We mentioned in the previous chapter that asynchronous transmission is used by modems only when the bit rate is low. However, PC serial ports produce data for asynchronous transmission only, by grouping the data into asynchronous data packets. (Synchronous serial interfaces are available, but they are not in common use.) To send the data synchronously, a high-speed modem must first convert the asynchronous data packets coming from the PCs serial port into data frames, by stripping out the start, parity and stop bits, and reframing the data. On the receiving end, the modem must then reformat the data into asynchronous form, for receipt by the remote PCs serial port. All this trouble is worth it, since synchronous data frames have a much higher data-to-nondata ratio than asynchronous data packets.
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