In designing an optical network, it is important to understand the following areas:

Applications and protocols: What traffic is going to be transported on the network?

Current network: Will new fiber or the existing fiber plant be used?

Optical power budget: Determine your loss plan.

Interoperability: A safe rule is not to mix equipment from different vendors on a per-leg basis. Whereas SONET is more mature and interoperability is more predominant, DWDM has a number of areas of non-standard contention between vendors, ranging from the spacing of wavelengths to the wavelength number scheme. Today, almost all DWDM implementations are single vendor.

Design the network to carry the traffic. In looking at applications, are you transporting voice, data, or a combination of both? Is the data ATM, Ethernet, frame relay, and so on? Are you transporting LAN-to-LAN traffic in a MAN or designing for long-haul WAN transport? Asking these questions should enable you to determine whether SONET transport, or a native type of transport such as gigabit Ethernet, is the right choice. If you have a combination of voice and data, SONET is the best solution for voice transport that is not packetized, but on a pure MAN for Ethernet, native transport might be the right choice.

Remember not to over engineer your solution by supporting both because as you design for savings in protocol efficiency, your costs of supporting two different transport technologies could skyrocket. Put another way, keep it simple stupid (KISS). Understand that order provisioning and network management costs are real and the complexity of a system forces large amounts of money to be spent supporting these activities.

For legacy network providers, understand your current network. If you have existing fiber, can it support optical techniques like SONET and/or DWDM? The following is a list of the three principle types of single-mode fiber and their ITU specifications:

• Non-dispersion-shifted fiber (NDSF), G.652
• Dispersion-shifted fiber (DSF), G.653
• Non-zero-dispersion-shifted fiber (NZ-DSF), G.655

NZ-DSF is optimized to support DWDM, but DSF is not suitable for DWDM. NDSF, also referred to as Standard single mode(SM) fiber, accounts for the majority of the installed fiber in the MAN that can support DWDM.

Optical power budgets are a critical part of planning an optical network. Equipment vendors must provide engineering rules. Many factors can result in optical signal loss-from the distance your fiber travels, to the number of devices such as OADMs. A rule of thumb is to use a span budget of 25 dBm. The span budget is sum of the following,

Total system loss + (fiber length $\times$ 25 ) + fiber aging margin + connector/splice losses

Connector splice loss: 0.2 db if connectors are modern, single mode, and from the same vendor, and 0.3 db if connectors are from different vendors

Fiber loss: 0.25 db/km due to attenuation

Fiber aging: 2 db over the life of the system

If the sum is less than $25,$ we are within budget. If the sum is more, we need to look at including amplifiers or reducing the number of loss-inducing elements on the span.