for long distances. In addition, the bandwidth is enormous. c = λ f f = c λ df dλ = c λ 2 f = c λ λ 2 The wider the range, and the shorter the wavelength, the higher the bandwidth. Fiber optics often work at λ = 1.3 10 6 with λ = 0.17 10 6 leading to 30 THz bandwidth! Paolo Costa 02 - Physical Layer Transmission Media 27 / 94 Paolo Costa 02 - Physical Layer Transmission Media 28 / 94
Fiber Connections Optical Fiber vs Copper Wire (1/2) An interface consists of a receiver (photodiode) which transforms light into electrical signals, and/or a transmitter (LED or laserdiode) Passive interface: A computer is directly connected to the optical fiber Active interface: There s an ordinary electrical repeater connected to two fiber segments and the computer: The optical fiber cable in the foreground has the equivalent information-carrying capacity of the copper cable in the background. Paolo Costa 02 - Physical Layer Transmission Media 29 / 94 Paolo Costa 02 - Physical Layer Transmission Media 30 / 94 Optical Fiber vs Copper Wire (2/2) Guided Transmission: Recap Bandwidth: Fiber can support enormous bandwidths, exactly what we need with upcoming image-based applications (video-on-demand). Attenuation: Because the attenuation in fiber is less than in copper (can you imagine why?), we don t need to boost the signal as often. In practice, fiber requires an active repeater every 30 km, copper every 5 km. External influences: That s right, no more interference from other cables, radios, power failures, etc. Crosstalk (you hearing another conversation) is out of the question. Weight: Fiber simply doesn t weigh as much. Good for backs, bones, and the use of heavy maintenance equipment. Paolo Costa 02 - Physical Layer Transmission Media 31 / 94 Paolo Costa 02 - Physical Layer Transmission Media 32 / 94
Guided Transmission: Recap Attenuation Wireless Transmission (1/5) Wireless transmission is really great for all of us who can t sit still, or feel they have to be on-line all the time. It s also convenient when wiring is needed where it can t be done, or isn t really worth the trouble (jungles, islands, mountains), or because it s just user-unfriendly (homes). signal carried in electromagnetic spectrum no physical wire bidirectional propagation environment effects: reflection obstruction by objects interference Paolo Costa 02 - Physical Layer Transmission Media 33 / 94 Paolo Costa 02 - Physical Layer Wireless Transmission 34 / 94 Wireless Transmission (2/5) Wireless transmissions travel at the speed of light (c), uses a frequency (f ) which has a wavelength (λ): c = λ f The larger the wavelength is, the longer the distance it can travel without attenuation. Also, the dispersion of higher frequencies is much lower. Wireless Transmission (2/5) Naming Question Paolo Costa 02 - Physical Layer Wireless Transmission 35 / 94 The terms LF, MF, and HF refer to low, medium, and high frequency. Nobody expected to go above 10 MHz The higher bands were later named the Very, Ultra, Super, Extremely, and Tremendously High Frequency bands. Paolo Costa 02 - Physical Layer Wireless Transmission 36 / 94
Wireless Transmission (3/5) Wireless Transmission (4/5) The amount of information that an electromagnetic wave can carry is related to its bandwidth f = c λ λ 2 The wider the range, and the shorter the wavelength, the higher the bandwidth. wireless transmission will generally have a much lower bandwidth (in practice: 50-100 Mbps maximum). Fiber optics operate in the high frequency range, which explains the transmission rates of gigabits per second. Frequency hopping: Use a wide band, but let the transmitter hop from frequency to frequency (hundreds of times per second). Good for avoiding continuous interference and reducing the effect of reflected signals (you won t be listening to them). Direct sequence: Simply spread the signal over a wide frequency band (and allow several signals with different encoding/modulation techniques to be transmitted simultaneously). Paolo Costa 02 - Physical Layer Wireless Transmission 37 / 94 Paolo Costa 02 - Physical Layer Wireless Transmission 38 / 94 Wireless Transmission (5/5) Observation: Radio