What is a Network
A network consists of two or more computers that are linked in order to share resources (such as printers and CD-ROMs), exchange files, or allow electronic communications. The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.
A network consists of two or more computers that are linked in order to share resources (such as printers and CD-ROMs), exchange files, or allow 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)
Wide Area
Network (WAN)
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, school, or
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.
Wide Area
Networks (WANs) connect larger geographic areas, such as Florida ,
the United States
Using a WAN,
schools in Florida can communicate with places
like Tokyo
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. Network able 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 at a school 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 schools 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 school personnel, and
it facilitates the dissemination of general information to the entire school
staff. Electronic mail on a LAN can enable students to communicate with
teachers and peers at their own school. If the LAN is connected to the
Internet, students can communicate with others throughout the world.
Flexible
Access. School
networks allow students to access their files from computers throughout the
school. Students can begin an assignment in their classroom, save part of it on
a public access area of the network, then go to the media center after school
to finish their work. Students can also work cooperatively through the network.
Workgroup
Computing. Workgroup software (such
as Microsoft BackOffice) allows many users to work on a document or project
concurrently. For example, educators located at various schools within a county
could simultaneously contribute their ideas about new curriculum standards to
the same document and spreadsheets.
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
schools 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 school 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.
What is a Protocol
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: See the Topology and Cabling sections of this tutorial for more information.
- Ethernet
- LocalTalk
- 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 wireless access points, twisted pair, coaxial, or fiber optic
cable at a speed of 10 Mbps up to 1000 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
schools 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.
LocalTalk
LocalTalk is a
network protocol that was developed by Apple Computer, Inc. for Macintosh
computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense
Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that
a computer signals its intent to transmit before it actually does so. LocalTalk
adapters and special twisted pair cable can be used to connect a series of
computers through the serial port. The Macintosh operating system allows the
establishment of a peer-to-peer network without the need for additional
software. With the addition of the server version of AppleShare software, a
client/server network can be established.
The LocalTalk
protocol allows for linear bus, star, or tree topologies using twisted pair
cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission
is only 230 Kbps.
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 school 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.
Protocol Summary
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What is Networking Hardware
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.
- File Servers
- Workstations
- Network Interface Cards
- Switches
- 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 a computer that can store a lot of
information and share it very quickly. File servers should have at least the
following characteristics:
- 800 megahertz or faster microprocessor (Pentium 3 or 4, G4 or G5)
- A fast hard drive with at least 120 gigabytes of storage
- A RAID (Redundant Array of Inexpensive Disks) to preserve data
after a disk casualty
- A tape back-up unit (i.e. DAT, JAZ, Zip, or CD-RW drive)
- Numerous expansion slots
- Fast network interface card
- At least of 512 MB of RAM
Workstations
All of the user
computers connected to 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 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. Some computers, such as Mac Classics, use
external boxes which are attached to a serial port or a SCSI port. 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, LocalTalk connectors,
and Token Ring cards. According to a International Data Corporation study, Ethernet
is the most popular, followed by Token Ring and LocalTalk (Sant'Angelo, R.
(1995). NetWare Unleashed, Indianapolis ,
IN
Ethernet Cards
Ethernet cards
are usually purchased separately from a computer, although many computers (such
as the Macintosh) now include an option for a pre-installed Ethernet card.
Ethernet cards contain connections for either coaxial or twisted pair cables
(or both) (See fig. 1). 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. (See the Cabling section for more information on connectors.)
LocalTalk Connectors
LocalTalk is
Apple's built-in solution for networking Macintosh computers. It utilizes a
special adapter box and a cable that plugs into the printer port of a Macintosh
(See fig. 2). A major disadvantage of LocalTalk is that it is slow in
comparison to Ethernet. Most Ethernet connections operate at 10 Mbps (Megabits
per second). In contrast, LocalTalk operates at only 230 Kbps (or .23 Mbps).
Fig.2. LocalTalk connectors
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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.
