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Dark fiber is, as its name suggests, optical fiber supplied by a telephone company in a point-to-point configuration. The telco is not driving the signals over the fiber--that is done by the equipment attached at the ends of the fiber. An early use of point-to-point fiber was to supply a 10-Mbps Ethernet extension over campus-style distances. This was provisioned by the telco supplying optical equipment on either end that supported a 10-Mbps transport. Some users detach these devices and attach their own Gigabit Ethernet hardware, and go from 10 Mbps to 1 Gbps throughput immediately. Look closely at the fine print of your contract with the telco before attempting such a maneuver.

MPLS (now an open standard, but originally a Cisco tag-switching technology) is a different story. Now that IP is the protocol of choice for just about everything, including voice networks, there is concern about how IP traffic should be transported within carrier networks. ATM was seen as the way to combine all traffic types on one physical network, but if all data is IP traffic, then the overhead of ATM headers is unnecessary. A cleaner option is to use MPLS to access Sonet directly and bypass ATM. It will be a while before the dust settles on this one.