Network
Cabling
Cable
is the medium through which information usually moves from one network device
to another. There are several types of cable which are commonly used with LANs.
In some cases, a network will utilize only one type of cable, other networks
will use a variety of cable types. The type of cable chosen for a network is
related to the network's topology, protocol, and size. Understanding the
characteristics of different types of cable and how they relate to other
aspects of a network is necessary for the development of a successful network.
The following sections
discuss the types of cables used in networks and other related topics. - Unshielded Twisted Pair
(UTP) Cable
- Shielded Twisted Pair
(STP) Cable
- Coaxial Cable
- Fiber Optic Cable
- Wireless LANs
- Cable Installation
Guides
Unshielded
Twisted Pair (UTP) Cable
Twisted
pair cabling comes in two varieties: shielded and unshielded. Unshielded
twisted pair (UTP) is the most popular and is generally the best option for
school networks (See fig. 1).
Fig.1. Unshielded twisted pair
The
quality of UTP may vary from telephone-grade wire to extremely high-speed
cable. The cable has four pairs of wires inside the jacket. Each pair is
twisted with a different number of twists per inch to help eliminate
interference from adjacent pairs and other electrical devices. The tighter the
twisting, the higher the supported transmission rate and the greater the cost
per foot. The EIA/TIA (Electronic Industry Association/Telecommunication
Industry Association) has established standards of UTP and rated five
categories of wire.
Categories
of Unshielded Twisted Pair
|
Buy
the best cable you can afford; most schools purchase Category 3 or Category 5.
If you are designing a 10 Mbps Ethernet network and are considering the cost
savings of buying Category 3 wire instead of Category 5, remember that the
Category 5 cable will provide more "room to grow" as transmission
technologies increase. Both Category 3 and Category 5 UTP have a maximum
segment length of 100 meters. In Florida
Unshielded Twisted Pair
Connector
The
standard connector for unshielded twisted pair cabling is an RJ-45 connector.
This is a plastic connector that looks like a large telephone-style connector
(See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands
for Registered Jack, implying that the connector follows a standard borrowed
from the telephone industry. This standard designates which wire goes with each
pin inside the connector.
Shielded Twisted Pair (STP) Cable
A
disadvantage of UTP is that it may be susceptible to radio and electrical
frequency interference. Shielded twisted pair (STP) is suitable for
environments with electrical interference; however, the extra shielding can
make the cables quite bulky. Shielded twisted pair is often used on networks
using Token Ring topology.
Coaxial cable is the kind of copper
cable used by cable TV companies between the community antenna and user homes
and businesses. Coaxial cable is sometimes used by telephone companies from
their central office to the telephone poles near users. It is also widely
installed for use in business and corporation Ethernet and other types of local
area network.
Coaxial cable is called
"coaxial" because it includes one physical channel that carries the
signal surrounded (after a layer of insulation) by another concentric physical
channel, both running along the same axis. The outer channel serves as a
ground. Many of these cables or pairs of coaxial tubes can be placed in a
single outer sheathing and, with repeaters, can carry information for a great
distance.
Coaxial cable was invented in 1929 and first used
commercially in 1941. AT&T established its first cross-continental coaxial
transmission system in 1940. Depending on the carrier technology used and other
factors, twisted pair copper wire and optical fiber are alternatives to coaxial
cable.
Coaxial
cabling has a single copper conductor at its center. A plastic layer provides
insulation between the center conductor and a braided metal shield (See fig.
3). The metal shield helps to block any outside interference from fluorescent
lights, motors, and other computers.
Although
coaxial cabling is difficult to install, it is highly resistant to signal
interference. In addition, it can support greater cable lengths between network
devices than twisted pair cable. The two types of coaxial cabling are thick
coaxial and thin coaxial.
Thin
coaxial cable is also referred to as thinnet. 10Base2 refers to the
specifications for thin coaxial cable carrying Ethernet signals. The 2 refers
to the approximate maximum segment length being 200 meters. In actual fact the
maximum segment length is 185 meters. Thin coaxial cable is popular in school
networks, especially linear bus networks.
