Setting new sights with Sonet

April 1, 1997

Setting new sights with Sonet

An expanding set of services and solutions allows network planners and service providers to design flexible, customer-responsive networks and to improve management control using Sonet technology

bijan khosravi


As a technology that has relieved service providers from the grip of today`s bandwidth-hungry applications, Synchronous Optical Networking (Sonet) is assuming an even greater role in addressing the total needs of today`s service providers--creating opportunities to respond to customers more quickly and to become more aggressive in offering new services.

For example, accommodating present business demands has prompted many service providers to expand the transmission rates of some long-haul and local routes to 20 Gbits/sec. Yet consumer needs for higher bandwidth have spawned an adoption rate that is more than doubling each year and should continue doubling annually over the next several years. To support this explosion, industry analysts predict, bandwidth growth could exceed 100 Gbits/sec by the year 2000. As Sonet penetration and network capacity continue to increase, asynchronous-based services should continue to decrease, further driving the demand for Sonet interfaces (see Fig. 1).

At the core of this evolution toward Sonet is an expanding set of services and solutions that allows service providers to design flexible, customer-responsive networks. Service providers will also have the ability to provide customers with more control over the network. The result is a network solution that not only breaks capacity barriers, but is also more cost-effective and reliable.

One model of such a next-generation Sonet network would encompass a hierarchical configuration (see Fig. 2). It would allow for full optical connectivity and survivability to and from any point in the network through an efficient, high-performance configuration of network components. The multilayer model would also simplify network planning. This model separates the network into three layers: low-capacity access, high-capacity express, and super-express layers.

Access layer. The local access and transport area used to be key in defining system deployments and network boundaries at the local level. However, deregulation and the resulting influx of new service providers have all but eliminated these boundaries. These effects have redefined how the local network must be viewed.

Access rings at OC-3 (155-Mbit/sec), OC-12 (622-Mbit/sec), and potentially OC-48 (2.5-Gbit/sec) levels provide Sonet penetration to remote areas of the network as well as to individual customers of all sizes. They offer optical interface capabilities for full survivability and typically serve as the gateway for gigabit services.

Express layer. This layer connects key regional offices and routes traffic to other carriers` points of presence. The network design must ensure adequate capacity for existing services as well as provide the flexibility to accommodate larger volumes generated by new services.

By deploying OC-48 collector rings at the express layer, network planners and providers can support regional offices as well as large customers, either directly connected or integrated into the super-express ring layer. As the need for gigabit services increases, express-layer rings can eventually be upgraded into the super-express layer, further extending the application for OC-192 (10-Gbit/sec) and wavelength-division multiplexing systems (wdm) systems.

In addition to providing bandwidth whenever and wherever it is needed, the express layer must also be able to efficiently manage service. With advances in Sonet bandwidth-management technology, multiplexing is combined with certain functions of a built-in digital crossconnect system (dcs). This combination minimizes the need for stand-alone dcs capacity at network nodes for more cost-effective end-to-end services. It also paves the way for integrating express rings into the super-express layer at key offices, further reducing service costs.

Super-express layer. Rings at this layer haul large amounts of bandwidth from one portion of the network to another. The super-express layer typically represents the backbone portion of an interexchange carrier network, or in the case of a metropolitan area network, one or two large network-wide rings. To accommodate long distances between network elements, systems deployed at this layer must use new optical technologies such as optical amplifiers, post- and pre-amplifiers, and external modulators. Therefore, the super-express layer complements OC-48, OC-192, and dense wdm systems.

If the super-express layer is to cost-effectively and efficiently receive, manage, and distribute large amounts of bandwidth, its network design must

integrate high-capacity optical interfaces directly into Sonet systems,

include full-featured synchronous-transport-signal management capabilities,

eventually include optical-layer bandwidth management of wavelength channels.

This approach improves operational efficiency and reduces reliance on external management vehicles.

Survivability should be inherent to all layers of the network. At the super-express layer, planning should include thorough cost-modeling for different levels of survivability.

New applications

New network designs such as this multilayer model are a response to a rapidly expanding array of new high-bandwidth services and applications. Since the Telecommunications Act of 1996 has opened the telephony market to new entrants, local-service providers are exploring emerging residential applications such as broadcast and interactive video services. Existing hybrid fiber/coaxial-cable networks could potentially support end-user applications at speeds to 10 Mbits/sec, and this would affect backbone network capacity. In addition, local-service providers are tapping into the Internet services market as well as new business initiatives such as service coverage within and beyond local access and transport areas.

High-bandwidth business services such as local area network (lan) interconnect to the public network, imaging, computer-aided design and manufacturing, and multimedia continue to grow. These applications are creating the need for user interfaces at speeds as high as 600 Mbits/sec, making the need to support OC-3 and OC-12 interfaces all the way to the customer a near-term reality.

