The promise of intelligence, flexibility and choice finally is coming to the set-top box, with significant benefits for both cable operators and subscribers. As part of the CableLabs OpenCable initiative, the Data Over Cable Service Interface Specification (DOCSIS) Set-top Gateway (DSG) protocol defines a new transport mechanism for command and control signaling between the set-top box and the headend. By taking advantage of DSG as they migrate to a single network infrastructure that supports multiple services, cable operators can leverage their existing DOCSIS-based headend equipment, replace legacy video equipment as capital budgets allow and enable exciting new subscriber choices for digital cable services. Many operators are deploying or planning to deploy DSG this year with Broadband Solutions in Korea being one of the first to launch interactive TV services using DSG in March 2004. Constraints of legacy networks Proprietary video systems and historical architectures have created a legacy of separate networks for different cable services. Today, it’s common to have a DOCSIS-based network for data and voice services, a digital video broadcasting/asynchronous serial interface (DVB/ASI) network for digital video services and still another network for delivering analog video. Simplifying these networks, with their disparate standards and architectures, to improve operating efficiency, reduce expenses and more effectively scale services is a high priority for operators. The industry is well on the way to full convergence of data and voice networks and is making progress toward the migration of analog video to all-digital video services. However, considerable challenges remain in achieving a completely converged network architecture. One of the greatest challenges is the ability of cable operators to transition from today’s proprietary digital video architecture to a flexible, standards-based system. Because proprietary digital video technology extends to the set-top box and headend systems, it has slowed innovation, hampered the introduction of interactive services and kept capital and operating expenses high. A critical component of the digital video architecture is the command and control signaling transport mechanism. Existing video command and control signaling relies on a dedicated channel for out-of-band (OOB) signaling to transmit control messages downstream from the headend to each set-top box. Examples of control messages include conditional access entitlement, system information, electronic program guide and emergency alert system (EAS). Today, a very low-bandwidth upstream channel of about 15 kbps returns messaging traffic from the set-top box to the headend. The existing OOB transport requires separate, proprietary headend equipment such as OOB modulators and return-path demodulators, which provide one-tenth the bandwidth of the lowest-modulation DOCSIS channel at a much higher cost per port. Legacy video equipment is inefficient and costly to scale to meet the demands of current and emerging video applications that require capabilities for multicast and unicast transport. How DSG works DSG links existing digital video headend application systems and DOCSIS data networks to enable integrated transport of OOB signaling. The DSG protocol is implemented in both headend systems and set-tops. In the headend, a DOCSIS-based cable modem termination system (CMTS) combines the Internet protocol (IP) traffic sent to the set-top box with the OOB control messages and carries them securely and cost effectively over the existing “always-on” data channel. By using the CMTS as the OOB signaling gateway, operators can eliminate separate, proprietary OOB signaling systems. On the subscriber side, a DSG-compatible set-top box contains a DOCSIS cable modem to process the IP and OOB traffic separately from the video stream. All video is sent using quadrature amplitude modulation (QAM) over a separate channel to a separate tuner on the set-top box. The DSG set-top box does not depend on IP connectivity to receive OOB traffic from the CMTS, which makes the channel more robust and helps ensure uninterrupted video delivery in case of a failure at Layer 3 or above. However, IP connectivity is required to use the upstream return channel for interactive traffic. The figure shows a converged network architecture with a DSG-based CMTS for transporting OOB signaling. A CMTS communicates to a set-top box over a one-way IP datagram transport mechanism, known as a DSG tunnel. A DSG tunnel is carried over one or more downstream DOCSIS channels and is identified by an Ethernet media access control (MAC) address. In DSG basic mode, the MAC address is a well-known unicast or multicast address and is reserved and published by the conditional access system (CAS) or point-of-deployment (POD) provider and embedded in the POD or “smart card” of the set-top box. In DSG advanced mode, the MAC address is provided dynamically to the set-top box as part of the dynamic channel descriptor MAC management message. DSG advanced mode offers additional features such as traffic classification to specific tunnels and regionalization to deliver OOB to different service areas when all interfaces on the CMTS are bundled together in a single subnet. The CableLabs DSG Forum now is finalizing the DSG advanced mode.
The network-side interface between application servers in the video network, and the CMTS may be IP multicast or unicast. IP multicast provides an efficient mechanism to distribute the same IP datagram to many receivers and is preferred for distributing OOB content to the CMTS. The DSG configuration helps ensure that the IP multicast or unicast traffic is mapped to appropriate DSG tunnels, and in turn, to the right set of set-tops or to an individual set-top box. The application servers do not need to know the DSG client’s MAC address because they send content to a reserved IP address on the CMTS, in the case of unicast, or to a specified multicast IP address. Operators can use a source-specific multicast mechanism to help ensure that only authorized application servers send OOB messages to the set-tops. Technical advantages The DSG specification moves away from existing, proprietary OOB transport to open, standards-based technology. DSG offers several technical advantages, such as: Deployment scenarios DSG can be inserted into a traditional video network with the goal of maintaining video and high-speed data services under separate operational domains. A DSG-configured CMTS uses the existing cable video plant layout and introduces higher bandwidth via DOCSIS downstream and upstream channels to support bandwidth-intensive applications or a higher density of set-tops per headend or hub. This deployment scenario is best for headends or hubs that can accommodate increased set-top box density and higher bandwidth per set-top box for interactive applications. Increased revenue from new services will offset significantly the cost of giving subscribers new set-tops, making DSG migration an attractive proposition. A second deployment scenario yields the significant benefits of a converged DOCSIS-based network. Cable operators that offer interactive services can consolidate cable modem and set-top box traffic over a single DOCSIS network. Although the plant and spectrum topology are driven by data requirements (meaning fewer set-top boxes are addressed per segment), significant savings from the consolidation of equipment and operations into one network make this scenario a compelling option. Because current set-top box applications require less bandwidth downstream and upstream, OOB traffic in this scenario fits neatly within an existing data channel and consumes relatively little data throughput. However, as more sophisticated services such as video-on-demand (VOD) or gaming are launched, the ability to support advanced signaling will require much higher throughput and processing capacity. To meet this need, DSG brings the advanced monitoring and management capabilities of DOCSIS, as well as expanded throughput capacity, into the video infrastructure to accommodate increased traffic. Industry acceptance The United States and Korea are leading the market in adopting DSG technology. Throughout the coming year, cable operators will work with vendors of headend equipment, CASs and advanced set-tops to conduct evaluations and roll out DSG technology. The goal for this activity is to make a major step toward network convergence by determining the best solution for migrating legacy headend and set-top box equipment to DSG-based headend and set-top box equipment. DSG gives digital video networks a gateway to the continual innovations of DOCSIS, including advanced capabilities in spectrum monitoring and analysis, as well as network management. As cable operators will discover, DSG not only offers cost savings from consolidation of OOB, video and data traffic for a growing subscriber base; it also opens new revenue streams from interactive services. John Mattson is director of marketing with the Broadband Edge and Midrange Routing Group at Cisco Systems. Email him at Sanjay Dhar is a senior product manager with the Broadband Edge and Midrange Routing Group at Cisco Systems. Email at him at FIGURE: Converged Network Architecture with DSG

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