electronic communications. The computers on a network may be linked
through cables, telephone lines, radio waves, satellites, or infrared light
beams.
The three basic types of networks include:
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN)
Local Area Network
A Local Area Network (LAN) is a network that is confined to a relatively small area. It is generally limited to a geographic area such as a writing lab,
a building. Rarely are LAN computers more than a mile apart.
In a typical LAN configuration, one computer is designated as the file server. It stores all of the software that controls the network, as well as the
software that can be shared by the computers attached to the network. Computers connected to the file server are called workstations. The
workstations can be less powerful than the file server, and they may have additional software on their hard drives. On most LANs, cables are used to
connect the network interface cards in each computer. See the Topology, Cabling, and Hardware sections of this tutorial for more information on the
configuration of a LAN.
Metropolitan Area Network
A Metropolitan Area Network (MAN) covers larger geographic areas, such as cities or districts. By interconnecting smaller networks within a
large geographic area, information is easily disseminated throughout the network. Local libraries and government agencies often use a MAN to connect
to citizens and private industries.
Wide Area Network
Wide Area Networks (WANs) connect larger geographic areas, such as Nevada, the United States, or the world. Dedicated transoceanic cabling or
satellite uplinks may be used to connect this type of network.
Using a WAN, people in Nevada can communicate with places like Tokyo in a matter of minutes, without paying enormous phone bills. A WAN is
complicated. It uses multiplexers to connect local and metropolitan networks to global communications networks like the Internet. To users, however,
a WAN will not appear to be much different than a LAN or a MAN.
Advantages of Installing a Network
Speed. Networks provide a very rapid method for sharing and transferring files. Without a network, files are shared by copying them to floppy
disks, then carrying or sending the disks from one computer to another. This method of transferring files (referred to as sneaker-net) is very
time-consuming.
Cost.
Networkable versions of many popular software programs are available
at considerable savings when compared to buying individually
licensed copies. Besides monetary savings, sharing a program on a network allows for easier upgrading of the program. The changes have to be
done only once, on the file server, instead of on all the individual workstations.
Security.
Files and programs on a network can be designated as "copy inhibit,"
so that you do not have to worry about illegal copying of
programs. Also, passwords can be established for specific directories to restrict access to authorized users.
Centralized Software Management.
One of the greatest benefits of installing a network is the fact that
all of the software can be
loaded on one computer (the file server). This eliminates that need to spend time and energy installing updates and tracking files on independent
computers throughout the building.
Resource Sharing.
Sharing resources is another area in which a network exceeds stand-alone
computers. Most businesses cannot afford enough
laser printers, fax machines, modems, scanners, and CD-ROM players for each computer. However, if these or similar peripherals are added to
a network, they can be shared by many users.
Electronic Mail.
The presence of a network provides the hardware necessary to install
an e-mail system. E-mail aids in personal and
professional communication for all personnel, and it facilitates the dissemination of general information to the entire staff.
Flexible Access. networks allow people access their files from computers
throughout the operation.
Workgroup Computing.
Workgroup software (such as Microsoft BackOffice) allows many users
to work on a document or project
concurrently.
Disadvantages of Installing a Network
Expensive to Install.
Although a network will generally save money over time, the initial
costs of installation can be prohibitive. Cables,
network cards, and software are expensive, and the installation may require the services of a technician.
Requires Administrative Time.
Proper maintenance of a network requires considerable time and expertise.
Many businesses have installed a
network, only to find that they did not budget for the necessary administrative support.
File Server May Fail.
Although a file server is no more susceptible to failure than any other
computer, when the files server "goes down," the
entire network may come to a halt. When this happens, the entire business may lose access to necessary programs and files.
Cables May Break.
The Topology chapter presents information about the various configurations
of cables. Some of the configurations are
designed to minimize the inconvenience of a broken cable; with other
configurations, one broken cable can stop the entire network.
A protocol is a set of rules that governs the communications between
computers on a network. These rules include guidelines that regulate the
following
characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer.
See the Topology and Cabling sections of this tutorial for more information.
The most common protocols are:
Ethernet
Token Ring
FDDI
ATM
Ethernet
The Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision
Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the
computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes,
two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random
amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and
retransmitting is very small and does not normally effect the speed of transmission on the network.
