Download what is a network?

WHAT IS A NETWORK?
A network can be described as a group of 2 or more computer systems linked together. A
Network can be via permanent connections, such as cables, or temporary connections made
through telephone or wireless links. It can be as small as a few computers, printers and
other peripherals, or it can consist of many small and large computers distributed over a
vast geographic area.
WHY USE A NETWORK?
1. To share hardware, such as printers
2. To allow communication between users (email, instant messaging)
3. To share data files
TYPES OF NETWORKS
Local Area Networks (LAN’s) – The computers are geographically close together, like the
Aquinas school network
Wide Area Networks (WAN’s) – The computers are further apart and are connected by
telephone lines.
WHAT IS A LOCAL AREA NETWORK?
A LAN is a computer network that covers a relatively small area. In practice, the total
distances covered range from a few metres (a group of computers in a single room) to over
2 km (University campus). Most LAN’s connect workstations and personal computers within
one organisation.
There are two categories of LAN:
Client/Server Network – This is like the Aquinas network. A number of servers are at the
heart of the network, and they control network activity including User IDs and passwords. A
client/server network has good security but is expensive to set up and maintain.
Peer-to-Peer Network – This is a LAN which has no server. A small business or family
household may have several computers linked together to share a printer or Internet
connection. Peer-to-Peer networks have low security but are cheap to set up because there
is no server to buy.
WHAT IS A WIDE AREA NETWORK?
Most LAN’s are limited to single buildings or groups of buildings, however, one LAN can be
connected to other LAN’s over any distance via telephone lines. A system of LAN’s
connected together in this way is called a Wide Area Network. Computers connected to a
wide-area network are often connected through the public telephone system. They can also
be connected through leased lines or satellites. The largest WAN in existence is the Internet.
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NETWORK HARDWARE
Network Interface Card - Each computer in the network
has a special card often referred to as a Network
Interface Card (NIC), which is inserted into each
computer and then allows these computers to be linked
together by cables. Every NIC has a unique number built
into it when it is manufactured. This unique Address can
identify a computer on a network.
Server – A Server is a powerful computer which stores user files,
programs and generally controls all activity on the network. On large
networks several server may be used, each doing a specific task. File
Servers, Email Servers, Internet Servers, Print Servers, Multimedia
Servers can all be found on large networks.
Workstation – This is simply a computer on the network. A
workstation is sometimes called a Node.
Hub – A hub is a device which allows a number of workstations to connect to a server. A hub
will have many (12 – 36) ports (connection sockets) on the front where network cables from
the workstations plug in. One of the ports will be a connection to the server. When the
Server has to send a file to a computer the file passes through the Hub. The Hub broadcasts
the file to all the computers,
but only the computer it was
intended for accepts the file.
Switch – A Switch looks like a
Hub and does the same job.
The advantage of a Switch is that the switch has a processor and memory inside. The Switch
learns where every device on the network is and when the Server wants to send a file to a
computer the Switch sends it to the correct destination only and does not broadcast it to
every computer. This makes a switched network more efficient.
Router – A Router is a device which is used to connect networks
together. A Router forwards packets of data from one network to
another as they pass round the Internet. You may have a wireless
router in your house to connect your computer to the Internet
Firewall - A firewall is hardware (or software) that checks information
coming from the Internet, and then either blocks it or allows it to pass
through to your computer. A hardware firewall protects all the computers
on a network. A software firewall must run on all the individual PCs on the network. A firewall is used
on a network to prevent unauthorised people from getting access to the files on the network. Firewalls
can’t stop viruses attached to emails getting through. Hardware firewalls are commonly combined
with a router into a single device.
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TRANSMISSION MEDIA
Cables:
CAT 6 – LANs are connected using CAT 6 cables. CAT is short for
‘category’ and these cables are also known as UTP cables – Unshielded
Twisted Pair. CAT 6 cables transmit data at 1000Mega Bits per Second
(Mb/sec) but are limited to 100m in length. CAT 6 cables are cheap and
easy to install.
Fibre Optic – For longer distances (many
kilometres) Fibre Optic cables can be used. Each cable is about
the same thickness as a human hair and hundreds or thousands
are bundled together to make a cable. Fibre Optic cables can
carry data for 100km at up to 40 Gb/sec, so they are very fast,
high capacity and do not suffer from interference.
Wireless
Wireless networking uses Wi-Fi to send data between devices without
using cables. It is commonly used in home networks. A Wireless Access
Point will be linked using CAT 6 cables to a router or the network switch.
Devices such as laptops, tablets and smartphones can then connect
wirelessly to the Access Point. The advantage is obvious - cables are not
required to link devices together. Wireless devices are also now inexpensive
and easy to set up. The disadvantages are that the range is limited to around
30m indoors a depends on the number and type of obstructions (walls) between the access
point and device, security is not as good and the speed of transmission is not as fast as a
wired connection.
Wireless Security – A wireless network can use WEP (Wired Equivalent Privacy) to encrypt
data as it is sent wirelessly. A user must know the WEP Key (security code) of the access
point before they can connect.
WEP is not very secure and has been replaced by WPA (Wi-Fi Protected Access) and now
WPA2. WPA2 provides stronger encryption than earlier methods and allows data to be
transmitted between devices more securely.
Wireless Standards – All networking devices follow the 802.11 standards. This means that
any networking device will work with any other device using the same standards. 802.11n is
the most recent. Before this were 802.11g and 802.11b. Each new version allows data to be
transmitted between devices faster and over greater distances, with more security.
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NETWORK SOFTWARE
The Network Operating System (NOS) runs on the network Servers. The
NOS controls User Logins with User IDs and passwords and is also
responsible for network security and generally controlling access to all
User Home Drives, applications and data files. The main NOS’s in use
today are Windows Server 2008 and Apple OS X.
A NOS is more robust than a desktop OS such as Windows XP – it does not crash or stop
working. This is really important for large companies whose business depend on Servers
working 24 hours, 365 days without stopping, such as large banks.
NETWORK SECURITY
Data and files on a network are very valuable. Schools and large businesses could not
operate without their networks. To secure their files network managers use several types of
security in combination to prevent criminals accessing their files.
Physical
Logical
Door Locks
User IDs
Alarms
Passwords
Security Cameras
User Access Rights
Fire Proof Rooms
Anti Virus Software
Firewalls
Encryption
Back Up Procedures
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NETWORK TOPOLOGY
Topology is the term used to describe the layout of the network.
Bus topology – All devices are connected to a central cable called the bus or backbone. Bus
networks are relatively inexpensive and easy to install for small networks. Bus networks are
not used for modern networks.
Node
Server
Node
Backbone
Terminator
Node
Node
The advantages of a bus typology are:


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Short cabling
Simple wiring
Easily extendible
A failure of any one node affects only that computer - the rest of the network functions
normally.
The disadvantages are:

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Difficult to diagnose faults
A break in any cable stops the whole network from working
Star topology – All devices are connected to a central
switch.
The advantages of a star network are:


A failure of any one node affects only that computer
- the rest of the network functions normally.
Relatively easy to install and manage;
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The disadvantages are:
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Initial costs for the central switch may be high as this is required no matter how many
nodes there will be
Problems at the switch close the whole system down
More cabling is required that in a bus network.
Tree Network (Hybrid)
These topologies can also be mixed.
For example, a Tree network consists
of a high-bandwidth bus called the
backbone which connects a collection
of star networks.
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