Thick
coaxial cable is also referred to as thicknet. 10Base5 refers to the
specifications for thick coaxial cable carrying Ethernet signals. The 5 refers
to the maximum segment length being 500 meters. Thick coaxial cable has an
extra protective plastic cover that helps keep moisture away from the center conductor.
This makes thick coaxial a great choice when running longer lengths in a linear
bus network. One disadvantage of thick coaxial is that it does not bend easily
and is difficult to install.
Coaxial Cable Connectors
The
most common type of connector used with coaxial cables is the
Bayone-Neill-Concelman (BNC) connector (See fig. 4). Different types of
adapters are available for BNC connectors, including a T-connector, barrel
connector, and terminator. Connectors on the cable are the weakest points in
any network. To help avoid problems with your network, always use the BNC
connectors that crimp, rather than screw, onto the cable.
Fiber Optic
Cable
Fiber
optic cabling consists of a center glass core surrounded by several layers of
protective materials (See fig. 5). It transmits light rather than electronic
signals eliminating the problem of electrical interference. This makes it ideal
for certain environments that contain a large amount of electrical
interference. It has also made it the standard for connecting networks between
buildings, due to its immunity to the effects of moisture and lighting.
Fiber
optic cable has the ability to transmit signals over much longer distances than
coaxial and twisted pair. It also has the capability to carry information at
vastly greater speeds. This capacity broadens communication possibilities to
include services such as video conferencing and interactive services. The cost
of fiber optic cabling is comparable to copper cabling; however, it is more
difficult to install and modify. 10BaseF refers to the specifications for fiber
optic cable carrying Ethernet signals.
- Outer insulating jacket is made of Teflon or PVC.
- Kevlar fiber helps to strengthen the cable and
prevent breakage.
- A plastic coating is used to cushion the fiber
center.
- Center (core) is made of glass or plastic fibers.
Fiber Optic Connector
The
most common connector used with fiber optic cable is an ST connector. It is
barrel shaped, similar to a BNC connector. A newer connector, the SC, is
becoming more popular. It has a squared face and is easier to connect in a
confined space.
Ethernet Cable Summary
|
Wireless LANs
Not
all networks are connected with cabling; some networks are wireless. Wireless
LANs use high frequency radio signals, infrared light beams, or lasers to
communicate between the workstations and the file server or hubs. Each
workstation and file server on a wireless network has some sort of
transceiver/antenna to send and receive the data. Information is relayed
between transceivers as if they were physically connected. For longer distance,
wireless communications can also take place through cellular telephone technology,
microwave transmission, or by satellite.
Wireless
networks are great for allowing laptop computers or remote computers to connect
to the LAN. Wireless networks are also beneficial in older buildings where it
may be difficult or impossible to install cables.
The
two most common types of infrared communications used in schools are
line-of-sight and scattered broadcast. Line-of-sight communication means that
there must be an unblocked direct line between the workstation and the
transceiver. If a person walks within the line-of-sight while there is a
transmission, the information would need to be sent again. This kind of
obstruction can slow down the wireless network.
Scattered
infrared communication is a broadcast of infrared transmissions sent out in
multiple directions that bounces off walls and ceilings until it eventually
hits the receiver. Networking communications with laser are virtually the same
as line-of-sight infrared networks.
Wireless
LANs have several disadvantages. They provide poor security, and are
susceptible to interference from lights and electronic devices. They are also
slower than LANs using cabling.
Installing Cable - Some
Guidelines
When running cable, it is
best to follow a few simple rules: - Always use more cable than you need. Leave plenty
of slack.
- Test every part of a network as you install it.
Even if it is brand new, it may have problems that will be difficult to
isolate later.
- Stay at least 3 feet away from fluorescent light
boxes and other sources of electrical interference.
- If it is necessary to run cable across the floor,
cover the cable with cable protectors.
- Label both ends of each cable.
- Use cable ties (not tape) to keep cables in the
same location together.
No comments:
Post a Comment