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Physical Layer

Physical Layer. IS250 Spring 2010 chuang@ischool.berkeley.edu. Summary. Physical layer is concerned with the communication of data encoded as signals transmitted over a medium Fundamental techniques: encoding, modulation, multiplexing

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Physical Layer

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  1. Physical Layer IS250 Spring 2010 chuang@ischool.berkeley.edu

  2. Summary • Physical layer is concerned with the communication of dataencoded as signals transmitted over a medium • Fundamental techniques: encoding, modulation, multiplexing • Channel capacity influenced by hardware bandwidth, encoding scheme, transmission impairments (noise and attenuation)

  3. Outline • Fundamental concepts • Data, signal, transmission (Ch. 5) • Transmission media (Ch. 7) • Multiplexing (Ch. 11) • Transmission impairments (Ch. 8.2) • Data encoding (Ch. 6, 10) • Channel capacity (Ch. 7)

  4. Communication System • Transmitter, receiver, medium http://i.ehow.com/images/GlobalPhoto/Articles/4996474/illustration-main_Full.jpg

  5. Communication System • Transmitter, receiver, medium • Data, Signal, Transmission • Data: entities that convey meaning (can be digital or analog) • Signals: electric or electromagnetic representations of data (can be digital or analog) • Transmission: communication of data by propagation and processing of signals

  6. Data and Signal • Digital data, digital signal • Analog data, digital signal • Digital data, analog signal • Analog data, analog signal Data

  7. Transmission Media • Guided (wired): twisted pair, coaxial cable, optical fiber • Unguided (wireless): RF, microwave (terrestrial & satellite), infra-red

  8. Frequencies you may be using today • Radio: 535-1605kHz (AM); 88-108MHz (FM) • TV: 54-88MHz; 174-216MHz; 470-806MHz • Cell phones: 850, 900, 1800, 1900MHz • Cordless phones: 900MHz, 2.4GHz, 5.8GHz • Wi-Fi: 2.4GHz (802.11b/g); 5GHz (802.11a) • Q: how do radio/tv stations and receivers, cell phones and towers, etc., share the airwaves? • Q: how are 500 channels of TV programming sent over the cable?

  9. Multiplexing • Combining multiple data streams into a single signal • Allows resource sharing (e.g., of a communication channel) • Many different forms of multiplexing • Time division multiplexing (TDM) • GSM, SONET • Frequency division multiplexing (FDM) • Applications: Broadcast radio/TV, DSL • Wave division multiplexing (WDM) for fiber optic communication • Orthogonal FDM (OFDM) used in DSL, 802.11, 802.16, etc. • Spread spectrum • Flavors: Frequency hopping (FHSS), direct sequence (DSSS) • Transmitter & receiver coordinates via pseudo-random number generator • Basis for CDMA (code-division multiple access) technologies • Spatial multiplexing • e.g., wireless MIMO antennae used in 802.11n

  10. Outline • Fundamental concepts • Data, signal, transmission (Ch. 5) • Transmission media (Ch. 7) • Multiplexing (Ch. 11) • Transmission impairments (Ch. 8.2) • Data encoding (Ch. 6, 10) • Channel capacity (Ch. 7)

  11. Transmission Impairments • Signal received may differ from signal transmitted • Analog transmission: degradation of signal quality • Digital transmission: bit errors • Causes • Attenuation • Noise Source: http://www.telebyteusa.com/primer/fig9.gif

  12. Attenuation and Noise • Attenuation • Signal strength falls off with distance • Received signal strength: • must be enough to be detected • must be sufficiently higher than noise to be received without error • Attenuation is an increasing function of frequency • Noise: additional signals inserted between transmitter and receiver • Thermal: thermal agitation of electrons (also called “white noise”) • Intermodulation: signals that are the sum and difference of original frequencies sharing a medium • Crosstalk: signal from one line is picked up by another • Impulse: irregular pulses or spikes that are high in amplitude and short in duration, e.g., external electromagnetic interference

  13. Analog v. Digital Transmission • Digital transmission better than analog transmission in supporting long distance communication. Why? • Analog signal transmitted without regard to content • Signal is subject to attenuation and noise • Amplifiers can be used to boost signal strength, but noise is also amplified • Digital transmission involves processing of content • Signal is subject to attenuation and noise • Repeaters can be used to boost signal strength • Repeater receives signal, extracts bit pattern, retransmits clean signal without noise • Attenuation is overcome, and noise is not amplified

  14. Outline • Fundamental concepts • Data, signal, transmission (Ch. 5) • Transmission media (Ch. 7) • Multiplexing (Ch. 11) • Transmission impairments (Ch. 8.2) • Data encoding (Ch. 6, 10) • Channel capacity (Ch. 7)

