What is the highest bandwidth available over dark fiber?
Commercially available router interfaces currently run up to 10 gigabits per second (Gbps), using either 10 Gigabit Ethernet or OC-192. Future plans call for 40 Gbps, but this equipment is still under development. Higher transmission rates also place much more stringent demands on the fiber path, and not all paths may be usable at those rates. However, with wavelength multiplexing techniques multiple signals can be transmitted across the same fiber, dramatically increasing the total bandwidth available. Coarse wavelength division multiplexing (CWDM) allows for 8 or 16 channels, each of which can carry up to 10 Gbps. Since each channel runs at a lower rate, a fiber span that might not be able to carry 40 Gbps traffic could handle 160 Gbps total across sixteen 10 Gbps channels. Another variation, dense WDM (DWDM), uses very closely spaced channels in a higher wavelength band.
