Editing Frame synchronization (section)

==Frame synchronizer==
[[Image:PCM Stream.jpg|frame|none|PCM stream prior to frame synchronization]]

In [[telemetry]] applications, a ''frame synchronizer'' is used to locate frame boundaries within a serial [[pulse-code modulated]] (PCM) binary stream.

The frame synchronizer immediately follows the bit synchronizer in most telemetry applications.  Without frame synchronization, [[decommutation]] is impossible.

[[Image:Frame Synced Stream.jpg|frame|none|Frame-synchronized PCM stream]]

The frame synchronizer searches the incoming bit-stream for occurrences of the frame synchronization pattern.  If the pattern persists for long enough that an accidental match is implausible, the synchronizer declares the data synchronized and available for decoding.  If a large number of mis-matches occur, the synchronizer declares a loss of synchronization.

The search can be sequential (only consider one starting point at a time), or multiple candidate starting points may be considered at once.  Advanced techniques continue searching even while synchronization is established, so that, if synchronization is lost, by the time the loss is noticed a new frame start position has been found.<ref>{{cite patent
 |country=US |number=5621773 |status=patent |gdate=1997-04-15
 |title=Method and Apparatus for Fast Synchronization of T1 Extended Superframes
 |assign=[[LSI Logic Corporation]]
}}</ref>

[[Image:Commutation.svg|frame|none|Different types of commutation within a frame synchronized PCM stream]]

It is not uncommon to have multiple levels of frame synchronization, where a series of frames is assembled into a larger "superframe" or "major frame".  Individual frames are then "minor frames" within that superframe.  Each frame contains a subframe ID (often a simple counter) which identifies its position within the superframe.  A second frame synchronizer establishes superframe synchronization.  This allows subcommutation, where some data is sent less frequently than every frame.