Networks
VCE Software
Development ¾
What is a network?
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A network is two or
more computers
connected together
for the purpose of:
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sharing resources
(printers, files,
software, hardware)
communication
How are they classified?
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Their size: LAN, WAN
Their configuration: client-server, peer-to-peer
(no servers)
The rules they use to exchange data:
protocols
How they are linked together: cable, wireless
Their logical shape (topology): bus, tree, star,
ring, mesh
Types of networks:
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Local Area Networks
(LAN)
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One location
Wide Area Networks
(WAN)
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Centralised WAN: Servers
in one location
Distributed WAN: Servers
distributed throughout
the network (the Internet
is an example of this)
LAN
WAN
Network configurations
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Client-server
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File server
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Application server
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Files stored and accessed
from central location
Applications installed and
executed from central
location.
Often used in combination
with file servers
Peer-to-peer
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No servers
Applications installed on
each PC, files shared
Protocols
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A protocol is a convention or a
standard that controls or
enables the connection,
communication and data
transfer between computers on
a network
Contains rules governing the
syntax, semantics and
synchronization of
communication
May be implemented by
hardware, software or a
combination
Like two people agreeing to
speak the same language…
Communication protocols
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Communication protocols are specifically
designed to control data transfer over a
communications channel
Some common examples:
 TCP/IP: Two protocols that control
communication across the Internet and These examples are
some networks
often referred to as the
 POP3, IMAP, SMTP: Protocols that can
‘Internet protocols’
be used to send and receive email
 HTTP: Method of publishing and
retrieving web pages
 FTP: Protocol for sending and receiving
files
Network Protocols
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Rules and conventions for
communication between
network devices
The commonly used are
Ethernet, Wi-Fi
Others exist but are not as
widely used
Ethernet
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Uses an access method called
CSMA/CD (Carrier Sense Multiple
Access/Collision Detection)
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CS – Computers (or nodes) ‘listen’ to
the network to see if there are any
signals being transmitted
MA – All computers (or nodes) can
‘hear’ what is being transmitted on
the network at any one time
CD – Collision detection: two
computers (or nodes) could transmit
at the same time, causing a collision
Ethernet
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What happens when there is a
collision?
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Each computer (or node) waits a
random amount of time and then
retransmits
The random time delay is usually
enough to ensure the collision does
not reoccur
The Ethernet specified speed is
100Mb/s
The Fast Ethernet standard
increases this to 1000Mb/s
Wi-Fi
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Based on the 802.11x
standard
Cannot do collision
detection and instead
uses RTS/CTS
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RTS: Request to send –
packet sent by the sender
CTS: Clear to send –
packet sent by the
intended recipient that
alerts all other nodes to
be ‘quiet’
TCP/IP
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TCP/IP consists of two
protocols: TCP and IP protocol
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TCP (Transport Control Protocol)
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Used to divide data that needs to be
transmitted into a number of packets,
each with a sequence number
The TCP at the destination address
assembles the data and notifies the
sender if any packets are not received
within a certain time (‘timeout’)
IP (Internet Protocol)
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Defines how information gets sent
between network nodes
TCP/IP: IP Address
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In order for packets to be able
to be sent across the Internet or
a network, each node must have
a unique IP address (just like a
street address)
Domain Name Servers (DNS)
translate URLs to IP addresses
An IP address consists of 4
bytes (32 bits)
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For example: 255.255.255.255
TCP/IP: Packets
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TCP divides data to be sent
into packets
Each packet contains:
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A header consisting of:
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a destination address
a source address
the packet number
the total number of packets
a checksum
and some other things…
The data itself (known as the
payload)
A trailer
TCP/IP: From source to
destination
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If a packet arrives at a
destination with an incorrect
checksum, it is discarded
The path from the source to
destination is in flux, therefore
packets ‘get lost’, are corrupted
or are duplicated
If a ‘timeout’ occurs or a packet
is corrupted, a request for a
resend of that packet is sent
After several unsuccessful
attempts at receiving a packet,
the connection is broken
Network Hardware
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Network hardware
includes:
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Network Interface
Controllers (NICs)
Switches / Hubs
Repeaters
Bridges
Routers
Transmission media
(cabling)
Network interface controller
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NICs provide a physical
connection from a
computer (or node) to the
network
Can be integrated on the
motherboard
Each has a unique 48-bit
MAC address
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Stored in ROM
Media Access Control
Acts like a name for the card
No two the same
http://en.wikipedia.org/wiki/Network_card
Hubs
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Connects a number of
NICs together
Consists of a number of
ports
Signals arriving at one
port and sent to all
other ports
Large number of
collisions: a “dumb
device”
http://en.wikipedia.org/wiki/Ethernet_hub
Switches
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Sends data only to the port
