WK 3 physical2

WK 3 physical2 - Physical Layer 2 Multiplexing, Master...

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Unformatted text preview: Physical Layer 2 Multiplexing, Master subtitle Click to edit Spread Spectrum style 11 Multiplexing Communication Networks of NYU © Kang Xi, Polytechnic Institute 22 Objectives n The purposes and requirements of multiplexing The principles of FDM, TDM, WDM and statistical multplexing DSL n n Communication Networks of NYU © Kang Xi, Polytechnic Institute 33 Framework n n Regard a user-user communication as a stream Multiplexer (Mux) ¨ Combine streams multiple n De-Multiplexer (DeMux) ¨ Pick out one/more streams out of the aggregation. © Kang Xi, Polytechnic Institute 44 Communication Networks of NYU Benefit n Wired networks ¨ Each wire (copper, fiber) carries multiple streams ¨ Total # of wires reduced: low cost ¨ Dedicated links between users are not necessary n Wireless networks ¨ Channels are naturally shared, dedicated transmission media unavailable ¨ Enables simultaneous wireless communication with no interference Communication Networks of NYU © Kang Xi, Polytechnic Institute 55 FDM: Example n Mux Example ¨ Two speakers ¨ One listener ¨ Listener can hear either ¨ Key: speakers have different voice (frequency) ¨ Note: not 100% FDM DeMux Communication Networks of NYU © Kang Xi, Polytechnic Institute 66 Frequency Division Multiplexing [globalspec.com] Communication Networks of NYU © Kang Xi, Polytechnic Institute 77 FDM: Principle n n n n n n n Different signals have non-overlapped spectrum a(t) = sin(t) b(t) = 5sin(t) a(t)+b(t): cannot pick out a(t) a’(t)= a(t) * sin(10t) b’(t)= b(t) * sin(20t) a’(t)+b’(t): can be separated © Kang Xi, Polytechnic Institute 88 Communication Networks of NYU FDM: Frequency Domain View n Mux ¨A number of baseband signals ¨ Modulate each signal to a high frequency ¨ Add them together n DeMux ¨ Bandpass filters ¨ Demodulators Communication Networks of NYU © Kang Xi, Polytechnic Institute 99 WDM WDM [M. Yang & C. Zhang] Input 1 Transmission fiber λ1 Input 2 λ2 λ3 Input 3 Inputs can be rate and protocol independent n Each input gets its own dedicated channel on the transmission fiber © Kang Xi, Polytechnic Institute 1010 Communication Networks of NYU TDM: Background n n High channel capacity in the time domain ¨ E.g. ¨ E.g. ¨ Split 10G 622M, 155M Low end-to-end requirement n Solution the channel into multiple slots in the time domain © Kang Xi, Polytechnic Institute 1111 Communication Networks of NYU TDM TDM Input 1 Input 2 [M. Yang & C. Zhang] Transmission Link Input 3 Inputs 1-3 places a cell every second n Resulting transmission link outputs 3 cells every second n Communication Networks of NYU © Kang Xi, Polytechnic Institute 1212 Simple TDM n n n A T B: 3 Mb/s A1 ¹ B1: 1 Mb/s A2 ¹ B2: 1 Mb/s Communication Networks of NYU © Kang Xi, Polytechnic Institute 1313 Distinguish two streams? Each one get 1.5 Mb/s Framing, indicate the position. Each get 1 Mb/s. A frame Communication Networks of NYU © Kang Xi, Polytechnic Institute 1414 Framing n Use a special sequence for the frame header (e.g., 01110011 in the yellow blocks) Scramble (randomize) the payload. © Kang Xi, Polytechnic Institute 1515 n Communication Networks of NYU TDM: Framework buffer buffer buffer buffer buffer Communication Networks of NYU © Kang Xi, Polytechnic Institute 1616 E1 Voice channel Sample frequency: 8 kHz Encoding: 8 bits Communication Networks of NYU © Kang Xi, Polytechnic Institute 1717 E1 Frame Communication Networks of NYU © Kang Xi, Polytechnic Institute 1818 The Problem of TDM n n n Fixed bandwidth allocation Low channel utilization Poor performance for bursty traffic Communication Networks of NYU © Kang