Due to the requirements of some infrastructure i have supported, we have needed to utilize high bandwidth, but low cost wan links. Some dark fibres were between brisbane and sydney, but most were in the Brisbane/Ipswich area. For quite a number of links we were able to get away with running either LR or ZX optics depending on link loss to get connections up and running. This was great for data, but as soon as you want multiple services for redundancy, or Fibre Channel etc. you can be better off looking at WDM.
My first experience was joining two sites via a common middle point on their path. The middle point needed to be serviced by either end, and the two far ends needed to communicate directly rather than using layer 2. The best solution at the time was to deploy a CWDM solution, patching through a wavelength in the middle site to allow one piece of fibre to service two links. You can use BX optics to get the same effect, but CWDM scales better if you are looking to add more services.
Which is what happened, coupled with the need to multiple 10Gbps links drove my involvement in deploying Cisco ONS15216 OADM’s and building DWDM links. The DWDM installation included doing things like bench testing hardware, to ensure stable operation, doing loss calculations. As we were using the middle hop, we needed to do things like add in loss on the middle site for their wave lengths, and on other links, we needed to use Cisco EDFA3’s to amplify the signal to get rid of errors and poor performance.
I got the opportunity to install DWDM on links just over 40Km, i was also the primary support for faults relating to the DWDM network and other Fibre links.