Switch
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 switch/hub. Most switches are active,
that is they electrically amplify the signal as it moves from one device to
another. Switches no longer broadcast network packets as hubs did in the past,
they memorize addressing of computers and send the information to the correct
location directly. Switches are:
- Usually configured with 8, 12, or 24 RJ-45 ports
- Often used in a star or star-wired ring topology
- Sold with specialized software for port management
- Also called hubs
- Usually installed in a standardized metal rack that also may
store netmodems, bridges, or routers
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
school 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
- 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
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
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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 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
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 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 . 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.
- 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
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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.
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
- Star-Wired Ring
- Tree
- Considerations When Choosing a Topology
- Summary Chart
Linear Bus
A linear bus
topology consists of a main run of cable with a terminator at each end . All nodes (file server, workstations, and peripherals) are connected
to the linear cable. Ethernet and LocalTalk 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 schools 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 . In Figure 4, the 5-4-3 rule is
adhered to. 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 schools is unshielded twisted pair,
which is most often used with star topologies.
Summary Chart:
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What is a Network
Operating System?
Unlike
operating systems, such as DOS and Windows, that are designed for single users
to control one computer, network operating systems (NOS) coordinate the
activities of multiple computers across a network. The network operating system
acts as a director to keep the network running smoothly.
The two major types of network operating
systems are: - Peer-to-Peer
- Client/Server
Peer-to-Peer
Peer-to-peer
network operating systems allow users to share resources and files located on
their computers and to access shared resources found on other computers. However,
they do not have a file server or a centralized management source (See fig. 1).
In a peer-to-peer network, all computers are considered equal; they all have
the same abilities to use the resources available on the network. Peer-to-peer
networks are designed primarily for small to medium local area networks.
AppleShare and Windows for Workgroups are examples of programs that can
function as peer-to-peer network operating systems.
Advantages of a peer-to-peer network:
- Less initial expense - No need for a dedicated server.
- Setup - An operating system (such as Windows XP) already in place
may only need to be reconfigured for peer-to-peer operations.
Disadvantages of a peer-to-peer network:
- Decentralized - No central repository for files and applications.
- Security - Does not provide the security available on a
client/server network.
Client/Server
Client/server
network operating systems allow the network to centralize functions and
applications in one or more dedicated file servers. The file
servers become the heart of the system, providing access to resources and
providing security. Individual workstations (clients) have access to the
resources available on the file servers. The network operating system provides
the mechanism to integrate all the components of the network and allow multiple
users to simultaneously share the same resources irrespective of physical
location. Novell Netware and Windows 2000 Server are examples of client/server
network operating systems.
Advantages of a client/server network:
- Centralized - Resources and data security are controlled through
the server.
- Scalability - Any or all elements can be replaced individually as
needs increase.
- Flexibility - New technology can be easily integrated into
system.
- Interoperability - All components (client/network/server) work
together.
- Accessibility - Server can be accessed remotely and across
multiple platforms.
Disadvantages of a client/server network:
- Expense - Requires initial investment in dedicated server.
- Maintenance - Large networks will require a staff to ensure
efficient operation.
- Dependence - When server goes down, operations will cease across
the network.
Examples of network
operating systems
The following list includes some of the
more popular peer-to-peer and client/server network operating systems. - AppleShare
- Microsoft Windows Server
- Novell Netware
Glossary
10Base2 -
Ethernet specification for thin coaxial cable, transmits signals at 10 Mbps
(megabits per second) with a distance limit of 185 meters per segment.
10Base5 -
Ethernet specification for thick coaxial cable, transmits signals at 10 Mbps
(megabits per second) with a distance limit of 500 meters per segment.
10BaseF -
Ethernet specification for fiber optic cable, transmits signals at 10 Mbps
(megabits per second) with a distance limit of 2000 meters per segment.
10BaseT -
Ethernet specification for unshielded twisted pair cable (category 3, 4, or 5),
transmits signals at 10 Mbps (megabits per second) with a distance limit of 100
meters per segment.
100BaseT
- Ethernet specification for unshielded twisted pair cabling that is used to
transmit data at 100 Mbps (megabits per second) with a distance limit of 100
meters per segment.
1000BaseTX
-Ethernet specification for unshielded twisted pair cabling that is used to
trasmit data at 1 Gbps (gigabits per second) with a distance limitation of 220
meters per segment.
