What is a Network?
A network
consists of two or more computers that are linked in order to share resources
(such as printers and CDs), 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.
Two very
common types of networks include:
You may
also see references to a Metropolitan Area Networks (MAN), a Wireless LAN
(WLAN), or a Wireless WAN (WWAN).
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.
Computers
connected to a network are broadly categorized as servers or workstations.
Servers are generally not used by humans directly, but rather run continuously
to provide "services" to the other computers (and their human users)
on the network. Services provided can include printing and faxing, software
hosting, file storage and sharing, messaging, data storage and retrieval,
complete access control (security) for the network's resources, and many
others.
Workstations
are called such because they typically do have a human user which interacts
with the network through them. Workstations were traditionally considered a
desktop, consisting of a computer, keyboard, display, and mouse, or a laptop,
with with integrated keyboard, display, and touchpad. With the advent of the
tablet computer, and the touch screen devices such as iPad and iPhone, our
definition of workstation is quickly evolving to include those devices, because
of their ability to interact with the network and utilize network services.
Servers
tend to be more powerful than workstations, although configurations are guided
by needs. For example, a group of servers might be located in a secure area,
away from humans, and only accessed through the network. In such cases, it
would be common for the servers to operate without a dedicated display or
keyboard. However, the size and speed of the server's processor(s), hard drive,
and main memory might add dramatically to the cost of the system. On the other
hand, a workstation might not need as much storage or working memory, but might
require an expensive display to accommodate the needs of its user. Every
computer on a network should be appropriately configured for its use.
On a
single LAN, computers and servers may be connected by cables or wirelessly.
Wireless access to a wired network is made possible by wireless access points
(WAPs). These WAP devices provide a bridge between computers and networks. A
typical WAP might have the theoretical capacity to connect hundreds or even
thousands of wireless users to a network, although practical capacity might be
far less.
Nearly
always servers will be connected by cables to the network, because the cable
connections remain the fastest. Workstations which are stationary (desktops)
are also usually connected by a cable to the network, although the cost of
wireless adapters has dropped to the point that, when installing workstations
in an existing facility with inadequate wiring, it can be easier and less
expensive to use wireless for a desktop.
See the Topology, Cabling, and Hardware sections of this tutorial for more information on
the configuration of a LAN.
Wide Area
Networks (WANs) connect networks in larger geographic areas, such as Florida,
the United States, or the world. Dedicated transoceanic cabling or satellite
uplinks may be used to connect this type of global network.
Using a
WAN, schools in Florida can communicate with places like Tokyo in a matter of
seconds, without paying enormous phone bills. Two users a half-world apart with
workstations equipped with microphones and a webcams might teleconference in
real time. A WAN is complicated. It uses multiplexers, bridges, and routers to
connect local and metropolitan networks to global communications networks like
the Internet. To users, however, a WAN will not appear to be much different
than a LAN.
User access
control.
Modern networks almost always
have one or more servers which allows centralized management for users and for
network resources to which they have access. User credentials on a
privately-owned and operated network may be as simple as a user name and
password, but with ever-increasing attention to computing security issues,
these servers are critical to ensuring that sensitive information is only
available to authorized users.
Information
storing and sharing.
Computers allow users to create
and manipulate information. Information takes on a life of its own on a
network. The network provides both a place to store the information and
mechanisms to share that information with other network users.
Connections.
Administrators, instructors, and
even students and guests can be connected using the campus network.
Services.
The school can provide services,
such as registration, school directories, course schedules, access to research,
and email accounts, and many others. (Remember, network services are generally
provided by servers).
Internet.
The school can provide network
users with access to the internet, via an internet gateway.
Computing
resources.
The school can provide access to
special purpose computing devices which individual users would not normally
own. For example, a school network might have high-speed high quality printers
strategically located around a campus for instructor or student use.
Flexible
Access.
School networks allow students to
access their information from connected devices 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.
