Fiber-optic cable is different than twisted-pair and coax cable because it transmits data using light signals instead of electrical impulses. When you look at the layout of the cable, it appears similar to coax, but it has a glass or plastic fiber as it inner conductor instead of a copper wire. Figure 7-4 shows you what the inside of a fiber-optic cable looks like.
Figure 7-4: An inside view of fiber-optic cable.
In Figure 7-4, notice that the inner glass core is sometimes called buffer coating, and the entire cable has another strong jacket around it. The outer jacket is designed to be thick enough to protect the inner fiber from being broken when the cable is handled (with care, that is).
Although it has a higher price tag than electrical cables, fiber-optic cable can also handle greater bandwidth, which means that it can transfer more data over longer distances. Fiber-optic cable is largely immune to electromagnetic interference (EMI) and other sources of noise that affect electrically conductive cables. One factor that adds to the expense of fiber-optic cable is the care required during installation. A knowledgeable technician must carefully polish each glass fiber with specialized tools and then add special connectors to the cable.
You often find fiber-optic cable installed between buildings in campus environments or between floors in a building. You rarely see fiber pulled to the desktop because of the expense involved — you must use fiber-optic NICs, and two cables must be attached to each workstation because one cable transmits outbound signals and the other receives inbound signals. Although the appetite for bandwidth is always increasing, don't expect your desktop to have a high-fiber diet anytime soon!
In some locations, such as hospitals, it's necessary to run fiber-optic cable to some desktops because X-ray and MRI equipment can interfere with electrical cables. Also, the bandwidth requirements for medical imaging equipment can be so extreme that conventional electrical cables can't handle the traffic involved.
TECNICAL STUFF For light signals to pass through a fiber-optic cable, a transmitter has to be attached to one end of the cable and a receiver attached to the other end. This is why two cables are needed to permit any one device to send and receive signals. On the transmitting end, an injection laser diode (ILD) or a light-emitting diode (LED) sends light pulses down the cable. These light pulses reflect within the glass core and bounce against the mirror-like cladding through the length of the cable until they reach a photo diode receiver at the cable's other end. Notice that data flows in only one direction. The receiver converts incoming light pulses into electrical signals and passes the data to the NIC.
Because of the way that light pulses travel through fiber-optic cable, splicing two such cables requires great care so that the cable's signal-carrying capabilities are not reduced. Otherwise, a light pulse may arrive at the splice but may not make it through to the other end of the cable. We call this situation a bad splice, but your users will call it much worse names!
Fiber-optic cable is the most expensive type, but it offers the highest bandwidth and the most room for future bandwidth growth.
A final note about cabling
TECNICAL STUFF If you're going to install cable yourself instead of hiring a cable contractor, here are a few final notes we'd like to share with you:
Obtain a copy of the blueprints for your building or floor and make sure that all the electrical devices and outlets are clearly marked.
This map assists you in placing cable away from electrical devices or motors that can interfere with your network. You don't want to install cable near elevator motors, transformers, or other heavy-duty electrical devices (unless you're using fiber-optic cable, and even then, you need to protect it from potential sources of damage or wear and tear).
Obtain all relevant local, state, and federal building code regulations and make sure that your plans conform to such ordinances.
You need to evaluate these requirements before purchasing any cable because some codes require you to purchase plenum-rated,
fire-retardant cable. Other codes require you to use plenum-rated cable only when you run cable through locations that are likely to catch fire rapidly. In any case, it behooves you to know the rules before you purchase and install a network.
Plenums are the air-handling spaces between the ceiling of one floor and the bottom of the floor above where cable is often strung. Fires spread more rapidly in these areas because air carries fire rapidly;
therefore, plenum-rated cable is mandated for use in such spaces to keep fire and smoke from spreading through a building.
Determine which parts of the network you can build and maintain on your own and which parts you need to subcontract.
For example, if you have the time and inclination to build cables and also have the time to troubleshoot the network when those cables don't work, so be it. We recommend that you buy as much prefabricated cable as possible and make cables only when you absolutely must.
Why? Because companies that make cables do it all the time, and they're good at it. If you make cables for your network only occasionally, you may introduce problems.
Try to hire a contractor to install cable for your LAN.
The wiring is your network's infrastructure. If the wiring is not installed properly, it can cause endless network snafus. Wiring contractors should provide you with bids, install the cabling, label all cables, test and certify all cables, and provide you with final documentation. You're responsible to keep them on track. Don't assume that a contractor will follow local ordinances unless you make him or her do it. Put all expectations in writing and keep tabs on the work.