Page 58 - M. Abrahim Thesis

Chapter (3) Networking Media and Data Link Layers Protocols

3.3.1 Fiber-Optic Cables

Fiber-optic cable is a networking medium that uses modulated light for data
transmissions through thin strands of glass. Signals that represent data bits are converted into
beams of light. It is important to recognize that while electricity is required to generate and
interpret the fiber-optic signals at the end devices, no electricity is in the cable itself as there is
with copper media. In fact, fiber-optic cable components are very good insulators. Many
characteristics of fiber-optic media are superior to copper. Every fiber-optic cable used for
networking consists of two glass fibers encased in separate sheaths. One fiber carries
transmitted data from device A to device B; the second fiber carries data from device B to device
A. A fiber for data goes in each direction, similar to two one-way streets going in opposite
directions. This arrangement provides a fullduplex communication link. Just as copper twisted-
pair uses separate wire pairs to transmit (Tx) and receive (Rx), fiber-optic circuits use one fiber
strand to transmit and one to receive, as illustrated in Figure (3-5). Typically, these two fiber
cables are in a single outer jacket until they reach the point at which connectors are attached.

Figure 3-5 shown a fiber-optic circuit Structure

At this point, the two fiber cables are separated. No need for twisting or shielding exists
because no light escapes when it is inside a fiber, which means no crosstalk issues exist with
fiber. It is common to see multiple fiber pairs encased in the same cable. This arrangement allows
a single cable to be run between data closets, floors, or buildings. One cable can contain 2, 4, 8,
12, 24, 48, or more separate fibers. With copper, one UTP cable has to be pulled for each circuit.
Fiber can carry many more bits per second and carry them farther than copper can.

As illustrated in Figure 3-6, five parts typically make up each fiberoptic cable:

• The core

• The cladding
• A buffer
• A strengthening material
• An outer jacket.

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