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Physical Layer

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. 1. 0 1 1 0 0 0 1 0. A Bit Stream and Its Harmonics.

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Physical Layer

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  1. 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

  2. 1 0 1 1 0 0 0 1 0 A Bit Stream and Its Harmonics 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

  3. Harmonicsand 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

  4. 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=2H log2V bps. That is,a sampling rate that doubles the bandwidth of the channel would be enough. 4 Ying-Dar Lin@CIS.NCTU

  5. Shannon’s Theorem for a Noisy Channel S N For a noisy channel with a low-pass filter of bandwidth H and whose signal-to-noise ratio is S/N, max data rate=H log2(1+ ) bps, regardless how many signal levels are used and how frequent samples are taken. Note: =10 10 log10 =10dB; =100 10 log10 =20dB, =100 10 log10 =30dB; =2 10 log10 =3dB S N S N S N S N S N S N S N S N 5 Ying-Dar Lin@CIS.NCTU

  6. Transmission Media 2 6 Ying-Dar Lin@CIS.NCTU

  7. 100 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024 Electromagnetic Spectrum f(Hz) ratio Microwave Infrared UV X-ray Gamma Ray visible light higher frequency: higher bandwidth (f=c  / 2 ) harder to produce and modulate more difficult to propagate through buildings (less omnidirectional) more dangerous to living things 7 Ying-Dar Lin@CIS.NCTU

  8. Wireless Transmission I/S/M:Industrial/Scientific/Medical 8 Ying-Dar Lin@CIS.NCTU

  9. Analog and Digital Communication • Information (content) • Data (form) • Signaling (encoding) • Transmission (processing) 9 Ying-Dar Lin@CIS.NCTU

  10. Amplifier,Repeater,Modem,Codec Analog signal Amplifier Analog signal (may suffer distortion) (analog or digital data) (analog transmission) Analog signal Amplifier Analog signal (no distortion) (digital data) (digital transmission) (data recovered , signal regenerated) Digital signal Repeater Digital signal (no distortion) (analog or digital data) (digital transmission) (signal recovered and regenerated) Digital data Modem Analog signal (binary voltage pulses) Analog data Codec Analog signal (analog signal) 10 Ying-Dar Lin@CIS.NCTU

  11. Modulation and Multiplexing • Attenuation, delay distortion, noise (analog vs. digital signal) • Amplitude/frequency/phase modulation and QAM (for modem) • PCM (for codec) and Manchester encoding • Frequency/wavelength/time-division multiplexing 11 Ying-Dar Lin@CIS.NCTU

  12. Attenuation,Delay Distortion,Noise ( Analog vs. Digital Signal ) 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. Analog signal is more robust to attenuation and delay distortion. Thus, modulation techniques to transform digital signal to analog signal are required. 12 Ying-Dar Lin@CIS.NCTU

  13. Amplitude/Frequency/Phase Modulation and QAM (for modem) 0 no signal 1 • Amplitude modulation: (Amplitude shift keying) • Frequency modulation: (Frequency shift keying) • Phase modulation (Phase shift keying) • QAM(Quadrature Amplitude Modulation): ITU V.32 (9600 bps):4 bits/baud ITU V.32 bis (14,400 bps):6 bits/baud 0 (lower frequency) 1 (higher frequency) phase change when 0 1 or 1 0 45o         15o (4 bits/baud modulation) (16 combinations of amplitude and phase shift)         13 Ying-Dar Lin@CIS.NCTU

  14. PCM (for codec) and Manchester Encoding • PCM (Pulse Code Modulation) 8 bits/sample x 8000 samples/sec=64 kbps • Manchester encoding: • Differential Manchester encoding: 0 1 1 0 0 transition 1 lack of transition transition 1 1 no transition 14 Ying-Dar Lin@CIS.NCTU

  15. Frequency/Wavelength/Time-Division Multiplexing source 1 channel 1 (f1) M U X source 2 channel 2 (f2) FDM:Using part of the resource all of the time source n channel n (fn) source 1 M U X source 2 1 2 n 1 2 n source 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

  16. Delay Analysis transmission time propagation time queueing delay processing time End-to-end delay= (packet transmission time +propagation time +queueing delay +processing time) at hop i i 16 Ying-Dar Lin@CIS.NCTU

  17. Example Systems Carrier P/S or C/S status Telephone twisted pair C/S daily use ADSL twisted pair both trials FTTC fiber&twisted pair any trials FTTH fiber any trials HFC fiber&coax both trials N-ISDN twisted pair both operational B-ISDN fiber/coax/twisted pair hybrid(ATM) trial testbeds (SONET/T3/T1) Cellular microwave almost C/S daily use(AMPS, CT2, DECT,GSM,IS-54, IS-95,etc.) Satellite microwave C/S Geo:operational Leo:launching(Iridium) Internet various P/S daily use 17 Ying-Dar Lin@CIS.NCTU

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