transmission (VLF VHF) is extremely popular for its cheapness and range. Also, waves just go all over the place. (a) In the VLF, LF and MF (AM radio) bands, radio waves follow the curvature of the earth (b) In the HF band, they bounce off the ionosphere Paolo Costa 02 - Physical Layer Wireless Transmission 39 / 94 Microwave transmissions (100 MHz - 10 GHz) Microwave transmission is also popular for telecommunication systems It is good for long distances (repeater every 10-100 km) but requires line-of-sight transmission: unlike radio waves, they don t pass through buildings they don t tolerate the curvature of the earth (e.g., San Francisco - Amsterdam) they propagate in straight lines high signal-to-noise ration high frequency high bandwidth Problem the density in the spectrum requires higher frequency (up to 10 GHz) hard for unguided transmissions waves are few centimeters long and are absorbed by rain Paolo Costa 02 - Physical Layer Wireless Transmission 40 / 94
Multipath Fading Communication Satellites Observation: Satellites are attractive because they provide a relatively simple model of communication: one signal up can be broadcast to many receivers downwards. Taking Mother Nature into account (i.e., avoiding belts around the earth consisting of highly-charged particles that would destroy a satellite), there are three types of satellites: Some waves may be refracted and may take slightly longer to arrive The delayed waves may arrive out of phase with the direct wave and thus cancel the signal it is weather and frequency dependent Paolo Costa 02 - Physical Layer Wireless Transmission 41 / 94 Paolo Costa 02 - Physical Layer Communication Satellites 42 / 94 Geostationary Orbit Satellites Feature: GEO satellites are placed at 35,800 km above the earth where their rotational speed is the same as that of the earth. The effect is that they appear to remain motionless in the sky. VSATs: Very Small Aperture Terminals simple systems that output 1 Watt at 19.2 kbps but can download as much as 512 kbps. To allow the VSATs to communicate with each other, hubs are used: Medium-Earth Orbit Satellites Example: The Global Positioning System (GPS) orbit at 18,000 km. It takes about 6 hours for a satellite to circle the earth. They are not used for telecommunications. Paolo Costa 02 - Physical Layer Communication Satellites 43 / 94 Paolo Costa 02 - Physical Layer Communication Satellites 44 / 94
Low-Earth Orbit Satellites (1/2) Essence: We throw in a relatively large number of low-orbit satellites which jointly cover the surface of the earth; when you are out of your current satellite s spot beam, you should be in that of the next satellite (Iridium): Low-Earth Orbit Satellites (2/2) Alternative: In Globalstar, much of the complexity is handled by ground stations that pick up a connection from a satellite, and pass it on to the one closest to the receiver: Note: Iridium uses 66 satellites, each having a maximum of 48 cells (i.e., spot beams), totaling 1628 cells. Observation: This approach is virtually the same as that of cellular radio, except that the cells are moving instead of the subjects. Paolo Costa 02 - Physical Layer Communication Satellites 45 / 94 Observation: This scheme avoids much of the complexity for (managing) inter-satellite communication. Paolo Costa 02 - Physical Layer Communication Satellites 46 / 94 Where (not) to use Satellites Bandwidth: Fiber wins, but not everyone has access to all the available bandwidth. Satellites may make it easier to transfer data anyway Mobility and remote locations: Satellites win, although it isn t clear whether simple cellular techniques may do just fine Broadcasting: Satellites win easily: broadcasting essentially comes for free Fast and reliable: Give credits to fiber: satellites are pretty bad due to inherent high latency (230 ms round-trip for geostationary satellites), and too much Mother Nature (rain!) The Local Loop When it comes the telephone system, from a networking perspective the local loop (a.k.a. the last mile) is the most interesting to look at. The general structure is as follows: Paolo Costa 02 - Physical Layer Communication Satellites 47 / 94 Paolo Costa 02 - Physical Layer Telephone System 48 / 94
