Modems are devices that convert digital data from computers into analog signals that can be transmitted over telephone lines, and vice versa. They allow computers to connect to networks over long distances using existing telephone infrastructure. Switches improve network performance over hubs by only forwarding data to the specific port of the intended recipient, reducing collisions. Routers connect multiple networks like a local area network to a wide area network such as the Internet.

1. CHP 4:
COMPUTER NETWORK

2. Network devices
 Some of the computer network are:
 MODEM
 Network hubs
 Switches
 Bridges
 Routers
 HTTP proxy servers

3. modem
 Modem
 Before the days of broadband Internet connections, most computers
connected to the Internet via telephone lines (dial-up connections).
The problem with using telephone lines is that they are designed to
carry voices, which are analogue signals. They are not designed for
digital data.
 The solution was to use a special device to join the digital computer to
the analogue telephone line. This device is known as a modem.
A modem contains a DAC and an ADC.
The DAC in the modem is required so that the digital computer can send
data down the analogue telephone line (it converts digital data into
noises which is exactly what the telephone line is designed to carry.)
The ADC in the modem is required so that the analogue signals (noises)
that arrive via the telephone line can be converted back into digital

4. modems
 Modems are devices which converts a computers
digital signal into analogue signal for transmission
over existing telephone line.
 It also does the reverse process, in that it converts
analogue signal from the telephone line into digital
signals to enable the computer to process the data.
 Modems are used to allow computers to connect o the
network over long distance using existing telephone
network.
 Dial up modem operate at a transmission speed of 60
kilobits per seconds.
 modern modem (asymmetric digital subscriber line)
are much faster and operate at 11 ,000 kilobits/second
 Cable modems also exist which allow cable television
providers to offer internet access as well s receiving tv
signals.

5. Modem cont..
 If you have ever used a dial-up connection, you have probably heard the noises
sent by the modem down the telephone line.
They sound like a horrible screeching beeping sound
 So, simply put, a modem is required because computers are digital devices
and the telephone system is analogue. The modem converts from digital
to analogue and from analogue to digital

6. switches
 Like hubs but more efficent.
 Devices connect to switches via twisted-pair cabling, one cable for each
 device. The difference between hubs and switches is in how the devices deal
 with the data that they receive. Whereas a hub forwards the data it receives
 to all of the ports on the device, a switch forwards it only to the port that
 connects to the destination device. It does this by learning the MAC address
 of the devices attached to it, and then by matching the destination MAC
 address in the data it receives. Figure 3.1 shows how a switch works.
 Data sent by
 one node
 Data is forwarded
 only to the
 destination node.

7. Switch function
 How a switch works.
 By forwarding data only to the connection
that should receive it, the switch
 can improve network performance in two ways.
First, by creating a direct
 path between two devices and controlling
their communication, it can greatly
 reduce the number of collisions on the
network. As you might recall, collisions
 occur on Ethernet networks when two devices
attempt to transmit at
 exactly the same time.

8. Network hubs
 Network hubs are hardware devices that can have a
number of devices/computers connected to them .its
main task is to take any data received via one of the ports
and then send out this data received from all the ports.
Each computer/device will receive the data whether
relevant or not.

9. bridges
 Definition: A bridge is a network device that
typically links together two different parts
of a LAN.
 Whereas a router is usually used to link a LAN
to a WAN (such as the Internet), a bridge links
independent parts of a LAN so that they act
as a single LAN.

10. Bridges…

11. routers
 Router
 A router is a network device that connects together two or
more networks.
A common use of a router is to join a home or business
network (LAN) to the Internet (WAN).
The router will typically have the Internet cable plugged into
it, as well as a cable, or cables to computers on the LAN.
 Alternatively, the LAN connection might be wireless (WiFi), making
the device a wireless router. (A wireless router is actually a router
and wireless switch combined)

12. http proxy servers
 Proxy Server
 A proxy server is a computer setup to share a
resource, usually an Internet connection
 It also acts as a buffer between a WAN and a LAN.
 Other computers can request a web page via the proxy
server. The proxy server will then get the page using its
Internet connection, and pass it back to the computer who
asked for it.
 Any page retrieved web pages from the internet is stored on
the server, which means that if a different computer request
the same web page it is available immediately thus
considerably speeding up the browsing process.

13. EMAIL.
 E-Mail
 E-mail is a system that allows messages to be sent and
received by computers. E-mail is the most common form
of electronic communication.
E-mail messages are text-based, but other types of file
can also be sent as ‘attachments’.
E-mails that are received wait in a user's inbox until the
user is ready to read them. (Unlike a telephone call, the
user is free to ignore e-mails until they have time to deal
with them.)

14. An e-mail message usually has the following parts:
 To send and receive e-mail, you need to have an e-mail address.
An address is made up of two parts: a username and an e-mail
provider, with an '@' symbol in the middle:
username@provider
 To The address(es) of the person who the message is for Subject
A short sentence describing what the message is about Message
The text of the message. This can be as long as you like
 An e-mail may also include the following parts:
 CC The address(es) of people to copy the e-mail to (Carbon Copy)
BCC The address(es) of people to copy the e-mail to without
anyone else knowing (Blind Carbon Copy) Attachments Files
linked to the message (images, documents, etc)

15. FORMAT OF EMAIL

16. Advantages and disadvantages
of e-mail
 Advantage of Email:
 The speed of sending and receiving emails.
 The low cast, since stamps, paper and envelopes
are not needed.
 Not needing to leave home.
 Disadvantages:
 The possibility of virus threats and hacking.
 The need of email of address completely correct.
 The inability to send bulky objects via emails.

17. E-mail spam
 E-Mail Spam
 'Spam' is the name given to unsolicited e-mails (ones sent without
being asked for).
Spam e-mails usually contain adverts for products...
 Spam is a huge problem. It is estimated that 97% of all e-mail messages
sent is spam. That's several hundred billion spam e-mails every day!
If it wasn't for e-mail spam filters (which separate out spam messages
from genuine messages, or 'ham') our e-mail systems would be
unusable.
 You might wonder who actually responds to spam e-mails, and why the
spammers would bother to send them.
The answer is all to do with the massive numbers of spam messages that
are sent...

18. E-mail spam
 Spammers send billions of messages every
day. Even if only 0.01% of people click on the
link in a spam message, that still means
thousands of links are being clicked. And
spammers are paid for every click.