Collaborative software 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,
spreadsheets, or website.
Expensive
to Install.
Large campus networks can carry
hefty price tags. Cabling, network cards, routers, bridges, firewalls, wireless
access points, and software can get expensive, and the installation would
certainly require the services of technicians. But, with the ease of setup of
home networks, a simple network with internet access can be setup for a small
campus in an afternoon.
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.
Servers
Fail.
Although a network 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. Good network
design practices say that critical network services (provided by servers)
should be redundant on the network whenever possible.
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.
Security
and compliance.
Network security is expensive. It
is also very important. A school network would possibly be subject to more
stringent security requirements than a similarly-sized corporate network,
because of its likelihood of storing personal and confidential information of
network users, the danger of which can be compounded if any network users are
minors. A great deal of attention must be paid to network services to ensure
all network content is appropriate for the network community it serves.
A protocol is a set of rules that governs the communications between
computers on a network. In order for two computers to talk to each other, they
must be speaking the same language. Many different types of network protocols
and standards are required to ensure that your computer (no matter which
operating system, network card, or application you are using) can communicate
with another computer located on the next desk or half-way around the world.
The OSI (Open Systems Interconnection) Reference Model defines seven layers of
networking protocols. The complexity of these layers is beyond the scope of
this tutorial; however, they can be simplified into four layers to help
identify some of the protocols with which you should be familiar (see fig 1).
|
OSI Layer
|
Name
|
Common Protocols
|
|
7
|
Application
|
HTTP | FTP | SMTP | DNS | Telnet
|
|
6
|
Presentation
|
|
|
5
|
Session
|
|
|
4
|
Transport
|
TCP | SPX
|
|
3
|
Network
|
IP | IPX
|
|
2
|
Data Link
|
Ethernet
|
|
1
|
Physical
|
|
Fig 1. OSI model related to common network protocols
Figure 1 illustrates how some of the major protocols would correlate to the
OSI model in order to communicate via the Internet. In this model, there are
four layers, including:
- Ethernet
(Physical/Data Link Layers)
- IP/IPX
(Network Layer)
- TCP/SPX
(Transport Layer)
- HTTP,
FTP, Telnet, SMTP, and DNS(combined Session/Presentation/Application
Layers)
Assuming you want to send an e-mail message to someone in Italy, we will
examine the layers "from the bottom up" -- beginning with Ethernet
(physical/data link layers).
The physical layer of the network focuses on hardware elements, such as
cables, repeaters, and network interface cards. By far the most common protocol
used at the physical layer is Ethernet. For example, an Ethernet network (such
as 10BaseT or 100BaseTX) specifies the type of cables that can be used, the
optimal topology (star vs. bus, etc.), the maximum length of cables, etc. (See
the Cabling section for more information on Ethernet standards related to the
physical layer).
The data link layer of the network addresses the way that data packets are
sent from one node to another. 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.
Ethernet
The original Ethernet standard was developed in 1983 and had a maximum speed
of 10 Mbps (phenomenal at the time) over coaxial cable. The Ethernet protocol
allows for bus, star, or tree topologies, depending on the type of cables used
and other factors. This heavy coaxial cabling was expensive to purchase,
install, and maintain, and very difficult to retrofit into existing facilities.
The current standards are now built around the use of twisted pair wire.
Common twisted pair standards are 10BaseT, 100BaseT, and 1000BaseT. The number
(10, 100, 1000) ands for the speed of transmission (10/100/1000 megabits per
second); the "Base" stands for "baseband" meaning it has
full control of the wire on a single frequency; and the "T" stands
for "twisted pair" cable. Fiber cable can also be used at this level
in 10BaseFL.
Fast Ethernet
The Fast Ethernet protocol supports transmission up to 100 Mbps. 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 standards include:
- 100BaseT
- 100 Mbps over 2-pair category 5 or better UTP cable.
- 100BaseFX
- 100 Mbps over fiber cable.