Modulation Techniques (1/3) Modulation techniques (2/3) Problem: How can we encode our signals when we can effectively use only a single frequency (or better: small frequency range)? Answer: Apply modulation techniques Change the amplitude (strength) of the signal: changing amplitude means a binary 1, constant amplitude a binary 0. Use different frequencies to encode your bits (these frequencies can be put on top of your base frequency). Change the phase of the wave (cf. sine and cosine) to do signal encoding. Paolo Costa 02 - Physical Layer Telephone System 49 / 94 (a) A binary signal (c) Frequency modulation (b) Amplitude modulation (d) Phase modulation Paolo Costa 02 - Physical Layer Telephone System 50 / 94 Modulation techniques (3/3) Observation: Modulation is strongly related to not being able to set a (wide-frequency-ranges) DC signal value on the wire as direct encoding of binary signals: becomes Paolo Costa 02 - Physical Layer Telephone System 51 / 94 Modem Modem derives from modulator-demodulator) modulates an analog carrier signal to encode digital information demodulates such a carrier signal to decode the transmitted information Used for digital data transmission through the public telephone network replacing the analog infrastructure would be too costly Two different frequencies are used to enable traffic in both direction (full duplex) Paolo Costa 02 - Physical Layer Telephone System 52 / 94
Increasing Transmission Rates Let s Recap... To get to higher and higher speeds, it is not possible to just increase the sampling rate Nyquist says that even with a perfect 3000 Hz line, there is no point in sampling more than 6000 Hz = most modems sample 2400 times / sec and focus on getting more bits per sample The number of samples per second is measured in baud an n-baud line transmits n symbols / sec Baud is NOT the modem s speed (but they are related) e.g., if only two symbols (e.g., 0 volt and 1 volt) are sent on a 2400 baud line, the bit rate is 2400 bps if the voltages 0, 1, 2, and 3 volts are used, each symbol consists of 2 bits and the bit rate becomes 4800 bps instead of using 4 different voltages, it is possible to use 4 phase shifts [Quadrature Phase Shift Keying (QPSK)] Bandwidth The range of frequency that pass through a given medium with minimum attenuation (measured in Hz) Baud Rate The number of samples (symbols) / sec made and is constrained by the bandwidth (Nyquist s theorem). The modulation technique (e.g., QPSK) determines the number of bits per symbol. Bit (Data) Rate The amount of information sent over the channel during a second (= number of symbols / sec number of bits / symbol) Paolo Costa 02 - Physical Layer Telephone System 53 / 94 Paolo Costa 02 - Physical Layer Telephone System 54 / 94 Modulation Techniques All modern modems use a combination of modulation techniques to transmit multiple bits per baud. Modulation Techniques All modern modems use a combination of modulation techniques to transmit multiple bits per baud. (a) Quadrature Phase Shift Keying (QPSK) The phase of a dot is indicated by the angle The amplitude is the distance from the origin 4 phases (45, 135, 225, and 315 degrees) with constant amplitude 2 bits / symbol = 4800 bps (assuming 2400 baud line) Paolo Costa 02 - Physical Layer Telephone System 55 / 94 (b) Quadrature Amplitude Modulation (QAM-16) 4 amplitudes and 4 phases 16 (2 4 ) possible combinations leading to 4 bits per symbol used to transmit 9600 bps over a 2400 baud line (c) QAM-64 8 amplitudes and 8 phases 64 (2 6 ) possible combinations leading to 6 bits per symbol used to transmit 14400 bps over a 2400 baud line Paolo Costa 02 - Physical Layer Telephone System 56 / 94
