The local cable plant (all the devices connected to a fabric at one site) should be using 50u (u = micron) multimode...
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cabling (850 nanometer wavelength), which allows a distance of up to 300 meters using 2 gigabit (Gb) lasers (GBICS), and 500 meters using 1 Gb lasers. So, if all you are trying to do is connect servers to SAN switches and SAN switches to storage, then your best option is to use standard multimode SAN cables as described above.
Dark fiber is a term used to describe 9u single-mode cabling. Single-mode cables have a narrower core, which limits light defraction and absorption during transmission, which means greater distances can be traveled before db loss is an issue. Single-mode transmission also uses a 1300 nanometer (nm) wavelength source, which also allows for greater distance. Single-mode cables can be run up to 10 kilometers before repeaters are required. So, if you are trying to connect two SAN islands together over greater distances, than dark fiber (9u single-mode 1300 nm) is the better choice.
If there is currently no dark fiber owned by your company between buildings though, installing or leasing dark fiber can be VERY expensive. It can cost the same as leasing an OC-48 connection from a Telco. If you own the fiber though, you can connect up dense wave division multiplexor (DWDM) equipment at either end, and actually slice up your dark fiber into multiple wavelengths (usually 32 or more) that will allow for massive bi-directional bandwidth between sites.
If you don't need all that bandwidth and just want to connect two SAN networks together for, say, data replication between sites for disaster recovery, then using a standard leased IP connection could also work for you. All you would require is an FC to IP bridge at each location. The bridge can run either the FC-IP or iFCP protocol to tunnel your FC frames across the IP network to the other location. I would recommend a leased T3 or above for the IP network, although a T1 may do if the amount of data is low, and the bridge also does compression.
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