Introduction to Data Communications
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If the receive station's clock is higher in frequency, the samples will be spaced closer together (higher frequency - shorter period). In the above example, we transmitted the following data: 0100 1010 but we received the data: 0100 0101. The samples are out of synchronization with the transmitting data. We would have an error in receiving data.

Clocks are controlled by crystals (abbreviated: Xtal). Crystals are metal cans that hold a piezo-electric element that resonates at a certain frequency when a voltage is applied to it. If you drop a crystal or a printed circuit board (PCB) that has a crystal on it, the crystal can fracture inside the metal can. Either it will stop working or change its frequency, both result in a malfunctioning circuit! Crystals are also temperature sensitive and change frequency with temperature!

Receive clock frequency lower than transmitted frequency:

If the receiving station's clock is lower in frequency than the transmitted frequency, then the samples become farther apart (lower frequency - wider period). Again the samples become out of sync with the transmitted data!

The transmitted data is 0100 1010 but the receive data is 0101 0101! Again we would have receive data errors.

This is a basic problem with asynchronous communications, both transmitter and receiver require a very stable clock to work properly. At high frequencies (which result in high transfer rates), clock stability is critical and asynchronous transmission is very difficult to accomplish. Because of this inherent problem with asynchronous transmission, it is used at low frequency/slow transfer rates.

Introduction to Data Communications
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