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Modem

Modem. Modem connection. PC. PC. modem. modem. 110011. 110011. DCE. DCE. DTE. DTE. Modem type. Asynchronous, Synchronous Half-Duplex, Full-Duplex. Modulation method. FSK 1 bit per baud. max speed =2400 bps PSK, DPSK up 3 bit per baud

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Modem

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  1. Modem

  2. Modem connection PC PC modem modem 110011 110011 DCE DCE DTE DTE

  3. Modem type • Asynchronous, Synchronous • Half-Duplex, Full-Duplex

  4. Modulation method • FSK 1 bit per baud. max speed =2400 bps • PSK, DPSK up 3 bit per baud • QAM combines ASK and PSK, encoding 4-7 bits per baud • TCM same as QAM, but adds extra bits for error correction

  5. Signal Constellation distance = sqrt(C2+D2) point defines a legimate signal change C C angle = sin-1 sqrt(C2+D2) D • distance defines signal抯 amplitude • angle defines phase shift

  6. Constellation samples V.22 600 baud 1200 bps PSK V.22bis 600 baud 2400 bps QAM V.32 2400 baud 9600 bps TCM

  7. V.29 Signal Constellation 90 • 8 phases with 4 amplitudes QAM • 2400 baud, 9600 bps 1010 3*sqrt(2) 0010 1000 1011 sqrt(2) 0011 5 0000 3 0 180 0111 1111 0001 1001 0101 0110 1101 0100 1110 1100 270

  8. Some standards • V.32bis • 2-dimensional TCM with 128 pts. • transmission rate up to 14,400 bps • V.32ter • 2-dimensional TCM with 512 points • Transmisiion rates up to 19,200 bps • V.34 • 4-dimensional TCM with 768 points • Transmisiion rates up to 28,800 bps

  9. Error correcting • LAPD (Link Access Protocol D) based on HDLC sychronous protocol • MNP (Micrcom Networking protocol)trademark of Microcom • TCM modulation with error detecting code • V.42CRC error correcting standard

  10. MNP protocol • Microcom Networking Protocol (MNP)Modem protocol developed by Micocom, Inc. • Divided into classes 1-10 MNP modems negotiate to operate at the highest supported class • Considered a de facto industry standard

  11. V.42 Recommendation • V.42 • Defines error-checking standard • uses CRC for each data block • V.42bis • Defines modem compression using the Lempel Ziv method

  12. 9,600 bps modem modem Down speed 7,200 bps modem modem 4,800 bps modem modem Fallback

  13. Smart modem • Introduces by Hayes Microcomputer Products • known as Hayes-comptible modems • Properties • standard modem with microprocessor • automatic dialling • provides command in ASCII form • Hayes command • known as Hayes-comptible modems • AT DT 9,9985800

  14. Hayes Response Codes • 0 OK • 1 CONNECT • 2 RING • 3 NO CARRIER • 4 ERROR • 5 CONNECT 1200 • 6 NO DIALTONE • 7 BUSY • 8 NO ANSWER • 10 CONNECT 2400

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