Traditional radio frequency identification (RFID) technologies allow tags to communicate with a reader but not among themselves. By enabling peer communications between nearby tags, the emerging networked tags represent a fundamental enhancement to today's RFID systems. They support applications in previously infeasible scenarios where the readers cannot cover all tags due to cost or physical limitations. This paper is the first study on identifying state-free networked tags, which is a basic, fundamental function for most tagged systems. To prolong the lifetime of networked tags and make identification protocols scalable to large systems, energy efficiency and time efficiency are most critical. Our investigation reveals that the traditional contention-based protocol design will incur too much energy overhead in multihop tag systems. Surprisingly, a reader-coordinated design that significantly serializes tag transmissions performs much better. In addition, we show that load balancing is important in reducing the worst-case energy cost to the tags, and we present a solution based on serial numbers.
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