Hi everyone,
I've recently executed some experiments on the performance of Atmel's
extended operating mode on the ATmega256RFR2 (which afaik is basically
the same as the AT82RF2xx transceivers) in multihop collection traffic
scenarios, with unslotted CSMA/CA. For those, we used our own framework
for WSNs (CometOS). Result is, that the extended operating mode can
severely impact performance (in terms of PRR) in the presence of large
fragmented datagrams sent over 6LoWPAN. However, I expect that there
will be an impact in any traffic scenario with "enough" traffic to get
some node's queue filled to some extent (resulting in repeated
transmissions towards the same node).
Problem is the property of the TX_ARET mode to discard any incoming
transmissions during the backoff phase of the CSMA mechanism. This leads
to a large number of lost frames on longer paths (4 hops are enough to
produce a large number of losses), even with rather high backoff
exponents and the maximum number of retries. This problem caused me to
reimplement our mac driver for the ATmega256RFR2 using the basic
operating mode.
As far as I can see, the at86rf2xx driver in the riot repo uses the same
extended operating mode and therefore will very likely have the very
same weakness. If the time is available, it would probably be a good
idea to change the driver to use the basic operating mode or
alternatively include a note for users explaining the impact of the
extended operating mode for said traffic scenarios. The frame solely
caused by the extended operating mode can render results of experiments
virtually useless (speaking from my own, sad experience ;-/ )
Kind Regards,
Andreas Weigel
PS: The results of my experiments are published as "Hardware-Assisted
802.15.4 Transmissions and Why to Avoid Them"
(http://link.springer.com/chapter/10.1007%2F978-3-319-23237-9_20). If
anyone without free access to the resource is interested in the paper,
please let me know and I can provide you the whole article.
--
________________________________________________________________________
Hamburg University of Technology andreas.wei...@tuhh.de
Institute of Telematics (E-17) Tel.: (+49) 40 42878-3746
Am Schwarzenbergcampus 3 Fax: (+49) 40 42878-2581
21073 Hamburg, Germany http://www.ti5.tu-harburg.de/staff/weigel
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