Performance analysis of anti-collision protocols for RFID systems
Abstract
Recently RFID technology has made its way into end-user applications, enabling automatic item identification without requiring line of sight. In particular passive tags provide a promising, low cost and energy-efficient solution for inventory applications. However, their large-scale adoption strictly depends on the efficiency of the identification process. A major challenge is how to arbitrate channel access so that all tags are able to answer the reader inquiries and identify themselves over time. This paper stems from the observation that a variety of anti- collision protocols for RFIDs have been proposed in the literature.
However, a thorough simulation comparison among them and a clear identification of the mechanisms resulting in better end- to-end performance is lacking. The objective of our work has been to fill this gap. This paper presents the results of a detailed ns2-based comparative evaluation of representatives of all the classes of anti-collision protocols so far proposed. Simulation
results show that end-to-end performance of the different classes of protocols in terms of metrics such as the time needed for tags identification differ significantly over what previously found by experiments which only focused on the number of reading cycles for tag identification. Our thorough performance evaluation has highlighted that different solutions are to be used in different application scenarios and that decreasing the collisions (rather than idle times) is the way to go to further improve anti-collision protocols performance.
INTRODUCTION
A basic RFID system consists of a reader and a set of tags. The reader inquiries tags that are able to communicate on the radio channel, returning their ID. Tags are typically passive devices, which answer to reader’s query by back- scattering the received signal. One of the main objectives of a RFID system is the identification of all the tags present in the area covered by the reader. The challenges related to tags identification depend on the reference scenarios, which may include one or more readers, and a variable or stationary set of tags. The coexistence of multiple readers in the same area may cause collisions among readers interfering with each other or with tags. Moreover, collisions may occur among tags simultaneously transmitting to the same reader, indepen- dently of the presence of one or more readers. Collisions are addressed by specific solutions for the multi-reader problem (through frequency allocation mechanisms) and the single- reader problem (through collision arbitration schemes). The other distinguishing factor is given by scenario variability. In case of stationary applications scenarios (i.e., consecutive readings of the same, slightly changed, set of tags), the reader This work was partially supported by the MIUR International FIRB RBIN047MH9. may adapt the reading procedure according to information gathered through sequential identification processes. Instead, for applications involving new or highly variable tag popula- tions (i.e., most of or all tag IDs change in time), each iden- tification is based on a single-reading process, that operates without any knowledge on the environment. In this paper we focus on collision arbitration in single reader scenarios, in which it is not possible to apply an adaptive process (that is also the case of the first reading in adaptive protocols). The main representative protocols in this class follow techniques that were studied in the past for the multiple access problem. Specifically, a first group of protocols is inspired by slotted aloha protocols [1], while a second group of protocols draw on serial tree algorithms [2] (also called walking tree algorithms). In the aloha-based protocols, time is slotted and slots are grouped into frames. Frame size varies over time and is communicated by the reader at the beginning of each frame in the query message. Tags then randomly select one slot in the current frame: they will answer the reader query with their ID only when such slot comes. At the end of each frame, identified tags become silent. Only colliding tags go on to the next frame. Protocols in this class differ in the way tags are grouped into frames and tag population is estimated [3][4]. A detailed description of the main representatives of aloha-based protocols is given in Section II. Tree-based protocols proceed more deterministically: they iteratively query a subset of tags which match a given property until all tags are identified. These protocols are called tree-based because the identification process can be represented as a tree where the root is the set of tags to be identified, intermediate nodes represent groups of colliding tags answering the same request from the reader, and the leaves correspond to single-tag responses. Tree-based protocols differ in the way tags are queried (e.g., based on a counter stored in the tags, or the binary structure of tag ID’s). A detailed description is given in Section II. Although several anti-collision protocols have been pro- posed in the literature, an in depth underst anding of their relative performance is still lacking. In particular preliminary comparison between different solutions have failed to consider end-to-end performance metrics reflecting important aspects such as the time needed to complete the identification process. There is also the need to have a more thorough understand- ing of the impact of different parameter settings, tags ID distributions on the relative protocol performance. Goal of this paper is to fill this gap, performing an in-depth analysis.
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