QUESTION:
What is the difference between a typical BITS/SSU feed and an IRIG-B feed?
ANSWER:
a) A BITS feed is typically provides a frequency reference ( as opposed to absolute time). Many formats exist and all follow some regular telecommunication signal format such as DS1 (T1) or E1. Variations of these include a 1.544 MHz (or 2.048 MHz) signal either as a sine-wave or a square-wave. The composite-clock ("CC") format is a 64 kbps signal that appears on the cable pair as a pulse train of return-to-zero pulses ( a pulse for a "1"; absence of pulse for a "0") with alternate mark inversion. In Central Office applications CC is used to time various channel banks and "DS0" terminal equipment and to provide an "octet marker" (essentially an 8 kHz marker) a bipolar violation is introduced. BITS feeds are used in telecommunications central offices to time all the various equipment, and hence the need for a variety of formats.
b) IRIG-B, on the other hand, is a feed that can deliver absolute time (and frequency as well). Most commonly it uses an amplitude modulated 1 kHz sine-wave. Two amplitudes allow the encoding of binary information. The "frame" length is 1000 bits and the rising edge associated with the first bit of the frame is considered the time marker. The message encoded in the frame provides the time at the time marker. One of the beauties of the IRIG-B format is that it is in the audio band and can be recorded on audio tape. IRIG-B has limited use in Telecommunications (compared to BITS) but is used in a variety of other industries. For example, the Dept of Defense uses IRIG-B to distribute time to the various elements in their "ranges" where they use this as a means to ascertain, quite precisely, the timing of different events.
There are some devices that do provide both types of outputs.
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