Asynchronous voice band modem

1
Asynchronous voice band modem

• overview
• features

2
Inside the modem Hardware

• Line Interface
• Lightning protector
• Ring voltage detector
• Off-hook relay (also used for pulse dialing)
• Isolation transformer
• Analog/Digital Conversion
• Digital Signal Processor
• Control Processor
• UART or UART emulation

3
Inside the modem Firmware

• Line Control
• Signal Processing - V.34/V.90
• HDLC - V.42
• Compression - V.42bis and MNP
• Command Processor
• UART emulation or control

4
PC Serial Ports (UART)

• NS8250
• dual character buffer
• interrupt for every byte
• NS16550A
• 16 byte FIFO buffer for transmit
• 16 byte FIFO for receive
• interrupts on specified FIFO level or timer
• fewer interrupts less critical timing

5
Serial Port Speed Buffering

• DTE port speed is fixed (115200 bps)
• DCE link speed is fixed (33600 bps)
• Data compression ratio is variable
• Flow control is required
• All modem applications must use hardware flow
  control (RTS/CTS)

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Modem Control Signals

• FROM DTE
• Transmit data
• DTR
• RTS (Ready to receive)
• TT (Terminal Timing)

• FROM DCE
• Receive data
• DSR - Modem is ON
• DCD - connected
• CTS (Clear to Send)
• RI (Ring Indication)
• ST (Send Timing)
• RT (ReceiveTiming)

7
Asynchronous modem glossary
CCITT InternationalTelegraph and Telephone
Consultative Commitee (now ITU-T) ITU-T Internatio
nal Telecommuncations Union Telecommunication
Standardization Sector (former CCITT) V.nn ITU-T
(CCITT) recommended standards
modulation standards Bell 103 V.21 110 150
300 bps Bell 212A V.22 1200 bps V.22bis 2400
bps V.32 2400 4800 9600 bps V.32bis 7200
9600 12000 14400 bps V.Fast preliminary
V.34 V.34 28800 ... 14400 bps V.34bis 33600
... 14400 bps V.90 56.700 bps modulation
fallback V.90 --gt V.34 --gt V.32bis --gt V.32 --gt
V.22bis --gt V.22
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Asynchronous modem glossary
speed buffering interface speed connection
speed flow control RTS/CTS hardware control by
special wires XON/XOFF software control through
special characters (Ctrl-Q/Ctrl-S) ASL Au
tomatic Speed Leveling during retraining err
or control MNP 1 2 3 4 Microcom Networking
Protocol (de facto standard) V.42 Link
Access Protocol for Modems (LAPM) error-cont
rol fallback V.42 --gt MNP4 --gt MNP3 --gt MNP2 --gt
MNP1 --gt none
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Asynchronous modem glossary
data compression MNP3 4 protocol optimization
(108-120) MNP5 V42bis true compression (up to
400 depends on data) compression
fallback V.42bis --gt MNP5 --gt none DTE Data
Terminal Equipment (computer) DCE Data
Communication Equipment (modem) DTE-DCE
signaling DTR Data Terminal Ready set by
DTE DSR Data Set Ready set by modem RTS
CTS Ready To Send Clear To Send used to
handshake on sped buffering CD Carrier
Detect set by modem (remote modem is
connected)