Better climates ahead

There are technologies on the horizon, however, that will improve optical layer efficiency and provide a new layer of restoration capability. This new optical layer will provide the restoration and reconfiguration capabilities for the network while the Sonet and services layers perform the multiplexing and customer interface functions (Figure 1).

The all-optical network inevitably will depend on wavelength division multiplexing, optical add/drop multiplexers (ADMs), and optical cross-connect systems.

Today's world Fixed DWDM systems increase fiber capacity by multiplexing and demultiplexing optical signals on a single optical fiber (Figure 2). Typical components used in these fixed WDM systems are optical combiners, optical splitters, wavelength selective filters, optical amplifiers and specific wavelength sources.

The most common network applications are point-to-point with access to all wavelengths at the terminal locations. More advanced systems incorporate wavelength add/drop at intermediate sites, but they often resemble back-to-back terminals more than ADMs. Because fixed systems require physical changes to add and remove channels, management becomes tedious and difficult.

True optical ADMs will allow provisioning optical channels in a manner similar to time slot assignment, and reassignment of optical channels in a way that resembles time slot interchange. Since these programmable ADMs will be provisioned on a wavelength basis, carriers can more easily add new sites requiring access to the network, and the burden for network planners can be reduced.

The evolution of optical networks will lead to more advanced systems that provide wavelength routing capability. Migration to the all-optical layer also provides new methods of protection for network restoration.

As technology breakthroughs are made in the arena of optical gates and matrices, optical cross-connect systems will begin to appear. There will be two basic types of cross-connect systems: line side and tributary side. The tributary side, or Type 1 optical cross-connect system, will function similar to the broadband Sonet cross-connects available today. The line side, or Type 2 optical cross-connect system, will support high-level network restoration and network reconfiguration on high-speed transport systems.

As customers purchase new services, the amount of traffic passing through service provider locations will increase, and high traffic volumes will have to be managed. To date, electronic broadband cross-connects have met the demands of the network, but the complexity of these systems and their matrix sizes eventually will reach the limit of feasibility.

Optical cross-connect systems can reduce the burden on digital cross-connect systems by using a higher level of traffic routing at the optical wavelength level. Signals that can be routed at rates above STS-1 can be handled efficiently in the optical layer.

An optical matrix is inherently smaller than its electronic cousin, requires less power, switches at higher speeds and handles large quantities of bandwidth with less complexity. Because a significant portion of the bandwidth explosion is due to customer-driven requirements for larger pipes, these connections can be managed more efficiently with an optical matrix.

On the horizon Network restoration for fully restorable services currently is accomplished by two basic methods: mesh protection using DCSs and ring protection using Sonet multiplexers. Both methods have positive and negative aspects-especially in the areas of restoration time, network and equipment cost, and management-and each will have its place in the optical network.

Compared with existing technology, however, fiber rings will experience a significant improvement in efficiency. Currently, all ADMs in a Sonet ring must operate at the same data rate. This leads to inefficiencies and additional cost in the transport network, especially since some routes will be significantly more dense than others.

If the route between two nodes on a ring only requires OC-48 capacity, but the route between two different nodes on the same ring requires OC-192, the entire Sonet ring would have to be installed as OC-192.

But with optical rings, switching occurs at the wavelength level, and any data rate can be supported by that wavelength. This will allow for one part of the ring to operate with wavelengths at the OC-192 rate and another segment to operate at the OC-48 rate.

Deploying increased bandwidth capability only when and where it is needed provides a significant network cost advantage over the current method of overlaying multiple rings using fixed WDM.

Additionally, optical rings can support the existing backbone, whether asynchronous or Sonet, and ease the migration to the all-optical network. Because the optical ring protects against fiber failures-including cable cuts and repeater equipment outages-the time division multiplexing systems need only support equipment protection.

This can be provided easily by 1-to-N protected systems, reducing the demand for capacity on the transport network. The value for N specifies the number of traffic-carrying fibers sharing a single backup, and N can be optimized to provide excellent system availability.

By implementing optical rings for high-level network restoration and optical cross-connect systems for wavelength routing and management, carriers can significantly reduce the cost of providing a network with the highest availability.

Some components that will be required in the all-optical network are being deployed in today's network. Optical amplifiers that support multiple wavelengths and standards-based wavelength channel plans, which are the basic building blocks of fixed WDM systems, are also critical in the migration to the all-optical network.

To protect the long-term investment made in these systems, carriers should consider several critical factors. The most important aspect is the wavelength channel plan, which should use as much of the available spectrum as possible.

Wider spectrums allow for more channels to be inserted into the available window. As bit rates and unregenerated spans increase, channel spacings will need to be as large as possible. Larger channel spacings mitigate non-linear effects such as four-wave mixing and allow increased channel capacity.

Due to the information bandwidth and laser tolerance, channels operating at 10 Gb/s may not be supported by a channel plan with less than 100 GHz spacing. To support a more robust channel plan, optical amplifiers need to have flat gain over the entire spectrum. These amplifiers are available today and provide cost-effective support of fixed WDM systems.

Other technologies currently commercially available to support provisionable optical ADMs include tunable filters and sources, optical switches and ring-protection algorithms.

Planning the route The availability of core technologies to support optical cross-connect systems in a cost-effective manner is still several years away. The major challenge is the optical matrix itself. Some companies have demonstrated prototypes that provide up to 16-by-16 capability, but the operating characteristics do not meet system requirements and functionality is limited.

The best way to be positioned for the inevitable technology breakthrough is to deploy optical ADM architectures that migrate gracefully to an optical cross-connect system. Optical ADMs can be designed to perform similarly to smaller cross-connects; migrations to larger cross-connect systems would involve an increase in the matrix itself.

Today's hybrid networks are segregated into separate systems to support voice and data services. These separate networks lead to inefficiencies in the transport layer because each network requires its own transmission equipment.

Optical networks will provide a common method for transport regardless of signal format. The service provider will be able to assign the end user a set of wavelengths and route those wavelengths through the network independently, transparent to the customer's information.

Quickly responding to customer service requirements is key to increasing a service provider's revenue stream. With its transparency to service type, the optical network will give network operators the performance needed to satisfy existing customers and the flexibility required to capture new ones.

Tim Krause is Director of Product Marketing and Business Development for Alcatel Network Systems Inc., Richardson, Texas.