No, actually, after OC12 (622Mbps) you'll see exclusively single-mode cabling. Single mode fiber is 8.3 micron "conductors", where multi-mode glass is usually either 62.5 micron or 50 micron.
The transmitters (and to some extent, the receivers) are the limiting factor.
Multimode generally uses an LED to generate the signal - which can only switch on and off so fast. The light is less coherent, and relys bascally on the light "bouncing" through the glass from wall-to-wall (/\/\/\/\/\/\/\/ ...).
Single mode transmitters are usually some flavor of laser/laser diode, which (may/usually) has a faster switching rate. The laser light is coherent (a single tight pencil of light) that is more like a direct, straight ine through the glass ( ======== ).
Up to OC12, either single or multimode fiber may be used, depending on the transmitter (the choice is usually a matter of the distance you need to cover - single mode / laser can achieve greater distances).
An organization may also implement single mode fiber as their inside plant to "future proof." GENERALLY SPEAKING, using single mode for shorter distances requires some sort of optical attenuator to reduce the signal strength (otherwise you may burn out the receiver or the receiver overloads and distorts the signal).
You can find devices that will do 10 Meg Ethernet over single mode, you can find devices that will do OC12 over multimode ... there are reasons for each ... but GENERALLY SPEAKING, Multi-Mode is used for indoor / local plants, and single mode is used for distance and extremely high speeds (regardless of the distance).
FWIW
Scott
