Uncertain capacity in newly emerged Cognitive Radio Networks(CRN) renders specific performance analysis for the secondary users (SUs), particularly in the aspects of Qualityof-Service (QoS). The effective bandwidth concept is employed to analyze the SUs' performance. Considering the time-varying arrival and service process, we analytically obtain the QoS metrics, such as the approximated delay-violation probability and mean time delay. The accuracy of the approximation is demonstrated via simulation. With the extension to multiple secondary users, we then consider the inevitable sensing errors in CNR in a cross-layer framework, and introduce the promising collaborative sensing method among SUs. By integrating to upper link layer, we then study the collaborative gains in terms of mean time delay.