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 two
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 for specific security features. Also, access can be
established for specific directories 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 personnel, and it facilitates the dissemination of
general information to the entire staff. Electronic mail on a LAN can enable to
communicate with peers at their own organisation. If the LAN is connected to
the Internet, one can communicate with others throughout the world.
·
Flexible Access. Networks
allow students to access their files from computers throughout the organization.
one can begin an assignment in their domain, save part of it on a public access
area of the network, then go to the other center to finish their work. Persons
can also work cooperatively through the network.
·
Workgroup Computing. Workgroup software allows many users to work on a document or
project concurrently. For example, educators located at various schools within
a country 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.
The most
common protocols are: - 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.
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.
- 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:
- Faster microprocessor (Pentium 4 or Xeon,
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 1 GB 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.
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). 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.
Ethernet card.
From top to bottom:
RJ-45, AUI, and BNC 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.
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).
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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, 24 or
48 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 net modems, 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 (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
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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
Category 6 cable
Category 6 cable, commonly referred to as Cat 6, is a cable
standard for Gigabit Ethernet and other network protocols that is backward
compatible with the Category 5/5e and Category 3 cable standards. Cat-6
features more stringent specifications for crosstalk and system noise. The
cable standard provides performance of up to 250 MHz and is suitable for 10BASE-T
/ 100BASE-TX and 1000BASE-T (Gigabit Ethernet). It is expected to suit the 10GBASE-T
(10Gigabit Ethernet) standard, although with limitations on length if
unshielded Cat 6 cable is used.
The cable contains four twisted copper wire pairs, just
like earlier copper cable standards. Although Cat-6 is sometimes made with 23 gauge
wire, this is not a requirement; the ANSI/TIA-568-B.2-1 specification states
the cable may be made with 22 to 24 AWG gauge wire, so long as the cable meets
the specified testing standards. When used as a patch cable, Cat-6 is normally
terminated in 8P8C modular connectors, often incorrectly referred to as "RJ-45"
electrical connectors. Cat-6 connectors are made to higher standards that help
reduce noise caused by crosstalk and system noise. Attenuation, NEXT (Near End
Crosstalk), and PSNEXT (Power Sum NEXT) are all significantly lower when
compared to Cat-5/5e.
Some Cat-6 cables are too large and may be difficult to
attach to 8P8C connectors without a special modular piece and are technically
not standard compliant. If components of the various cable standards are
intermixed, the performance of the signal path will be limited to that of the
lowest category. As with all cables defined by ANSI/TIA/EIA-568-B, the maximum
allowed length of a Cat-6 horizontal cable is 100 meters (330 ft) in
length, depending upon the ratio of cord length:horizontal cable length.
The cable is terminated in either the T568A scheme or the
T568B scheme. It doesn't make any difference which is used, as they are both
straight through (pin 1 to 1, pin 2 to 2, etc). Mixing T568A-terminated patch
cords with T568B-terminated horizontal cables (or the reverse) does not produce
pinout problems in a facility. Although it may very slightly degrade signal
quality, this effect is marginal and certainly no greater than that produced by
mixing cable brands in-channel. The T568B Scheme is by far the most widely used
method of terminating patch cables.
Crossover is used for hub to hub, computer to computer,
wherever two-way communication is necessary. All gigabit ethernet equipment,
and most new 10/100Mb equipment, supports automatic crossover, meaning that
either a straight-through or crossover cable may be used for any connection.
However, older equipment requires the use of a straight-through cable to
connect a switch to a client device, and a crossover cable to connect a switch
to a switch or a client to a client. Crossover cables can be constructed by
wiring one end to the T568A scheme and the other end with the T568B scheme.
This will ensure that the Transmit (TX) pins on both ends are wired through to
the Receive (RX) pins on the other end.
|
8P8C Wiring (TIA/EIA-568-A T568A)
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|||
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Pin
|
Pair
|
Wire
|
Color
|
|
1
|
3
|
1
|
 white/green |
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2
|
3
|
2
|
 green |
|
3
|
2
|
1
|
 white/orange |
|
4
|
1
|
2
|
 blue |
|
5
|
1
|
1
|
 white/blue |
|
6
|
2
|
2
|
 orange |
|
7
|
4
|
1
|
 white/brown |
|
8
|
4
|
2
|
 brown |
|
8P8C Wiring (TIA/EIA-568-B T568B)
|
|||
|
Pin
|
Pair
|
Wire
|
Color
|
|
1
|
2
|
1
|
white/orange |
|
2
|
2
|
2
|
orange |
|
3
|
3
|
1
|
white/green |
|
4
|
1
|
2
|
blue |
|
5
|
1
|
1
|
white/blue |
|
6
|
3
|
2
|
green |
|
7
|
4
|
1
|
white/brown |
|
8
|
4
|
2
|
brown |
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
- 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.
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 (See fig. 1). All nodes (file server, workstations, and peripherals) are
connected to the linear cable. Ethernet and LocalTalk networks use a linear bus
topology.
Fig. 1. 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 (See
fig. 2).
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.
Fig. 2. Star topology
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 (See fig. 3). 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 (See fig. 4). Tree topologies allow for the expansion of an
existing network, and enable schools to configure a network to meet their
needs.
Fig. 4. Tree topology
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.
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:
|
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.
Fig. 1. Peer-to-peer network
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 (See fig. 2). 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.
Fig. 2. Client/server network
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.
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