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Old-web POCSAG notes plus a wider paging index at braddye.com/paging.html. Capcodes, batch math, the stuff the ITU PDF assumes you know.
http://www.braddye.com/pocsag.html
Modes 1600/2, 3200/2, 3200/4, 6400/4. Sync + 11 blocks per frame. Battery-save by sleeping between assigned frames. No encryption.
https://handwiki.org/wiki/FLEX_(protocol)
Feb 1998 table: POCSAG 2400 2-FSK async; FLEX 6400; ReFLEX-25 6400/9600 on 25/12.5 kHz; ReFLEX-50; InFLEXion. Best one-page comparison that is not a vendor slide.
https://archive.org/details/Hewlett-Packard_Journal_Vol._49_No._1_1998-02_Hewlett-Packard
Radio Paging Code No. 1. Official POCSAG recommendation. 512 / 1200 / 2400 bit/s, 32-bit codewords, preamble plus batches.
https://www.itu.int/rec/R-REC-M.584/en
What the signal looks like. 2-FSK, ±4.5 kHz typical, VHF / UHF / 929–932 MHz commercial, 439.9875 MHz ham DAPNET.
https://www.sigidwiki.com/wiki/POCSAG
Readable walkthrough. 576-bit 1010 preamble, frame sync, eight frames of two 32-bit words, function bits 20–21 pick numeric vs alpha.
https://pic.hallikainen.org/techref/pager/pocsag.html
Motorola one-way successor to POCSAG. 1600 / 3200 / 6400 bit/s, 2- or 4-level FSK, 128 frames every four minutes.
https://www.notepage.net/learning-center/what-is-flex-paging.htm
European Radio Message System. 6250 bit/s four-level FSK. Dead commercially; listed so the protocol shelf is complete.
https://en.wikipedia.org/wiki/ERMES
FLEX one-way, ReFLEX two-way, InFLEXion voice. The Motorola family tree in one page.
https://en.wikipedia.org/wiki/FLEX_(protocol)
Short history and the three bit rates. Start here, then open M.584.
https://en.wikipedia.org/wiki/POCSAG
