1. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 1 Modems Goal: Carry data over telephone lines Modulator / Demodulator Baud rate / Bit rate / Bandwidth relationship Amplitude Frequency 300 600 30003300 2400 Hz for data 3000 Hz for voice Utilize this bandwidth as efficiently as possible

2. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 2 Theoretical Bit Rates ModulationHalf-DuplexFull-Duplex ASK24001200 FSK<2400<1200 2-PSK24001200 4-PSK, 4-QAM48002400 8-PSK, 8-QAM72003600 16-QAM96004800 32-QAM120006000 64-QAM144007200 128-QAM168008400 256-QAM192009600

3. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 3 Bell 103/113 Full-duplex on 2-wire lines Similar to CCITT V.21 specifications FSK modulation, asynchronous Data rate: 300bps Amplitude Frequency 300 2225 Mark 33002025 Space 1270 Mark 1070 Space Upstream Downstream

4. 13 - Winter 2005 ECE 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

5. 13 - Winter 2005 ECE 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 DibitsPhase Shift 0045º 01135º 11225º 10315º

6. 13 - Winter 2005 ECE 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

7. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 7 Modem Features Proprietary Modems –Modulation –Error control –Data compression Fallback rates Protocol negotiation Echo cancellation Scrambling

8. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 8 V.22 1200 or 600 bps Asynchronous or synchronous Full duplex, 4-PSK DibitsPhase Shift 0090º 010º0º 11270º 10180º At 1200bps

9. 13 - Winter 2005 ECE 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 DibitsPhase Shift 0090º 010º0º 11-90º 10180º X X Quadrant Change Bits within Quadrant

10. 13 - Winter 2005 ECE 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

11. 13 - Winter 2005 ECE 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

12. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 12 V.42bis and MNP-5 Data 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

13. 13 - Winter 2005 ECE 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)

14. 13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 14 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  2 m values (Inverse) PCM m=8, baud rate = 8000  Theoretical limit = 8 x 8000 = 64 kbps V.90 Client D/A A/D Local Office PSTN V.90 Server Internet

15. 13 - Winter 2005 ECE 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)