Despite the fact that the third generation (3G) mobile system is expected to provide a wide range of services, including data access, it has been designed primarily for voice services and it provides QoS guarantee in terms of bit rate and delay. These QoS metrics are more suitable for a real time streaming service rather than the dominant data service in the Internet. The paper proposes new QoS metrics, and response time, for Internet data access in the Universal Mobile Telecommunication System (UMTS). The new metrics reflect the user's expectation of QoS. Networks can change delay and bit rate according to the current situation while keeping the response time constant. A proxy based architecture is presented to support the new QoS guarantee. The proxy caches files and helps the wireless network to schedule file transfer according to the system capacity and user demand. Theoretical analysis and simulation show that the new scheme can improve user-level performance and network utilization.