Sonet networks help service providers survive convergence
To meet open competition, network service providers are expected to implement the value-added capabilities of Sonet broadband networks to deliver new and improved multimedia communications
robert k. koslowsky and s. timothy boatwright
Northern Telecom ltd.
Because telecommunications technologies such as synchronous optical network (Sonet) and wavelength division multiplexing (WDM) are rapidly becoming commercially available, network service providers are being challenged to deliver new and improved voice, video and data applications over fiber-optic networks. Passage of the Telecommunications Act of 1996 means open competition in the local and long-distance telephony markets, and network service providers must, therefore, strive to upgrade their fiber-optic broadband communications networks.
According to industry analysts, the key broadband network strategies for achieving competitiveness in the new telecommunications era include:
The implementation of Sonet networks and the use of industry standard protocols for delivering new and improved voice, video and data services
The planning of networks that involve end-to-end solutions rather than point-to-point network element deployments for bandwidth-capacity relief
The extension of the network`s reach and capacity by optimizing utilization of the installed fiber plant
The incorporation of network management capabilities that enable centralized management to increase performance, reduce costs and multiply service access.
By adhering to these strategies, network service providers can ensure deployment of a competitive Sonet infrastructure. The overbuilding of capacity-exhausted spans for lowest initial cost will not deliver a manageable and expandable network. A step-by-step network plan, implemented within a three- to five-year construction budget, is needed to achieve a competitive advantage for service providers.
To ensure market competitiveness, service providers should seek planning services from network and equipment vendors to simplify the complexity of Sonet technology deployment. One approach involves the layout of a service provider`s central office location, fiber-optic cable routing and data traffic demands for the next few years. This approach becomes viable if the following objectives are realized:
Maintain a 5% to 10% bandwidth reserve at all times to absorb unforeseen traffic demand
Eliminate traffic bottlenecks and stranded capacity
Pre-position the core Sonet network for both cable-cut and node-failure protection to ensure transmission survivability
Group traffic so that inter-ring hand-offs are less than 15%.
To achieve these objectives, network planning should include sensitivity analyses for incremental bandwidth, site additions and year-by-year deployments. This up-front planning activity helps achieve a durable and reliable Sonet infrastructure. Otherwise, a faulty network deployment strategy could result in costly future network adds and changes.
Although a network that contains adequate bandwidth reserve appears efficient in terms of cost, equipment and ring interconnections, the network could still perform in a disjointed manner; that is, various systems are crisscrossing throughout the service area. For this situation, the network becomes difficult for operations personnel to manage and, most likely, proves inflexible to handle traffic growth and shifting traffic demand patterns.
In Sonet network ring configurations, several communications functions mandate special attention. For example, overlapping rings make it difficult to write provisioning rules. Moreover, matched nodes (rings joined at two points) cannot be implemented easily. In addition, inter-ring hand-offs might be difficult to do for certain office locations. The possibility of future ring service exhaust must also be addressed. These problems can be alleviated by constraining the Sonet network`s ring architecture.
By anticipating and planning network growth and changes, network service providers can realize a competitive Sonet infrastructure that is easy to expand, manage and adjust.
Bandwidth needed
Bandwidth-hungry applications, such as video supertrunking, the Internet, distance learning and medical imaging, are driving up demands on network capacity requirements. Service providers are, therefore, using every available resource to increase the capacity and reach of their existing networks. Some providers are responding to possible network service exhaust by adding extra capacity, figuring that multimedia services are expected soon.
In this regard, service providers should first study how to modify their networks to handle the transmission of gigabit speeds and then evaluate the available ways to implement them. Gigabit-fiber technology can deliver both network capacity and reach gains. In fact, before the end of this year, a combination of 10-gigabit-per-second OC-192 equipment and optical amplifier technology is expected to be operational.
One such gigabit approach is to use WDM technology to deliver as much as four to eight times the capacity of an existing network by running multiple wavelengths over the same fiber-optic cable. Another approach is to increase network capacity by moving up to more-powerful transmission technology, such as upgrading to 10-Gbit/sec signal rates over a single fiber-optic cable.
The appropriate high-capacity solution might incorporate a combination of these two transmission methods. For network routes growing at a rate of a few 44.736-megabit-per-second DS-3 circuits per month, activity studies indicate that 2.5-Gbit/sec OC-48 WDM transmission is probably the most economical delivery method. For network routes growing at 12 or more DS-3 circuits per month, the initial deployment of OC-192 transmission is most likely the better delivery choice. In the long term, OC-192 WDM transmission is expected to provide unprecedented capacity and reliability for advanced network services (see Table 1).
The long stretch
To upgrade an existing network, service pro viders should also investigate the installation of optical line amplifiers. These de vices have been shown to extend the reach of an embedded fiber plant more cost-effectively than do regenerators. They can also support both OC-48 and OC-192 bit rates, thereby extending the life of the service providers` network investment.
In another approach, several service providers are deploying bidirectional optical line amplifiers to support their network performance requirements (see Fig.1). One typical installation incorporates a four-port coupler with an optical amplifier to facilitate the deployment of cascaded bidirectional optical line amplifiers. These special amplifiers support both linear 1:n and bidirectional line-switched ring network protection schemes.
With a greater tolerance for dispersion, cascaded optical line amplifiers can stretch signal regeneration spacing from, for example, 80 to 320 kilometers. This extension could easily decrease equipment costs by as much as 17%. Other network benefits include decreased deployment costs, enhanced system availability and improved reliability (for example, one four-port coupler replaces four three-port couplers); and the network`s WDM capacity increases and its reach is elongated.
To use Sonet network and element management in the local exchange arena, service providers are confronted with stringent requirements to support large commercial customers. They should activate DS-1 and DS-3 private line services within a short time--48 hours--and demonstrate circuit performance over time for premium services.
By incorporating network element layer management, Sonet network technology enables STS, virtual tributary and DS-0 connection capabilities that can be activated in minutes or seconds. For service providers, these capabilities translate into improved service responsiveness. For example, high-speed private line services could be activated in days, rather than weeks.
In addition, advanced network management capability can provide better diagnostic capabilities at centralized management centers. It can achieve complete network management, regardless of the mix of manufacturers` network elements.
Historical trends show that every four years, bandwidth deployment generally quadruples in a given portion of service providers` networks. Furthermore, Sonet network deployment in the local loop demands more analysis than just point-to-point asynchronous replacement (see Table 2).
For service providers, the overlay of private networks to increase service delivery is not needed. For example, an OC-3 unidirectional path-switched-ring (UPSR) platform can support both multiple service interfaces (such as DS-1, DS-3, OC-3/3c) and multiple destination connections (see Fig. 2). The interface mixes are survivable by connecting the traffic traveling through the ring network, which is terminated as an OC-3 circuit, to a crossconnect device. This approach is required because of the difficulty in predicting end-user network changes.
Some end users might want a new long-distance provider (new destination), more private line services (more DS-1 or DS-3 circuits) or the addition of asynchronous transfer mode capability (such as an OC-3c circuit). The OC-3 UPSR technology can support a 1.87-Gbit/sec STS-36 switch matrix so that numerous tributaries and rings can be accommodated without signal blocking problems. Furthermore, the central office-based network element can deliver the full OC-3 bandwidth to the crossconnect device for appropriate destination routing. u
Robert K. Koslowsky is director, transport marketing, and S. Timothy Boatwright is manager, emerging markets in broadband networks, at Northern Telecom Ltd. in Atlanta.