Implementing the Gen 2 MAC on the Intel-UW WISP

Abstract

We present the implementation and evaluation of the Gen 2 anti-collision protocol on the Intel-UW WISP. Because WISPs are wirelessly powered and highly energy constrained, appropriate MAC protocols are an open challenge. The previous approach to medium access on the WISP was to transmit data at every opportunity. This strategy consumes very little energy per data transmission, and works well when only one WISP is active at a time. However, when many tags are present and attempting to transmit simultaneously, their transmissions collide and no WISP can communicate with the reader. The Gen 2 RFID standard speci?es an anti-collision mechanism where tags randomize their transmissions across a number of slots, though this approach has been considered too energy intensive for use on the WISP. Through experimentation with a deployment of WISPs, we show that our Gen 2 MAC implementation allows all tags to communicate at a high rate, generally achieving better than 10 responses per second when close to a reader. This is in contrast to WISPs using the prior MAC approach where no tag can respond to the reader more than once per second in the same scenario. We additionally show that even for a single tag, the use of slotting does not degrade performance. This means that the full Gen 2 MAC is appropriate for use on the WISP, as it enables tags to communicate effectively in deployments consisting of one or many tags.