  15. Encoding Techniques • Digital data, digital signal • Analog data, digital signal • Digital data, analog signal • Analog data, analog signal Data

  16. 3.2v 0v 1. Digital Data, Digital Signal • Digital signal as discrete, discontinuous voltage pulses • Binary data encoded into signal elements • Bit duration (function of data rate), voltage levels have to be specified • Example 1: RS-232 • Example 2: USB • USB uses NRZI (non-return-to-zero inverted) encoding • Presence of transition encodes a “1” • Absence of transition encodes a “0” • Data rates: 1.5Mbps, 12Mbps, 480Mbps

  17. 2. Analog Data, Digital Signal • Step 1: convert analog data into digital data via samplingand quantization (e.g., pulse code modulation) • Example: 4-bit PCM • Analog data input (in red) • 16 quantized levels can be represented using 4 bits • Therefore each sample converted into 4 binary bits • Digital data output: 1001101111001101111011101111… • Step 2: digital data can then be transmitted using digital encoding schemes (previous slide) • Variations: delta PCM, adaptive DPCM

  18. 3. Analog Data, Analog Signals • Example: broadcast radio, TV • Carrier signal modulated by analog data • Types of analog modulation • Amplitude modulation (AM) • Frequency modulation (FM) • Phase modulation (PM) • Why modulate analog signals? • Higher frequency can give more efficient transmission • Permits frequency division multiplexing by using different carrier frequencies for different channels (see slide on multiplexing) carrier data

  19. 4. Digital Data, Analog Signal • Example: using a modem (modulator-demodulator) to send data over analog public telephone system • Digital Modulation very similar to Analog Modulation: • ASK (amplitude shift keying): values represented by different amplitudes of carrier • Usually, one amplitude is zero, i.e., detect presence or absence of carrier • FSK (frequency shift keying): values represented by different frequencies (near carrier) • PSK (phase shift keying): phase of carrier signal shifted to represent data • Can be combined: e.g., QAM (quadrature amplitude modulation) is combination of ASK and PSK

  20. Outline • Fundamental concepts • Data, signal, transmission (Ch. 5) • Transmission media (Ch. 7) • Multiplexing (Ch. 11) • Transmission impairments (Ch. 8.2) • Data encoding (Ch. 6, 10) • Channel capacity (Ch. 7)

  21. Channel Capacity • Hardware cannot change signal states (e.g., voltage levels) instantaneously  transmission systems have limited bandwidth • Bandwidth (B): maximum rate that the hardware can change a signal (measured in Hertz, or cycles per second) • Data rate (D): rate at which data can be communicated (measured in bits per second) • Channel capacity (C): maximum data rate, which is determined by hardware bandwidth

  22. Channel Capacity • Nyquist (1928): D < 2B • the number of independent pulses that could be put through a telegraph channel per unit time is limited to twice the bandwidth of the channel • Hartley (1928): D < 2B log2(K) • where K is the number of distinct messages that can be sent • Nyquist result is special case of K=2

  23. Exampledial-up modem w QAM (Comer 10) • B = 2400Hz • V.32 modem: • K = 32 • D < 2*2400*log232 = 24000bps • V.32bis modem: • K = 128 • D < 2*2400*log2128 = 33600bps • But these modems can only support data rates of 9600bps and 14400bps, respectively. Why?

  24. Shannon’s Theorem (1948) • Channel capacity in the presence of noise: C = B log2(1+S/N) Where • C is effective channel capacity • B is hardware bandwidth • S/N is the Signal-to-Noise Ratio

  25. Decibels (dB) • Engineers like to express signal-to-noise ratio in decibels (dB) using the following quantity: 10log10(S/N) • Example: a signal-to-noise ratio of 100 is expressed as 20dB • Example: a signal-to-noise ratio of 30dB is the same as 10^(30/10) or 1000

  26. Application • Conventional telephone system • Engineered for voice • Bandwidth is 3000Hz • SNR ~= 30dB • Effective capacity is: 3000log2(1+1000) ~= 30000bps • Conclusion (Comer, p.130): dial-up modems have little hope of exceeding 28.8Kbps • Q: So what about those 56k modems?

  27. Implications • Nyquist/Hartley: encoding more bits per cycle will improve data rate • Shannon: no amount of clever engineering can overcome the fundamental physical limits of a real transmission system

  28. Summary • Physical layer is concerned with the communication of dataencoded as signals transmitted over a medium • Fundamental techniques: encoding, modulation, multiplexing • Channel capacity influenced by hardware bandwidth, encoding scheme, transmission impairments (noise and attenuation)

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