background image
86 Chapter 3: Switched/Campus LAN Solutions
Cable Design Choices
A chain is only as strong as its weakest link, and in networking, a network connection is only
as strong as its weakest cable. Cables have different limitations, depending on the topology. The
CCDP should ensure that Layer 1, the physical layer, is not the weakest link of the network.
Choosing the right cable depends on several factors, including installation logistics, shielding,
transmission speed, and security requirements.
When making design choices, the CCDP should ask the following questions:
·
What distances must the cable cover?
·
What are the network's security needs?
·
How much bandwidth is required?
·
What transmission speeds must be supported?
Types of Cable
There are three primary types of cable for Ethernet: coaxial, twisted-pair, and fiber-optic.
Coaxial Cable
During the first days of Ethernet, coaxial cable represented the de facto standard for LAN
network cabling. Coaxial cable is composed of a core made of solid copper surrounded by
insulation, a braided metal shielding, and an outer cover. A dual-shielded cable consists of one
layer of foil insulation and one layer of braided metal shielding. Quad shielding, which consists
of two layers of foil insulation and two layers of braided metal shielding, is also available.
Shielding is the stranded mesh that surrounds the cable. Shielding protects transmitted data
from being distorted by stray electronic signals. The core of a coaxial cable, which can be
solid or stranded, carries the data. The core is surrounded by a dielectric insulating layer
that separates it from the wire mesh. The braided wire drowns outs noise and other
undesirable signals.
Coaxial cable has a copper core surrounded by wire mesh that absorbs noise and crosstalk.
Figure 3-5 depicts coaxial cable.
Figure 3-5
Coaxial Cable
87200333.book Page 86 Wednesday, August 22, 2001 2:18 PM