- 100BaseSX
-100 Mbps over multimode fiber cable.
- 100BaseBX
- 100 Mbps over single mode fiber cable.
Gigabit Ethernet
Gigabit Ethernet standard is a protocol that has a transmission speed of 1
Gbps (1000 Mbps). It can be used with both fiber optic cabling and copper. (see
the
Cabling
section for more information).
- 1000BaseT
- 1000 Mbps over 2-pair category 5 or better UTP cable.
- 1000BaseTX
- 1000 Mbps over 2-pair category 6 or better UTP cable.
- 1000BaseFX
- 1000 Mbps over fiber cable.
- 1000BaseSX
-1000 Mbps over multimode fiber cable.
- 1000BaseBX
- 1000 Mbps over single mode fiber cable.
The Ethernet standards continue to evolve. with 10 Gigabit Ethernet (10,000
Mbps) and 100 Gigabit Ethernet (100,000 Mbps),
Ethernet Protocol Summary
|
Protocol
|
Cable
|
Speed
|
|
Ethernet
|
Twisted Pair, Coaxial, Fiber
|
10 Mbps
|
|
Fast Ethernet
|
Twisted Pair, Fiber
|
100 Mbps
|
|
Gigabit Ethernet
|
Twisted Pair, Fiber
|
1000 Mbps
|
Older Network Protocols
Several very popular network protocols, commonly used in the 90's and early
21st century have now largely fallen into disuse. While you may hear terms from
time to time, such as "Localtalk" (Apple) or "Token Ring"
(IBM), you will rarely find these systems still in operation. Although they
played an important role in the evolution of networking, their performance and
capacity limitations have relegated them to the past, in the wake of the
standardization of Ethernet driven by the success of the Internet.
The network layer is in charge of routing network messages (data) from one
computer to another. The common protocols at this layer are IP (which is paired
with TCP at the transport layer for Internet network) and IPX (which is paired
with SPX at the transport layer for some older Macintosh, Linus, UNIX, Novell
and Windows networks). Because of the growth in Internet-based networks, IP/TCP
are becoming the leading protocols for most networks.
Every network device (such as network interface cards and printers) have a
physical address called a MAC (Media Access Control) address. When you purchase
a network card, the MAC address is fixed and cannot be changed. Networks using
the IP and IPX protocols assign logical addresses (which are made up of the MAC
address and the network address) to the devices on the network, This can all
become quite complex -- suffice it to say that the network layer takes care of
assigning the correct addresses (via IP or IPX) and then uses routers to send
the data packets to other networks.
The transport layer is concerned with efficient and reliable transportation
of the data packets from one network to another. In most cases, a document,
e-mail message or other piece of information is not sent as one unit. Instead,
it is broken into small data packets, each with header information that
identifies its correct sequence and document.
When the data packets are sent over a network, they may or may not take the
same route -- it doesn't matter. At the receiving end, the data packets are
re-assembled into the proper order. After all packets are received, a message
goes back to the originating network. If a packet does not arrive, a message to
"re-send" is sent back to the originating network.
TCP, paired with IP, is by far the most popular protocol at the transport
level. If the IPX protocol is used at the network layer (on networks such as
Novell or Microsoft), then it is paired with SPX at the transport layer.
Several protocols overlap the session, presentation, and application layers
of networks. There protocols listed below are a few of the more well-known:
- DNS -
Domain Name System - translates network address (such as IP addresses)
into terms understood by humans (such as Domain Names) and vice-versa
- DHCP -
Dynamic Host Configuration Protocol - can automatically assign Internet
addresses to computers and users
- FTP -
File Transfer Protocol - a protocol that is used to transfer and
manipulate files on the Internet
- HTTP -
HyperText Transfer Protocol - An Internet-based protocol for sending and
receiving webpages
- IMAP -
Internet Message Access Protocol - A protocol for e-mail messages on the
Internet
- IRC -
Internet Relay Chat - a protocol used for Internet chat and other
communications
- POP3 -
Post Office protocol Version 3 - a protocol used by e-mail clients to
retrieve messages from remote servers
- SMTP -
Simple Mail Transfer Protocol - A protocol for e-mail messages on the
Internet
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.
