ieee802.15.4 specifies 40 symbols as LIFS value and 12 symbols as SIFS
value. Furthermore, the 2.4Ghz DSSS mode has a symbol rate of
62.5Ksymbols/s. To have the LIFS and SIFS in the code match the timings
from the specification, the TIMER_FREQ must match the symbol rate of
62.5Ksymbol/s such that one tick of the timer equals one symbol in time.
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