Networks (Part 2)
Use of a Modem
The different modems
Baud Rate
Bandwidth
Server and Client machines
Network Topologies (Bus, Ring and Star)
Recap
 What
is a Network?
 What are the two main types of
Networks?
 What is the difference between LAN
and WLAN?
 What are the advantages of a
Network?
 What is the WWW?
What is a Modem

A modem is on of the main requirements
of a network

A modem could be an Analogue to Digital
and a Digital to Analogue device

The work modem comes from two
words Modulate and Demodulate which
basically refer to what the task of a
modem
Use of a Modem

The modem allows the computer to be
connected to a telephone line so that
data can be sent and received

Without a modem there would be no
communication available

The modem is responsible from changing
analogue data to digital data for the
computer to understand and vice versa
Different Modems

Due to technology advances many
different modems have been developed
These are some of them;
1. External PSTN Modem
2. Internal PSTN Modem
3. ADSL Modem
4. Cable Modem

1. External PSTN Modem

PSTN: Public Switched Telephone Network

This modem is most common modem used
in our homes

This modem is simply plugged into the
computer via the serial port, with its own
power cable

The telephone cable is plugged into the
modem
2. Internal PSTN Modem

This modem is also a common modem
found in homes

It is the same as the external PSTN
modem, but it is directly connected to the
motherboard

Due to being connected to the
motherboard it is found inside the tower
hence it shares power with the computer
3. ADSL Modem

ADSL stands fro Asymmetric Digital
Subscriber Line

The ADSL modem is an external modem
and taking over the other modems

It plugs to the PC using either USB

The ADSL modem still uses the PSTN line
but has a two way connection this makes the
service much faster than normal dial-up
4. Cable Modem

The cable modem does not use a
telephone line

Instead of a telephone line it uses the
cables that are used for Televisions

The cables are used to to connect to the
ISP (Internet Service Probider)
How Modems Work

The telephone-lines use analogue technology which is
not compatible with computers, the modem is used
to convert analogue signals into digital signals
1.
When information is being sent, the digital data
sent by the computer is delivered to the modem
2.
The modem converts it to analogue data which in
turn is transferred to the telephone line
3.
When information is being received by the PC,
information from the telephone line is converted
from analogue to digital by the modem
Digital data is
passed to the
modem.
Modem
receives
analogue data.
10101011
11010100
00….
10101011
11010100
00….
The modem converts it
into analogue, since the
telephone line is
analogue.
Data is converted
back into digital
format.
Baud Rate

The baud rate of modem determines how
fast information is sent or received by the
modem

Most dial-up modems have a speed of
56kbps

Most modern modems such as ADSL can
reach speeds of 4096Kbps
Client - Server Machines

Client-server relationship is a method of
network organization

In this relationship work stations make
use of resources available in the server

One computer has the role of central
resource manager for the network.
The Server

The server is a computer offering a
service

Services could be a set of resources such
as files, printers, databases, CD-ROM

The server handles the requests by a
client which asks to make use of these
resources

Some types of servers are …
1. File Server

The file server is the central disk storage
for any work stations on the network

The file server software identifies each
user’s files separately so that other users
cannot use them

Users can access their own files from any
client station on the network using a
username and passwork
2. Printer Server

This server allows all the work stations to
use a printer
3. CD – ROM Server

This allows all the work stations to obtain
data from a CD-ROM disk
Database Server

This server manages a large database.

Work stations can access data in the
database and, if authorized, can edit the
database.

The database processing is usually carried
out by the server, with the query being sent
by a work station of the server and the
results assembled by the server and
returned to the client station
Bandwidth

One of the main problems with client-server
technologies is bandwidth.

Bandwidth determines the amount of data that can
travel through the network at a given moment.

For example in a LAN the bandwidth’s capacity is
100Mbits/sec, that means that at any time the
maximum amount of data that can pass through the
network is 100Mbits.

This may lead to some problems if the data that
needs to be passed through the number is greater
than this limit. In that case the communication
between the computers will be corrupted.
Network Topologies
1.
2.
3.
Bus Topology
Ring Topology
Star Topology
What is a Topology

A topology is the layout of the network

The topology of a network is basically
how the computers are connected to
each other and the network

The topology used depends on the need
of the network and this changes
depending on the building
1. Bus Topology
A bus network allows work stations to be
connected to the network in a line
fashion
 Work stations will be set up one after
each other (just like bus stops)

COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
2. Ring Topology
A ring network has each of its computers
connected to a network ring
 Signals are sent in a round about fashion

COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER
COMPUTER

In both bus and ring network setups, it is
usually preferable to connect the computers
to the network using spurs.

This is preferred because if a computer
breaks down, the rest of the network will
not be affected.

In the case of the network setups without
the spurs, precautions must be taken to
prevent a breakdown of the whole network,
if a single computer breaks down.
3. Star Topology
In a star network all devices are
connected to one central computer
 The central computer (called hub) is a
computer which has separate connections
to each computer.

COMPUTER
COMPUTER
COMPUTER
SERVER
COMPUTER
(HUB)
COMPUTER
COMPUTER
COMPUTER
COMPUTER