Fiberoptic cable comes in two main types, multimode and single-mode. Multimode fibers are sized by the core dimension and the cladding dimension. The most common multimode cable is the 62.5/125 micron. An enhanced version is the 50/125 micron. Multimode transmission uses inexpensive LEDs. Light in multimode transmission
is propagated in several modes; i.e., multimode. Transmission distance is limited. Connecting multimode is easier and faster. Multimode is used to the desktop and in the building backbone. Single-mode transmission is by laser. Large amounts of data over long distances may be handled. Single-mode fiber is found in long-haul work, campus distribution, and in large-system building backbones.
Hybrid cable may be purchased with varying counts of single-mode and multimode fibers, such as a 12/6 cable or a 24/12 cable. The first number being the quantity of multimode fiber, the second being the number of single-mode fibers. (See Figs. 18.718.9.)
Fiber selection, particularly the selection of multimode or enhanced multimode cable, is based on network design, because the network gear will be ordered with a specific multimode light source for the cable. There are different types of singlemode fibers, but the higher grades are presently used only in long-haul work. the selection of the number of fibers is based on network design, but more importantly on cost, availability, future growth, and future network type. Fibers are cheap compared to the jacket cost and the installation cost, so, if the planned network needs eight-multimode fibers, good practice would be to use a 12/6 hybrid cable; leaving four spare multimode and six spare unterminated single-mode. The single-mode fibers could be used if they upgrade from Ethernet to ATM, or for a special circuit.
Cost is also a big factor in cable selection. The prices from distributors fluctuate greatly based on the availability of the product. The IT owner will take advantage of these wide price shifts to purchase the best equipment at the lowest total cost.
Cable jackets are based on their application and there are many from which to choose. Outdoor cables may be gel-filled to repel water, but the gel is hazardous and the cable must terminate within 50 ft of entering the building. Cables needing protection from rodents and lightning can have jacketing of aluminum, steel, or stainless steel. Strength members in the cable are available in steel and nonmetallic.
The NEC requires specific cable construction for specific applications in buildings.
Plenum, riser, and tray are a few of the rated applications. Because the costs vary so widely with distributor stock, it is good practice to check costs during design and during construction to review cable selection.
Installation of fiberoptic cables takes a little more care than other cables. The installer can not exceed the manufacturers published tensile strength or the minimumbending rating. Tensile strength ratings include pulling and vertical rise. Typically, only two 90 bends are permissible (180 total) between pull points when installing communications cable in conduit. Innerduct, thin corrugated plastic tubing, is often used to protect fiberoptic cable above ceilings and in cable tray. It is used in large ducts to segregate cables and to allow for future installations. A tension meter or a rated breakaway knuckle is used to pull fiberoptic cables.
Fiber connections are made by splicing or mechanical connector. Splicing is only permitted in splice trays or LIUs (Figs. 18.10 and 18.11). Splicing provides a lower light loss than connectors. The best method of splicing is called fusion splicing, but it is expensive and should only be used in construction where the high number of connectors would impact the light-loss budget. As a facility owner, a cheaper splice method is often preferred for single splices where the light-loss budget is not threatened.
Testing of fiberoptic cables is done by two methods. The optical time domain reflectometer (OTDR) is used primarily after initial installation. It sends light down the cable, and reflects light back wherever anomalies exist, providing a profile of the cable for the entire length. OTDR testing is expensive but worthwhile when purchasing under a construction contract. Fiberoptic meters are usually used to determine light loss, and suffice most of the time, but if a problem exists where the location cannot be found, an OTDR may be rented to find the exact point.
Fiberoptic meters can be standalone equipment or an accessory to a UTP cable meter.
There is one type of media converter worth mentioning, it changes multimode to copper at the workstation. Two versions of the media converter are the Alcatel and the 3M. The Alcatel version fits in a junction box and has two 8-pin modular jacks. The 3M version plugs into a fiber jack at the workstation and is then plugged in on, or under, the desk. The expense for the two versions is allocated differently in accounting. The Alcatel version would typically be a capital expense in the construction budget; the 3M product, typically an office expense. Previously, making a media conversion in the PC would be an expense for the IT department, along with the PC. Since fiberoptic cable is presently comparable in price with copper UTP, we may finally see a fiber that is practical for use up to the desktop.