This needs to be a sprite
This section provides information on the following components:
One or more network servers is a part of nearly every local area
network.These are very fast computers with a large amount of
RAM and storage space,
along with a one or more fast network interface card(s). The network operating
system provides tools to share server resources and information with network
users. A sophisticated permissions-handling system is included, so that access
to sensitive information can be carefully tailored to the needs of the users.
For small networks, a singe network server may provide access control, file
sharing, printer sharing, email, database, and other services.
The network server may be responding to requests from many network users
simultaneously. For example, it may be asked to load 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 and quickly share large amounts of information. When configuring such
a server, budget is usually the controlling factor. The following guidelines
should be followed:
- Fastest
processor(s)
- Large
amount of RAM
- multiple
large, fast hard drives
- Extra
expansion slots
- Fast
network interface card(s)
Optionally (if no other such devices are available on the network):
- A RAID
(Redundant Array of Inexpensive Disks) to preserve large amounts of
data(even after a disk failure)
- A
back-up unit (i.e. DAT tape drive, removable hard drives, or CD/DVD/BluRay
burner)
Computers that humans use are broadly categorized as 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 large storage hard drives, because files can be saved on the
file server. Almost any computer can serve as a network workstation.
Laptops and other mobile devices are becoming more and more common. These
devices typically have modest internal storage, but enough power to serve as a
workstation for users on the go. These machines nearly always have a wireless
adapter to allow quick network connections without cumbersome cabling. In a
school environment with good wireless coverage, a mobile device user can move
about the campus freely, and remain continuously connected to the network.
The network interface card (NIC) provides the physical connection between
the network and the computer workstation. Most NICs are internal, and they are
included in the purchase of most computers. 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 most common network interface connections are Ethernet cards and
wireless adapters.
Ethernet Cards
Ethernet cards are usually included with a computer, although additional
ethernet cards can be purchased and installed on most computers,. Ethernet
cards can 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. Only the RJ-45 connector is found on most modern ethernet cards
(See the
Cabling
section for more information on connectors.)
Fig. 1. Ethernet card.
From top to bottom:
RJ-45, AUI, and BNC connectors
Wireless Adapters
Wireless adapters are found in most portable devices, such as laptops, smart
phones, and tablet devices. External wireless adapters can be purchased and
installed on most computers having an open USB (Universal Serial Bus) port, or
unused expansion slot. (See the
Cabling section for more
information on connectors.)
An ethernet switch 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. The predecessor of the switch was the hub, which
broadcasted all inbound packets out all ports of the device, creating huge
amounts of unnecessary network traffic. Modern switches build a port map of all
IP address which respond on each port, and only broadcasts on all ports when it
doesn't have a packet's target IP address already in its port map. Switches
are:
- Usually
configured with 8, 12, or 24 RJ-45 ports
- Often
used in a star or tree topology
- Available
as "managed" or "unmanaged", with the later less
expensive, but adequate for smaller networks
- direct
replacements for hubs, immediately reducing network traffic in most
networks
- Usually
installed in a standardized metal rack that also may store network servers,
bridges, or routers
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.
