The ISM frequency bands (902∼918 MHz (region 2 only), 2.4 GHz ∼ 2.483 GHz, and 5.725∼5.875 GHz) are unlicensed bands available for use by commercial technologies. In particular, the 2.4 GHz ∼ 2.483 GHz band is very popular and is currently used by WiFi, Zigbee, Bluetooth, and RFID technologies. Moreover, microwave ovens, hand held phones, and other wireless devices operate in the same frequency range, often interfering with each other. The effects of the coexistence of different standards are complicated and significant. However, network simulators do not model the effect of interference from colocated devices using different technologies. In particular, they model external noise, including interference from technologies outside the primary network being simulated, as a flat Gaussian component. In this paper, we propose a methodology of modeling external interference, by considering the realistic characteristics interference generated by co-located wireless standards. The traditional view that noise is flat in space and time, is inaccurate when one considers interference from co-located technologies. The noise signals are generated from their sources, attenuate over distance and experience fading. In addition, external interference exists only when its sources are active. Based on these characteristics, we propose a realistic and configurable noise model that captures such behavior. We demonstrate that this model can be well-fitted to real noise data. Moreover, we also demonstrate the impact of the model on the performance of network protocols as estimated by simulation.
Abstract. We consider the impact of transmission errors on the backoff algorithm behavior in the IEEE 802.11 protocol. Specifically, since the backoff algorithm assumes that all packet losses are due to collisions, it unnecessarily backs off when a packet is lost due to a transmission error. Two performance problems arise as a result: (1) low throughput, due to unnecessary loss of transmission time; and (2) unfairness when two competing links have different transmission error rates. In this paper, we characterize this problem and propose three solutions to it. The solutions aim to provide discrimination between transmission errors and collisions such that the sender can back off appropriately. The first algorithm relies on receiver discrimination and feedback; the receiving radio can in many instances differentiate between collisions and transmission errors. The second algorithm estimates the clear channel quality, and backs off if the observed quality deviates from the clear channel quality (indicating collisions). The third algorithm develops the probability of collision as a function of the number of observed idle slots during contention, and uses this probability to control the backoff algorithm. We show via simulation that the techniques significantly improve both performance and fairness of IEEE 802.11 in the presence of transmission errors.
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