Frame synchronization: Difference between revisions

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{{context|date=February 2013}}

While receiving a stream of [[data frame|framed]] [[data]], '''frame synchronization''' is the process by which incoming frame alignment signals (i.e., distinctive [[bit]] sequences/a [[syncword]]), are identified (i.e., distinguished from data bits) permitting the data bits within the frame to be extracted for decoding or retransmission. This is sometimes referred to as ''framing''.

In [[telecommunications]], '''frame synchronization''' or '''framing''' is the process by which, while receiving a stream of fixed-length [[Frame (networking)|frames]], the receiver identifies the frame boundaries, 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 (e.g., Ethernet's [[Ethernet_frame#End_of_frame_–_physical_layer|end of stream symbol]]). Loss of carrier signal can be interpreted as a packet-end delimiter in some cases. 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.

While receiving a stream of [[data frame|framed]] [[data]], '''frameFrame synchronization''' is theachieved processwhen by whichthe incoming frame alignment signals (i.e., distinctive [[bit]] sequences/a [[syncword]]), are identified (i.e.that is, distinguished from data bits), permitting the data bits within the frame to be extracted for decoding or retransmission. This is sometimes referred to as ''framing''.

== Framing ==

{{Redirect|Flag sequence|the emojis|Emoji flag sequence}}

[[Image:Frame Synced Stream.jpg|frame|none|Frame Synchronized PCM Stream -- Telemetry Application]]

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 Synchronizedsynchronized PCM Streamstream --— Telemetrytelemetry Applicationapplication]]

The transmitter and the receiver must agree ahead of time on which frame [[synchronization in telecommunications|synchronization]] scheme they will use.

The most common frame synchronization schemes are:

The most commonCommon frame synchronization schemes are:

;Framing bit: A common practice in [[telecommunication]]s, for example in [[T-carrier]], is to insert, in a dedicated [[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 indicate the beginning or end of a frame. They occur at specified positions in the frame, do not carry information, and are usually repetitive.

;Syncword framing: Some systems use a special [[syncword]] at the beginning of every frame.

;Framing bit: A common practice in [[telecommunicationtelecommunications]]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.

;CRC-based framing: Some telecommunications hardware uses [[CRC-based framing]].

;Syncword and flag sequence framing: Rather than a single bit, some systems use a multi-bit [[syncword]] in each frame, or a '''flag sequence''' that marks the beginning and end of each frame. [[High-Level Data Link Control]] and similar systems use flag sequences.<ref>{{Cite IETF |rfc=1662 |title=PPP in HDLC-like Framing |last=Simpson |first=William A. |date=July 1994 |publisher=Internet Engineering Task Force}}</ref>

;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 Streamstream prior to frame synchronization]]

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

===Television===

{{further|Time base corrector}}

A '''frame synchronizer''' is a device used in [[live television]] [[television production|production]] to match the timing of an incoming [[video]] source to the timing of an existing video system. They are often used to "time in" consumer video equipment to a professional system but can be used to stabilize any video. The frame synchronizer essentially takes a picture of each [[Film frame|frame]] of incoming video and then immediately outputs it with the correct synchronization signals to match an existing video system. A [[genlock]] signal is required to provide a means for video synchronizing with the house reference.

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

===Telemetry===

[[Image:PCM Stream.jpg|frame|none|PCM Stream prior to frame synchronization]]

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

[[Image:CommutationFrame Synced Stream.jpg|frame|none|Different types of commutation within a frame Frame-synchronized PCM stream]]

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

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.

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

The frame syncronisation 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 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:Frame Synced StreamCommutation.jpgsvg|frame|none|FrameDifferent 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==

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* [[Self-synchronizing code]]

* [[Superframe]]

* [[Generator locking]]

==References==

{{Reflist}}

{{FS1037C MS188}}

===Scientific articles===

* [https://ieeexplore.ieee.org/abstract/document/1091127 J. L. Massey. "Optimum frame synchronization ". IEEE trans. comm., com-20(2):115-119, April 1972].

* [https://ieeexplore.ieee.org/abstract/document/1094813 R Scholtz. "Frame synchronization techniques", ''IEEE Transactions on Communications'', 1980].

* P. Robertson. "Optimal Frame Synchronization for Continuous and Packet Data Transmission", PhD Dissertation, 1995, Fortschrittberichte VDI Reihe 10, Nr. 376 [https://web.archive.org/web/20110718231623/http://www.kn-s.dlr.de/People/Robertson/Papers/diss.pdf PDF]

1995, Fortschrittberichte VDI Reihe 10, Nr. 376 [http://www.kn-s.dlr.de/People/Robertson/Papers/diss.pdf PDF]

==External links==

* [http://www.ulyssix.com Telemetry Ground Station Processing Electronics telemetry link]

* [http://www.algolith.com/products/broadcast/frame-synchronization/index.html Algolith Frame Synchronizer Solutions]

[[Category:Data transmission]]