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Physical Layer Sending raw bits across “the wire”. Issues: What’s being transmitted. Transmission medium. Basic Concepts Signal: electro-magnetic wave carrying information. Time domain: signal as a function of time. Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities. Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave). Time Domain Periodic signals: Same signal pattern repeats over time. Example: sine wave Amplitude (A) Period (or frequency) (T = 1/f) Phase(φ) s ( t ) = A sin( 2 Π ft + φ ) s (t + T ) = s (t ) Frequency Domain Signal consists of components of different frequencies. Spectrum of signal: range of frequencies signal contains. Absolute bandwidth: width of signal’s spectrum. Example: s (t ) = sin( 2Π f1t ) + 1 / 3 sin( 2Π (3 f1 )t ) S(f) 1 2 3 Spectrum of S(f) extends from f1 to 3f1. Bandwidth is 2f1. f Bandwidth and Data Rate Data rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal). Example: 2Mbits/sec, or 2Mbps. Digital signal has infinite frequency components, thus infinite bandwidth. If data rate of signal is W bps, good representation achieved with 2W Hz bandwidth. Baud versus Data Rate Baud rate: number of times per second signal changes its value (voltage). Each value might “carry” more than 1 bit. Example: 8 values of voltage (0..7); each value conveys 3 bits, ie, number of bits = log2V. Thus, bit rate = log2V * baud rate. For 2 levels, bit rate = baud rate. Data Transmission 1 Analog and digital transmission. Example of analog data: voice and video. Example of digital data: character strings Use of codes to represent characters as sequence of bits (e.g., ASCII). Historically, communication infrastructure for analog transmission. Digital data needed to be converted: modems (modulator-demodulator). Digital Transmission Current trend: digital transmission. Cost efficient: advances in digital circuitry (VLSI). Advantages: Data integrity: better noise immunity. Security: easier to integrate encryption algorithms. Channel utilization: higher degree of multiplexing (time-division mux’ing). Transmission Impairments Cause received signal to differ from original, transmitted signal. Analog data: quality degradation Digital data: bit errors. Types of impairments: Attenuation. Delay distortion. Noise. Attenuation 1 Weakening of the signal’s power as it propagates through medium. Function of medium type Guided medium: logarithmic with distance. Unguided medium: more complex (function of distance and atmospheric conditions). Attenuation 2 Problems and solutions: Insufficient signal strength for receiver to interpret it: use amplifiers/repeaters to boost/regenerate signal. Error due to noise interference (level is not high enough to be distinguished from noise): use amplifiers/repeaters. Attenuation increases with frequency: special amplifiers to amplify high-frequencies. Delay Distortion Speed of propagation in guided media varies with frequency. Different frequency components arrive at receiver at different times. Solution: equalization techniques to equalize distortion for different frequencies. Noise Noise: undesired signals inserted anywhere in the source/destination path. Different categories: thermal (white), crosstalk, impulse, etc. Decibel and Signal-to-Noise Ratio Decibel (dB): measures relative strength of 2 signals. Example: S1 and S2 with powers P1 and P2. NdB = 10 log10 (P1/P2) Signal-to-noise ratio (S/N): Measures signal quality. S/NdB = 10 log10 (signal power/noise power) Channel Capacity 1 Rate at which data can be transmitted over communication channel. Noise-free channel: Nyquist Theorem Limitation of data rate is signal’s bandwidth. Given channel bandwidth W, highest signal rate (or baud rate) is 2W. From receiver’s point of view: sampling at rate 2W can reconstruct signal. Channel Capacity 2 Using data rate, C = 2W log2V, where V is number voltage levels. Same bandwidth, increasing number of signal levels, increases data rate, but more complex signal recognition at receiver and more noise-prone. This is a theoretical upper bound, since channels are noisy. Channel Capacity 3 Noisy channel: Shannon’s Theorem Given channel with W (Hz) bandwidth and S/N (dB) signal-to-noise ratio, C (bps) is C = W log2 (1+S/N) Theoretical upper bound since assumes only thermal noise (no impulse noise, etc). Transmission Media Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. Types