Unit interval (data transmission)
The unit interval is the minimum time interval between condition changes of a data transmission signal, also known as the pulse time or symbol duration time.
A unit interval (UI) is the time taken in a data stream by each subsequent pulse (or symbol).
Time is a physical quantity. The UI is used as a unit of measurement of time and represents a definite predetermined time interval. When we say 10 unit intervals (or 10 UI), we actually mean 10 times the definite predetermined time called "UI".
When UI is used as a measurement unit of a time interval, the resulting measure of such time interval is dimensionless. It expresses the time interval in terms of UI.
This measurement unit is extensively used in the jitter literature, for instance in various ITU-T Recommandations [1], or in the tutorial from Ransom Stephens [2].
Very often, but not always, the UI coincides with the bit time, i.e. with the time interval taken to transmit one bit (binary information digit). The two coincide in fact in NRZ transmission; they do not coincide in a 2B1Q transmission, where one pulse takes the time of two bits.
For example, in a serial line with a baud rate of 2.5 Gb/s, a unit interval is 1/(2.5 Gb/s) = 0.4 ns/baud. The widespread use of UI in the jitter measurements comes from the need to apply the same requirements or results to cases of different symbol rates. This happens when the phenomena investigated are not independent from the symbol duration time but closely related to it. This is the typical case of timing jitter in serial communications or in clock distribution.
See also
References
- ^ ITU-T G.825 TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Digital networks . Quality and availability targets (03/2000)
- ^ Tektronics Jitter 360° Knowledge Series rom http://www.tek.com/learning/ .
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