TV distribution is changing rapidly as traditional analog systems are replaced by their digitized counterparts, which are more interactive and use bandwidth more efficiently. Existing digital distribution systems generally rely on installed communal cabling to deliver TV signals to household set-top boxes. Although this approach has the advantage of requiring a low initial capital investment, it cannot guarantee future-proof converged services such as high-definition video, voice, and high-bandwidth Internet access. By adopting a delivery platform such as an Internet protocol (IP) multimedia subsystem, various voice and video services can be run on top of an IP bearer network. The key challenge is that IP makes the stringent quality of service (QoS) and availability requirements of multimedia services difficult to meet. Building an all-service delivery platform on which more value-added options can be carried is becoming essential for carriers and vendors alike. The 3TNet project, supported by China's high-tech 863 Program, is a pioneering research initiative to achieve this goal.1,2
Instead of using an all-IP or all-optical architecture, 3TNet employs a hybrid solution, with circuit switching in the core network and packet switching for distribution and access. On the one hand, this greatly simplifies QoS provisioning and traffic engineering in the core, enabling it to take advantage of mature optical transport technologies. On the other hand, it also preserves the ease of use, ubiquity, and scalability of IP technology. In September 2006, five years after launching this project, a large-scale field trial was carried out on the resulting 3TNet testbed. As many as 15,000 users across eastern China witnessed the success of this effort.3
Figure 1. Headend (HE) devices encode digital TV signals into separate multicast streams and push them to the optical network. There, IP multicast streams are aggregated and delivered to residential offices in point-to-multipoint connections. Finally, the access module sends the appropriate channels to household set-top boxes. FE: Front end. LAN: Local area network. LSP: Label-switched path.
The main challenge in building the hybrid network is getting the circuit-switched core network to interoperate successfully with the entirely different packet-switched distribution and access networks, while at the same time maintaining satisfactory efficiency. 3TNet addresses this problem through the innovative solutions of dynamic optical circuit switching and optical multicasting.
In 3TNet, the core optical network consists of 13 synchronous digital hierarchy cross-connects, each equipped with a generalized multiprotocol label-switching (GMPLS) control plane entity. By incorporating a calling interface into the operator's billing and order support system, dynamic end-to-end Ethernet virtual links can be invoked by routers and media servers. The calling interface is implemented according to the Optical Internetworking Forum's User Network Interface 1.0 signaling specification with Ethernet extension.4 A set of metrics for assessing the performance of dynamic circuit switching for 3TNET is now being discussed by the Internet Engineering Task Force's Common Control and Measurement Plane working group.5
While dynamic circuit switching is generally designed for point-to-point services such as content delivery and video on demand, multicasting through point-to-multipoint connections is also needed. As illustrated in Figure 1, control devices (headends) encode TV channels into separate IP multicast streams and push them to the edge of the optical network. Point-to-multipoint (P2MP) connections are set up in the optical network to carry the IP multicast streams from the central office to residential locations. Since a P2MP connection provides more bandwidth (e.g., 1000Gbps) than a single IP multicast stream needs, channels are aggregated to increase bandwidth efficiency.6 The P2MP feature in the optical network is realized by extending the standard point-to-point signaling protocols of GMPLS.4 Using IP multicast for access retains the flexible and scalable IP group management. The coarse granularity of the P2MP connection reduces the number of multicast states, increasing the stability and scalability of the core network. Our experimental results show that this type of multicast can provide much better QoS performance than all-IP multicast.7
Providing a total bandwidth of more than 40Mbps per user, 3TNet is capable of delivering standard and high-definition TV signals to households. Because IP technology is retained on the user side, the network is also capable of carrying high-speed Internet access, as well as other IP-based services that a typical residential user may need. This is important given the continual rise in bandwidth-consuming applications with ever more stringent QoS requirements. Also, as the security and privacy concerns of end users increase, it is desirable to incorporate more sophisticated mechanisms into the network that allow people to use and manage their own private end-to-end lines or networks.
The authors would like to acknowledge the support of the National Natural Science Foundation of China under grants 60602010 and 60502004, and of the 863 Program under grants 2006AA01Z247 and 2006AA01Z245.
Weiqiang Sun, Weisheng Hu
State Key Lab on Advanced Optical Communication Systems and Networks
Shanghai Jiao Tong University
Weiqiang Sun is a lecturer in the State Key Lab on Advanced Optical Communication Systems and Networks at Shanghai Jiao Tong University in China. He received his PhD degree from the University of Science and Technology of China in 2004. His has published more than 20 papers in major international conference proceedings and journals.
Weisheng Hu is a professor and director of the State Key Lab on Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University. His interests lie in the areas of generalized automatic switched optical networks and optical packet switching. He is the author or coauthor of over 100 journal and conference papers.