Winter 2005

1
Modems

• Goal Carry data over telephone lines
• Modulator / Demodulator
• Baud rate / Bit rate / Bandwidth relationship

2
Theoretical Bit Rates
Modulation Half-Duplex Full-Duplex
ASK 2400 1200
FSK lt2400 lt1200
2-PSK 2400 1200
4-PSK, 4-QAM 4800 2400
8-PSK, 8-QAM 7200 3600
16-QAM 9600 4800
32-QAM 12000 6000
64-QAM 14400 7200
128-QAM 16800 8400
256-QAM 19200 9600
3
Bell 103/113

• Full-duplex on 2-wire lines
• Similar to CCITT V.21 specifications
• FSK modulation, asynchronous
• Data rate 300bps

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
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

Dibits Phase Shift
00 45º
01 135º
11 225º
10 315º
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
Modem Features

• Proprietary Modems
• Modulation
• Error control
• Data compression
• Fallback rates
• Protocol negotiation
• Echo cancellation
• Scrambling

8
V.22

• 1200 or 600 bps
• Asynchronous or synchronous
• Full duplex, 4-PSK

Dibits Phase Shift
00 90º
01 0º
11 270º
10 180º
At 1200bps
9
V.22bis

• 2400 bps
• Asynchronous or synchronous input
• Data synchronized between modems
• Full duplex, 16-QAM
• Includes 1200 bps at V.22

Dibits Phase Shift
00 90º
01 0º
11 -90º
10 180º
X X X X
QuadrantChange
Bits withinQuadrant
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
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
V.42bis and MNP-5Data Compression Protocols

• Maximum compression of V.42 41
• Maximum compression of MNP-5 21
• 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
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
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 m8, baud rate 8000? Theoretical
  limit 8 x 8000 64 kbps

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)