Depending on the RAS version, RAS supports X.25, ISDN, Null Modem, and asynchronous modem communication. RAS does not support synchronous communication and therefore cannot communicate with a synchronous serial port. Synchronous communication synchronizes the transmission by controlling the timing and the duration of data signals.
Asynchronous communication means that each byte is framed with a start and stop bit. RAS sends data asynchronously to the serial port, and the serial port sends data asynchronously to the modem. The asynchronous modem then strips the start and stop bit from each byte (a byte is also referred to as character) and converts the characters into blocks that are then sent synchronously (not asynchronously) to the other modem by using an error control protocol. The other modem disassembles these synchronous blocks, and frames each character with a start and stop bit before sending it on to the serial port of the RAS server. The serial port then sends it to the RAS server service. This process occurs during a communication call similar to that shown in Figure E.1.
Leased line communication using asynchronous serial ports and either asynchronous or synchronous modems can be implemented under some circumstances, as long as RAS can treat the connection as an asynchronous null modem connection. Some modems support both synchronous and asynchronous communication.