Higher Speeds With many dots, even a small amount of noise can result in an erroneous decode of phase / amplitude Add extra bits to perform error correction (Trellis Code Modulation) Higher Speeds 56K modem Standard modems stop at 33,600 because the Shannon limit for a telephone system is about 35 Kbps (a) V.32 (b) V.32 bis V.32 modem uses 32 (2 5 ) dots to transmit 4 data bits and 1 parity bit 9,600 bps with error correction at 2400 baud rotation was done only for engineering reason V.32 bis modem uses 128 (2 7 ) dots to transmit 6 data bits and 1 parity bit 14,400 bps with error correction at 2400 baud (fax modems) Higher speeds (28,800 and 33,600 bps) are achieved using 12 and 14 bits / symbol (V.34 and V.34 bis) Further improvements are made by also using compression techniques Paolo Costa 02 - Physical Layer Telephone System 57 / 94 A call from Computer to ISP 1 goes through two local loops each of them adds noise ISP 2, instead, has a digital trunk less noise = maximum data rate now becomes 70 kbps V.90 modems use 8 bits per symbol (7 data bit and 1 control bit) over a 8,000 baud line (assuming 4,000 Hz band for phone cable) 56,000 bps in downstream 33,000 bps in upstream (using V.34 to reduce noise) Paolo Costa 02 - Physical Layer Telephone System 58 / 94 Beyond 56 K Cable TV industry was offering speeds up to 10 Mbps and satellite companies were planning to offer 50 Mbps Telephone companies realized that they need to offer higher speeds to their customers to remain competitive 56 kbps were not appealing any longer The new service went under the name of xdsl (Digital Subscriber Line) the most popular is ADSL (Asymmetric DSL) Digital Subscriber Lines Traditional telephone cables are artificially limited to a 3000 Hz bandwidth good enough to carry human voice a filter in the end office attenuates all frequencies below 300 Hz and above 3400 Hz With xdsl, the incoming line of a customer is connected to a different kind of switch, that does not have any filter the limiting factor then becomes the physics of the local loop The service can be offered only within near proximity Paolo Costa 02 - Physical Layer Telephone System 59 / 94 Paolo Costa 02 - Physical Layer Telephone System 60 / 94
Asymmetric DSL Asymmetric DSL Basics Data Rate The spectrum of the local loop is about 1.1 Mhz spectrum We can divide it into 256 channels of 4,000 Hz each (like in traditional phone systems) each one plays a different role Within each channel a modulation scheme similar to V.34 is used 15 bits / baud on 4000 baud line = with 224 downstream channels, the downstream bandwidth is 13.44 Mbps! In practice, the signal-to-noise ratio is never good enough but 8 Mbps is possible on short-runs over high quality loops channel 1-5 are not used to avoid interference since most users download more data than they upload 80 % of channels are allocated to downstream this explains the A of Asymmetric different combinations are possible. Paolo Costa 02 - Physical Layer Telephone System 61 / 94 Paolo Costa 02 - Physical Layer Asymmetric DSL Asymmetric DSL Architecture Hardware Telephone System 62 / 94 ADSL Model 4: 6: 13: 17: The NID (Network Interface Device) is a small plastic box, marking the border between company s and customer s property The splitter is a filter that separates the voice band (0-4000 Hz) from the rest The ADSL modem can be seen as 250 QAM modems operating in parallel at different frequencies The DSLAM (DSL Access Multiplexer) recovers the signal into a bit stream and sends off to the ISP Paolo Costa 02 - Physical Layer Telephone System 63 / 94 CPU 8 Mb RAM Ethernet port Telephone port 5: 7: 16: JTAG interface Flash memory USB port DSLAM Paolo Costa 02 - Physical Layer Telephone System 64 / 94
Asymmetric DSL ADSL2(+) The bandwidth of the local loop has been expanded to 2.2 MHz ADSL2 allows to a maximum of 12 Mbps download speed ADSL2+ allows to a maximum of 25 Mbps, with a 3 7 km distance to be crossed by copper. Maximum upload speed is 1.2 3.5 Mbps Highly dependent on the cable length Wireless Local Loop Problem: Suppose you want to start an ISP but using the local loop is out of the question because it is owned by your competitor. Solution: Set up a wireless direct connection between one of your antennas and your subscribers (in a so-called sector): e.g., WiMax (IEEE 802.16 standard) A sector can operate at 36 Gbps downstream bandwidth and 1 Mbps upstream, to be shared by subscribers. The range is about 2 5 km. Practice: single users get 10 Mbps over 10 km. Paolo Costa 02 - Physical Layer Telephone System 65 / 