Networks
Software Development Unit 2
2010
What is a network?

A network is two or
more computers
connected together
for the purpose of:


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:

Local Area Networks
(LAN)


One location
Wide Area Networks
(WAN)


Centralised WAN: Servers
in one location
Distributed WAN: Servers
distributed throughout
the network (the Internet
is an example of this)
LAN
WAN
Network configurations

Client-server

File server


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


No servers
Applications installed on
each PC, files shared
Protocols




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


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



Rules and conventions for
communication between
network devices
The commonly used are
Ethernet, Wi-Fi
Others exist but are not as
widely used
Ethernet

Uses an access method called
CSMA/CD (Carrier Sense Multiple
Access/Collision Detection)



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

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
10Mb/s
The Fast Ethernet standard
increases this to 100Mb/s
Wi-Fi


Based on the 802.11x
standard
Cannot do collision
detection and instead
uses RTS/CTS


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

TCP/IP consists of two
protocols: TCP and IP protocol

TCP (Transport Control Protocol)



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)

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)

For example: 255.255.255.255
TCP/IP: Packets


TCP divides data to be sent
into packets
Each packet contains:

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

Network hardware
includes:



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Network Interface
Controllers (NICs)
Switches / Hubs
Repeaters
Bridges
Routers
Transmission media
(cabling)
Network interface controller



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




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



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


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


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


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


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

http://searchnetworking.techtarget.co
m/sDefinition/0,,sid7_gci211806,00.h
tml




http://www.fotosearch.com/resu
lts.asp?keyword=coaxial+cable
&category=&searchtype=sss

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

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!


WAP (Wireless Application
Protocol)
WPA, WPA2 are better
Wireless LANs

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


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

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

All messages are routed
through the central
computer which acts as a
traffic cop to prevent
collisions.
http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/RingNetwork-Topology-Diagram.htm

Any connection failure between a
node and the hub will not affect the
overall system.

Common network topology

Easy and cheap to implement
Tree Topology
 In a hierarchical network, one computer,
usually the mainframe, controls the
entire network. It can be described as a
multiple star network with various levels
of computers with their own smaller
network, and so on down the
hierarchical chain. The network branches
out similar to a tree.


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

These networks are a lot easier to
repair than other topologies because
you can isolate the problem and it will
not have too much of an effect on the
rest of the network.
Server
Switch
Switch
Mesh Topology

In a mesh topology system every
computer is connected to every other
computer. Designed to offer the
maximum amount of reliability for data
transit and fault-tolerance.
The major problem is the amount of
cabling necessary to create this
topology, plus each link from one
device to another requires an individual
network card.
 Not only are physical components
wasted, but the overall capacity to carry
data is grossly under-utilised unless all
computers are transmitting to one
another almost constantly.


Typically, a mesh network is used by
governments and the defence forces, so
if one computer or a number of
components are knocked out by warfare,
enough links still exist for all of the
remaining computers to be able to talk
to each other.
A mesh topology 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
Hybrid Topology

A Hybrid topology can be formed by
using elements from more than one of
four topologies - star, ring, bus, mesh.
Ring

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
http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/RingNetwork-Topology-Diagram.htm

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
Other Network Components
Cont

Network Operating System (NOS)
Network Operating Systems
The most popular NOSes are:
•Novell Netware (better, more expensive at first,
cheaper over time)
•Microsoft Server 2003 (now dominant)
They offer similar services.
Network Operating System
(NOS)

Server is a generic term applied to any
machine running a ‘service’ application.
Eg access to shared files (file server) or
access to shared printers (print server).

These tasks, and many more, are
controlled by the Network Operating
System.

A NOS operates in the same way to the
operating system of a PC; however it
has special capabilities that enable it to
connect computers and peripheral
devices, such as printers, to a LAN.

