Synchronous, Asynchronous, Isochronous. What Does it Mean?
By Stephen Bucaro
These are all methods used to transfer streams of data. Waveform diagrams can be used
to illustrate these different transmission modes. A waveform diagram shows how the signal
might appear on an oscilloscope screen, which produces a diagram with voltage on the
vertical axis and time on the horizontal axis.
Bits in a synchronous data stream must be transferred in sync with a clock signal. The control
signals for the data are derived from a clock signal. Synchronous data transfer systems
usually have an error detection mechanism. If an error is detected the data can be resent.
Bits in an asynchronous data stream can be transferred at random intervals and the data rate
of the stream is not required to be constant. Asynchronous systems use a start bit to signal the
beginning of a data transmission. A stop bit is used to signal the end of a data transmission.
Asynchronous data transfer systems usually have an error detection mechanism. If an error is
detected the data can be resent.
Isochronous (pronounced "eye-sock-ron-us")
An isochronous data transfer system combines the features of an asynchronous and synchronous
data transfer system. An isochronous data transfer system sends blocks of data asynchronously, in
other words the data stream can be transferred at random intervals.
Each transmission begins with a start packet. Once the start packet is transmitted, the data
must be delivered with a guaranteed bandwidth. Isochronous data transfer is commonly used for
where data must be delivered within certain time constraints, like streaming video.
Isochronous systems do not have an error detection mechanism (acknowledgment of receipt of
packet) because if an error were detected, time constraints would make it impossible to resend the data.
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