Information Technology Reference
In-Depth Information
three standards are 1000BaseSX, 1000BaseLX, and 1000BaseSLX (an emerging
standard). Within the gigabit speed range, they are often referred to simply by the
suffix letters SX, LX, and SLX. The three suffixes actually refer to the wavelength
of light that is transmitted.
As shown in earlier chapters, the wavelengths commonly used for fiber-optic
transmission are 850, 1350, and 1550 nm, with the last two being used primarily
on single-mode fiber. So, you can say that the SX on 850 is the “short” wavelength,
the LX the “long” wavelength, and the SLX the “super-long” wave mode.
Conveniently, the letters also correspond to the relative transmission distances of
each operating mode. Thus, SX is short distance, LX is longer distance, and SLX is
super-long distance. Convenient, isn't it? The new ELX and ZX standards offer
extreme distances, way outside the needs of LAN wiring.
Understanding Fiber Performance at Gigabit Ethernet Speeds.
Several factors affect
the bandwidth and range of Gigabit Ethernet fiber links. Most of these were dis-
cussed in Chapter 5; however, they gain special importance at these much higher data
rates. Chief among these factors is the bandwidth of the transmitting optics that
“launch” the beam of light along the fiber. In lower-speed transmissions, inexpensive
light-emitting diodes (LEDs) are used. However, LEDs do not have the speed to sup-
port gigabit transmission bandwidths, so more expensive laser diodes must be used.
Another problem that affects link performance is the spectral width of the
transmit optics. An ideal light source for fiber-optic transmission would have zero
spectral width; it would produce a pure light of just one wavelength. In practice,
these solid-state light sources produce light that is distributed across a band of
wavelengths, approximated as a bell-shaped curve, with the peak at the desired
nominal wavelength. An LED has a relatively broad spectral width of about 150
nm. That is, a device that operates at 850 nm actually produces light near 850 nm
that may range from 775 to about 900 nm. A typical laser diode source has a spec-
tral width of about 5 nm. See Fig. 12.2.
The special width is important because of a phenomenon called
chromatic disper-
sion
. As you know from looking at a prism, different colors of light are refracted dif-
ferently when passing through the glass. The fiber core of a multimode graded-index
fiber will refract the different wavelengths of light accordingly, and the components of
a transmitted wave front will arrive at the destination receiver at slightly differing times.
At the high data rate of Gigabit Ethernet, this signal distortion can make the signal
unusable as the distance increases. For standard LEDs, with a very wide spectrum to
begin with, this usable distance is far too short to be used for a gigabit connection.
Laser diodes have a much narrower bandwidth, on the order of 5 to 10 nm,
and therefore experience much less of the chromatic dispersion phenomenon.
