Field Guide · concept

Also known as: deframer, frame extraction, frame recovery

Deframing is the receiver step that finds where each frame begins and ends in a continuous symbol or bit stream — locating sync words, aligning to time slots, and slicing out the bits that belong to one frame.1 It is the inverse of the transmitter’s framing: the demodulator and clock recovery produce an unbroken stream of symbols with no inherent marks showing where a message starts, and deframing restores that structure so the payload can be decoded.

stream …011 [SYNC] hdr payload crc [SYNC] hdr… correlate for sync one aligned frame → decode
Deframing correlates for the sync word, then cuts one aligned frame out of the running stream.

How it works

The stream that reaches the deframer has bits but no punctuation. Deframing puts the punctuation back:

  • Find the sync word. Every frame carries a known fixed pattern — an access code or frame sync word. A sliding correlator runs the stream past that pattern and flags each position where they match above a threshold. That match is the frame’s anchor point.
  • Establish bit alignment. The sync-word position also resolves byte/dibit phase: from the anchor, the receiver knows which bit is bit 0 of the header, so subsequent fields land in the right place.
  • Slot the bits. In TDMA systems, frames arrive interleaved across time slots; the deframer uses the sync position and the known frame geometry to assign each burst to its slot before assembling a logical channel.
  • Extract the frame body. Counting the standard-defined field lengths from the anchor, the deframer copies out header, payload, and check bits and hands them to FEC decoding and parsing.

In practice

Robust deframing tolerates errors in the sync word itself: at low SNR the pattern arrives with bit flips, so correlation uses a threshold that accepts a few mismatches rather than requiring an exact hit, and often maintains a flywheel — once locked, it predicts where the next sync should be and trusts that timing even if one instance is corrupted. It must also reject false matches, which is why sync words are chosen for low autocorrelation sidelobes (like Barker codes).

Relevance to SDR

Deframing sits between symbol recovery and payload decode in every framed protocol — P25’s Frame Sync, DMR’s SYNC patterns, TETRA’s training sequences, HDLC flags in AX.25. Getting it right is the difference between a locked, decoding receiver and a stream of noise.

GopherTrunk deframes across all its protocols. internal/dsp/sync provides a generic Correlator that slides a soft-symbol pattern to flag sync positions, and each radio under internal/radio (P25, DMR, NXDN, TETRA, D-STAR, YSF) has its own sync and framing code that takes those anchors, resolves slot and bit alignment, and extracts frame bodies for decoding. The packet framing layout the transmitter used is what tells the deframer where each field lives.

Sources

  1. Frame synchronization — Wikipedia, on locating frame boundaries via a known sync sequence. ↩

See also