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The data over cable service interface specifications (DOCSIS) of the Multimedia Cable Network System (MCNS) organization intends to support IP traffic over HFC (hybrid fiber/coax) networks with significantly higher data rates than analog modems and integrated service digital network (ISDN) links. The high speed access enables the delivery of high quality audio, video and interactive services. To support quality-of-service (QoS) for such applications, it is important for HFC networks to provide effective medium access and traffic scheduling mechanisms. We consider an HFC network that has a shared upstream channel for transmission from stations assigned with different service priorities to the headend. We first present a multilevel priority collision resolution scheme with adaptive contention window adjustment. This collision resolution scheme separates and resolves collisions for different classes of critically delay-sensitive and best effort traffics, thus achieving the capability of preemptive priorities. To enhance the performance of our scheme, we adopt a novel methodology in which the headend dynamically selects the optimal backoff window size according to the estimate of the number of contending stations for each priority class. A data transmission traffic scheduling policy with multiple priority queues is also employed in the headend. This scheduling strategy is used to satisfy bandwidth requirements of higher priority traffics. A set of simulation scenarios is conducted using OPNET to demonstrate the performance efficiency of the proposed scheme.