Feature

Meeting the demands of video-on-demand

As consumers in greater numbers adopt video-on-demand services, cable operators must make sure their networks can rise to the occasion.

By Robin Andrew, BTI Photonic Systems -- EDN, 9/14/2005

Cable MSOs (multiservice operators) are in a race with telcos to roll-out differentiated VOD (video-on-demand) services by leveraging their high bandwidth HFC (hybrid-fiber-coax) access networks. With VOD availability now nearing 100% in some markets (see table) MSO operators now face a twofold challenge. They must figure out how to scale their core networks to match this increased market penetration. And they must also extend their service footprint at the network edge to effectively reach remaining tier-two markets and regions.

The technology options MSOs have before them can be mystifying. And the choices they make will impact not only their own plans but also vendors of infrastructure gear and in turn consumers.

MSOs have invested a great deal over the last 10 years on last-mile access distribution. The access network is now well served by HFC networks that the MSOs have upgraded to 750/850 MHz using 256 QAM (quadrature amplitude modulation). A 750-MHz distribution plant serving 400 to 1000 homes has sufficient bandwidth to deliver more than 500 standard broadcast and VOD digital television channels using MPEG-2 encoding. The next challenge lies on the network edge and in the core, where MSOs will need new architectures in order to deliver and distribute this volume of content to the access network (Figure 1).

Delivering a VOD stream from a head-end to secondary or distribution hubs, and then on to the QAM access network, involves a dramatic growth in the volume of GbE (gigabit Ethernet) traffic in the core and edge of the network. MSOs can use either a distributed or centralized video-server architecture. In a distributed architecture, the network aggregates GbE from QAM access units at collocated Ethernet switches/routers in key secondary hubs. In a centralized architecture, the network backhauls all traffic directly to the core and drops it at centralized video head-ends. Either approach requires heavy bandwidth scaling at the edge.

Today, these edge transport networks remain predominantly SONET (synchronous optical networking)-based, with some deployment of DWDM (dense wavelength division multiplexing) in the metro core. Cable operators have recently deployed GbE at the edge with an independent overlay and backhauled over separate fiber to the core.

As VOD service penetration grows in the access network, the delivery of GbE transport is quickly becoming a bottleneck at the metro edge and core. Photonic-layer network architectures can offer an effective solution to address this bottleneck while at the same time leveraging the MSOs' current RPR (resilient packet ring) technologies and supplementing their plans for ROADM (reconfigurable optical add/drop multiplexer) deployments in the core.

Scalability and footprint

MSOs face four key network challenges in their bid to deliver scalable and cost-effective GbE transport for VOD service connectivity:

Transport costs:As adoption of high-speed-data and digital-video services increases, cable operators are looking for cost-effective transport schemes to meet the growing demands of their VOD networks. The technical complexities of transport across the core and edge of the network mean that backhaul costs increase greatly for widespread coverage. GbE is the cost-efficient transport mechanism of choice for multiple services across metro networks. To ensure their profit margins, MSOs are indicating cost targets for the VOD transport network of approximately $10 per video stream. With 240 MPEG-2 video streams encapsulated in a GbE transport signal, this correlates to $2400 per GbE for transport costs.

Carrier-grade technology:VOD operators require carrier-grade technology in their networks if they are to meet today's service demands while also designing in future scalability to reach tier-two markets. In addition to performance, systems deployed in service-provider networks must be able to provide:

• Built-in discovery and inventory of network assets
• Fault detection and isolation for troubleshooting
• Office alarms with the ability to provision thresholds
• Current and historical performance metrics to manage service-level agreements
• Hot-swappable service modules for in-service upgrades and additions.

Carrier-grade management and operations: VOD networks, which have both connectivity and GbE backhaul challenges, require network architectures that easily fit into existing infrastructures while remaining cost-effective. Equipment that offers integrated management including performance monitoring, inventory management, fault-detection, and service provisioning can reduce operational costs in addition to providing carrier-grade reliability.