94 Paolo Costa 02 - Physical Layer Telephone System 66 / 94 Multiplexing: FDM Problem: Considering that the bandwidth of a channel can be huge, wouldn t it be possible to divide the channel into sub-channels? Frequency Division Multiplexing: Divide the available bandwidth into channels through frequency filtering, and apply modulation techniques per channel: Multiplexing: FDM Example Frequency Division Multiplexing is used by Radio and TV broadcasting Station Channel Frequency Nederland 1 22 479,25 Nederland 2 25 503,25 Nederland 3 26 511,25 RTL 4 28 527,25 RTL 5 36 591,25 SBS 6 37 599,25 RTL 7 38 607,25 Veronica / Jetix 39 615,25 Net 5 40 623,25 Tien 41 631,25 Casema Service Kanaal 29 535,25 TV West 44 655,25 Eén 42 639,25 Ketnet / Canvas 43 647,25 Discovery Channel 46 671,25 National Geographic / CNBC 47 679,25 Animal Planet 48 687,25 Eurosport 54 735,25 MTV 50 703,25 Nickelodeon / The BOX 51 711,25 TMF 53 727,25 BBC 1 55 743,25 BBC 2 56 751,25 Paolo Costa 02 - Physical Layer Telephone System 67 / 94 Paolo Costa 02 - Physical Layer Telephone System 68 / 94
Multiplexing: WDM Wavelength Division Multiplexing: Actually the same as FDM, but used for fiber optics. Multiplexing: TDM Basics Time Division Multiplexing: Simply merge/split streams of digital data into a new stream. Data is handled in frames a fixed series of consecutive bits: Light waves have their own frequency range; they are simply combined and separated using standard (de)fraction properties This is a full-digital solution in contrast to FDM and WDM Paolo Costa 02 - Physical Layer Telephone System 69 / 94 Paolo Costa 02 - Physical Layer Telephone System 70 / 94 Multiplexing: TDM PCM Analog signals need to be digitalized before being eligible for TDM PCM (Pulse Code Modulation) Multiplexing: TDM T1 system Example: The T1 system samples at 8000 Hz, and encodes each sample as a 7-bit number (i.e. 128 different values). With some extra control bits, we merge samples into 193-bit frames, every 125 µsec: The magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in binary code But because the quantized values are only approximations, it is impossible to recover the original signal exactly 8 bits / sample are used for digitalized voice on the phone 16 bits / sample are used for Audio CD Paolo Costa 02 - Physical Layer Telephone System 71 / 94 Observation: T1 supports a total of 193 1 125 106 = 1.544 Mbps Paolo Costa 02 - Physical Layer Telephone System 72 / 94
Multiplexing: TDM QoS TDM also makes it easy to offer individual senders higher bandwidth, by simply putting more data into a frame: Switching Packet vs. Circuit Circuit switching: Make a true physical connection from sender to receiver. This is what happens in traditional telephone systems. Packet switching: (1) Split any data (i.e. message) into small packets, (2) route those packets separately from sender to receiver, and (3) assemble them again. or to combine several trunks into higher-bandwidth trunks: Paolo Costa 02 - Physical Layer Telephone System 73 / 94 Paolo Costa 02 - Physical Layer Telephone System 74 / 94 Switching Message Switching Switching Comparison Message (Store-and-forward) switching: a message is completely received at a router, stored, and then put into an outgoing queue for further routing analogous to packet switching but here a message must be fully received before being forwarded Note Check also what discussed in the Introduction. Paolo(a) Costa circuit-switching; (b) store-and-forward; 02 - Physical Layer (c) packet-switching Telephone System 75 / 94 Paolo Costa 02 - Physical Layer Telephone System 76 / 94
