High-quality multiservice transmission on a single network using DWDM
With the growing popularity of applications such as cable television, telemedicine, distance education, and business conferencing/interactive media, competitive service providers are looking to optimize their current network infrastructures for the delivery of video traffic. Designed with voice and data in mind, these networks are not well equipped to meet the stringent quality requirements of video transmission.
The "bookends" for video-service performance are broadcast services, which require high signal performance, bandwidth, and reliability, and videoconferencing, which consumes the lowest bandwidth, has the lowest price, and carries with it a lower-quality expectation (to date). While each video application has different network and service requirements based on a tradeoff between price and performance, customer expectations are continually increasing-quality is a significant differentiator for service providers looking to gain new business for their video services.
Service providers who attempt to "shoehorn" video traffic into their current telecommunications architecture will face a critical problem. Most uncompressed, native data rates do not map well into the standard telecommunications hierarchy. Full-bandwidth D1 video (serial digital component) operates at 270 Mbits/sec, while the basic business-quality videoconferencing system consumes 128 kbits/sec-less than 1/2000 of D1 video requirements. For comparison's sake, a residential TV signal runs at approximately 150 Mbits/sec. (The table indicates the diverse bandwidth requirements of other types of video services.)
A dense wavelength-division multiplexing (DWDM)-based, optical-networking solution can overcome this hurdle to enable the delivery of video as well as voice and data over a single integrated platform.
Traditionally, service providers have chosen between two options for accommodating new services in their existing networks (see Fig. 1). They can compress the video stream into a standard network payload, which increases the transport cost and degrades the signal quality, or they can build an independent, service-specific network overlay, which maintains signal integrity but incurs the cost of operating multiple, independent technology infrastructures.
For example, to transmit video cost-effectively and efficiently in the interoffice loop over the public network, compression is used to manipulate the required bandwidth into the standard network payloads (i.e., DS-3). Various forms of compression exist to reduce the amount of bandwidth consumed by a video signal, including Linear PCM, MPEG-1, MPEG-2, and Wavelet. While compression makes more efficient use of network bandwidth, it comes with a high equipment cost and significant signal degradation.
Generally, the attractiveness of signal compression is in direct proportion to the distance the signal travels. The shorter the signal distance, the less likely a service provider is to choose compression. Thus, in the local loop, video is almost always transmitted by means of a network overlay. For the short distances involved, it is simply more economical to couple the electrical signal to an optical fiber than it is to process the video to reduce its transmission bandwidth. At the same time, the video signal quality can be maintained because uncompressed video offers completely lossless transmission. In addition, the Synchronous Optical Network (SONET) bandwidth that was allocated to video services in the original fiber plant can be used to offer higher revenue-generating services such as Internet access. Video typically generates only one-third of the potential revenue per SONET bit when compared to data-services revenue.
Despite the advantages an overlay network offers in comparison to signal compression, it still carries the drawbacks associated with the operation of multiple, independent networks. An optical networking solution based on DWDM enables flexible, economical delivery of voice, data, video, and other services at wire speed over a single integrated platform. Multiple, protected wavelengths are used as a high-capacity, flexible, service delivery vehicle.
A transport network based on DWDM enables service providers to carry signals economically in their native form at the optical layer. Here, traditional protocols such as SONET OC-n and nonstandard services such as video (D1 video, HDTV), Gigabit Ethernet, and mainframe networking (Escon/Ficon/Fibre Channel) can be transported simultaneously, eliminating the need for service-specific network overlays and increasing the network's economies of scale (see Fig. 2). Because services are carried at wire speed on a wavelength, compression and network overlays are no longer required to fit nonstandard optical services into the telecommunications infrastructure.
As the number of video services required by customers increases, DWDM can be used to increase the effective bandwidth of fiber, thereby reducing the bandwidth bottleneck and eliminating the need for compression. DWDM technology provides increased throughput by allowing multiple wavelengths of light, each carrying a separate service, to be multiplexed onto a single fiber. The optical layer becomes the focal point for the convergence of a multiservice network.
The first applications of DWDM were purely for fiber relief of point-to-point spans in long-distance networks. Since the dominant networking costs are fiber infrastructure and equipment at intermediary sites, service providers who offer long-distance networking realized substantial value from a solution that carries the maximum number of wavelengths on a single fiber.
With the introduction of optical add/drop multiplexers (OADM), DWDM allows individual wavelengths to be added, dropped, or passed through a node, truly providing optical networking with a wavelength. This functionality is similar to SONET ADMs, except that wavelengths replace time slots as the unit of transport.
In a DWDM solution that offers protocol and bit-rate independence, wavelengths can be individually turned up to a higher speed or a different protocol without a single change to the DWDM infrastructure. Bit-rate and protocol-independent interfaces on DWDM equipment offer an easy evolutionary path to accommodate increased capacity requirements or next-generation interfaces. As an example, a service provider's customer may request a technology upgrade from D1 video to HDTV. DWDM technology can address the new service request without new tributary interfaces or another independent transport system, which could potentially lead to fiber shortages. This flexible platform for service delivery enables rapid service introduction, minimizes field dispatches, and lowers overall operations cost, giving a service provider a unique competitive advantage.
The era of network convergence is here and the vision of a single network capable of providing voice, high-speed data, and various forms of video service is becoming a reality. There will no longer be the need to build multiple network overlays for each service type. Instead, a single network will merge the many overlays currently deployed into one seamless network with a common infrastructure and management. This point of convergence is the optical layer, enabled by DWDM technology and the deployment of metropolitan DWDM solutions.
A DWDM-based solution for service convergence enables service providers to reduce capital and operations, administration, and maintenance costs by maximizing the use of their existing fiber plant and eliminating multiple network overlays. The ability to carry multiple services on wavelengths in a single fiber increases economies of scale. The management simplicity of a single multiservice network as opposed to multiple network overlays dominated by service-specific, point-to-point systems also reduces costs and increases service providers' responsiveness to new service requests.
As customer requirements keep pace with rapid application developments, service providers must find a single platform that is optimized to deliver all types of traffic, including those not yet on the market. They must be able to turn up these services quickly and easily to differentiate their offerings from competitors. A DWDM-based solution provides a robust, scalable, multiservice platform that gives service providers the flexibility to mix services on a fiber and bring them to market quickly in response to unpredictable demand.
Paul Schoenau is network planner, OPTera solutions, at Nortel Networks (Brampton, ON, Canada). Brendan Smith is vice president, business development, at Artel Video Systems (Marlborough, MA).