Pager signal (FLEX)
A faster pager protocol developed by Motorola, at 1600, 3200 or 6400 bits per second.
What does it sound like?
A continuous, rough digital buzz or chatter.
Who uses it?
Commercial paging services and some hospitals and public agencies.
Where and when can I hear it?
In the US mostly 929-932 MHz.
The science
FLEX organizes time into a four-minute cycle of 128 frames, each 1.875 seconds long: a 115-millisecond sync signal followed by 11 interleaved data blocks of 160 milliseconds. It runs at 1600, 3200 or 6400 bits per second using two-level or four-level FSK; in four-level FSK each symbol is one of four frequencies and so carries two bits. Codewords use the same BCH(31,21) plus parity code as POCSAG, and interleaving spreads them out so a burst of up to 16 consecutive bit errors at 1600 bits per second (64 at 6400) can still be corrected.
Why use it?
The faster rates and tight time frames let one channel serve far more pagers; a 2000 trade article credited FLEX with quadrupling the capacity of other paging protocols. Because a pager is assigned to decode only certain frames, it can sleep in between, which extends battery life.
Lore
FLEX is Motorola's protocol, and by 2000, 229 paging operators in 48 countries were using it commercially. Japan's national paging air-interface standard, set by ARIB in 1995, is FLEX-TD, which is built on FLEX.
Easily confused with
- Pager signal (POCSAG): POCSAG bursts start with a long preamble