[Tinyos] Strange Ranges of Random Backoff Delays

[Anton]

Hello!

There is something in the 
/tinyos-2.x/tos/chips/cc2420/csma/CC2420CsmaP.nc (lines 218 and 225), 
which I don't understand.

Communicating wireless modules should wait some random delay before 
performing Clear Channel Assess.
According to the IEEE 802.15.4 standard, that delay should be   
random(2^BE-1)   backoff periods, where BE can vary from 3 to 5 (the 
default values macMinBE and macMaxBE, IEEE 802.15.4-2006, pages 
163-164). This means that for CC2420 the maximum backoff delay is 31*320 
= 9920 microseconds (when BE=5), and the maximum initial backoff delay 
is 7*320 = 2240 microseconds (since initially BE=3).

But according to CC2420CsmaP.nc file the maximum initial backoff delay 
can be around 9765 microseconds (line 218) and maximum congestion 
backoff delay can be around 2441 microseconds (line 225). So, the range 
of congestion backoff delay is shorter than that of initial backoff delay.


Of course, if BE (mentioned in the standard) is initially set to 5, the 
range of the initial backoff delay should be 9920 microseconds. But in 
such a case, all subsequent congestion backoff delays which might happen 
should take on some value within the same range (according to standard 
CSMA-CA algorithm).

Could anybody explain it?

Thanks.

Anton.