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Modems ( MOD ulator- DEM odulator) Telephone Line Usage: 3 khz Bandwidth– 6 kBaud Max

Modems ( MOD ulator- DEM odulator) Telephone Line Usage: 3 khz Bandwidth– 6 kBaud Max 1200 b/s – FSK (Frequency Shift Keying) Must use multi-bit symbols for higher data rates (multilevel PSK, QAM). Type 202 modem: Half Duplex FSK 1200 hz = “mark” =“1” 2200 hz = “space” = “0”

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Modems ( MOD ulator- DEM odulator) Telephone Line Usage: 3 khz Bandwidth– 6 kBaud Max

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  1. Modems (MODulator-DEModulator) Telephone Line Usage: 3 khz Bandwidth– 6 kBaud Max 1200 b/s – FSK (Frequency Shift Keying) Must use multi-bit symbols for higher data rates (multilevel PSK, QAM) Type 202 modem: Half Duplex FSK 1200 hz = “mark” =“1” 2200 hz = “space” = “0” -9 dbm -12 dBm on 600 ohm twisted pair line FM discriminator or PLL to demodulate 387 hz on/off keyed reverse channel ACK/NAK Telephone Line 3.3 khz Bandwidth 387hz 1200hz “mark” 2200hz “space” 3500hz 300hz

  2. Full Duplex FSK Telephone Line 3.3 khz Bandwidth 1070hz “mark” 1270hz “space” 2025hz “mark” 2200hz “space” 3500hz 300hz Originator Receiver “mark” Tx Oscillators Originator 1070 1270 Line 2025 2200 Rx Decoders Rx Mark “space” 2025 High Band 2200 1070 Low Band Receiver Rx Space 1270

  3. Continuous phase FSK (MSK: Minimum Shift Keying) Transitions between frequencies are done at 0, p radians of phase; Therefore the bit time must be an even multiple of one half cycle for both mark and space frequencies: Example: fm= 1500, Nm = 3 fB = 1000 fs = 2000, Ns = 4 Or fs = 2500, Ns = 5 TB = 1uS space space mark space space mark

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