Physical Layer Harmonic bandwidth and bit rate Transmission
Physical Layer Harmonic, bandwidth and bit rate Transmission media Analog and digital communication. Modulation and multiplexing Delay analysis Example systems 1 Ying-Dar Lin@CIS. NCTU
A Bit Stream and Its Harmonics 1 0 1 1 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 one harmonic 0 T 1 2 Ying-Dar Lin@CIS. NCTU
Harmonics and Data Rate • For a bit rate of 300 bps , the time to transmit the 8 -bit stream is 8/300 and the frequency of the first harmonic is 300/8 Hz. • Given a bandwidth(cut off frequency)of 3000 Hz, the number of highest harmonic passed through is 3000/(300/8)=80. • However, if the bit rate is driven to 9600 bps, the number is dropped to 3000/(9600/8)=2, which results in bit stream signal difficult to recognize. • Lesson: Limiting the bandwidth limits the data rate. However, multi-bit-baud coding schemes can achieve higher bit rates. 3 Ying-Dar Lin@CIS. NCTU
Nyquist’s Theorem for a Noiseless Channel For a noiseless channel with a low-pass filter of bandwidth H and signal of V discrete levels max data rate=2 H log 2 V bps. That is, a sampling rate that doubles the bandwidth of the channel would be enough. 4 Ying-Dar Lin@CIS. NCTU
Shannon’s Theorem for a Noisy Channel For a noisy channel with a low-pass filter of bandwidth H and whose signal-to-noise ratio is S/N, S max data rate=H log 2(1+ N ) bps, regardless how many signal levels are used and how frequent samples are taken. Note: S =10 N S =100 N 10 log 10 S =10 d. B; N S =30 d. B; N 5 S =100 N S =2 N S 10 log 10 =20 d. B, N S 10 log 10 =3 d. B N Ying-Dar Lin@CIS. NCTU
Transmission Media 2 6 Ying-Dar Lin@CIS. NCTU
Electromagnetic Spectrum 100 102 104 106 108 f(Hz) ratio 1010 1012 Microwave 1014 Infrared 1016 1018 1020 1022 UV X-ray 1024 Gamma Ray visible light higher frequency: 4 higher bandwidth ( f=c / 2 ) 4 harder to produce and modulate 4 more difficult to propagate through buildings (less omnidirectional) 4 more dangerous to living things 7 Ying-Dar Lin@CIS. NCTU
Wireless Transmission I/S/M: Industrial/Scientific/Medical 8 Ying-Dar Lin@CIS. NCTU
Analog and Digital Communication 4 Information (content) 4 Data (form) 4 Signaling (encoding) 4 Transmission (processing) 9 Ying-Dar Lin@CIS. NCTU
Amplifier, Repeater, Modem, Codec Amplifier Analog signal (analog or digital data) Analog signal (may suffer distortion) (analog transmission) Amplifier Analog signal (no distortion) (digital transmission) (data recovered , signal regenerated) Digital signal (analog or digital data) Repeater Digital signal (no distortion) (digital transmission) (signal recovered and regenerated) Digital data (binary voltage pulses) Modem Analog signal Analog data (analog signal) Codec Analog signal 10 Ying-Dar Lin@CIS. NCTU
Modulation and Multiplexing 4 Attenuation, delay distortion, noise (analog vs. digital signal) 4 Amplitude/frequency/phase modulation and QAM (for modem) 4 PCM (for codec) and Manchester encoding 4 Frequency/wavelength/time-division multiplexing 11 Ying-Dar Lin@CIS. NCTU
Attenuation, Delay Distortion, Noise ( Analog vs. Digital Signal ) r. Digital signal has a series of Fourier components (frequencies) which are attenuated by different amount and travel at different speeds. Impulse noise wipes out bits in digital signal. r. Analog signal is more robust to attenuation and delay distortion. r. Thus, modulation techniques to transform digital signal to analog signal are required. 12 12 Ying-Dar Lin@CIS. NCTU
Amplitude/Frequency/Phase Modulation and QAM (for modem) q Amplitude modulation: 0 keying)1 no signal (Amplitude shift 0 (lower frequency) q Frequency modulation: (higher frequency) (Frequency shift keying)1 q Phase modulation phase change when 0 1 or 1 0 (Phase shift keying) q QAM(Quadrature Amplitude Modulation): ITU V. 32 (9600 bps): 4 bits/baud 45 ITU V. 32 bis (14, 400 bps): 6 bits/baud 15 o o (4 bits/baud modulation) (16 combinations of amplitude and phase shift) 13 Ying-Dar Lin@CIS. NCTU
PCM (for codec) and Manchester Encoding n PCM (Pulse Code Modulation) 8 bits/sample x 8000 samples/sec=64 kbps n Manchester encoding: 0 1 n Differential Manchester encoding: 0 1 1 transition lack of transition 0 1 1 transition no transition 14 Ying-Dar Lin@CIS. NCTU
Frequency/Wavelength/Time-Division Multiplexing source 1 source 2 source n M U X channel 1 (f 1) channel 2 (f 2) channel n (fn) 1 2 n 1 2 FDM: Using part of the resource all of the time n TDM: Using all of the resource part of the time WDM: optical, similar to FDM except it is completely passive. 15 Ying-Dar Lin@CIS. NCTU
Delay Analysis transmission time propagation time queueing delay processing time End-to-end delay= i (packet transmission time +propagation time +queueing delay +processing time) at hop i 16 Ying-Dar Lin@CIS. NCTU
Example Systems Telephone ADSL FTTC FTTH HFC N-ISDN B-ISDN Cellular Carrier P/S or C/S status twisted pair C/S daily use twisted pair fiber&twisted pair fiber&coax twisted pair fiber/coax/twisted pair (SONET/T 3/T 1) microwave both any both hybrid(ATM) almost C/S Satellite microwave C/S Internet various P/S 17 trials operational trial testbeds daily use(AMPS, CT 2, DECT, GSM, IS-54, IS-95, etc. ) Geo: operational Leo: launching(Iridium) daily use Ying-Dar Lin@CIS. NCTU
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