Information Technology Reference
In-Depth Information
Gigabit Wiring Standards for Fiber
The fiber specification in TIA-568-C allows the use of either 62.5/125 µm or 50/125 µm
fiber and single-mode fiber in horizontal and backbone cabling. However, the standard
contains minimal specifications for bandwidth, requiring 160 MHz• km at 850 nm and
500 MHz• km at 1300 nm and does not get into the complexities of laser-launched mul-
timode optics. Fiber does not have the luxury of splitting the gigabit signal among mul-
tiple fibers, as with copper, so the bandwidth component is quite critical. Some
manufacturers are offering higher-bandwidth fiber to support gigabit applications. You
will typically find this fiber promoted as gigaspeed, to emphasize its capabilities.
The link budget for a 100 m horizontal fiber link is very modest, considering
the length. For example, at 850 nm, the standard requires a maximum cable atten-
uation of 3.75 dB/km, which translates to about a 0.34 dB loss in the 90 m horizon-
tal run. It turns out that the losses of the connectors and adapters are much more
significant in the overall fiber link performance. In recognition of this, the cabling
standard permits a 0.75 dB coupling loss for the SC connector termination, and a
total 1.5 dB loss for a cross-connect to another fiber. Again, this might result in a
link loss of as much as 7.87 dB for the fiber equivalent of the copper cable channel,
with a 90 m horizontal drop and user and equipment patch cords totaling 10 m in
length. The fiber would contribute 0.37 dB, the six SC connectors 4.5 dB, and the
two coupling adapters a total of 3.0 dB. See Fig. 12.4.
In practice, the link loss is far less. A typical connector termination loss is 0.15
to 0.20 dB, and coupling losses are about 0.5 dB. Redoing the calculations using the
minimum values, you could expect the link loss to be only 2.27 dB. As a matter of
fact, Gigabit Ethernet over fiber functions very well with a 3.5 dB loss budget.
Outlet
adaptor
Horizontal
cable run
User cord
*
.75 dB
.75 dB
.75 dB
0.34 dB
1.5 dB
TR
adaptor
Equipment cord
0.03 dB for
10 m max.
total length
*
*
.75
.75
.75
1.5
FIGURE 12.4
Elements of 100 m fiber link loss.
