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 300hz 3500hz 1200hz “mark” 2200hz “space” 387hz

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

T B = 1uS mark space mark Continuous phase FSK (MSK: Minimum Shift Keying) Transitions between frequencies are done at 0,  radians of phase; Therefore the bit time must be an even multiple of one half cycle for both mark and space frequencies: Example: f m = 1500, N m = 3 f B = 1000 f s = 2000, N s = 4 Orf s = 2500, N s = 5