APPENDIX A

FLEX OVERVIEW

FLEX SIGNAL STRUCTURE

A FLEX signal transmitted on the radio channel consists of a series of four-minute cycles, each cycle having 128 frames at 1.875 seconds per frame. A pager may be assigned to process any number of the frames. Battery saving is performed for frames that are not assigned. The FLEX signal can assign additional frames to the pager using collapse, fragmentation, temporary addressing, or carry-on information within the FLEX signal. Each FLEX frame has a synchronization portion followed by an eleven block data portion, each block lasting 160 milliseconds.

The synchronization portion indicates the rate at which the data portion is transmitted, 1600, 3200 or 6400 bits per second. The 1600 bps rate is transmitted at 1600 symbols per second using 2 level FSK modulation and consists of a single phase of information at 1600 bps, “phase A”. The 3200 bps rate is transmitted at either 1600 symbols per second using 4 level FSK modulation or 3200 symbols per second using 2 level FSK modulation and consists of two concurrent phases of information at 1600 bps, “phase A” and “phase B”. The 6400 bps rate is transmitted at 3200 symbols per second using 4 level FSK modulation and consists of four concurrent phases of information at 1600 bps, “phase A”, “phase B”, “phase C”, and “phase D”.

Each block has eight interleaved words per phase; thus, there are eighty-eight words, numbered 0–87, per phase in every frame. Each word has information that allows for bit error correction and detection contained within an error correcting code. The eighty-eight words in each phase are organized into a block information field, an address field, a vector field, a message field, and idle field. The boundaries between the fields are independent of the block boundaries. Furthermore, at 3200 and 6400 bps, the information in one phase is independent of the information in a concurrent phase, and the boundaries between the fields of one phase are unrelated to the boundaries between the fields in a concurrent phase. The synchronization portion consists of: a first sync signal at 1600 bps, a Frame Information Word having the Frame Number 0–127 (7 bits) and the Cycle Number 0–14 (4 bits), and a second sync signal at the data rate of the interleaved portion. The block information field may contain block information words for determining time and date information and certain paging system information.