One of the largest challenges facing network planners who are implementing optical networks is the task of designing and planning the optical layer. Only when providers begin to plan the optical network do some of the more intricate and difficult issues arise.

Span Designs

Ideally, the optical network will provide end-to-end services entirely in the optical domain, without ever converting signals into electrical format. Unfortunately, for at least a decade, it is probable that technology will not progress to the point that it is possible to transmit signals for long distances without electrical regeneration. Even as optical regenerators become commercially viable, network spans will still need to be designed to maintain signal quality throughout the entire signal path.

Planners must design optical networks so that signals traveling on the fiber between one network element site and another, called a span, maintain their quality. Many factors must be taken into account, including the optical signal-to-noise ratio (OSNR) (see Figure 13), chromatic dispersion (see Figure 14), and a myriad of nonlinear effects introduced by the interaction of the fiber with the optical signal. The challenge of designing optical networks increases with the introduction of optical cross-connects and add/drop multiplexers, which could dynamically change a signal path to travel across a different physical route.

Figure 13. Optical Signal to Noise Ratio

Figure 14. Chromatic Dispersion

Wavelength Routing Plans

The basic element in the optical network is the wavelength. As many wavelengths of signals are transported across the network, it becomes important to manage and switch each one individually. One of the benefits of optical networks is that they allow the network architecture to be different for each wavelength. For example, one wavelength may be established in the network to be part of a ring configuration, while another wavelength, using the same physical network, can be provisioned as a point-to-point system. The flexibility of provisioning the network one wavelength at a time has led to two definitions of end-to-end services: wavelength paths (WPs) and virtual wavelength paths (VWPs).

Wavelength Path

The simplest implementation of a wavelength service in the optical network is a WP. Using a WP, a signal enters and exits the optical layer at the same wavelength, without ever changing to a different wavelength throughout the network. Essentially a wavelength is dedicated to connect the two endpoints.

Virtual Wavelength Path

Although a WP is simple to implement, it can impose some limitations on the bandwidth available in the network and the cost of implementing it. One method by which to overcome this limitation is to use a VWP, in which a signal path can travel on different wavelengths throughout the network. By avoiding a dedicated wavelength for an end-to-end connection, the network can reuse and optimize wavelengths to provide the greatest amount of capacity.