The Mobile Telephone System Overview Advanced Mobile Phone System (AMPS) Overview We will discuss the following technologies: AMPS (Advanced Mobile Phone System) first generation of mobile phones analog voice a.k.a. (E)TACS GSM (Global System for Mobile communication) digital voice digital data (EDGE and GPSR) UMTS (Universal Mobile Telecommunication System) third generation (3G) high speed digital voice and data Paolo Costa 02 - Physical Layer Mobile Telephone System 77 / 94 The whole idea is to break up an area into small regional cells, each has their own frequency range no two adjacent cells have the same frequency. Pretty good for handling different densities cells may be split in sub-cells The problem is frequency allocation and energy emission Paolo Costa 02 - Physical Layer Mobile Telephone System 78 / 94 Advanced Mobile Phone System (AMPS) Channels The AMPS uses 832 full-duplex channels, each consisting of a pair of simplex channels 832 simplex channels from 824 to 849 Mhz 832 simplex channels from 869 to 894 Mhz each channel is 30kHz wide Thus, FDM is used to separate channels Channels are used for different tasks control (base to mobile) to manage the systems paging (base to mobile) to notify calls access (bidirectional) for call setup and channel assignment data (bidirectional) for voice Since the same frequencies cannot be reused in nearby cells, the actual number of voice channels per cell is about 45 Paolo Costa 02 - Physical Layer Mobile Telephone System 79 / 94 GSM Overview GSM: Global System for Mobile communications, is a full-blown digital cellular radio transmission system FDM is used with each mobile transmitting on one frequency and receiving on a higher one also, a single frequency pair is split by time-division multiplexing into time-slots shared by multiple mobiles GSM uses 124 downlink channels, and 124 uplink channels per cell, each channel multiplexed by TDM (GSM-900): this gives 8 124 = 992 full duplex channels. a lot of them are not used to avoid interference with neighboring cells Paolo Costa 02 - Physical Layer Mobile Telephone System 80 / 94
GSM Channels GSM Data Transmission There are also separate channels for: broadcasting cell info (so that a mobile station can see whether it has changed cells). cell maintenance (the base station has to know who s in its cell). call setup (incoming, and outgoing). EDGE (Enhanced Data rates for GSM Evolution) is GSM with more bits per baud. more bits per baud also mean more errors per baud EDGE introduces nine different schemes for modulation and error detection GPSR (General Packet Radio Service) is an overlay packet network on top of GSM it allows mobile stations to send and receive IP packets when GPSR is operating, some time slots on some frequencies are reserved for packet transmission the number of these slots are dynamically managed by the base-station according to the traffic conditions Paolo Costa 02 - Physical Layer Mobile Telephone System 81 / 94 Paolo Costa 02 - Physical Layer Mobile Telephone System 82 / 94 CDMA Overview Code Division Multiple Access (CDMA) allows transmissions to be interleaved, but avoids interference. no message collisions Principle: assign a chip sequence to a station, which is just an m-bit code (each bit in this code is called chip) to transmit a 1 bit, a station sends its chip sequence to transmit a 0 bit, it sends the one s complement of the chip sequence e.g., assuming m = 8, if A is assigned the chip sequence 00011011, it sends 1 by sending 00011011 and 0 by sending 11100100 Let s introduce bipolar notation: rewrite a binary 0 as -1, and a binary 1 as +1 so, 1 bit for station A becomes ( 1 1 1 + 1 + 1 1 + 1 + 1) Paolo Costa 02 - Physical Layer Mobile Telephone System 83 / 94 CDMA Math Let s indicate with S the m chip vector for station S and S for its negation Key: chip sequences S and T belonging to two stations S and T must be orthogonal i.e., the normalized inner product must be 0 S T = 1 m m S i T i = 0 note that if S T = 0, then also S T = 0 the normalized product of any chip sequence with itself is 1: S S = 1 m m S i S i = 1 m i=1 also note that S S = 1 i=1 m (S i ) 2 = 1 m i=1 m (±1) 2 = 1 Paolo Costa 02 - Physical Layer Mobile Telephone System 84 / 94 i=1
