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Page Scan In Different Detail
Original Post: Clock Phase (eGroups Msg.)
Date: 2000-08-02
Two main factors are used to set up a successful
page connection, the frequency sequence and phase of the paged device (the
phase is the position in the sequence)
- In the Page state the 'future' master knows the sequence of
frequencies in which the 'future' slave is going to listen to (PAGE
SCAN) (because it has collected the BD_ADDR when inquiring).
- The 'future' master uses an estimate of the current phase of the
slave, which may be computed from the last offset between the two
Bluetooth clocks. The precision of the estimation depends on various
factors (clock drift, time since last connection...).
However the two devices do not change frequency at
the same rate.
- The paging unit hops with a fast sequence (3200 hops/s, with 1/2 of
these for TX packets),
- While the scanning unit hops with a slow sequence (one hop every
1.28 s)
Therefore page packets from the paging unit will
quickly be picked up by the paged unit, as the paging unit will eventually
select the correct (hop) frequency that the paged unit is currently
scanning on. How long the paged unit remains in the page scan substate is
determined by the scan window (which must last for at least 16 page
frequencies , i.e. 10ms)
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