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Asynchronous voice band modem for IP data protocols

Asynchronous voice band modem for IP data protocols. overview features. 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

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Asynchronous voice band modem for IP data protocols

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  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. 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) Modem Control Signals

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