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 are the traffic directors of the global internet. All routers
maintain complex routing tables which allow them to determine appropriate paths
for packets destined for any address. Routers communicate with each other, and
forward network packets out of or into a network. Here's an example:
You want to search for something on the internet using a search engine. You
open a browser on your workstation. The browser opens to a blank page (not
usually the default, but appropriate for this example). You type
"http://www.google.com" into the URL (Universal Resource Locator)
address line of the browser. The browser software packages up the URL you
typed, and sends it with a request for an IP address to the DNS (Domain Name
Server) that has been set in your network adapter's configuration. The domain
server returns an IP, such as 74.125.67.103 (actual address returned by DNS for
google.com on June 7th, 2011). The browser ships the request for that IP
address off to the network card, which bundles the request into an ethernet
packet, destined for 74.125.67.103. The network card sends the packet to the
gateway of your network, which opens the header of the packet, and makes a
determination that the packet is traveling out of your network, in search of
74.125.67.103. Your network's router has routing tables which it has been
building from communicating with other routers, and potentially augmented with
"static routes", which are specific paths added by your network's
administrators to make the task of accessing certain networks easier, or
faster, or in some cases, not possible. In this case, I find that my router
knows about another router at my ISP(Internet Service Provider), which in turn
has several more routers that are all on networks of which I am just a small node,
much like finding an atom of a molecule of a piece of dust on a rock on a moon
of a planet of a sun of a galaxy of the universe. In any case, the packet gets
passed from router to router, each time moving out of the subnets of the packet
sender, towards a router that will know where the desired server is. The packet
finally reaches the router of the network at 74.125.67.103, which dutifully
delivers the packet to the server at that IP address. The server carefully
crafts a response, and sends a reply back, which follows the same process to
get the response "Yes. Go ahead" back to the requester. Whew. And
that's just the initial request.
While bridges know the addresses of all computers on each side of the
network, routers know the addresses other routers which in turn know about
their own networks. Routers can even "listen" to entire networks to
determine which sections are busiest -- they can then redirect data around
those sections until traffic congestion clears.
So, routers are network gateways. They move network packets from one network
to another, and many can convert from one network protocol to another as
necessary. Routers select the best path to route a message, based on the
destination address of the packet. 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.
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 forwarder
between the information on your LAN and the Internet. It also determines the
best route to send the data over the Internet.
A firewall is a networking device that is installed at the entrance to a LAN
when connecting a networks together, particularly when connecting a private
network to a public network, such as the internet. The firewall uses rules to
filter traffic into and out of the private network, to protect the private
network users and data from malevolent hackers.
Firewalls are either hardware or software, depending on their intended use.
A firewall used to protect a network is a hardware device that should be
installed in the network between the router and the network. Almost all
hardware firewalls will have at least two ports, labeled "Trusted"
and "Untrusted". These terms imply the true nature of the firewall's
responsibility to the private network. The public network is connected to the
untrusted network port, and the private network is connected to the trusted
port.
Firewall rules are usually simple, consisting of a verb, either allow or
deny, the direction of the traffic, either inbound or outbound, and an address
or other network traffic identifier. Firewall rules are cumulative, so general
rules may be specified, and exceptions added as necessary. Some examples are:
- Allow
outbound all (all private network users can do anything on the public
network)
- Deny
inbound all (default setting to prevent all traffic from the public or
untrusted port, to the private port)
- Allow
inbound port 80 (allow internet web traffic to come into network to find
web servers)
- Allow
inbound port 80 destined to 170.200.201.25 (allow inbound web traffic to a
specific web server on your private network)
- Deny
inbound from 201.202.1.1/24 (deny all inbound traffic from a specific IP
address or range of addresses)
Software firewalls are commonly included in modern workstation and server
operating systems. They operate in a similar way as hardware firewalls, except
that they filter traffic in and out of the machine itself. These software
firewalls are typically unnoticed by machine users, and only need attention
occasionslly when an internet-connected application don't work as expected. The
software firewall should always be considered a "suspect" in such
cases. The problem is easily resolved, by setting an exception rule in the
firewall for the software that is attempting to communicate.
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
- Cable
Installation Guides
- Wireless
LANs
- Unshielded
Twisted Pair (UTP) Cable
Twisted pair cabling comes in two varieties: shielded and unshielded.
