Frame synchronization: Difference between revisions

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{{context|date=February 2013}}

In [[telecommunication]], '''frame synchronization''' or '''framing''' is the process by which, while receiving a stream of fixed-length [[Frame (networking)|framed]] [[dataframes]], incomingthe framereceiver alignmentidentifies signalsthe (i.e.frame boundaries, apermitting distinctivethe [[bitdata]] sequences or [[syncword]]s) are identified (that is, distinguished from data bits), permitting the data bits within the frame to be extracted for decoding or retransmission.

When packets of varying length are sent, it is necessary to have an instantly recognizable packet-end delimiter. When a continuous stream of fixed-length frames are sent, a synchronized receiver can in principle identify frame boundaries forever. In practice, receivers can usually maintain synchronization despite transmission errors; [[bit slip]]s are much rarer than [[bit error]]s. Thus, it is acceptable to use a much smaller frame boundary marker, at the expense of a lengthier process to establish synchronization in the first place.

Frame synchronization is achieved when the incoming frame alignment signals are identified (that is, distinguished from data bits), permitting the data bits within the frame to be extracted for decoding or retransmission.

== Framing ==

If the transmission is temporarily interrupted, or a [[bit slip]] event occurs, the receiver must re-synchronize.

[[Image:Frame Synced Stream.jpg|frame|none|Frame synchronized PCM stream — telemetry application]]

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Common frame synchronization schemes are:

;Framing bit: A common practice in [[telecommunication]]s, for example in [[T-carrier]], is to insert, in a dedicated [[Time-division multiplexing|time slot]] within the frame, a noninformation bit or '''framing bit''' that is used for synchronization of the incoming data with the receiver. In a [[bit stream]], framing bits indicateare thepredictable beginning(do ornot endcarry ofinformation), a frame. Theyand occur at specified positions in the frame,. do notCorrect carryframing information,is andverified arewhen almost all framing bits (minus a small allowance for transmission errors) have their usuallypredicted repetitivevalues.

;Syncword framing: SomeRather than a single bit, some systems use a specialmulti-bit [[syncword]] at the beginning ofin everyeach frame.

;CRC-based framing: Some telecommunications hardware uses [[CRC-based framing]], where correct framing is verified when almost all frames have valid CRCs.

==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-align boundaries within a serial pulse code-modulated ([[Pulsepulse-code modulation|PCMmodulated]] (PCM) binary stream.

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

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

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

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

The frame synchronization pattern is a known binary pattern which repeats at a regular interval within the PCM stream. The frame synchronizer recognizes this pattern and aligns the data into minor frames or sub-frames. Typically the frame sync pattern is followed by a counter (sub-frame ID) which dictates which minor or sub-frame in the series is being transmitted. This becomes increasingly important in the decommutation stage where all data is deciphered as to what attribute was sampled. Different commutations require a constant awareness of which section of the major frame is being decoded.

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.

==See also==