All networking software has a range of
functions to perform once the
connection has been made:
These functions are:
1.
2.
3.
4.
Administration
File Management
Other Resource Management
Security
1.
Administration - the management of
users is the main task. Their
authorisation details need to be added,
deleted and organised into groups.
Backup procedures need to be run.
2.
File Management - the location and
transfers of files being used needs to
be managed between users and
storage locations.
3.
Other Resource Management such as printer management that
involves the spooling of print tasks
and the allocation of these tasks to
the appropriate printer.
4.
Security - the access rights of users
need to be set up and monitored.
Popular Network Operating
Systems

Unix - a multi-user, multi-tasking
operating system that can be used on
a variety of computers ranging from a
mainframe to a personal computer.

Novell NetWare - the most popular
of all NOS. provides users with a
regular interface that is self-sufficient
of the hardware being used at the
time. It is a stand-alone system which
is situated on the server.

Windows 2000, Windows Server
2003 & Windows Server 2008 - is
the major upgrade of Windows NT; it
allows for plug-and-play, which
enables the addition of peripheral
devices. It also uses ‘active directory’
which makes network administration
simpler and more stable.

Linux - is a version of Unix that runs
on a number of hardware platforms.
Because of its stability it is very
popular with Internet Service
Providers (ISPs) as an operating
system for web servers.

Mac OSX Server -is a line of computer
operating systems developed, marketed,
and sold by Apple Inc, the latest of
which is pre-loaded on all currently
shipping Macintosh computers.

Mac OS X is the successor to the
original Mac OS, which had been
Apple's primary operating system since
1984.

Unlike its predecessors, Mac OS X is a
Unix-based operating system built on
technology that had been developed
through the second half of the 1980s.
Servers
Robust central computers at the
heart of a network.
File servers are the most common
server type.
File Servers
File servers run the Network operating system
(NOS) which handles:
•authenticating users during login
•controlling users’ access to resources based on their
rights
•managing print queues
•doing backups
•running centralised software such as virus scanners
•running services like DHCP to give out IP addresses
to workstations
• controlling internet services
File Servers
Servers don’t really have anything special in terms of
hardware.
Expensive because of their high-quality components,
and “scalability” (expandability).
•Memory – servers love lots of RAM.
•Storage –need large and fast hard disks – often RAID
(discussed soon)
File Servers vs Desktops 1
•CPU Processing power – not very important in a file server
•Backup – most servers have inbuilt high-capacity tape
backup drives to protect against data loss. Tape drives usually
use QIC (Quarter Inch Cartridge) DAT (Digital audio tapes)
tapes.
Servers are the muscle men in
the computer world
File Servers vs Desktops 2
Connectivity – servers often have two or more gigabit NICs
to increase their data-throughput.
Robustness - servers run all day for years, and need rugged
high-quality components
Scalability –the ability to increase the size and power of
equipment and networks as required e.g. add 8 hard disks,
two power supplies, two NICs, two CPUs, lots of RAM etc.
Designing and engineering this expandability is expensive.
R.A.I.D.
Redundant Array of Independent [or
Inexpensive] Disks) arrays for reliability
and/or speed.
RAID uses a group of hard disks that work
as a single disk under a RAID controller.
Flavours of RAID: RAID0 to RAID10 (RAID
1 + RAID 0) offer reliability and/or speed
(at ever-increasing cost). Includes
mirroring (for reliability) and striping (for
speed).
RAID disks are usually "Hot Swap".
EXPENSIVE – needs justifying for small
organisation
3-disk RAID array
Server farms
On smaller networks, network services are
performed by software in a single server.
On busy LANs, multiple servers share the work…
•Login servers – authenticate users
•Proxy servers – cache downloads
•DHCP servers – allocate IP addresses
•Print servers –manage print job queues
•Web/FTP servers – serve web pages or files
•Email servers –handle email
Proxy Servers
A proxy server is a program on a special
server computer that is used to access
websites on behalf of other computers;
hence it acts as a proxy for the actual
Internet.
 When a computer wants a website, it is
retrieved by the proxy server and then
sent to the requesting computer after it
has passed the firewall program.

Security