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Modems

Utilize this bandwidth as efficiently as possible. Amplitude. 300. 600. 3000. 3300. Frequency. 2400 Hz for data. 3000 Hz for voice. Modems. Goal: Carry data over telephone lines Mo dulator / De modulator Baud rate / Bit rate / Bandwidth relationship. Theoretical Bit Rates.

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Modems

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  1. Utilize this bandwidth as efficiently as possible Amplitude 300 600 3000 3300 Frequency 2400 Hz for data 3000 Hz for voice Modems • Goal: Carry data over telephone lines • Modulator / Demodulator • Baud rate / Bit rate / Bandwidth relationship ECE 766 Computer Interfacing and Protocols

  2. Theoretical Bit Rates ECE 766 Computer Interfacing and Protocols

  3. Amplitude Downstream Upstream Frequency 300 1070Space 1270Mark 2025Space 2225Mark 3300 Bell 103/113 • Full-duplex on 2-wire lines • Similar to CCITT V.21 specifications • FSK modulation, asynchronous • Data rate: 300bps ECE 766 Computer Interfacing and Protocols

  4. Bell 202 • Half-duplex over 2-wire lines • FSK modulation, asynchronous • Data rate: 1200bps • Space at 1200Hz, Mark at 2200Hz • Uses ASK channel at 387Hz (5bps) for signaling, flow control, error control • Bit rate can be improved using specially conditioned lines ECE 766 Computer Interfacing and Protocols

  5. Bell 201 • Half-duplex over 2-wire lines at 1200bps or full-duplex over 4-wire lines at 2400bps • 4-PSK modulation, synchronous • 1200 baud in each direction  only half of the theoretical limit ECE 766 Computer Interfacing and Protocols

  6. Modem Features • Internal vs. external modems • Hayes compatible (intelligent) modem (AT command set) • Flow control • Synchronization • Asynchronous, synchronous, pseudo-synchronous (start and stop bits removed) • Error control • Retransmissions, forward error control • Modem level vs. file transfer level • Data compression • Effective data rate • Modem level vs. file transfer level ECE 766 Computer Interfacing and Protocols

  7. Modem Features • Proprietary Modems • Modulation • Error control • Data compression • Fallback rates • Protocol negotiation • Echo cancellation • Scrambling ECE 766 Computer Interfacing and Protocols

  8. V.22 • 1200 or 600 bps • Asynchronous or synchronous • Full duplex, 4-PSK At 1200bps ECE 766 Computer Interfacing and Protocols

  9. V.22bis • 2400 bps • Asynchronous or synchronous input • Data synchronized between modems • Full duplex, 16-QAM • Includes 1200 bps at V.22 X X X X QuadrantChange Bits withinQuadrant ECE 766 Computer Interfacing and Protocols

  10. V.32 • 9600 bps • Asynchronous or synchronous input • Data synchronized between modems • Echo cancellation • Full-duplex • 1800 Hz carrier, 2400 baud, 32-QAM • Trellis coding, 32-bit coding space, only 16 used at a time • V.32bis  14400 bps using 64-QAM ECE 766 Computer Interfacing and Protocols

  11. V.42 and MNP-4 Error Correction Protocols • V.42 CCITT • MNP – Microcom Network Protocol • Assemble packets • Send synchronously without start/stop bits • BCC sent with packet • Error recovery through retransmissions • V.42 uses LAP-M (link access procedure for modems) with MNP-4 alternative ECE 766 Computer Interfacing and Protocols

  12. V.42bis and MNP-5Data Compression Protocols • Maximum compression of V.42: 4:1 • Maximum compression of MNP-5: 2:1 • Unlike V.42, V.42bis does not have MNP-5 as alternative However, some modems have both • MNP-5 • Run length encoding • Adaptive frequency encoding by characters • V.42bis • Lempel-Ziv-Welch compression • Replaces strings with codes ECE 766 Computer Interfacing and Protocols

  13. 56K Modems – V.90 • Traditional modems have limitation of 33.6 kbps • ITU-T announced V.90 in 1998 • 56 kbps over PSTN, downstream direction • V.90 replaces proprietary 56K standards by 3Com (X2) and Rockwell (K56Flex) • Client side: Analog modem on analog local loop to PSTN • Server side: Digital modem with digital connection to PSTN (e.g. ISDN) ECE 766 Computer Interfacing and Protocols

  14. D/A V.90Server Local Office V.90 Client PSTN A/D Internet 56K Modems – V.90 • Quantization during A/D limits rate No user-to-user V.90 connection • Pulse Code Modulation (PCM) • PCM sends m bits  2m values • (Inverse) PCM m=8, baud rate = 8000 Theoretical limit = 8 x 8000 = 64 kbps ECE 766 Computer Interfacing and Protocols

  15. 56K Modems – V.90 • 1 bit used by PSTN  7 bits (64k  56k) • Noise, power limitations further reduces the number of distinct levels • Typically, rate is ca. 40 kbps • Uploads still limited to 33.6 kbps (V.34bis) ECE 766 Computer Interfacing and Protocols

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