that requires it – a “clever
hub”
Reduces collisions – and in
many cases can eliminate
them altogether
Much better than hubs –
traditionally were much
more expensive, but this is
no longer the case
http://en.wikipedia.org/wiki/Network_switch
Repeaters
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As a network signal
degrades as it travels
over large distances, a
repeater can be used
to boost the signal
Both switches and hubs
act as repeaters
http://www.unitedsale.net/products/image02/D-Link-DWL-G710-Range-Extender2.jpg
Bridges
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Network devices that
can be used to connect
two different physical
networks together
For example, a bridge
could be used to join
an ethernet network
and a fast ethernet
network or a star and a
bus topology
http://www.streamshop.ru/Modems/P6212.jpg
Routers
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Acts as a junction between two
or more networks
Directs traffic in the same way
as a switch, but can also
translate and filter (firewall)
Used to connect LANs to the
Internet
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For example, a router may
connect to the Internet and be
assigned an IP address by an ISP
IP addresses may exist within the
LAN that duplicate those on the
Internet, but the router ensures
these are private to the LAN
http://shopproducts.howstuffworks.co
m/Linksys+WRT54GS+Wireless+Ro
uter/SF-1/PID-20664068
Bridges vs Routers
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Bridges and routers work
in different ways (on
different OSI layers – look
up the definition of this if
you want to know more)
Bridges are not able to
convert from one protocol
to another where routers
can
http://images.amazon.com/images/P/
B000H1L5W4.01._AA280_SCLZZZ
ZZZZ_V61775592_.jpg
Network Cabling
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Cables transmit data from
one network node to
another
Several types of cable
exist, each having a
particular bandwidth
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Coaxial cable
UTP cable
Fibre optic cable
Different types can be
used within the same
network, but this can
affect bandwidth
Coaxial cable
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http://searchnetworking.techtarget.co
m/sDefinition/0,,sid7_gci211806,00.h
tml
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http://www.fotosearch.com/resu
lts.asp?keyword=coaxial+cable
&category=&searchtype=sss
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Consists of a round conducting
wire, surrounded by an
insulating spacer, surrounded
by a cylindrical conducting
sheath
Highly resistant to signal
interference
Not used much today
Bayonet style connectors
Often associated with bus
networks
Bandwidth: 10Mb/s
UTP Cable
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Unshielded Twisted Pair
(UTP) is the most popular
cabling used currently
Also comes in a Shielded
Twisted Pair version, that
prevent interference
Cables are twisted
together to decrease
interference
Length limit is 100 metres
Cheap
UTP Cable
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Connector on the end is
called an RJ45
CAT5 is most common
version and provides a
bandwidth of 100Mb/s
CAT5e can provide
1000Mb/s (or a gigabit/s)
CAT6a is a future
specification designed for
10Gb/s!
http://en.wikipedia.org/wiki/Image:U
TP_ethernet_cable_8pin_modular_ja
ck_endings.jpg
http://en.wikipedia.org/wiki/Category
_5_cable
Fiber Optic Cable
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Transmits light using the concept of critical angle
As a result, cannot bend through tight angles
Not affected by interference
Can carry multiple signals at the same time
Very hand bandwidth possible
Expensive and difficult to install
http://www.fotosearch.com/PD
S084/aa031778/
Wireless LANs
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Wireless network hardware
is available
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Wireless router
Wireless NIC
Wireless Access Point
(equivalent to a switch)
Relatively cheap and easy to
install
Encryption a must!
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WAP (Wireless Application
Protocol)
WPA, WPA2 are better
Wireless LANs
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802.11x is a set of standards
defining the way signals are
formatted, the frequencies they
use and ultimately, transfer speed
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802.11a: (1999), max 54Mb/s
802.11b: (1999), max 11Mb/s
802.11g: (2003), max 54Mb/s
802.11n (draft): (2007), max 540Mb/s
Typical speeds are often half the
max speeds due to other network
traffic
Most devices are backwards
compatible, for example a device
might be 802.11bg
http://en.wikipedia.org/wiki/I
mage:Linksys_Router.png
Topology
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The arrangement of nodes
within a network
Main types :
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Bus
Star
Ring*
Tree
Mesh*
* Not examinable
http://en.wikipedia.org/wiki/I
mage:NetworkTopologies.png
Bus
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Contains a backbone
which usually runs the
length of the network
Ends are normally
terminated using special
connectors
Often associated with
coaxial cable
http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/BusNetwork-Topology-Diagram.htm
Star
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In a star network, a
central point is used to
connect all other
network nodes
The central point is
usually a hub or a
switch
Common network
topology
Easy and cheap to
implement
http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/RingNetwork-Topology-Diagram.htm
Ring*
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(not examinable)
All network nodes are
connected in a ring
A token is passed around the
network from one computer to
another
Only the node that holds the
token can transmit data
No collisions
Used to be implemented using
coaxial cable and is sometimes
used with fibre optic cable
today
Two rings transmitting data in
different directions can be
used
http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/RingNetwork-Topology-Diagram.htm
Tree
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A tree topology
combines star
and bus
topologies
Usually this takes
the form of star
networks linked
by a network
backbone (or a
bus network)
http://compnetworking.about.com/od/networkdesign/ig/ComputerNetwork-Topologies/Tree-Network-Topology-Diagram.htm
Mesh*
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(not examinable)
A mesh topolgy is one
in which paths between
network nodes are
often changing as the
status of nodes changes
The Internet is a perfect
example of a mesh
topology
http://compnetworking.about.com/od/networkdesign/ig/ComputerNetwork-Topologies/Mesh-Network-Topology-Diagram.htm