Jeremy Gummeson, Pengyu Zhang, and Deepak Ganesan
RFID-scale sensors present a new frontier for distributed sensing. In contrast to existing sensor deployments that rely on battery-powered sensors, RFID-scale sensors rely solely on harvested energy. These devices sense and store data when not in contact with a reader, and use backscatter communication to upload data when a reader is in range. Unlike conventional RFID tags that only transmit identifi ers, RFID sensors need to transfer potentially large amounts of data to a reader during each contact event. In this paper, we propose several optimizations to the RFID network stack to
support ecient bulk transfer while remaining compatible with existing Gen 2 readers. Our key contribution is the design of a coordinated bulk transfer protocol for RFID-scale sensors that maximizes channel utilization and minimizes energy lost due to idle listening while also minimizing collisions. We present an implementation of the protocol for the Intel WISP, and describe several parameters that are tuned using empirical measurements that characterize the wireless channel. Our results show that the burst protocol improves goodput in comparison to vanilla EPC Gen 2 tags, improves energy-efficiency, allows multiple RFID sensors to share the channel, and also coexists with passive, non-sensor tags.
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Computational RFIDs (CRFIDs) have recently emerged as a new class of pervasive sensing platforms. They are equipped with small but non-trivial energy buffers and can perform storage, sensing and computation. Because CRFID sensors have small energy buffers and often work in mobile environments, it is important for them to maximize communication goodput while minimizing the amount of energy per unit data during the short contact durations with readers. However, the EPC Gen 2 protocol adopted by commercial RFID readers is mainly optimized for large numbers of tags that each transfers a small amount of data. It leads to inefficient power usage and low throughput for bulk data transfers between CRFIDs and readers.
This paper proposes Flit, a new bulk transmission protocol for CRFIDs. Flit uses several interesting techniques to efficiently utilize the energy buffer of an energy harvesting CRFID node, while still remaining compatible with existing RFID readers. First, it enables each sensor to transfer data in a burst by responding to all slots in a query round (rather than just assigned slot). Second, Flit reduces potential for collisions by using explicit burst notifiers that are echoed by RFID readers to coordinates across sensors. Third, Flit duty-cycles the RFID sensor when another CRFID is in the middle of a burst, avoiding energy wastage due to overhearing of reader messages. Flit is evaluated on Intel WISP platforms. The results show significant performance improvements for a class of traffic.
This is a very solid system paper that systematically explores different ways to improve CRFID-reader throughput. There are several issues that this paper does not address. First, Flit explicitly treats every slot as available for burst transfer after every duty cycle. While this approach maximizes the throughput for the link between a CRFID and reader, the system performance may not scale well for a large number of CRFIDs. For the same reason, coexisting passive RFIDs may experience increased communication latency. Second, Flit does not guarantee error-free communication across multi-message bursts. Communication reliability is important for CRFID systems because errors could change the semantics of transmitted data. This issue can be addressed, to some extent, by implementing reliable data transfer mechanisms at a higher layer.
There has been a growing interest for exploring the use of CRFIDs in a few emerging pervasive sensing applications. This paper lays foundation for building data-intensive CRFID applications that go beyond conventional RFID-based passive identification. The increasing adoption of CRFIDs will likely open up new research opportunities in security, privacy, and mobile sensing.
Flit is a bulk transmission protocol for Computational RFIDs (CRFIDs) that increases the throughput and energy efficiency of small populations of CRFIDs. These improvements are achieved by reducing the number of un-utilized slots in communication rounds and by reducing the number of messages a given CRFID overhears while the reader interrogates a different CRFID.
One limitation of Flit is that it overloads Gen 2 messages for inter-CRFID coordination; this technique could result in collisions between Flit and non-Flit tags. Our protocol exploits the Gen 2 RN16 message, a 16-bit random number, which is used as a burst notifier that coordinates the activities of a population of CRFIDs. The value of this burst notifier is statically selected prior to deployment; this leads to a requirement that a range of RN16s need to be reserved for use as notifiers To minimize the likelihood of overlap between a static notifier used by Flit and a random value chosen by a non-Flit tag, the size of this range should be kept small. Depending on the number of anticipated passive tags, this could make it difficult to uniquely address CRFIDs. While this assumption is quite important in a general context, it may be relaxed when considering a particular deployment environment. For example, Flit would behave quite poorly in a warehouse environment were many thousands of passive tags are being used for supply chain monitoring because of the resulting channel contention. However, Flit would behave quite well in an environment where reading a passive tag is a more spurious event, rather than the common case. An example of this would be monitoring patient health in a hospital, where there would likely be small numbers of passive, non-Flit tags present. In designing Flit, we focus on a deployment environment similar to the latter scenario and focus on small numbers of CRFIDs (up to 5) and evaluate the impact of small populations of passive tags (up to 30) that are representative of those that may be spuriously read during the course of a deployment.
Another limitation of Flit is that it does nothing to guarantee the semantic correctness of large blocks of information that span multiple EPCs but were sent from a single CRFID. A reader requesting data from CRFIDs would need to disambiguate from which CRFID a given EPC belongs to; this is necessary as RN16s do not guarantee uniqueness for reasons previously discussed. Additionally, each message would need to contain a sequence number so that the reader knows how to correctly reconstruct a CRFID’s individual EPC messages into a single longer message. In designing Flit, our goal was to provide a lightweight protocol that maximizes throughput; a higher-layer protocol would utilize a portion of the bits in a 56-bit EPC message for addressing and acknowledgement. While this would reduce the effective application throughput, it would enable a class of applications that require robust communications.
By exploiting short message primitives and reducing coordination overhead, Flit improves the efficiency of communication between CRFIDs and conventional RFID readers. Our protocol allows existing readers designed for inventory applications to be used for a new class of applications. This flexibility will increase the adoption of CRFIDs for sensing tasks and will bootstrap the development of readers better optimized for communication with sensors.