of media: Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber. Unguided: “wireless” transmission (atmosphere, outer space). Guided Media: Examples 1 Twisted Pair: 2 insulated copper wires arranged in regular spiral. Typically, several of these pairs are bundled into a cable. Cheapest and most widely used; limited in distance, bandwidth, and data rate. Applications: telephone system (home-local exchange connection). Unshielded and shielded twisted pair. Examples 2 Coaxial Cable Hollow outer cylinder conductor surrounding inner wire conductor; dielectric (non-conducting) material in the middle. Applications: cable TV, long-distance telephone system, LANs. +’s: Higher data rates and frequencies, better interference and crosstalk immunity. -’s: Attenuation and thermal noise. Examples 3 Optical Fiber Thin, flexible cable that conducts optical waves. Applications: long-distance telecommunications, LANs. +’s: greater capacity, smaller and lighter, lower attenuation, better isolation, Unguided, Wireless Media Microwave: directional, LOS transmission. Satellite: directional, LOS, large delay, high bandwidth. Radio: omnidirectional (broadcast), single hop (cellular), multi-hop (ad hoc net’s). Infrared: directional, LOS transmission, cannot penetrate obstacles and used outdoors. Data Encoding Transforming original signal just before transmission. Both analog and digital data can be encoded into either analog or digital signals. Digital/Analog Encoding Encoding: g(t) (D/A) Encoder g(t) Digital Medium Source Source System Decoder Destination Destination System Modulation: g(t) g(t) (D/A) Modulator Source Source System Analog Medium Demodulator Destination Destination System Encoding Considerations Digital signaling can use modern digital transmission infrastructure. Some media like fiber and unguided media only carry analog signals. Analog-to-analog conversion used to shift signal to use another portion of spectrum for better channel utilization (frequency division mux’ing). Digital Transmission Terminology Data element: bit. Signaling element: encoding of data element for transmission. Unipolar signaling: signaling elements have same polarization (all + or all -). Polar signaling: different polarization for different elements. More Terminology Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec. Modulation rate = baud rate (rate at which signal levels change). Digital Transmission: ReceiverSide Issues Clocking: determining the beginning and end of each bit. Transmitting long sequences of 0’s or 1’s can cause synchronization problems. Signal level: determining whether the signal represents the high (logic 1) or low (logic 0) levels. S/N ratio is a factor. Comparing Digital Encoding Techniques Signal spectrum: high frequency means high bandwidth required for transmission. Clocking: transmitted signal should be selfclocking. Error detection: built in the encoding scheme. Noise immunity: low bit error rate. Digital-to-Digital Encoding Techniques Nonreturn to Zero (NRZ) Multilevel Binary Biphase Scrambling NRZ Techniques Use of 2 different voltage levels. NRZ-L: positive voltage represents one binary value; negative voltage, the other. NRZI (Nonreturn to zero, invert on ones): transition (low-to-high or high-to-low) represents “1”; no transition, “0”. NRZI is an example of differential encoding: decoding based on comparing polarity of adjacent signal elements. Multilevel Binary Use more than 2 signal levels. Bipolar-AMI: “0”: no signal; “1”: positive and negative pulse; consecutive “1”s alternate in polarity: avoid synchronization loss. Pseudoternary: opposite representation. Long sequence of 0’s or 1’s still a problem for bipolar-AMI and pseudoternary respectively. Biphase Manchester: transition in the middle of bit period. Carries data and provides clocking. Low-to-high: “1”. High-to-low: “0”. Differential Manchester: Mid-bit transition only provides clocking. “0”: transition in the beginning of bit interval. “1”: no transition. Scrambling Avoid long sequences of 0’s or 1’s. Bipolar with 8-zeros substitution (B8ZS) Inserts transitions when transmitting 8 consecutive “0”s. High-density bipolar-3 zeros (HDB3) Inserts pulses when transmitting 4 consecutive “0”s. Receiver must recognize insertions and re-generate original signal. Digital-to-Analog Encoding Transmission of digital data using analog signaling. Example: data transmission of a PTN. PTN: