FLEX (protocol)

FLEX (protocol)

FLEX is a communications protocol developed by Motorola and used in many pagers. FLEX provides one-way communication only (from the provider to the pager device), but a related protocol called ReFLEX provides two-way messaging.

Transmission of message data occurs in one of four modes: 1600/2, 3200/2, 3200/4, or 6400/4. All modes use FSK modulation. At 1600/2 this is on a 2 level FSK signal transmitted at 1600 bits per second. At 3200/2, this is a 2 level FSK signal transmitted at 3200 bits per second. At 3200/4, this is a 4 level FSK signal transmitted at 1600 symbols per second. Each 4 level symbol represents two bits for a bit rate of 3200 bits per second. At 6400/4, this is a 4 level FSK signal transmitted at 3200 symbols per second or 6400 bits per second.

Data is transmitted in a set of 128 frames that takes 4 minutes to complete. Each frame contains a sync followed by 10 data blocks. The data blocks contain 256, 512 or 1024 bits for 1600, 3200 or 6400 bits per second respectively.

A BCH type ECC is used to improve the integrity of the data. The standard has been designed to allow the pager's receiver to be turned off for a high percentage of the time and therefore save on battery usage.