On Jul 11, 2007, at 11:26 AM, Chonggang Wang wrote:
Thanks for the posting, Phil. Yes, There is a backoff in CSMA-CA.
However the backoff is not that much to make 250 Kbps down to 35
Kbps. The reason is below.
According the IEEE 802.15.4 standard 2003 version, the average
backoff time will be (2^BE * aUnitBackOffPeriod * aUnitSymbolTime) /
2 . Also according to the standard, aUnitBackOffPeriod = 20 and
aUnitSymbolTime = 1/SymoblRate = 1/ 62.5K = 16 us (for 2.45 GHz
PHY). In this experiement, there is no collision, therefore BE =
macMinME. macMinME is between 0 and 3. If it is 0, there is no
backoff. Let us choose the maximum 3 and BE=3.
Therefore, for each packet, the average backoff time is about
(8*20*16)/2 = 1.28 ms. In my experiment, the UDP packet length is
110 Bytes and consider 10 bytes or so overhead, it is 120 bytes in
total, and the consumed time is 120*8/250K = 3.84 ms.
Therefore the achieved throughput should be around (3.84/(3.84
+1.28))*250K = 187.5 Kbps. Am my understanding right?
187.5 Kbps is still far larger than 35 Kbps. I still do not know
the reason.
TinyOS does not use an 802.15.4 MAC. Take a look at the MAC backoff
code.
Phil
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