CDMA Simultaneous Transmissions CDMA Example When two stations transmit at the same time, their signals add linearly remember that no FDM or TDM is used e.g., if three stations output +1 and one station outputs -1, the result is +2 To recover the bit stream of an individual station, say C, the receiver computes the inner product of the received chip sequence and the chip sequence of C e.g., assume two stations A and C transmitting a 1 bit at the same time that B transmits a 0 bit the receiver sees the sum S = A + B + C and computes: S C = (A + B + C) C = A C + B C + C C = 0 + 0 + 1 = 1 the first two terms vanish thanks to the orthogonality Paolo Costa 02 - Physical Layer Mobile Telephone System 85 / 94 Paolo Costa 02 - Physical Layer Mobile Telephone System 86 / 94 CDMA Example CDMA Analogy Let s make an analogy: TDM is comparable to all people being in the middle of the room but talking at different times Hidden Assumptions the receiver must known the chip sequence sender and receiver are synchronized power levels of all stations are the same as perceived by the receiver Paolo Costa 02 - Physical Layer Mobile Telephone System 87 / 94 FDM is comparable to people being in widely separated groups, each group holding separate conversations but at the same time CDMA is comparable to everybody being in the middle of the room, talking at the same time but in different languages Paolo Costa 02 - Physical Layer Mobile Telephone System 88 / 94
CDMA Bandwidth UMTS The amount of information grows from b bits / sec to mb bits / sec This can be done only if the available bandwidth is increased by a factor m CDMA uses a form of spread spectrum communication e.g., if we have 1-MHz band available for 100 stations with FDM, each one would have 10 khz and could send at 10 kbps (assuming 1 bit per Hz) with CDMA, each station uses the full 1 MHz, so the chip rate is 1 megachips / second (chip is a bit in the chip sequence= if we use less than 100 bit per chip sequence, the effective bandwidth for CDMA is higher than FDM and the channel allocation problem is solved UMTS adopts a CDMA scheme: it runs a 5 MHz bandwidth and is compatible with GSM it offers 2 Mbps for indoor stationary basestation, 384 kbps for people walking, and 144 kbps for connections in cars It is supposed to deliver the following services: High-quality voice transmissions Messaging (e-mail, fax, SMS, chat,...) Multimedia (playing music, viewing videos, films, televisions,...) Internet access on mobile phones? Paolo Costa 02 - Physical Layer Mobile Telephone System 89 / 94 Paolo Costa 02 - Physical Layer Mobile Telephone System 90 / 94 Wi-Fi: IEEE 802.11g OFDM Frequency division multiplexing (FDM) transmits multiple signals simultaneously over a single transmission medium each signal travels within its own unique frequency range (carrier) Orthogonal FDM s (OFDM) distributes the data over a large number of carriers that are spaced apart at precise frequencies the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other At the center frequency of any particular subcarrier all other subcarriers attain a null point no guardbands required no special filter to demodulate each frequency carriers Paolo Costa 02 - Physical Layer Mobile Telephone System 91 / 94 Cable Television TV cable companies are actively working to increase their market They use a mix of fiber (for the long-haul runs) and coaxial cable (to the house) = HFC (Hybrid Fiber Coax) Issue: every cable is shared among multiple users (a) while in the telephone systems every house has its own private cable (b) Paolo Costa 02 - Physical Layer Cable Television 92 / 94
Cable Television Principle There s a lot of unused bandwidth that can be allocated to sending bits over the wire: Cable Television Modem Internet access require a cable modem The downstream data always comes from one source, which makes it easier to handle. Upstream data requires that subscribers contend for available slots: Because downstream (television) starts at 54 MHz, there is limited bandwidth that can be used for upstream data most traffic, however, is likely to be downstream (like in ADSL) QAM-64 (or QAM-256 if the cable quality is really high) is used on each downstream 6-Mhz channel 36 Mbps (of which 27 Mbps for payload) data rate For upstream QAM-64 does not work well (too noisy) QPSK is used 2 bits / baud instead of 6 or 8 bits of QAM Paolo Costa 02 - Physical Layer Cable Television 93 / 94 Each modem is assigned a minislot to use to request upstream bandwidth this may be subject to contention retry mechanism Security is also a concern Paolo Costa 02 - Physical Layer Cable Television 94 / 94