Asynchronous voice band modem for IP data protocols

1
Asynchronous voice band modem for IP data
protocols

• 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
• HDLC - V.42
• Compression - v.42bis
• 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)

6
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)