The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over twisted pair, coaxial, or fiber optic cable at a speed
of 10 Mbps.
Fast Ethernet
To allow for an increased speed of transmission, the Ethernet protocol has developed a new standard that supports 100 Mbps. This is commonly
called Fast Ethernet. Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards. In addition,
category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet is becoming common in business that have been recently wired.
Gigabit Ethernet
The most recent development in the Ethernet standard is a protocol that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for
backbones on a network at this time. In the future, it will probably be used for workstation and server connections also. It can be used with both fiber
optic cabling and copper. The 1000BaseTX, the copper cable used for Gigabit Ethernet, is expected to become the formal standard in 1999.
Token Ring
The Token Ring protocol was developed by IBM in the mid-1980s. The access method used involves token-passing. In Token Ring, the computers
are connected so that the signal travels around the network from one computer to another in a logical ring. A single electronic token moves around the
ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If a
computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the
computer for which the data is meant. At this point, the data is captured by the receiving computer. The Token Ring protocol requires a star-wired ring
using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the increasing popularity of Ethernet, the
use of Token Ring in business environments has decreased.
FDDI
Fiber Distributed Data Interface (FDDI) is a network protocol that is used primarily to interconnect two or more local area networks, often over large
distances. The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of
the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete
ring. A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbps.
ATM
Asynchronous Transfer Mode (ATM) is a network protocol that transmits data at a speed of 155 Mbps and higher. ATM works by transmitting all
data in small packets of a fixed size; whereas, other protocols transfer variable length packets. ATM supports a variety of media such as video,
CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable.
ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed
access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local
area networks.
Networking hardware includes all computers, peripherals, interface
cards and other equipment needed to perform data-processing and
communications within the network. CLICK on the terms below to learn more about those pieces of networking hardware.
This section provides information on the following components:
File Servers
Workstations
Network Interface Cards
Concentrators/Hubs
Repeaters
Bridges
Routers
File Servers
A file server stands at the heart of most networks. It is a very fast computer with a large amount of RAM and storage space, along with a fast network
interface card. The network operating system software resides on this computer, along with any software applications and data files that need to be
shared.
The file server controls the communication of information between the nodes on a network. For example, it may be asked to send a word processor
program to one workstation, receive a database file from another workstation, and store an e-mail message during the same time period. This requires
Workstations
All of the computers connected to the file server on a network are called workstations. A typical workstation is a computer that is configured with a
network interface card, networking software, and the appropriate cables. Workstations do not necessarily need floppy disk drives or hard drives
because files can be saved on the file server. Almost any computer can serve as a network workstation.
Network Interface Cards
The network interface card (NIC) provides the physical connection between the network and the computer workstation. Most NICs are internal, with
the card fitting into an expansion slot inside the computer. Laptop computers can now be purchased with a network interface card built-in or with
network cards that slip into a PCMCIA slot. Network interface cards are a major factor in determining the speed and performance of a network.
It is a good idea to use the fastest network card available for the type of workstation you are using.
The three most common network interface connections are Ethernet cards
and Token Ring cards.
Ethernet Cards
Ethernet cards are usually purchased separately from a computer, although many now include an option for a
pre-installed Ethernet card. Ethernet cards contain connections for either coaxial or twisted pair cables (or both). If it is designed for
coaxial cable, the connection will be BNC. If it is designed for twisted pair, it will have a RJ-45 connection. Some Ethernet cards also contain an AUI
connector. This can be used to attach coaxial, twisted pair, or fiber optics cable to an Ethernet card. When this method is used there is always an
external transceiver attached to the workstation.
Token Ring Cards
Token Ring network cards look similar to Ethernet cards. One visible difference is the type of connector on the back end of the card. Token Ring
cards generally have a nine pin DIN type connector to attach the card to the network cable.
Concentrators/Hubs
A concentrator is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology,
twisted-pair wire is run from each workstation to a central concentrator. Hubs are multislot concentrators into which can be plugged a number of
multi-port cards to provide additional access as the network grows in size. Some concentrators are passive, that is they allow the signal to pass from
one computer to another without any change. Most concentrators are active, that is they electrically amplify the signal as it moves from one device to
another. Active concentrators are used like repeaters to extend the
length of a network.