Asynchronous Transfer Mode (ATM) - A network protocol that transmits data at a speed of
155 Mbps and higher. It is most often used to interconnect two or more local
area networks.
AppleTalk
- Apple Computer's network protocol originally designed to run over LocalTalk
networks, but can also run on Ethernet and Token Ring.
AUI Connector (Attachment Unit Interface) - A 15 pin connector found on
Ethernet cards that can be used for attaching coaxial, fiber optic, or twisted
pair cable.
Bit -
Binary digit in the binary numbering system. Its value can be 0 or 1. In an
8-bit character scheme, it takes 8 bits to make a byte (character) of data.
Bridge -
Devices that connect and pass packets between two network segments that use the
same communications protocol.
Client/Server - A networking system in which one or more file servers (Server)
provide services; such as network management, application and centralized data
storage for workstations (Clients).
CSMA/CA -
Carrier Sense Multiple Access Collision Avoidance is a network access method in
which each device signals its intent to transmit before it actually does so.
This prevents other devices from sending information, thus preventing
collisions from occurring between signals from two or more devices. This is the
access method used by LocalTalk.
CSMA/CD -
Carrier Sense Multiple Access Collision Detection is a network access method in
which devices that are ready to transmit data first check the channel for a
carrier. If no carrier is sensed, a device can transmit. If two devices
transmit at once, a collision occurs and each computer backs off and waits a
random amount of time before attempting to retransmit. This is the access
method used by Ethernet.
Coaxial Cable - Cable consisting of a single copper conductor in the center
surrounded by a plastic layer for insulation and a braided metal outer shield.
Concentrator - A device that provides a central connection point for cables
from workstations, servers, and peripherals. Most concentrators contain the
ability to amplify the electrical signal they receive.
DIN - A
plug and socket connector consisting of a circular pattern of pins in a metal
sleeve. This type of connector is commonly seen on keyboards.
Dumb Terminal - Refers to devices that are designed to communicate exclusively
with a host (main frame) computer. It receives all screen layouts from the host
computer and sends all keyboard entry to the host. It cannot function without
the host computer.
Ethernet
- A network protocol invented by Xerox Corporation and developed jointly by
Xerox, Intel and Digital Equipment Corporation. Ethernet networks use CSMA/CD
and run over a variety of cable types at 10 Mbps (megabits per second).
Expansion Slot - Area in a computer that accepts additional input/output boards
to increase the capability of the computer.
Fast Ethernet - A new Ethernet standard that supports 100 Mbps using category 5
twisted pair or fiber optic cable.
Fiber Distributed Data Interface (FDDI) - A network protocol that is used
primarily to interconnect two or more local area networks, often over large
distances.
Fiber Optic Cable - A cable, consisting of a center glass core surrounded by layers
of plastic, that transmits data using light rather than electricity. It has the
ability to carry more information over much longer distances.
File Server
- A computer connected to the network that contains primary files/applications
and shares them as requested with the other computers on the network. If the
file server is dedicated for that purpose only, it is connected to a
client/server network. An example of a client/server network is Novell Netware.
All the computers connected to a peer-to-peer network are capable of being the
file server. Two examples of peer-to-peer networks are LANtastic and Windows
for Workgroups.
Gigabit Ethernet - An Ethernet protocol that raises the transmission rates to 1
Gbps (gigabits per second). It is primarily used for a high speed backbone of a
network.
Hub - A
hardware device that contains multiple independent but connected modules of
network and internetwork equipment. Hubs can be active (where they repeat
signals sent through them) or passive (where they do not repeat but merely
split signals sent through them).
Infrared
- Electromagnetic waves whose frequency range is above that of microwaves, but
below that of the visible spectrum.
Internet
- A global network of networks used to exchange information using the TCP/IP
protocol. It allows for electronic mail and the accessing ad retrieval of
information from remote sources.
LAN
(Local Area Network) - A network connecting computers in a relatively small
area such as a building.
LocalTalk
- Apple Corporation proprietary protocol that uses CSMA/CA media access scheme
and supports transmissions at speeds of 230 Kbps (Kilobits per second).
MAN
(Metropolitan Area Network) - A network connecting computers over a large
geographical area, such as a city or school district.
