WDM has emerged as a critical enabling technology for operators delivering new bandwidth-intensive services over existing access networks. The clearest example of the adoption of WDM is in video on demand (VoD), where Ethernet over WDM has emerged as the transport technology of choice for cable operators delivering on-demand video services over existing cable networks.
The low cost and inherent flexibility of the Ethernet over WDM architecture are allowing a VoD business model that really works. In the United States, WDM has been deployed to transport VoD services by six of the top seven cable operators. In Europe, high-speed data services are driving the scalable provisioning of new bandwidth over existing cable networks, and here again WDM is being leveraged to accommodate rapidly growing cable-modem traffic over existing analogue video fibre networks.
Telewest's high-capacity 100-GHz deployment is a good example of how DWDM is being used here in Europe to meet growing customer demand for high-speed data services over an existing fibre network. The use of DWDM enables the delivery of new narrowcast broadband services that will mean new revenue. Today, the most important such service is Internet access via cable modem. In the future, as customer preferences and business models evolve, the same infrastructure could be used to support VoD, interactive gaming, and other custom broadband services.
Within the cable industry, we see WDM being adopted to support two main architectures/network topologies.
In the first architecture ("traditional"), WDM is used to augment the bandwidth interconnecting headend and hubs (a.k.a. secondary headends), often in a ring topology. Here, the WDM wavelengths are used to scale the bandwidth between central headend and hub locations to carry expanding traffic associated with new services such as cable-modem traffic and VoD content over existing fibres. In this architecture, the WDM wavelengths are terminated at the hub locations (i.e., received by an optical-to-electrical optical receiver), and the traffic is then transported between the hub and the optical node at the edge of the fibre network over traditional single- or dual-wavelength technology.
In the second newer architecture, WDM is used not only to interconnect headend and hubs, but also to deliver narrowcast (i.e., cable modem or VoD) traffic to the optical node at the edge of the fibre access network. In this topology, WDM is exploited for its bandwidth advantages as well as for the way in which wavelengths carrying different traffic payloads can be passively directed to different optical-node locations. This architecture eliminates the need for the optical-electrical-
optical translation at the hub locations, consequently enabling hubs to be reduced from being full buildings to simple cabinets that contain only passive optical components used to direct different wavelengths to their appropriate destination.
However, there are interesting differences in the relative importance of various broadband services in different parts of the world. These differences seem to stem from differences in culture and government policy, differences in competitive dynamics, and differences in the availability of content. In Korea and Japan, the governments have encouraged broadband deployments, and interesting new IP-based education, gaming, and video communications applications are emerging as people achieve a critical mass of broadband connectivity. In the United States, intense competition between cable and satellite operators has encouraged the cable operators to aggressively deploy VoD as a means of service differentiation. In the United Kingdom and Europe, high-speed Internet connectivity seems to be more compelling for customers than new video-based services. From a technology perspective, however, the underlying network mechanics and trends are the same: more Ethernet (carrying data and/or video)—and WDM to carry the Ethernet in parallel with legacy broadcast services.
Where fibre has been installed and with the aid of WDM and scalable transmission technology, network operators can now flexibly provision new bandwidth over their network in a matter of weeks, allowing "just in time" transmission equipment expense and optimised return on new investment. Technologies such as WDM that can enable broadband service providers to deliver new services and tap into new revenue streams over an existing physical network infrastructure are of tremendous economic value. The growing demand for a wide array of new broadband services translates into significant opportunities for broadband service providers.
Patrick Harshman, President
Broadband Access Networks, Harmonic Inc.