Network scalability at the core and edge: Cable operators must address the VOD scalability problem—the tendency to congest networks with bandwidth-intensive video streams—at the core and the edge of the network. In order for operators to serve individual secondary hub sites at the network edge, approximately four to eight GbE streams are required with scalability to 10-Gbps Ethernet or greater for dense secondary or tier two sites. The core requires 150 Gbps, with 20 distribution hubs across the metro.

Today's way

To determine the best way to deliver GbE connectivity for VOD networks, we need to review the available technical approaches today. A number of technology strategies exist for providing VOD GbE connectivity across the metro, including ROADM-based, metro-DWDM systems, resilient-packet-ring approaches, and metro photonic-layer systems.

But before we look at these technologies, let's briefly discuss stand-alone photonic components and their role in initial VOD network rollouts. Simple to deploy and provision, photonic components initially found use in small GbE overlays. Examples of stand-alone components include GbE regenerators, amplifiers, media converters, and mux/demuxes in individual "pizza boxes." The relatively low cost of stand-alone building blocks enables ad-hoc network growth—for example, where only one link and one server need to be deployed—without significant investment in standardizing on a new WDM system. However, because these components lack fault management and remote-monitoring capabilities, scaling such deployments as VOD service demands increase proves difficult.

Cable operators need an approach that marries the value of stand-alone photonic components with a carrier-grade architecture to enable cost-effective scaling. But before we go there, it is worthwhile to discuss how RPR and ROADM technologies play into MSO VOD solutions.

Resilient Packet Ring: RPR is an alternative packet technology that includes grooming at Layer 2 for increased bandwidth flexibility. RPR is now a mature technology and readily available from a number of vendors. Stemming from a packet-based transport protocol, RPR is an optical packet transport ring for voice and data that interacts with multiple nodes on a ring. In RPR-based networks, voice and data packets get combined and placed on rings for transmission. An operator can deliver multiple services on the same network at the same time.

One of the primary benefits of RPR is more effective management of existing shared infrastructure. Operators can deploy RPR over existing SONET rings, which enables the combining of Ethernet packets within GbE pipes for transport with SONET-grade reliability. Dependant on the vendor, RPR solutions can also integrate into existing operational management systems and can be deployed as upgrades to existing SONET platforms.

RPR is an effective solution for multiservice delivery, as well as for increasing bandwidth utilization within existing SONET networks. But it has limited scaling potential. As already discussed, VOD networks involve transport of multiple GbE streams at the edge and potentially hundreds of streams in the metro core. Restricted by a fixed bandwidth between 2.5 and 10 Gbps, RPR approaches better suit data-service deployments than they do the massive bandwidth-scaling required for video. In cases where the number of GbE pipes may reach hundreds of streams, the packet overhead of RPR becomes prohibitive and overwhelms the economics.

ROADMs: ROADM-based, metro-DWDM technologies are now attracting a lot of attention because they promise compelling benefits for highly scaleable VOD networks. These systems enable full bandwidth flexibility to seamlessly reroute GbE streams on demand anywhere within the metro network. Next-generation ROADMs coupled with tunable-laser sources will provide dynamic reconfiguration and therefore be capable of providing the bandwidth required for VOD service connectivity.

The main value of a ROADM lies in its ability to support remote provisioning without the need to reengineer optical links. Such a capability enables network operators to reroute GbE circuits in response to time-of-day needs and unpredictable growth. The latest ROADM-based systems also provide extensive fault-management capabilities, which enable rapid isolation and protection against faults. Cable operators are pursuing today's ROADM-based approach primarily as a means of efficiently scaling the underlying infrastructure in the core of the network.

ROADM-based metro-DWDM systems suit the scaling to the needs of the core, but this automation comes at a cost. Although prices are dropping as the technology matures, the systems carry a price premium over traditional metro-DWDM systems. In addition, some ROADM vendors implement proprietary control schemes that result in significant training and overhead.