Unshielded twisted pair (UTP) is the most popular and is generally the best
option for 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 six categories
of wire (additional categories are emerging).
|
Category
|
Speed
|
Use
|
|
1
|
1 Mbps
|
Voice Only (Telephone Wire)
|
|
2
|
4 Mbps
|
LocalTalk & Telephone (Rarely used)
|
|
3
|
16 Mbps
|
10BaseT Ethernet
|
|
4
|
20 Mbps
|
Token Ring (Rarely used)
|
|
5
|
100 Mbps (2 pair)
|
100BaseT Ethernet
|
|
1000 Mbps (4 pair)
|
Gigabit Ethernet
|
|
5e
|
1,000 Mbps
|
Gigabit Ethernet
|
|
6
|
10,000 Mbps
|
Gigabit Ethernet
|
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
Although UTP cable is the least expensive cable, it may be susceptible to
radio and electrical frequency interference (it should not be too close to
electric motors, fluorescent lights, etc.). If you must place cable in
environments with lots of potential interference, or if you must place cable in
extremely sensitive environments that may be susceptible to the electrical
current in the UTP, shielded twisted pair may be the solution. Shielded cables
can also help to extend the maximum distance of the cables.
Shielded twisted pair cable is available in three different configurations:
- Each
pair of wires is individually shielded with foil.
- There
is a foil or braid shield inside the jacket covering all wires (as a
group).
- There
is a shield around each individual pair, as well as around the entire
group of wires (referred to as double shield twisted pair).
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 has been 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 screw, onto the cable.
Fig. 4. BNC connector
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.
The center core of fiber cables is made from glass or plastic fibers (see
fig 5). A plastic coating then cushions the fiber center, and kevlar fibers
help to strengthen the cables and prevent breakage. The outer insulating jacket
made of teflon or PVC.
Fig. 5. Fiber optic cable
There are two common types of fiber cables -- single mode and multimode.
Multimode cable has a larger diameter; however, both cables provide high
bandwidth at high speeds. Single mode can provide more distance, but it is more
expensive.
|
|
Cable Type
|
|
10BaseT
|
Unshielded Twisted Pair
|
|
10Base2
|
Thin Coaxial
|
|
10Base5
|
Thick Coaxial
|
|
100BaseT
|
Unshielded Twisted Pair
|
|
100BaseFX
|
Fiber Optic
|
|
100BaseBX
|
Single mode Fiber
|
|
100BaseSX
|
Multimode Fiber
|
|
1000BaseT
|
Unshielded Twisted Pair
|
|
1000BaseFX
|
Fiber Optic
|
|
1000BaseBX
|
Single mode Fiber
|
|
1000BaseSX
|
Multimode Fiber
|
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.
More and more networks are operating without cables, in the wireless mode.
Wireless LANs use high frequency radio signals, infrared light beams, or lasers
to communicate between the workstations, servers, 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, portable devices,
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 standards and speeds
The Wi-Fi Alliance is a global, non-profit organization that helps to ensure
standards and interoperability for wireless networks, and wireless networks are
often referred to as WiFi (Wireless Fidelity). The original Wi-Fi standard
(IEEE 802.11) was adopted in 1997. Since then many variations have emerged (and
will continue to emerge). Wi-Fi networks use the Ethernet protocol.
|
Standard
|
Max Speed
|
Typical Range
|
|
802.11a
|
54 Mbps
|
150 feet
|
|
802.11b
|
11 Mbps
|
300 feet
|
|
802.11g
|
54 Mbps
|
300 feet
|
|
802.11n
|
100 Mbps
|
300+ feet
|
Wireless Security
Wireless networks are much more susceptible to unauthorized use than cabled
networks. Wireless network devices use radio waves to communicate with each
other. The greatest vulnerability to the network is that rogue machines can
"eves-drop" on the radio wave communications. Unencrypted information
transmitted can be monitored by a third-party, which, with the right tools
(free to download), could quickly gain access to your entire network, steal
valuable passwords to local servers and online services, alter or destroy data,
and/or access personal and confidential information stored in your network
servers. To minimize the possibility of this, all modern access points and
devices have configuration options to encrypt transmissions. These encryption
methodologies are still evolving, as are the tools used by malicious hackers,
so always use the strongest encryption available in your access point and
connecting devices.
