Tradeoffs abound in optical transmission system design, and it's critical for carriers to understand their priorities based on network/service characteristics, and select systems that meet their needs at the lowest first and per-channel cost. Now more than ever, it is important for a carrier to have a good view of their network and its needs prior to launching into the search for a new long-haul transmission system. There are a number of tradeoffs that can be made in system design, and having a clear view of network topology, traffic requirements and operational needs can help in finding the best system for a particular carrier's network and application. There is no "one size fits all" system, and each one has been designed with a particular set of costs and capabilities in mind. Of course, lowering costs are foremost in every carrier's mind today when it comes to long-haul transmission systems. A few years ago, the conventional wisdom was to lower costs by putting more and more bandwidth on a fiber, sharing common equipment over more channels and driving down per-wavelength dollars. Today, first wavelength costs are equally important in laying new systems, making sure that cost saving are realized within an achievable time window. Often, achieving the optimum fully-loaded wavelength cost can lead to prohibitive first wavelength costs, and vice-a-versa. Operational costs also play a harder-to-quantify but important role when evaluating system options, and operational savings mechanisms such as power and footprint are considered up-front in decision-making. There are a few network characteristics that are key to setting requirements on a long-haul transmission system: 1) The amount of available fiber, and the fiber type and characteristics 2) The network topology, including distance required between add-drops, service needs at add-drops and number of on/off ramps on a system 3) The service topology, including average distance of wavelengths between customers or grooming points in the network 4) The level of flexibility required of the system, including granularity of add/drops, the need for network reconfiguration at a wavelength level, the need to support a variety of rates and services 5) The amount of automation required of the system, including seamless system turn-up, fault location to a root cause, and wavelength provisioning.
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