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Wireless networks require strong security mechanisms due to their open medium. However, security effects system performance, and therefore impacts quality of service (QoS) of communications. To analyze the impact of security on system performance, we conduct a detailed experimental study on a wireless IP testbed with security at different layers. We study their impact on different types of data streams such as TCP and UDP with regard to authentication time and cryptographic overhead. Specifically, we experiment with the most widely used security protocols such as WEP, IPsec, 802.1x with RADIUS, and SSL. We classify security protocols into individual and hybrid policies. Then, a new metric, relative security index, is introduced to analyze security strength and overhead tradeoffs quantitatively. Our results demonstrate that the stronger the security, the more signaling and delay overhead; whereas, the overhead does not necessarily increase monotonically with the security strength. Also, we notice that authentication time is a more significant factor than cryptographic cost regarding their contributions towards QoS degradation in wireless networks.