A NOTE ON ENCRYPTION: As of this writing WEP (Wired Equivalent Privacy)
encryption can be easily hacked with readily-available free tools which
circulate the internet. WPA and WPA2 (WiFi Protected Access versions 1 and 2)
are much better at protecting information, but using weak passwords or
passphrases when enabling these encryptions may allow them to be easily hacked.
If your network is running WEP, you must be very careful about your use of
sensitive passwords or other data.
Three basic techniques are used to protect networks from unauthorized
wireless use. Use any and all of these techniques when setting up your wireless
access points:
Encryption.
Enable the strongest encryption
supported by the devices you will be connecting to the network. Use strong
passwords (strong passwords are generally defined as passwords containing
symbols, numbers, and mixed case letters, at least 14 characters long).
Isolation.
Use a wireless router that places
all wireless connections on a subnet independent of the primary private
network. This protects your private network data from pass-through internet
traffic.
Hidden SSID.
Every access point has a Service
Set IDentifier (SSID) that by default is broadcast to client devices so that
the access point can be found. By disabling this feature, standard client
connection software won't be able to "see" the access point. However,
the eves-dropping programs discussed previously can easily find these access
points, so this alone does little more than keep the access point name out of
sight for casual wireless users.
Advantages of wireless networks:
- Mobility
- With a laptop computer or mobile device, access can be available
throughout a school, at the mall, on an airplane, etc. More and more
businesses are also offering free WiFi access ("Hot spots").
- Fast
setup - If your computer has a wireless adapter, locating a wireless
network can be as simple as clicking "Connect to a Network" --
in some cases, you will connect automatically to networks within range.
- Cost -
Setting up a wireless network can be much more cost effective than buying
and installing cables.
- Expandability
- Adding new computers to a wireless network is as easy as turning the
computer on (as long as you do not exceed the maximum number of devices).
Disadvantages of wireless networks:
- Security
- Be careful. Be vigilant. Protect your sensitive data with backups,
isolated private networks, strong encryption and passwords, and monitor
network access traffic to and from your wireless network.
- Interference
- Because wireless networks use radio signals and similar techniques for
transmission, they are susceptible to interference from lights and
electronic devices.
- Inconsistent
connections - How many times have you hears "Wait a minute, I just
lost my connection?" Because of the interference caused by electrical
devices and/or items blocking the path of transmission, wireless
connections are not nearly as stable as those through a dedicated cable.
- Speed -
The transmission speed of wireless networks is improving; however, faster
options (such as gigabit Ethernet) are available via cables. If you are
only using wireless for internet access, the actual internet connection
for your home or school is generally slower than the wireless network
devices, so that connection is the bottleneck. If you are also moving
large amounts of data around a private network, a cabled connection will
enable that work to proceed much faster.
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.
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.
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.
A star topology is designed with each node (file server, workstations, and
peripherals) connected directly to a central network hub, switch, or
concentrator (See fig. 2).
Data on a star network passes through the hub, switch, or concentrator
before continuing to its destination. The hub, switch, 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 when 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, switch, or concentrator fails, nodes attached are disabled.
- More
expensive than linear bus topologies because of the cost of the hubs, etc.
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. 3). Tree topologies allow for the expansion of an
existing network, and enable schools to configure a network to meet their
needs.
Fig. 3. 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.
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
that 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.
NOTE: 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 would translate to a 7-6-5 rule.The speed of networking
switches is vastly improved over older technologies, and while every effort
should be made to limit network segment traversal, efficient switching can
allow much larger numbers of segments to be traversed with little or no impact
to the network.
