  Computational RFID (CRFID) devices are emerging platforms that can enable
perennial computation and sensing by eliminating the need for batteries.
Although much research has been devoted to improving upstream (CRFID to RFID
reader) communication rates, the opposite direction has so far been neglected,
presumably due to the difficulty of guaranteeing fast and error-free transfer
amidst frequent power interruptions of CRFID. With growing interest in the
market where CRFIDs are forever-embedded in many structures, it is necessary
for this void to be filled. Therefore, we propose Wisent-a robust downstream
communication protocol for CRFIDs that operates on top of the legacy UHF RFID
communication protocol: EPC C1G2. The novelty of Wisent is its ability to
adaptively change the frame length sent by the reader, based on the length
throttling mechanism, to minimize the transfer times at varying channel
conditions. We present an implementation of Wisent for the WISP 5 and an
off-the-shelf RFID reader. Our experiments show that Wisent allows transfer up
to 16 times faster than a baseline, non-adaptive shortest frame case, i.e.
single word length, at sub-meter distance. As a case study, we show how Wisent
enables wireless CRFID reprogramming, demonstrating the world's first
wirelessly reprogrammable (software defined) CRFID.