Modem
(Modulator/Demodulator) - Devices that convert digital and analog signals.
Modems allow computer data (digital) to be transmitted over voice-grade
telephone lines (analog).
Multiplexer
- A device that allows multiple logical signals to be transmitted
simultaneously across a single physical channel.
Network Modem - A modem connected to a Local Area Network (LAN) that is
accessible from any workstation on the network.
Network Interface Card (NIC) - A board that provides network communication capabilities
to and from a computer.
Network Operating System (NOS) - Operating system designed to pass information and
communicate between more than one computer. Examples include AppleShare, Novell
NetWare, and Windows NT Server.
Node -
End point of a network connection. Nodes include any device attached to a network
such as file servers, printers, or workstations.
Peer-to-Peer Network - A network in which resources and files are shared without a
centralized management source.
Physical Topology - The physical layout of the network; how the cables are
arranged; and how the computers are connected.
Protocol
-A formal description of a set of rules and conventions that govern how devices
on a network exchange information.
RAID
(Redundant Array of Inexpensive Disks) - A configuration of multiple disks
designed to preserve data after a disk casualty.
RAM (Random
Access Memory) - The working memory of a computer where data and programs are
temporarily stored. RAM only holds information when the computer is on.
Repeater
- A device used in a network to strengthen a signal as it is passed along the
network cable.
Router -A
device that routes information between interconnected networks. It can select
the best path to route a message, as well as translate information from one
network to another. It is similar to a superintelligent bridge.
SCSI (Small Computer Serial Interface) - An interface controller that allows
several peripherals to be connected to the same port on a computer.
Segment -
Refers to a section of cable on a network. In Ethernet networks, two types of
segments are defined. A populated or trunk segment is a network cable that has
one or more nodes attached to it. A link segment is a cable that connects a
computer to an interconnecting device, such as a repeater or concentrator, or
connects a interconnecting device to another interconnecting device.
Sneaker-Net
- Refers to a manual method of sharing files in which a file is copied from a
computer to a floppy disk, transported to a second computer by a person
physically walking (apparently wearing sneakers) to the second computer, and
manually transferring the file from floppy disk to the second computer.
Speed of Data Transfer - The rate at which information
travels through a network, usually measured in megabits per second.
Star Topology - LAN topology in which each node on a network is connected
directly to a central network hub or concentrator.
Star-Wired Ring - Network topology that connects network devices (such as
computers and printers) in a complete circle.
Tape Back-Up - Copying all the data and programs of a computer system on
magnetic tape. On tape, data is stored sequentially. When retrieving data, the
tape is searched from the beginning of tape until the data is found.
Terminator - A device that provides electrical
resistance at the end of a transmission line. Its function is to absorb signals
on the line, thereby keeping them from bouncing back and being received again
by the network.
Token - A
special packet that contains data and acts as a messenger or carrier between
each computer and device on a ring topology. Each computer must wait for the
messenger to stop at its node before it can send data over the network.
Token Ring
- A network protocol developed by IBM in which computers access the network
through token-passing. Usually uses a star-wired ring topology.
Topology
- There are two types of topology: physical and logical. The physical topology
of a network refers to the configuration of cables, computers, and other
peripherals. Logical topology is the method used to pass the information
between workstations. Issues involving logical topologies are discussed on the
Protocol chapter
Transceiver
(Transmitter/Receiver) - A Device that receives and sends signals over a
medium. In networks, it is generally used to allow for the connection between
two different types of cable connectors, such as AUI and RJ-45.
Tree Topology - LAN topology similar to linear bus topology, except that tree
networks can contain branches with multiple nodes.
Twisted Pair - Network cabling that consists of four pairs of wires that are
manufactured with the wires twisted to certain specifications. Available in
shielded and unshielded versions.
USB (Universal Serial Bus) Port - A hardware interface for low-speed peripherals such as
the keyboard, mouse, joystick, scanner, printer, and telephony devices.
WAN (Wide
Area Network) - A network connecting computers within very large areas, such as
states, countries, and the world.
Workgroup
- A collection of workstations and servers on a LAN that are designated to
communicate and exchange data with one another.
Workstation
- A computer connected to a network at which users interact with software
stored on the network.
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