Upstream QoS Scheduling for Voice-over-IP Service in CATV Access Networks

摘要

This paper presents a study of upstream scheduling in CATV access networks for providing quality of service (QoS) to realtime services. Although the current high-speed data service over CATV networks is basically a best-effort access to the Internet, real-time services such as voice over IP and video over IP are expected to be widely spread. In order to ensure the quality of these real-time services, upstream medium access control (MAC) is crucial because the CATV uplink has limited resource and is shared by hundreds of users. The DOCSIS (Data-Over-Cable Service Interface Specifications), a de-facto standard of PHY/MAC protocols for CATV networks, provides various access methods for real-time services to reduce the network latency. Based on the DOCSIS, we enhance its QoS related features and investigate its effect on the QoS of voice-over-IP traffic. By combining priority based scheduling scheme with these access methods, the access delay of voice traffic can be controlled small enough even under congestion. The number of simultaneous voice flows is evaluated to keep the voice packet loss rate low. We also devise a policing mechanism to prevent violation of the traffic contract. It is confirmed that the throughput of non-conformant flows is kept at the reserved bandwidth under network congestion while the QoS of conformant flows is unaffected.