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.
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
| Type |
Use |
| Category 1 |
Voice Only (Telephone Wire) |
| Category 2 |
Data to 4 Mbps (LocalTalk) |
| Category 3 |
Data to 10 Mbps (Ethernet) |
| Category 4 |
Data to 20 Mbps (16 Mbps
Token Ring) |
| Category 5 |
Data to 100 Mbps (Fast
Ethernet) |
|
Click above image to see
Straight through and crossover diagram
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, Category 5
cable is required for retrofit grants. 10BaseT refers to the
specifications for unshielded twisted pair cable (Category 3, 4, or 5)
carrying Ethernet signals. Category 6 is relatively new and is used for
gigabit connections.
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.
Fig. 2. RJ-45
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
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.
Fig. 3. Coaxial
cable
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.
Fig. 4. BNC connector
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.
Fig.5. Fiber optic
cable
Facts about fiber optic cables:
-
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
| Specification |
Cable Type |
Maximum length |
| 10BaseT |
Unshielded Twisted Pair |
100 meters |
| 10Base2 |
Thin Coaxial |
185 meters |
| 10Base5 |
Thick Coaxial |
500 meters |
| 10BaseF |
Fiber Optic |
2000 meters |
| 100BaseT |
Unshielded Twisted Pair |
100 meters |
| 100BaseTX |
Unshielded Twisted Pair |
220 meters |
|
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.
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