Field Guide · algorithm

Also known as: FLEX coding, FLEX framing, Motorola FLEX

FLEX protocol coding is the framing and forward-error-correction of Motorola’s FLEX paging — the high-throughput successor to POCSAG that multiplexes many pagers onto one channel with time-framed, interleaved, BCH-protected data.1 A receiver locks the 32-bit sync marker 0xA6C6AAAA, reads a mode code, reads a frame-info word, then de-interleaves an 88-codeword phase and BCH-decodes each word before any address or message can be read.2

sync A6C6AAAA mode 16b FIW 32b phase · 88 words (2816 bits) FLEX codeword: 21 info (bits 0..20) 10 parity par info in the LOW bits — the bit-reverse of POCSAG, which puts info in the high bits bit-reversal preserves Hamming distance, so the same BCH(31,21) primitive decodes both
FLEX syncs on a fixed marker, reads a mode code and frame-info word, then de-interleaves an 88-word phase; each word is the POCSAG BCH(31,21) code with its bits reversed so the information lands in the low positions.

How it works

GopherTrunk’s Decoder is a four-state machine driven one wire bit at a time. In hunt-marker it slides a 32-bit window for 0xA6C6AAAA; like the paging protocols generally, it accepts the marker’s bit-inverse too and remembers whether it locked on the complemented sense. It then reads a 16-bit mode code (the speed/level word — 0x870C marks the 1600 bps two-level mode GopherTrunk decodes; other modes are recognised and cleanly skipped), and a 32-bit frame-info word from which it extracts the cycle number (fiw>>4)&0x0F and frame number (fiw>>8)&0x7F. Finally it collects the phase: 88 codewords of 32 bits each, PhaseBits = 2816 in total.

The phase is block-interleaved, so bits belonging to one codeword are scattered across the 2816-bit span to spread a fading burst into isolated single errors. The de-interleave maps the running bit counter c to codeword index ((c>>5)&0xFFF8)|(c&7) and bit position (c>>3)&31 within that word — reversing the standard’s permutation to reassemble each 32-bit codeword before it is decoded.

Frame and phase structure

Once the phase is BCH-decoded, its 88 information words are walked as a small self-describing structure:

Word Role Fields GopherTrunk reads
word 0 Block Information Word (BIW) voffset = (biw>>10)&0x3F, aoffset = ((biw>>8)&0x03)+1
words [aoffset, voffset) address words short capcode = word − 0x8000
word voffset + (i−aoffset) Vector Information Word (VIW) type (viw>>4)&0x07, first message word (viw>>7)&0x7F, length (viw>>14)&0x7F
message-word span message body 7-bit alphanumeric or 4-bit numeric symbols

Each address’s vector word names where its message words live, so the decoder can jump straight to a page’s text. Idle address slots (all-zero or all-ones 21-bit words) are skipped.

The bit-reversed BCH

FLEX uses the same BCH(31,21) code as POCSAG — generator 0x769, correcting up to two bit errors, plus an overall even-parity bit — but lays the codeword out the other way round. FLEX puts the 21 information bits in the low positions (0..20) with the 10 parity bits above them (21..30) and the even-parity bit at bit 31; POCSAG puts information in the high bits. Because a bit-reversal of a codeword preserves Hamming distance, GopherTrunk reuses its tested POCSAG primitive by reversing 31 bits into and out of it (FLEXBCHDecode32) rather than maintaining a second BCH decoder. The package notes that the exact on-wire bit order is reproduced from the de-facto reference decoders (multimon-ng, ARIB STD-T44) and should be confirmed against a real capture — a best-effort calibration caveat the code carries explicitly.

Sources

  1. FLEX (protocol) — Wikipedia, on Motorola’s FLEX paging protocol, its FSK modes and frame structure. ↩

  2. BCH code — Wikipedia, on the error-correcting code shared by FLEX and POCSAG. ↩

See also