With scalability to potentially hundreds of GbE streams in the metro core, plus the benefits of service flexibility and automation, ROADMs justify the additional costs. They effectively connect hub and distribution sites centered around downtown areas of large metros. They are not, however, effective at the network edge, or as a feeder from secondary hubs, due to both their size profile and complexity.

Photonic layer systems

By combining the economic value of the photonic layer with carrier-grade functionality, an emerging class of photonic-layer products provides a scalable and cost-effective VOD architecture for delivery of GbE connectivity at the network edge. MSOs can exploit this architecture by deploying GbE-based optical transport in their existing infrastructure and also add new revenue-generating, on-demand services that differentiate them from their telco competitors.

Operators need an architecture at the edge that is more cost effective than ROADMs, yet able to adapt to the massive scaling of video better than RPR. VOD connectivity using a photonic-layer system delivers GbE backhaul with a very cost- and management-efficient transport architecture.

Photonic-layer systems are a new generation of metro-WDM systems optimized for metro-edge and access transport. They integrate key functionality for overlay of high-bandwidth service connectivity. They also provide the reach required to extend to tier-two cities and markets in a carrier-managed system that integrates with existing architectures and management networks. The functionality of these systems commonly includes CWDM coarse wavelength division multiplexing) and DWDM multiplexing, optical amplification, full network management, and wavelength conversion for the integration of "alien" or legacy wavelengths into existing networks.

Photonic-layer systems built for the edge are well suited for thin routes--they are typically optimized for four to eight WDM channels with scalability to 32 channels. The key difference over traditional core WDM systems is the tight integration of all these functions in a usable carrier platform that is both scaleable and easy to use.

Figure 2 shows a typical network architecture for VOD scalability including the photonic layer at the metro edge. In this example, the photonic layer provides a feeder for core ROADMs and interconnects the secondary hub with GbE for backhaul. Note that the edge could be 100 to 200 km away and may require optimized optical amplification to obtain the reach required.

Since photonic-layer systems are Layer 0/1 based, they are being deployed as complementary solutions to extend the scaling of RPR and Ethernet aggregation architectures. Operators can overlay GbE transport directly at the photonic layer using low-cost CWDM or DWDM. And if required, RPR-based SONET transport can be overlaid on a separate wavelength to provide aggregation of lower-bit-rate services. Photonic-layer systems also effectively complement core-based metro-DWDM and ROADM systems, acting as feeder systems to backhaul GbE traffic to the metro core.

An important requirement of photonic-layer systems for VOD connectivity is the ability to extend network reach to new markets and towns. Photonic-layer systems can integrate amplification and often dispersion compensation to extend the reach of WDM systems carrying Ethernet to 200 km and beyond—ideal for deployment of VOD to remaining outlying cities and towns.

The low cost and ease-of-use of photonic-layer systems enables operators to meet aggressive cost targets. Studies have shown that deployment of photonic-layer systems for backhaul of 16 wavelengths can result in transport cost of as low as $4 per video stream, as compared with $20 per stream in typical router-based, Ethernet-over-SONET and traditional WDM implementations today.

This economic value wouldn't mean much if these systems could not be easily integrated into core networks and existing management systems. Photonic-layer systems have the advantage of being Layer 0/1 based and easily integrated into the MSO's management framework. Without the complexity of ROADM- or SONET-based solutions, photonic-layer systems integrate simply into northbound TL-1 and SNMP-based management systems and provide intuitive simple craft interfaces. The benefits of a straightforward integrated system are reflected at all phases of the management lifecycle including installation, which typically takes less than two hours, to operation, which requires next to no training and simple maintenance.

VOD and other interactive video services dramatically increase the need for the network to massively scale. Photonic-layer systems are complementary to the technologies already deployed by MSOs in their VOD networks. Enabling scaling at the edge and complementing existing core RPR and ROADM investments, photonic-layer systems can help MSOs meet the VOD challenge.

Robin Andrew is strategic marketing managerat BTI Photonic Systems.