In addition, the impact of Internet services on networking is far-reaching. In the United States alone, the number of household Internet subscribers has more than doubled, from 6.2 million to 14.7 million, during 1996. Furthermore, collaboration between government and business interests is driving Internet usage in public schools. As subscriber usage grows, so does the popularity of applications requiring higher-quality graphics and video. The Internet surge is backed by an expanding list of Internet service providers, which topped 3000 in 1996. Most Internet service providers are racing to upgrade 1.55-Mbit/sec DS-1 and 45-Mbit/sec DS-3 facilities to OC-3 and OC-12 types.

For both Internet service providers and business customers, the use of the Internet protocol over Sonet is also becoming an increasingly attractive option and adding to the drive for more bandwidth. Since point-to-point protocol encapsulation has relatively low overhead, high throughput can be attained over Sonet using lower-cost line termination equipment.

Network connectivity

As new applications become readily available and affordable, network connectivity and end-to-end service transport become central to networking strategy. To simplify analysis and planning, the network can be viewed in two segments: the long-haul, high-capacity portion and the local transport network.

In 1996, much attention was paid to new, high-capacity solutions, specifically wdm and time-division multiplexing (tdm). From the conventional design point of view, these technologies are distinct alternatives, and the choice between them is an "either/or" decision. However, due to advances in high-capacity systems, both wdm and tdm have been demonstrated to provide complementary internetworking benefits, allowing the network to support multiple rates over a common platform and provide the lowest cost per bit.

For example, Sonet transmission equipment vendors are working with several carriers to successfully deploy both wdm and tdm systems within the same network. By leveraging tdm systems such as OC-192, a single Sonet system can deliver 10 Gbits/sec via high-speed lasers and electronics. Dense wdm systems increase capacity by transmitting multiple channels at different wavelengths; for instance eight OC-48 signals can be carried over a single fiber for capacities up to 20 Gbits/sec.

Add wdm to OC-192 and a single fiber could accommodate 80 Gbits/sec. The result is a flexible platform that takes advantage of OC-48, OC-192, and dense wdm without changing systems--a key advantage.

While plans for the long-haul portion of the network focus on building highly reliable, high-capacity routes, the local network must support a range of different services. Specifically, the local platform must be engineered to cost- effectively support the migration of network interfaces from legacy DS-3 services to Sonet hand-offs between carriers, as well as newer services such as those using Asynchronous Transfer Mode technology.

Traditionally, lans were point-to-point, single-service configurations. Multiplexing of switched and private line services was accomplished using asynchronous M13 multiplexers, Sonet multiplexers, and dcss, depending on the economics of the configuration. Now, networks must incorporate diverse fiber facilities to create a multifunctional platform that accommodates voice, data, video, and dynamic service changes. Therefore, Sonet ring systems such as the one illustrated in Fig. 2 are becoming the network architecture of choice due to their added survivability and capacity gains.

Network planning

To leverage the latest trends in Sonet technology, a combined set of techniques, skills, and methodologies is required for effective network planning. Such plans must address service area boundaries, fiber sizing, fiber routing, equipment types, equipment topology, equipment configurations, and recommended phasing. The ultimate objective is to determine the most economically feasible approach to providing network services. It is an exercise in creating new revenue streams while reducing costs and building a smooth evolution path into the future.

With expanded service capabilities and the associated network elements come the inherent complexities of network management. What is required is an integrated, end-to-end management platform--designed specifically for advanced Sonet networks--that provides higher levels of service, network reliability, and staffing efficiencies.

New, comprehensive, network- management systems simplify monitoring and troubleshooting. Because such systems provide a real-time view of the network system and equipment topology, service delivery can be accomplished in days by a single network operator rather than requiring multiple field technicians over the course of weeks.

These types of network-management capabilities give service providers an important competitive edge and expand market and pricing opportunities. At a minimum, Sonet serves as a marketing tool for ensuring greater reliability across existing network services. However, other revenue opportunities exist, such as premium service pricing based on Sonet survivability schemes. In addition, new high-speed capabilities open new markets, such as lan connections and video-related services. And since Sonet technology improves bandwidth usage and reduces the need for equipment, the cost of service decreases, allowing service providers to change their pricing models and increase profitability.

Plan ahead

As network planners and service providers balance new revenue opportunities with expansion efforts, a comprehensive Sonet-based network plan becomes critical. An emphasis on long-term planning rather than short-term requirements ensures that costs associated with "forklift upgrades" and equipment overlays are kept to a minimum. Having a total network plan that focuses on the future keeps the adverse effects of day-to-day planning to a minimum. The result is an agile network that leverages the best of Sonet`s expanding end-to-end service capabilities, allowing planners and providers to meet current service needs and open up new markets. u

Bijan Khosravi is director of product applications marketing at Nortel, Alpharetta, GA.

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