Handbook for Sound Engineers

1502 Chapter 39

Pair 2 Yellow and Black
Pair 3 Blue and Grey
Pair 4 Brown and Grey

39.11.16 Fiber Optic Cable

There are two basic varieties of fiber optic cable,
single-mode and multimode. Both are used in Ethernet
network designs. Two fibers are needed to make an
Ethernet connection, one fiber for transmit, and one for
receive, Fig. 39-49.

Multimode fiber is built of two types of glass
arranged in a concentric manner. Multimode fiber
allows many modes, or paths, of light to propagate
down the fiber optic path. The relatively large core of a
multimode fiber allows good coupling from inexpensive
LED light sources, and the use of inexpensive couplers
and connectors, Fig. 39-50.

Two sizes of multimode fiber are available.
62.5/125μm is used primarily in data communications,
and 50/100μm is used primarily in telecommunica-
tions applications. The standard for transmission of
100 Mbit Ethernet over 62.5/125μm multimode fiber is
called 100Base-FX. 100Base-FX has a 2 km distance
limitation.
Single-mode fiber optic cable is built from a single
type of glass. The cores range from 8μm to 10μm,
with 8/125μm being the most commonly used. There is
only a single path of light through the fiber, Fig.39-51.
While single-mode fiber cable costs approximately
the same as a multimode cable, the cost of the optical
transmitters and receivers is significantly more for a

single-mode installation than multimode. Single-mode

fiber has a core diameter that is so small that only a

single mode of light is propagated. This eliminates the

main limitation to bandwidth, but makes coupling light

into the fiber more difficult.

Although multimode fiber cable has a specific

distance limitation of 2 km, distance limitations of

single-mode fiber vary according to the proprietary

system in use. All are in excess of 2 km with some

allowing 100 km. There is currently no Ethernet stan-

dard for single-mode fiber.

39.11.16.1 Fiber Optic Connectors

There are two common types of fiber optic connectors,

SC and ST, Fig. 39-52. The ST, or straight tip,

connector is the most common connector used with

fiber optic cable, although this is no longer the case for

use with Ethernet. It is barrel shaped, similar to a BNC

connector, and was developed by AT&T. A newer

connector, the SC, is becoming more and more popular.

It has a squared face and is thought to be easier to

connect in a confined space. The SC is the connector

type found on most Ethernet switch fiber modules and

is the connector of choice for 100 Mbit and gigabit

Ethernet. A duplex version of the SC connector is also

available, which is keyed to prevent the TX and RX

fibers being incorrectly connected.

Figure 39-49. Cross-section through a multimode fiber
optic cable.

Figure 39-50. Possible light paths down a multimode fiber
optic cable. Note that there are multiple possible paths for
the light to take. This is why this is called multimode cable.

62.5 Mm core cladding^125 Mm

Figure 39-51. Possible light paths down a single-mode fiber

optic cable. Note that there is only one possible path for

the light to take. This is why this is called single-mode

cable.

Figure 39-52. The two most common fiber optic connec-

tors. The ST on the right has been the most popular. Today

most Ethernet fiber optic interfaces come equipped for the

SC on the left.

SC ST