Repeaters
Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically
amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used
when the total length of your network cable exceeds the standards set for the type of cable being used.
A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for
unshielded twisted-pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a
multi-port active concentrator. The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to
exceed the 100 meter limit.
Bridges
A bridge is a device that allows you to segment a large network into two smaller, more efficient networks. If you are adding to an older wiring scheme
and want the new network to be up-to-date, a bridge can connect the two.
A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges
can "listen" to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message
and, if necessary, broadcast it on the other side of the network.
The bridge manages the traffic to maintain optimum performance on both sides of the network. You might say that the bridge is like a traffic cop at a
busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges
can be used to connect different types of cabling, or physical topologies.
They must, however, be used between networks with the same protocol.
Routers
A router translates information from one network to another; it is similar to a superintelligent bridge. Routers select the best path to route a message,
based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct
traffic along back roads and shortcuts.
While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers
on the network. Routers can even "listen" to the entire network to determine which sections are busiest -- they can then redirect data around those
sections until they clear up.
If you have a business LAN that you want to connect to the Internet, you will need to purchase a router. In this case, the router serves as the translator
between the information on your LAN and the Internet. It also determines the best route to send the data over the Internet. Routers can:
Direct signal traffic efficiently
Route messages between any two protocols
Route messages between linear bus, star, and star-wired ring topologies
Route messages across fiber optic, coaxial, and twisted-pair cabling
What is 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
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 business networks.
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. Buy the best cable you can afford; most businesss 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.
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.
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
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
business 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 . 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. 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.
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.
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 businesss 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 are very expensive, provide poor security, and are susceptible to interference from lights and
electronic devices. They are also slower than LANs using cabling.
What is a Topology?
The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused
with logical topology which is the method used to pass information between workstations. Logical topology was discussed in the Protocol chapter .
Main Types of Physical Topologies
The following sections discuss the physical topologies used in networks and other related topics.
Linear Bus
Star
Tree
Linear Bus
A linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals)
are connected to the linear cable. Ethernet networks use a linear bus topology.
Advantages of a Linear Bus Topology
Easy to connect a computer or peripheral to a linear bus.
Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology
Entire network shuts down if there is a break in the main cable.
Terminators are required at both ends of the backbone cable.
Difficult to identify the problem if the entire network shuts down.
Not meant to be used as a stand-alone solution in a large building.
Star
A star topology is designed with each node (file server, workstations,
and peripherals) connected directly to a central network hub or concentrator
Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all
functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used
with coaxial cable or fiber optic cable.
Advantages of a Star Topology
Easy to install and wire.
No disruptions to the network then connecting or removing devices.
Easy to detect faults and to remove parts.
Disadvantages of a Star Topology
Requires more cable length than a linear topology.
If the hub or concentrator fails, nodes attached are disabled.
More expensive than linear bus topologies because of the cost of the concentrators.
The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology, called the star-wired ring.
Star-Wired Ring
A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the MAU (multistation access unit) of a star-wired
ring contains wiring that allows information to pass from one device to another in a circle or ring. The Token Ring protocol uses a
star-wired ring topology.
Tree
A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear
bus backbone cable . Tree topologies allow for the expansion of an existing network, and enable businesses to configure a network to meet
their needs.
Advantages of a Tree Topology
Point-to-point wiring for individual segments.
Supported by several hardware and software venders.
Disadvantages of a Tree Topology
Overall length of each segment is limited by the type of cabling used.
If the backbone line breaks, the entire segment goes down.
More difficult to configure and wire than other topologies.
5-4-3 Rule
A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent
out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through
adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected
through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A
populated segment is one which has one or more nodes attached to it . The furthest two nodes on the
network have 4 segments and 3 repeaters/concentrators between them.
This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling or a combination of a fiber backbone with UTP
cabling is used. If there is a combination of fiber optic backbone and UTP cabling, the rule is simply translated to 7-6-5 rule.
Considerations When Choosing a Topology:
Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators.
Length of cable needed. The linear bus network uses shorter lengths of cable.
Future growth. With a star topology, expanding a network is easily done by adding another concentrator.
Cable type. The most common cable in businesses is unshielded twisted pair, which is most often used with star topologies.
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College of Education, University of South Florida ©1997-99.