A subframe or major frame synchronization pattern is added to the minor
frame so the decommutator can distinguish subframe words, i.e., those
words that have a unique meaning in each minor frame. The figure below
shows that word 4 of each minor frame has a different meaning (measurands
A through E). Several methods are frequently employed to distinguish a
unique single position (word) in the major frame, or, in other words,
to identify the subframe.
The most
common subframe sync method is SubFrame IDentification (SFID). This
sync pattern occupies a word in each minor frame (typically, the
first word — see diagram above). The SFID sync acts as a counter
for all frame samples. The pattern increments or decrements to a specified
value and then resets itself, indicating that the last minor frame was
generated. The decommutator can thereby locate the start of the next
subframe
sampling.
Another minor frame identification method inverts the frame sync pattern
of the first minor frame in a subframe. This major frame synchronization
scheme is known as Frame Code Complement (FCC). Since the complementary
pattern exhibits the same correlation properties as the true pattern,
minor frame sync lock will not be compromised. Minimum sync overhead regarding
the number of words is also attained using the FCC method, although it
requires a longer time before you are assured correct data. The figure
below illustrates the FCC replacing the frame sync in words 11 and 12
of frame 1.
The Unique Recycle Code (URC) major frame synchronization technique is
similar to FCC except that the first sync word is not related to the standard
sync word. Of course, a hardware decommutator becomes more complicated
because it requires the ability to correlate two different sync words.
Frame Code Complement/Subframe Identification (FCC/SFID) seen in the
next figure uses the SFID word plus an alternating FCC frame sync word.
The frame sync pattern is placed in the usual location in the first minor
frame and its complement is placed in the second minor frame. This cycle
repeats. A normal SFID word occupies its position in every minor frame.
In Sync Code subframe synchronization, a single sync word occupies several
data words. This method has the latency drawback of FCC, plus it requires
processing to extract the subframe sync from the data frame (unless the
hardware decommutator incorporates this feature).
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