Information Technology Reference
In-Depth Information
will briefly describe the Ethernet fiber standards. More information is contained in
Chapter 11.
10BaseFx Wiring
The increasing use of fiber optics to transport Ethernet signals over extended dis-
tances or between buildings eventually led to the standardization of this method by
the IEEE committees. The 802.3 standard now recognizes several variations of the
fiber-optic link that started out (fiber -ptic inter-repeater link (FOIRL). The replace-
ment for FOIRL is known as 10BaseFL. This standard, as the original name implies,
is used to interconnect repeaters, although several manufacturers have used it to link
stations as well. Additionally, supplements to the 802.3 standard have added
10BaseFB and 10BaseFP.
The 10BaseFB specification is a “synchronous Ethernet” link between repeaters
that extends the limit for repeaters and segments in a single unbridged network.
Normal repeaters use a portion of the preamble of the Ethernet package to distin-
guish incoming data from noise and to synchronize with the data. Each repeater, in
effect, gobbles up some of these “sync” bits, which limits the number of repeaters a
packet can traverse.
Remember, a repeater does not store and forward a packet, so the lost bits can-
not be replaced in the outgoing signal. Synchronizing the links means that the repeater
must only distinguish data from noise and the entire preamble may be preserved.
The 10BaseFP specification is a passive star configuration for fiber optics. The
signal is shared with other fiber arms of the star via a unique system of optical dis-
tribution.
All of these 10BaseF standards use graded-index 62.5/125 µm fiber-optic cable
with two fibers per link (one for transmit and one for receive). Distance limits are 2
km for 10BaseFL and 10BaseFB, and 500 m for passive star 10BaseFP. The 10BaseFx
data rates, of course, are the same 10 Mbps as copper 10BaseT, and consequently
offer distance and electrical isolation as the primary advantages over copper.
Active point-to-point fiber-optic links offer few problems that are not quickly
revealed by the link status indicators. The 10BaseFP installation, however, does pres-
ent the same problems as the coax copper star of Arcnet, since the hub is passive and
has no indicators. Fiber-optic links offer unique advantages such as extended trans-
mission distances and immunity to interference. However, fiber is not indestructible
and may require some special handling during and after installation. For example,
fiber-optic cable is susceptible to moderate to severe performance degradation from
a one-time bending of the fiber below the bending minimum radius. While light may
still transfer past the microfracture, transmission loss may soar.
