[riot-devel] at86rf2xx radio driver -- potential severe performance issues for multi-hop communication

Andreas Weigel andreas.weigel at tuhh.de
Thu Oct 15 13:57:03 CEST 2015

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.weigel at 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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