CSE123A discussion session

Transcription

1 CSE123A discussion session 2007/01/26 Ryo Sugihara Review Physical layer (2/3): Coding sublayer Clock recovery Async/Sync coding (HW1-2) Example: Manchester coding Phase locked loops Eye pattern (HW1-3) Topics Physical layer (3/3): Media transmission sublayer Broadband coding, modulation Multiplexing Media (HW1-4) HW1 Fourier analysis, recap (HW1-1) 1

2 Where are we now? Today s topic Fourier analysis, Nyquist/Shannon limit Sublayering Coding/Decoding En/decode bit sequence bit sequence (coded) bit sequence Related to sync issue Physical layer Media transmission/reception (coded) bit seq. Waveform Media-dependent Wire, Fiber, Satellite.. Signal transmission Fundamental limits governed by physical laws Media-independent Waveform (deteriorated) waveform 2

3 Coding sublayer: Clock recovery Why do we need clock? Two issues Getting in sync For receiver to wake up Solution: Initial training bits (= preamble ) Staying in sync To cope with clock drift Solution: Use transitions Add some redundant transitions (by coding) Asynchronous coding Short frame: per-character transmission e.g.) ASCII code (char in 7bits) Ignore clock drift during the frame Time between characters is arbitrary But you need to have some interval between characters cf.) Time between bits is NOT arbitrary 3

4 Synchronous coding Long frame Cannot ignore the effect of clock drift within a frame Use transitions to resync while receiving data Adding redundant transitions Manchester: one per each bit 4-5 codes: one per 4 bits Comparison of Async/Sync coding Similarities? Both are synchronous and asynchronous Synchronous within a frame Asynchronous between frames Receiver doesn t know when the next frame comes Differences? Frame size Async: small, Sync: large Resync while receiving bits? Async: NO, Sync: YES 4

5 Synchronous coding Example: Manchester coding 2 signals for 1 bit 0: low-high, 1: high-low Always have transition Resync on every bit Receiving strategy (HW1-2) Asynchronous coding (see the lecture note for pseudocode) (1) 1bit 1/2bit (2) (3) start bit data Synchronous coding (Manchester) 1bit 1bit (4) (1) detect transition (2) wait 1/2 bit (3) sample (4) wait 1 bit; goto (3) (1) 1/4bit (2) (3) (~1bit) (4) 1/4bit (1) detect transition (2) wait 1/4 bit (3) sample (4) wait for transition; repeat from (2) 5

6 Oscilloscope Show amplitude over time Dot sweeps from left to right... and start from left again You can change the sweep speed Eye pattern (HW1-3) Eye pattern Easy way to visualize channel quality How? (blackboard) Good eye pattern? What is the ideal eye pattern? Resync on each transition ( Eager-beaver strategy ) Abrupt changes Drawbacks Susceptible to noise Receiver will not learn the sender s clock Feedback loop Make receiver learn the sender s clock How? By feedback control Gradually change clock speed See the difference between Expected clock tick Actual clock tick (measure using transition) If I m ahead, I ll slow down (& vice versa) Eventually gets to good sync with sender Takes some time to sync Reason for long preamble Phase locked loops 6

7 Sublayering Coding/Decoding En/decode bit sequence bit sequence (coded) bit sequence Related to sync issue Physical layer Media transmission/reception (coded) bit seq. Waveform Media-dependent Wire, Fiber, Satellite.. Signal transmission Fundamental limits governed by physical laws Media-independent Waveform (deteriorated) waveform Broadband coding, modulation Why do we need modulation? Modulation = changing the properties on a carrier wave of a certain frequency Modem = modulator-demodulator Depending on media, you need to use certain range of frequency e.g.) Only very high frequency wave can go through ionosphere (to reach the satellite) Also related to multiplexing FDM Broadband coding Baseband (e.g. Ethernet) Modulate baseband information on a carrier wave of a certain frequency 7

8 Modulation: examples (from Multiplexing Share single medium with multiple users freq. S1 S2 S1 freq. time S1 S2 time 8

9 Media Fiber optics Media: example (HW1-4) dispersion modal dispersion: direct signal, slow signal chromatic dispersion: due to the difference of speed for different colors ex.) direct = 50ns, slow = 70ns Sender Receiver intersymbol interference t?

10 Simple example Fourier analysis, recap (HW1-1) 1) Input: f ( t) = Asin( 2π ft +θ ) Output: g ( t) = sasin 2πft + θ + p ( ) Notice the freq. does not change Amp and phase change scale s phase shift p f freq. response freq. (Hz) phase response In general, f ( t) = A sin + i i ( 2πf t θ ) g( t) = si Ai sin 2πf it + θi + pi i Read si and pi for each fi i i ( ) f freq. (Hz) BACKUP 10

11 What does the Physical layer do? Where are we now? Important topics in transmission sublayer Fourier analysis A tool to understand channel characteristics Frequency response, phase response Describe wave as sum of sine waves Nyquist limit Fundamental limit on signalling rate signalling rate = How fast you send signal Max signalling rate (Baud rate) = 2 * bandwidth Shannon limit Fundamental limit on bit/sec Determined by bandwidth and SNR(signal-to-noise ratio) If you have wider bandwidth and/or better SNR, maximum achievable bps is higher Async/Sync coding Asynchronous coding Synchronous coding 11