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 Hz for data 3000 Hz for voice Utilize this bandwidth as efficiently as possible

13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 2 Theoretical Bit Rates ModulationHalf-DuplexFull-Duplex ASK FSK<2400< PSK PSK, 4-QAM PSK, 8-QAM QAM QAM QAM QAM QAM

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 Mark Space 1270 Mark 1070 Space Upstream Downstream

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

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

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

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

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

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

13 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 10 V 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  bps using 64-QAM

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

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 - 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 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)

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

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)