Network Media
Whatever type of network is used, some type of network media is needed to carry signals between computers. Two types of media are used in networks: cable-based media, such as twisted pair, and the media types associated with wireless networking, such as radio waves.
In networks using cable-based media, there are three basic choices:
-
Twisted pair
-
Coaxial
-
Fiber-
optic
Twisted-pair and coaxial cables both use
copper
wire to conduct the signals
electronically
; fiber-optic cable uses a glass or plastic conductor and transmits the signals as light.
For many years, coaxial was the cable of choice for most LANs. Today, however (and for the past 10
years
), twisted pair has proved to be far and away the cable media of choice, thus
retiring
coax to the confines of storage closets. Fiber-optic cable has also seen its popularity rise butbecause of costhas been primarily restricted to use as a network backbone where segment length and higher speeds are needed. That said, fiber is now increasingly common in server room environments as a server to switch connection method, and in building to building connections in what are termed as metropolitan area networks (MANs).
The following sections summarize the characteristics of each of these cable types.
Twisted-pair Cabling
Twisted-pair cabling has been around a very long time. It was originally created for voice transmissions and has been widely used for telephone communication. Today, in addition to telephone communication, twisted pair is the most widely used media for networking.
The popularity of twisted pair can be attributed to the fact that it is lighter, more flexible, and easier to install than coaxial or fiber-optic cable. It is also cheaper than other media alternatives and can achieve greater speeds than its coaxial competition. These factors make twisted pair the ideal solution for most network environments.
Two main types of twisted-pair cabling are in use today:
Unshielded Twisted Pair (UTP)
and
Shielded
Twisted Pair (STP)
. UTP is significantly more commonplace than STP and is used for most networks. Shielded twisted pair is used in environments in which greater resistance to EMI and attenuation is required. The greater resistance comes at a price, however. The additional shielding, plus the need to ground that shield (which requires special connectors), can significantly add to the cost of a cable installation of STP.
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A third type of twisted pair cable, called Screened Twisted Pair (ScTP) , is available, though not widely deployed. ScTP encases all of the wires in the cable within a single shield. This is in contrast to standard STP cable, which uses the same shield, but also encases each individual wire with shielding. ScTP is cheaper than STP, but it is still more expensive than UTP.
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STP provides the extra shielding by using an
insulating
material that is wrapped around the wires within the cable. This extra protection increases the distances that data signals can travel over STP but also
increases
the cost of the cabling. Figure 2.1 shows STP and UTP cabling.
There are several categories of twisted-pair cabling, with the early categories most commonly associated with voice transmissions. The categories are specified by the Electronics Industries Association/Telecommunications Industries Association (EIA/TIA). Table 2.1 shows the categories along with the speeds that they are used to support in common network
implementations
.
Table 2.1. UTP Cable Categories
|
Category
|
Common Application
|
|
1
|
Analog voice applications
|
|
2
|
1Mbps
|
|
3
|
16Mbps
|
|
4
|
20Mbps
|
|
5
|
100Mbps
|
|
5e
|
1000Mbps
|
|
6
|
1000Mbps +
|
|
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It should be noted that the figures provided in Table 2.1 refer to speeds that these cables are commonly used to support. Ratified standards for these cabling categories might actually specify lower speeds than those listed, but cable and network component manufacturers are always pushing the performance envelope in the quest for greater speeds. The ratified standards define minimum specifications. For more information on cabling standards, visit the TIA website at http://www.tiaonline.org/.
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Coaxial
Coaxial cable, or
coax
as it is commonly referred to, has been around for a long time. Coax found success in both TV signal transmission as well as in network implementations. Coax is
constructed
with a copper
core
at the center that carries the signal, plastic insulation, braided metal shielding, and an outer plastic covering. Coaxial cable is constructed in this way to add resistance to
attenuation
(the loss of signal strength as it
travels
over distance),
crosstalk
(the degradation of a signal caused by signals from other cables running close to it), and EMI (electromagnetic interference). Figure 2.2 shows the construction of coaxial cabling.
Networks can use two types of coaxial cabling: thin coaxial and thick coaxial. Both have
fallen
out of favor, but you might still encounter thin coax in your travels.
Thin Coax
Thin coax is much more likely to be seen than thick coax in today's networks, but it isn't common, either. Thin coax is only .25 inches in diameter, making it
fairly
easy to install. Unfortunately, one of the disadvantages of all thin coax types are that they are prone to cable breaks, which increase the difficulty when installing and troubleshooting coaxial-based networks.
There are several types of thin coax cable, each of which has a specific use. Table 2.2 summarizes the categories of thin coax.
Table 2.2. Thin Coax Categories
|
Cable
|
Type
|
|
RG-58 /U
|
Solid copper core
|
|
RG-58 A/U
|
Stranded wire core
|
|
RG-58 C/U
|
Military specification
|
|
RG-6
|
Used for cable TV and cable modems
|
Fiber-optic Cable
In many ways, fiber-optic media addresses the shortcomings associated with copper-based media. Because fiber-based media use light transmissions instead of electronic
pulses
, threats such as EMI, crosstalk, and attenuation become a nonissue. Fiber is well suited for the transfer of data, video, and voice transmissions. In addition, fiber-optic is the most secure of all cable media. Anyone trying to access data signals on a fiber-optic cable must physically tap into the media. Given the composition of the cable, this is a particularly difficult task.
Unfortunately, despite the advantages of fiber-based media over copper, it still does not enjoy the popularity of twisted-pair cabling. The moderately difficult installation and maintenance procedures of fiber often require skilled technicians with specialized tools. Furthermore, the cost of a fiber-based solution limits the number of organizations that can afford to implement it. Another sometimes hidden drawback of implementing a fiber solution is the cost of
retrofitting
existing network equipment. Fiber is incompatible with most electronic network equipment. This means that you have to purchase fiber-compatible network hardware.
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Fiber-optic cable, although still more expensive than other types of cable, is well suited for high-speed data communications. It eliminates the problems associated with copper-based media, such as
near-end
crosstalk, electromagnetic interference (EMI), and signal tampering.
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Fiber-optic cable itself is
composed
of a core glass fiber
surrounded
by
cladding
. An
insulated
covering then
surrounds
both of these within an outer protective sheath. Figure 2.3 shows the composition of a fiber-optic cable.
Two types of fiber-optic cable are available: single and multimode fiber. In
multimode
fiber, many beams of light travel through the cable bouncing off of the cable walls. This strategy actually weakens the signal, reducing the length and speed the data signal can travel. Single-mode fiber uses a single direct beam of light, thus allowing for greater distances and increased transfer speeds. Some of the common types of fiber-optic cable include the following:
-
62.5 micron core/125 micron cladding multimode
-
50 micron core/125 micron cladding multimode
-
8.3 micron core/125 micron cladding single mode
In the ever-increasing search for bandwidth that will keep pace with the demands of modern applications, fiber-optic cables are sure to play a key role.
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Understanding the types of fiber optics available, as well as their advantages and their limitations, is important for real-world application, as well as the Network+ exam.
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