Catch the wave-length.

Iowa Network Systems implements new optical network using WDM.

Remember the good old days when designing a SONET network involved only traffic requirements and span budgets? With the explosive deployment of wave division multiplexing (WDM) technologies and 10 Gbps SONET, designing a transport network is becoming increasingly involved. Yet, as WDM becomes the choice technology for core networks, planners are looking beyond WDM to the emerging optical layer as a supplement of the SONET transport layer.

From designing the WDM physical spans to planning for optical rings, Iowa Network Services, Inc. (INS) serves as an example as to what planners should consider when deploying WDM--a company that has designed and deployed an optical network and is looking toward its future as optical layer evolves.

Headquartered in Des Moines, Iowa, INS provides telecommunication services to its thousands of customers over more than 4,500 miles of fiber-optic network throughout the state. INS is taking advantage of its existing fiber plant to prepare itself for new data network services to its existing and new customers. It is capitalizing on its capabilities and facilities to provide a variety of services, including reselling bandwidth.

Over the years, INS has kept its network at the leading edge. Currently, it has an all-digital network based on the unsurpassed reliability of OC-48 rings in its backbone. Covering nearly the entire state, the backbone network is comprised of four fiber-optic rings. Northeast, southeast, northwest, and southwest rings interconnect at major hubs, providing access to every major population center in the state (Figure 1).

[Figure 1 ILLUSTRATION OMITTED]

THE CHALLENGE

As the paradigm in telecommunications moves to a data-centric network, INS's challenge in expanding its network was two-fold: increase capacity and provide flexible transport for a variety of-traffic types. In addition to providing more bandwidth to the enormous amount of business and residential customers, INS also wanted to generate more revenue from its existing fiber-optic plant.

Because any investment in its backbone network would need to last for many years, INS needed a way to grow without restricting itself to certain data rates or protocols. The ability to carry anything from SONET to TCP/IP, ATM, Ethernet, FDDI, and the plethora of other data formats was a requirement INS could not overlook.

Taken together, the capacity requirement and flexible transport meant INS wanted the ability to offer "wavelength services." A company often buys capacity, or wavelengths, from network providers so that it can complete a route without deploying its own network. The companies that sell capacity to meet these needs have become known as "carrier's carriers" because they provide enormous capacity to other long-haul carriers.

THE OPTICAL NETWORK SOLUTION: WDM

To increase its network capacity to meet its own internal needs, INS deployed WDM using Alcatel's Optinex 1640 Optical Add/Drop Multiplexer (OADM), a system capable of transporting 400 Gbps on a single fiber. By deploying the WDM on its backbone, INS exceeded its capacity requirements while avoiding the costlier option of deploying new fiber.

Optical amplification is deployed to minimize network cost. By placing one amplifier at a remote site, all WDM signals are regenerated optically, without the need for electrical regeneration for each of the individual channels. Optical amplifiers proved to be extremely useful in INS's network. Because today many of the sites have limited traffic requirements served by other systems, there is no need to access the 1640 OADM information electrically. Should any of these sites require more capacity in the future, it can be upgraded to an optical drop/insert or to a full optical terminal configuration.

When designing a WDM network, the optical signal-to-noise ratio (OSNR) tends to be the major limiting factor (Figure 2). When deploying its 400 Gbps WDM system, INS did not want to suddenly require many more optical amplifier sites to be maintained just to carry the additional traffic. New technologies allowed INS to grow from a 2.5 Gbps (OC-48) network to a 400 Gbps network without adding additional amplifier sites.

[Figure 2 ILLUSTRATION OMITTED]

Flat Gain Amplifiers: Optical amplifier technology has come a long way since it was first introduced, and some of the latest amplifiers push performance towards the edge quantum limits. When designing for its 400 Gbps WDM system, the performance of the new amplifiers, along with forward-error correction, INS found its network did not need a major overhaul. In fact, INS could use most of the same sites.

Forward-Error Correction (FEC): Used especially for higher bit-rate SONET signals, such as OC-192 (10 Gbps), forward-error correction allows the signals to be carried farther before needing electrical regeneration. Several types are available, and some optical network systems even use FEC based on submarine standards to provide eight times better tolerance to OSNR degradation. INS designed its network base on a 40-channel OC-48 network--but, by using the advanced FEC on the OC-192 channels, no additional amplifier sites are required. This design philosophy proved to be extremely cost effective and allows INS to grow its network, in both channel count and bit rate, in an extremely easy manner.

OPTICAL NETWORKING 

Although it is clearly the most efficient and reliable means of transport in large networks, SONET is certainly not the only format out there. As more carriers deploy ATM- and IP-based networks for Internet traffic, customers are demanding more flexibility from their carders. INS looked for a WDM system that would support this variety of traffic types, as well as traditional SONET. In WDM parlance, INS wanted an "open architecture" WDM system (Figure 3). An open architecture allows interconnection to SONET equipment from multiple vendors. The competitiveness of a multivendor environment normally drives network costs down. The added benefit of transporting any traffic type is also critical.

[Figure 3 ILLUSTRATION OMITTED]

The other type of WDM system, called an "embedded architecture" system, is often a lower first-cost solution, because it does not require each channel to be electrically regenerated by the WDM network element. This single-vendor architecture limits the competitive savings for growth channels. INS was in a common situation where its existing OC-48 rings, full of customer traffic, needed to move onto the WDM system.

In the end, INS selected a WDM system that supported both "open" and "embedded" architectures, so that it could roll its existing OC-48 rings onto the WDM using low-cost interfaces and grow each channel, as needed, from any vendor and in any data format. The existing OC-48 rings were rolled onto the system as the first channels, and future channels will be deployed as either "open" or "embedded" as required. This provides the cost benefits of both architectures.

The ability to transport a wavelength as a service appealed to INS because of its unique fiber plant throughout Iowa. Although initially designed as a WDM system, INS has a deployment plan which will allow it to migrate its WDM system to an alloptical ring.

By overlaying the WDM system on its existing OC-48 rings, INS ends up with an optical layer that can be closed into an optical ring (Figure 4), providing protection the same way its SONET OC-48 rings do today. This was an important part of planning implementation: instead of simply installing point-to-point WDM, deployment is based on a site and design strategy that will allow INS to close the optical ring sometime in the future. The optical ring would allow it to provide SONET-like protection to data traffic, without overbuilding additional SONET rings.

[Figure 4 ILLUSTRATION OMITTED]

LEARNING FROM EXPERIENCE

Some key design considerations can be taken as lessons from INS's experience. First, INS planned and deployed its network with growth in mind. Whether growing from OC-48 to OC-192, or growing in channels, the ability to do both without revisiting the entire network design proves to be the most cost effective in the end.

Second, INS kept its eye on data traffic as a force in the structure of telecommunications that will have an impact that is not yet clear. Planning for an optical layer instead of point-to-point WDM was the best long-term solution for INS.

Although INS was initially looking only for a WDM system to solve capacity needs, the company found that deploying a true optical network not only met its immediate needs but provided it with the ability to offer a whole new realm of services: wavelength services. By carefully selecting a platform and design that allows smooth growth from OC-48 to OC-192, as well as mixing any traffic type on any channel, INS prepared its network for the data-dominated world of traffic.

With a network that will also provide protection in the optical layer through optically switched rings, INS can offer reliability for data traffic that carriers traditionally only expect from SONET-based solutions. Looking beyond the "fiber plant" mentality that WDM tends to garner, INS deployed an optical layer, enabling it to stay a valued service provider, regardless of how data traffic will transform the electrical world.

Vislosky is an optical networks product specialist at Alcatel Network Systems in Richardson, Texas.

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