Luis,
As "thisisshanky" said, standard distances for 100BASE-FX are 2km over multimode and 15km over single mode, if you get Cisco modules with these ports built in.
For a 30km run, you should look at Fast Ethernet media converters. There are several manufacturers, but I recommend Transition Networks (www.transition.com). I have used them with several customers, and they work very well. Transition has models that convert from Cat5 copper to single-mode fiber and can run typical distances of 20km, 40km, 60km, or 80km, depending on your needs. These media converters can be installed standalone, or they can be part of a rack-mounted solution. Check out the website for more information.
I have used the 20km variety to connect buildings in school districts that have access to private or "dark" fiber. I have also used the 20km and 40km converters to connect school districts to their ISPs over fiber. (The ISPs monitor and control the bandwidth used through the switchports at their end of the link.) I even know someone who's using a pair of the 80km media converters to carry an Internet2 connection.
Transition also has Gigabit Ethernet media converters, if you would prefer to go that fast. Cisco's GBICs are a more elegant solution, however, if you have the slots available.
Optical power of the transmit port's LED or laser, as well as its wavelength, determine how far you can go, and how much the equipment will cost. However, the actual distance you can go with a particular media converter or GBIC depends ultimately on the quality of the fiber optic cable installation. I have pushed 20km converters to 25km or more, over good fiber; and I have one customer who uses a pair of 40km converters to push data across a 16km span that had excessive optical power loss. If you can, measure the dB loss of your 30km fiber span at 1300/1310nm and 1550nm wavelengths first, before you buy any equipment.
NOTE: Fast Ethernet media converters need to be attached to switch ports that can run at full duplex in order to cover any fiber distance greater than 412 meters. They must be switch ports and NOT hub ports, too -- ports on dual-speed hubs or Fast-Ethernet-only hubs operate at half-duplex, which restricts the diameter of the collision domain. If you must use a half-duplex hub, Transition makes a "bridging" media converter that acts as a two-port switch, so that you can run full duplex over the fiber.
Hope this helps.