Xi, Polytechnic Institute 1919 Statistical Multiplexing n n n n n Packet based channel-sharing Packet header indicates its affiliation Still time-domain multiplexing Non-periodic Packet size may be variable Communication Networks of NYU © Kang Xi, Polytechnic Institute 2020 Mux n n n Each stream has its own buffer Mux read from a certain buffer for transmission Mux determines the service time to each buffer Communication Networks of NYU © Kang Xi, Polytechnic Institute 2121 DeMux n n Examine the header of each packet Put packets into the corresponding buffer Communication Networks of NYU © Kang Xi, Polytechnic Institute 2222 Example Case 1: A2° B2 has no traffic Case 2: A2° B2 has light load Case 3: both have heavy load fair bandwidth allocation Communication Networks of NYU © Kang Xi, Polytechnic Institute 2323 DSL—Digital Subscriber Line Communication Networks of NYU © Kang Xi, Polytechnic Institute 2424 Link Bandwidth Twisted-wire remains the same Modem is different Communication Networks of NYU © Kang Xi, Polytechnic Institute 2525 DSL Bandwidth in Detail Communication Networks of NYU © Kang Xi, Polytechnic Institute 2626 Spread Spectrum Communication Networks of NYU © Kang Xi, Polytechnic Institute 2727 Objectives n n n Theoretical explanation Basic idea and framework FHSS, DSSS, CDMA Communication Networks of NYU © Kang Xi, Polytechnic Institute 2828 The Inventress: Hedy Lamarr 9 Nov., 1913 --- 19 Jan., 2000 Hollywood Star Communication Networks of NYU © Kang Xi, Polytechnic Institute 2929 Theory n Shannon Channel Capacity: ¨A wideband spectrum can be used to provide the same capacity under noisy environment. C = B log2 (1+ S/N) Communication Networks of NYU © Kang Xi, Polytechnic Institute 3030 Principle: Spectrum n Spreading n Despreading Communication Networks of NYU © Kang Xi, Polytechnic Institute 3131 Framework Communication Networks of NYU © Kang Xi, Polytechnic Institute 3232 Robust to Noise/Jamming Communication Networks of NYU © Kang Xi, Polytechnic Institute 3333 Frequency-Hopping Spread Spectrum (FHSS) n n n FSK ¨ Using f0/f1 for bit 0/1 multiple carrier frequencies FHSS ¨ Using ¨A Example control sequence [c1 c2 c3 c4] ¨ A number of frequencies 1 1 2 2 3 3 4 4 [f 0 =f 1 ; f 0 =f 2 ; f 0 =f 1 ; f 0 =f 1 ] ¨ For ck , use the k th frequency pair Communication Networks of NYU © Kang Xi, Polytechnic Institute 3434 FHSS Communication Networks of NYU © Kang Xi, Polytechnic Institute 3535 Direct Sequence Spread Spectrum (DSSS) Carrier frequency is fixed n Data is modified using a sequence n Spectrum spreading is realized before modulation n Communication Networks of NYU © Kang Xi, Polytechnic Institute 3636 DSSS Example Communication Networks of NYU © Kang Xi, Polytechnic Institute 3737 DSSS Communication Networks of NYU © Kang Xi, Polytechnic Institute 3838 IEEE 802.11 DSSS Communication Networks of NYU © Kang Xi, Polytechnic Institute 3939 Code Division Multiple Access (CDMA) Multiple users occupy the same spectrum n To a certain user, others look like noise n Communication Networks of NYU © Kang Xi, Polytechnic Institute 4040 CDMA: Two Users n A: ¨ ¨ ¨ n Orthoganal ¨ ¨ ¨ ¨ Data: a Code: x=[1 1] Signal: ax n xxT=2 yyT=2 xyT=0 yxT=0 n B: ¨ ¨ ¨ Signal: b Code: y=[1 -1] Signal: by To get A: ¨ a’ = sxT=axxT+byxT=2a n To get B: ¨ n Share channel: ¨ b’ = syT=2b Signal: s=ax + by © Kang Xi, Polytechnic Institute 4141 Communication Networks of NYU CDMA Communication Networks of NYU © Kang Xi, Polytechnic Institute 4242 ...
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This note was uploaded on 04/06/2011 for the course EE 5363 taught by Professor Kang during the Spring '09 term at NYU Poly.

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