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 ( 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)