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Multiplexing and Spreading Sharing Medium A link is divided into channels link, n channels Multiplexer D E M U X n signals (input) M U X n signals (output) Demultiplexer Frequency Division Multiplexing (FDM) analog multiplexing technique to combine sign Medium BW > Channel BW Each signal is modulated to a different carrier frequency E.g., broadcast radio Channel allocated even if no data Channel Channel Channel Conceptual View of FDM Time f1 f2 f3 nc ue q Fre y FDM: Multiplexing Process FDM: Demultiplexing Process Guard Bands Strips of unused bandwidth to prevent signals from overlapping FDM f kHz kHz kHz kHz f FDM (no guard band) kHz kHz kHz kHz f f FDM (with guard band) kHz kHz kHz f FDM: Example Five Five voice voice channels, channels, each each with with 3-kHz 3-kHz bandwidth, bandwidth, are are to to be be multiplexed multiplexed together together IfIf there there is is aa need need for for aa guard guard band band of of 1.5 1.5 kHz, kHz, what what is is the the minimum minimum bandwidth bandwidth of of the the link? link? kHz 1.5 kHz kHz 1.5 kHz kHz 1.5 kHz kHz 1.5 kHz kHz f 21 kHz FDM: Example Four Four digital-data digital-data channels, channels, 11 Mbps Mbps each, each, use use aa satellite satellite channel channel of of 11 MHz MHz Design Design an an appropriate appropriate configuration configuration using using FDM FDM Analog Hierarchy Used by AT&T 48 kHz 12 voice channels 2.52 MHz 600 voice channels supergroup master group 16.984 MHz 3600 voice channels Jumbo group F D M … F D 10 supergropusM kHz 240 kHz 60 voice channels group groups F D M kHz F D master groups M kHz … 12 voice channels 10 TDM of Different Data Rates B C A B C kbps B C B A MUX Data rate from one source may be faster than the others More than one time slot can be assigned to certain kbps frame sources A 16 kbps MUX 16 kbps kbps kbps kbps kbps kbps kbps Note: assume unit = bit 22 TDM of Different Data Rates B A A B A A B A B B B A B kbps kbps B A A B A A B A MUX A A A A A frame kbps Note: assume unit = bit 23 Bit Padding Different data rates may not be integer multiples of each other Solution – Bit padding ms B kbps Bit Padding kbps kbps A B A B A A B A MUX A A B A kbps Extra bit 3000 cycles/s B A A B A A B A A Note: assume unit = bit B A A B A A B 24 Digital Signal (DS) Hierarchy 64 kbps 1.544 Mbps 24 DS-0 DS-0 44.376 Mbps DS-2 DS-2 T D M T D M 6.312 Mbps DS-1 DS-1 … 24 channels DS-0 274.176 Mbps DS-3 T D M DS3 T D M DS4 kbps = 1,000 bps Mbps = 1,000 kbps 25 DS Services and T Lines DS-0, DS-1, etc, are services T lines are used to implement these services Service Line Rate (Mbps) Voice Channels DS-1 T-1 1.544 24 DS-2 T-2 6.312 96 DS-3 T-3 44.736 672 DS-4 T-4 274.176 4032 26 T Lines and Analog Signals 27 T-1 Frame Structure 28 E Lines European's version of T lines Also used in Thailand E Line Rate (Mbps) Voice Channels E-1 2.048 30 E-2 8.448 120 E-3 34.368 480 E-4 139.264 1920 29 Inverse Multiplexing low-speed lines high-speed line Inverse MUX Inverse DEMUX high-speed line 30 Spread Spectrum Spread signal to use larger bandwidth To prevent eavesdropping To reduce effect from interference 31 Frequency-Hopping SS "FHSS" – Frequency-Hopping Spread Spectrum Used in Bluetooth technology 32 FHSS Cycles 33 Direct-Sequence SS "DSSS" – Direct-Sequence Spread Spectrum Used in Wireless LANs 34 DSSS and Interference Amplitude Narrow Band Signal Narrow Band Interference Spread Spectrum Signal Frequency 35 DSSS Example 36 ... U X n signals (input) M U X n signals (output) Demultiplexer Frequency Division Multiplexing (FDM) analog multiplexing technique to combine sign Medium BW > Channel BW Each signal is... data Channel Channel Channel Conceptual View of FDM Time f1 f2 f3 nc ue q Fre y FDM: Multiplexing Process FDM: Demultiplexing Process Guard Bands Strips of unused bandwidth to prevent signals from... which means only 790 channels are available for cellular phone users. 11 Wavelength Division Multiplexing (WDM) analog multiplexing technique to combine optical sign WDM is a special case of FDM 1 1 2
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