voice signals ranging from 300Hz to 3400 Hz. Modems: convert digital data to analog signals and back. Techniques: ASK, FSK, and PSK. Amplitude-Shift Keying 2 binary values represented by 2 amplitudes. Typically, “0” represented by absence of carrier and “1” by presence of carrier. Prone to errors caused by amplitude changes. Frequency-Shift Keying 2 binary values represented by 2 frequencies. s ( t ) = A cos( 2 Π f 1 t ), "1 " s ( t ) = A cos( 2 Π f 2 t ), " 0 " Frequencies f1 and f2 are offset from carrier frequency by same amount in opposite directions. Less error prone than ASK. Phase-Shift Keying Phase of carrier is shifted to represent data. Example: 2-phase system. s ( t ) = A cos( 2 Π f c t + Π ), "1" s ( t ) = A cos( 2 Π f c t ), "0 " Phase shift of 90o can represent more bits: aka, quadrature PSK. Analog-to-Digital Encoding Analog data transmitted as digital signal, or digitization. Codec: device used to encode and decode analog data into digital signal, and back. 2 main techniques: Pulse code modulation (PCM). Delta modulation (DM). Pulse Code Modulation 1 Based on Nyquist (or sampling) theorem: if f(t) sampled at rate > 2*signal’s highest frequency, then samples contain all the original signal’s information. Example: if voice data is limited to 4000Hz, 8000 samples/sec are sufficient to reconstruct original signal. PCM 2 Analog signal -> PAM -> PCM. PAM: pulse amplitude modulation; samples of original analog signal. PCM: quantization of PAM pulses; amplitude of PAM pulses approximated by n-bit integer; each pulse carries n bits. Delta Modulation (DM) Analog signal approximated by staircase function moving up or down by 1 quantization level every sampling interval. Bit stream produced based on derivative of analog signal (and not its amplitude): “1” if staircase goes up, “0” otherwise. Parameters: sampling rate and step size. Analog-to-Analog Encoding Combines input signal m(t) and carrier at fc producing s(t) centered at fc. Why modulate analog data? Shift signal’s frequency for effective transmission. Allows channel multiplexing: frequencydivision multiplexing. Modulation techniques: AM, FM, and PM. Amplitude Modulation (AM) Carrier serves as envelope to signal being modulated. S AM (t ) = [1 + m(t )] cos(2Πf c t ) Signal m(t) is being modulated by carrier cos(2π fct). Modulation index: ratio between amplitude of input signal to carrier. Angle Modulation FM and PM are special cases of angle modulation. FM: carrier’s amplitude kept constant while its frequency is varied according to message signal. PM: carrier’s phase varies linearly with modulating signal m(t). Spread Spectrum 1 Used to transmit analog or digital data using analog signaling. Spread information signal over wider spectrum to make jamming and eavesdropping more difficult. Popular in wireless communications Spread Spectrum 2 2 schemes: Frequency hopping: signal broadcast over random sequence of frequencies, hoping from one frequency to the next rapidly; receiver must do the same. Direct Sequence: each bit in original signal represented by series of bits in the transmitted signal. Transmission Modes Assuming serial transmission, ie, one signaling element sent at a time. Also assuming that 1 signaling element represents 1 bit. Source and receiver must be in sync. 2 schemes: asynchronous and synchronous transmission. Asynchronous Xmission 1 Avoid synchronization problem by including sync information explicitly. Character consists of a fixed number of bits, depending on the code used. Synchronization happens for every character: start (“0”) and stop (“1”) bits. Line is idle: transmits “1”. Asynchronous Xmission 2 Example: sending “ABC” in ASCII 0 10000010 1 0 01000010 1 0 110000 1 1111… Timing requirements are not strict. But problems may occur. Significant clock drifts + high data rate = reception errors. Also, 2 or more bits for synchronization: overhead! Synchronous Xmission 1 No start or stop bits. Synchronization via: Separate clock signal provided by transmitter or receiver; doesn’t work well over long distances. Embed clocking information in data signal using appropriate encoding technique such as Manchester or Differential Manchester. Synchronous Xmission 2 Need to identify start/end of data block. Block starts with preamble (8-bit flag) and may end with postamble. Other control information may be added for data link layer. 8 -bit Control flag Data 8 -bit Control flag