Also known as: POCSAG
POCSAG (the CCIR Radiopaging Code No. 1) is the classic one-way paging protocol used worldwide to deliver numeric and alphanumeric messages to pagers. It is a simple asynchronous 2-level FSK scheme, standardised internationally as ITU-R Recommendation M.584, and still in active use by hospitals, fire/EMS, and industry.12 Its name comes from the Post Office Code Standardisation Advisory Group, the British committee that defined it.
Overview
POCSAG is deliberately austere, which is why it has outlived far more sophisticated systems. A transmitter sends a long preamble (a 576-bit alternating pattern that lets battery-saving pagers wake up and lock their bit clock), then a repeating structure of batches. Each batch begins with a fixed 32-bit sync codeword and contains eight frames of two 32-bit codewords each. A pager’s identity (capcode or RIC) selects which of the eight frame positions it must listen in, so most of the time a pager’s receiver can sleep and only power up for its slot — the trick that gives pagers their famously long battery life. A codeword is either an address codeword (marking that a message for a given capcode follows) or a message codeword (carrying the numeric or alphanumeric payload).
Technical characteristics
| Property | Value |
|---|---|
| Modulation | 2-FSK (±4.5 kHz typical deviation) |
| Bit rates | 512, 1200, 2400 bps (receiver auto-detects) |
| Codeword | 32 bits: 1 flag + 20 data + 10 BCH + 1 parity |
| Coding | BCH(31,21) + even parity |
| Batch | 1 sync codeword + 8 frames × 2 codewords |
| Content | Numeric (BCD), alphanumeric (7-bit), or tone-only |
How it works
Every 32-bit codeword carries only 20 or 21 bits of usable payload; the rest is a BCH(31,21) code plus an overall even-parity bit. BCH lets a receiver correct up to two bit errors and detect more in each codeword, which is what makes an uncoded, unacknowledged one-way link reliable enough for life-safety paging — there is no return channel to request a retransmission, so the forward error correction has to carry the whole burden. Numeric messages pack four bits per digit (BCD); alphanumeric messages pack seven-bit characters across consecutive message codewords. Because the three bit rates share the same codeword structure, a decoder typically locks the preamble, measures the symbol period, and then reads the rest at whichever of 512/1200/2400 bps it detected.
History
POCSAG was developed by a British Post Office working group and adopted by the CCIR (now ITU-R) in 1981 as Radiopaging Code No. 1, later maintained as Recommendation M.584.2 It rapidly displaced earlier, slower two-tone and five-tone paging and became the dominant global paging code through the 1980s and 1990s. Higher-capacity successors such as FLEX and the European ERMES system were built to handle traffic POCSAG’s 512/1200 bps rates could not, but POCSAG’s simplicity kept it in service long after commercial paging declined.
Deployment
POCSAG remains widely used where a robust, infrastructure-light one-way alert matters: hospital and clinical paging, fire and EMS dispatch, industrial and process-control alerting, and utility SCADA notifications. The amateur-radio DAPNET network runs POCSAG for operator paging. Because a POCSAG transmitter is little more than an FSK modulator, it is also a common beginner target for SDR decoding.
Decoding it with GopherTrunk
POCSAG is squarely in scope for GopherTrunk. The decoder FM-demodulates the channel, recovers the 2-FSK symbol stream, synchronises on the preamble and sync codeword, BCH-checks and error-corrects each codeword, and reassembles numeric and alphanumeric messages by capcode — no proprietary vocoder or encryption stands in the way (though some operators scramble content at the application layer, which GopherTrunk does not attempt to unscramble). See the POCSAG decoder page.
Sources
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POCSAG — Wikipedia, for the CCIR Radiopaging Code No. 1, its FSK bit rates, BCH coding, capcodes, and codeword/batch structure. ↩
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Recommendation ITU-R M.584 — ITU-R, the international standard for codes and formats for radiopaging that formalises POCSAG. ↩ ↩2