Page 55 - Handout Computer Network.

CHAPTER 3

NETWORKING MEDIA AND DATA LINK LAYERS

PROTOCOLS

3.1Overview

The function of the physical layer is to transmit data by defining the electrical, wireless,
or light specifications between the source and destination. After it reaches a building, low-
voltage electricity is carried to workstations, servers, and network devices via wires concealed
in walls, floors, and ceilings. Data, which can consist of such things as text, pictures, audio, or
video, travels through the wires and is represented by the presence of either electrical pulses
on copper conducting wires or light pulses in optical fibers. This chapter introduces the basic
theory of electricity, which provides a foundation for understanding networking at the physical
layer of the OSI model. This chapter also discusses different types of networking media that
are used at the physical layer, including shielded twisted-pair cable, unshielded twisted-pair
cable, coaxial cable, and fiber-optic cable.
3.2 Copper Media

Copper is the most common medium for signal wiring. Copper wires are the
components of a cable that carry the signals from the source computer to the destination
computer.

Two types of copper cable used for networks:

• Twisted-pair—Twisted-pair cables are composed of one or more pairs of
copper wires. Most data and voice networks use twisted-pair cabling.

• Coaxial—Coaxial cable has one center conductor of either solid or stranded
copper wire. Coaxial cable, once the choice for local-area network (LAN)
cabling, is now used primarily for video connections, high-speed connections
such as T3 (or E3) lines, and cable television.

3.2.1 Twisted-Pair Cable

Twisted-pair cable is a type of cabling that is used for telephone communications and
most modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The
pairs are twisted to provide protection against crosstalk, the noise generated by adjacent
pairs. The wire pairs are twisted for two reasons. First, when a wire is carrying a current, that
current creates a magnetic field around the wire. This field can interfere with signals on
nearby wires. Two basic types of twisted-pair cable exist: shielded twisted-pair (STP) and
unshielded twisted-pair (UTP).

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