- 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.
|
Physical
Topology
|
Common Cable
|
Common Protocol
|
|
Linear Bus
|
Twisted Pair
Coaxial
Fiber
|
Ethernet
|
|
Star
|
Twisted Pair
Fiber
|
Ethernet
|
|
Tree
|
Twisted Pair
Coaxial
Fiber
|
Ethernet
|
What is a Network Operating System?
Unlike operating systems, such as 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:
Nearly all modern networks are a combination of both. The networking design
can be considered independent of the servers and workstations that will share
it.
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. Nearly all modern desktop operating systems, such as Macintosh
OSX, Linux, and Windows, 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 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. UNIX/Linux and the Microsoft family of Windows Servers 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.
The following links include some of the more popular peer-to-peer and
client/server network operating systems.
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 transmit 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.
Backbone - A cable to which multiple nodes or workstations
are attached.
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.
BNC Connector (Bayone-Neill-Concelman) - Standard connector used
to connect 10Base2 coaxial cable.
Bridge - Devices that connect and pass packets between
two network segments that use the same communications protocol.
Byte - an 8-bit long binary value, which originally
mapped to text character values (between 0 and 255 decimal). For example, a
decimal value of 65, represented in a binary byte is "01000001" and
represents the capital letter "A". A byte also is the atomic value of
data storage, so a megabyte is the amount of memory required to store a million
bytes.
Cable - Transmission medium of copper wire or optical
fiber wrapped in a protective cover.
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.
E-mail - An electronic mail message sent from a host
computer to a remote computer.
End User - Refers to the human executing applications on
the workstation.
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 - An 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 legacy
client/server network is Novell Netware. All the computers connected to a
peer-to-peer network are capable of being the file server. Most modern
operating systems can operate as servers or as clients, greying the distinction
in the server architecture.
Firewall - A security device which inspects traffic
entering and leaving a network, and allows or disallows the traffic, depending
on rules describing acceptable use of the network, by filtering out unwanted
packets. The firewall is usually positioned as the gateway device to another
network, such as the internet. Many routers now contain firewalls. A personal
firewall is usually software that runs on a workstation or server to filter
unwanted traffic at the individual machine.
Gigabit Ethernet - An Ethernet protocol that raises the
transmission rates to 1 Gbps (gigabits per second). Most school, corporate, and
household networks provide gigabit ethernet to the workstations via cabled
connections.
Gigabyte (GB) - One billion bytes of information. One
thousand megabytes.
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.
Intranet - Network internal to an organization that uses
Internet protocols.
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.
Linear Bus - A network topology in which each node attaches
directly to a common cable.
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.
MAU (Multistation Access Unit) - A Token Ring wiring
hub.
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 Linux/Unix and Windows Server.
Node - End point of a network connection. Nodes include
any device attached to a network such as file servers, printers, or
workstations.
Node Devices - Any computer or peripheral that is connected to
the network.
PCMCIA - (later versions were PCMCIA2 and PC
Card) An expansion slot found in many laptop computers. Largely replaced by
USB in the 2000-2010 period.
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.
Point-to-Point - A direct link between two objects in a network.
Ports - A connection point for a cable.
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.
RJ-45 - Standard connectors used for unshielded
twisted-pair 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. Many routers now
contain firewalls. Home routers can contain firewall, router, switching (for
cabled connections), and a wireless access point.
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.
Switch - A "intelligent" type of hub, in that
it sends packets only to the intended ports, rather than all computers on the
network.
Tape Back-Up - A common server or network peripheral which
allows copying data and programs from a computer system to 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.
Thicknet - A thick coaxial cable that is used with a
10Base5 Ethernet LAN.
Thinnet - A thin coaxial cable that is used with a 10Base2
Ethernet LAN.
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/ USB2 Port - A hardware interface for peripherals from
keyboards to hard drives, widely used on all computers.
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.
