Emerging CATV networks are evolution to Hybrid Fiber/Coax plants. It is widely believed that the HFC network will play an important role in providing broadband accessing service to subscribers due to it's large excess bandwidth and high penetration to the home. Asymmetry broadband data service is the main service provided in the HFC broadband access networks now. It's performance will determine whether the subscribers be satisfied or not and affect the growth of HFC broadband access networks. The authors outline the mechanism of transmitting data in the upstream channel of the HFC network, and analyze how it affecting the performance of data service. The performance of data service and TCP protocol in HFC network is examined. An optimal scheme of bandwidth allocating algorithm for the upstream channel is proposed to enhance the performance of the data service over these networks.