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Computer networks

Chapter 4
by

Monica E

on 5 August 2014

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Transcript of Computer networks

Computer systems connected together to :
COMPUTER NETWORKS
SERVERS
LAN Topologies
file servers (save/load files)
application servers (Distribution of application software to each computer)
printer servers (ensure printing from devices on the network is done in a queue)
proxy servers (used as a buffer between LANs and WANs)
TYPES OF NETWORKS
LAN

WAN

WIFI

BLUETOOTH|
RING

BUS

STAR

TREE
Share resources (printers,software)
(Eg Basic school or home networks)

communicate with any other computer similarly connected (Eg Internet)
The Internet
Worldwide collection of networks
Intranets
It is a computer network based on internet technology that is designed to meet the internal needs of a company

Safer
Possible to prevent employees
accessing unwanted websites
It requires
password entry
It can only be accessed
from agreed points
It is
behind a firewall
(protection against hackers and viruses)
Information used in intranets is usually stored on
local servers
Network hubs
They have a number of devices/computer connected to them.

Main task: take any data received via one of the ports and then send out this data from all of the ports.
Network security
User Id


Passwords


Encryption


Authentication techniques
NETWORKS DEVICES
Modems

Networks hubs

Switches

Bridges

Routers

HTTP proxy servers
LAN: Local Area Network
A number of computers and devices which will be connected to HUBs or SWITCHES. One hub/switch will usually be connected to a ROUTER and MODEM (usually brodband) to allow the Lan to connect to the internet, so that it becomes part of a WAN.
WLAN - Advantages and Disadvantages
All computers can access the same services and resources
No cabling. Cost are reduced.
More flexible (users can move their laptops)
Easy to add computers or devices ( a Wlan adapter is needed. The device must be within the range of an AP)

Security is a problem. It is necessary to adopt complex data encryption techniques.
Problems of interference can affect the signal
Data transfer rate is slower than in a wired LAN
WLAN (Wireless LAN)
Devices, known as access points (
AP
s), are connected into the wired network at fixed locations. The Aps use
spread spectrum technology
(radio frecuency -30 to 50 metres) or infrared - 1 to 2 metres)
Aps: receives and transmit data between the WLAN and the wired network structure. End users access to WLAN through WLAN adapters.
|
WAN: Wide Area Network
LANs connected together through a
router
or
modem
.

Some companies will set up
private WAN
s (by
fibre optic cabling
or telephone wires restricted to company only). Expensive but Secure.
More common to use
ISP
(Internet Services Provider) for connection to the internet.

Harware needed: routers, modems and proxy servers.
WIFI
Any system where it is possible to connect to a network or to a single computer through
wireless communications
:
on WLAN
PDAs and other handled devices
laptop computer which are WIFI enabled
peripherals devices such as printers, keyboard and mouse

WiFi Hotspots
: places to access WiFi (free or paid)
Bluetooth
It is an example of
WPAN
(
wireless personal area networking
) technology.
Spread spectrum transmission (
radio waves
) is used to provide wireless links.
Small range (about 10 metres)
It behaves as a mini-LAN to allow communication and transfer data between computers, mobile phones, digital cameras, media players, printers.
RING NETWORKS
Advantages:
Work well under heavy loading
Possible to create large networks

Disadvantages:
If there is a fault in the wiring between two computers then the whole network will fail
Difficult to add new computers or devices
Bus
Advantages
Easy to add computers or devices
If one device or computer fails it does not affect the rest of the network
It requires less cabling
No need for a hub or switch

Disadvantages
Difficult to isolate faults
If the central line fails then the whole network fails
outdated topology
The performance is worse when more devices/computers are added
Star Networks
Advantages
Unaffected if one computer or devices fails
Easy to identify problems on the network
work on a faulty device does not affect the rest of the network
Easy to expand the network

Disadvantages
If the central hub breaks down, the whole network crashes
TREE or Hybrid
It has a central line connecting together a series of star networks.

The server is connected to this central line.

Flexibility

same advantages and disadvantages of bus and star networks
Increasingly popular
Modems
It allows computers to connect to networks (e.g. internet) over long distances using existing telephone lines.

It converts computer ´s digital signal into an analogue signal (
modulates
) for transmission over an existing telephone line.

It also does the reverse process, it converts analogue signal from a telephone line into digital signal (
demodulates
).
Modems
Dial-up
modems




ADSL
(Asymmetric digital subscriber line)

modems
Slow
They
tie up
the telephone line while accesing the internet
Faster
at downloading data than at upñoading (sending) data
They do
not tie up
the telephone line while accesing the internet
Switches
Similar to hubs.

More efficient in the distribution of data because the network traffic only goes to where it is needed.
Each device has a unique media access control (MAC) address.
Data packets sent to the switch will have the source MAC and the receiving device MAC.
Bridges
Connect one LAN to another LAN with the same procol (rules that determine the format and transmission of data).
The bridge examines each message and
passes on to the LAN
where the
target
device is connected.
Workstation addresses are not specified, so
data packets are sent to all the devices in the specified LAN
. Only the target device or workstation accepts this message.
Bridges
Routers
Connects LANs together and also connect them to Internet.
Using the
IP
(internet proctocol)
address
sends the data package to the target LAN (to the appropriate switch).This switch will send it to the target device using the
MAC address
.
HTTP Proxy Servers
It is a
buffer between WAN
(e.g. Internet) and a
LAN
.
Any page retreived from the Internet is stored on the proxy server. When a different computer request the same page it is available inmdiately
speeding up the process
.
www
(world wide web) is part of the Internet which the user can access by way of a web browser. (Internet Explorer, Chrome, Firefox, etc)
Website
: webpages linked together. Navigate using
hyperlinks
.
URL
s: Uniform resource location to locate a resource on Internet.

(
protocol://site address/path/filename
)
Accessing
to the Internet
Accesing to the Internet
ISP (Internet Service Provider) with an account, username and password.
TYPES OF Internet ACCESS:
Dial-up
(Dial-up modem)

Cable
(Cable modem)

Via DSL
(broadband modem)
Slowest
Via telephone line
Telephone line is tied up while while a dial-up modem is in operation
Dial up Modem
Cable Modem
Local cable television operators give a user access to the internet through their own cable network.
Broadband modem
Fastest
download speed.
Often
with
wireless
interface
ISP
usually has a download/update limit
Broadband is
always on
The fast transfer rate allows systems such as voice over IP (
VOIP
) and online
chat
rooms to be
used effectively
.
Communication methods
converting data into a code by scrambling it or encoding it. An encription software and key is needed.
It does not prevent illegal access. It only makes the data useless to somebody
Something you know: Pin/Password, PIN
Something belonging to you: bank card
Something unique to you: your fingerprints
Full transcript