Frame synchronization

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In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished from data bits), permitting the data bits within the frame to be extracted for decoding or retransmission.

Framing[edit]

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

Frame synchronized PCM stream — telemetry application

The transmitter and the receiver must agree ahead of time on which frame synchronization scheme they will use.

Common frame synchronization schemes are:

Framing bit
A common practice in telecommunications, 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.
CRC-based framing
Some telecommunications hardware uses CRC-based framing.

Frame synchronizer[edit]

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.

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, decommutation is impossible.

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.

See also[edit]

References[edit]

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

Scientific articles[edit]

  • J. L. Massey. "Optimum frame synchronization ". IEEE trans. comm., com-20(2):115-119, April 1972.
  • 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 PDF

External links[edit]