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7. Network System
Textbook:
Chapter 1 (p.1-6, 15, 19-21), 4, 9 (p.172-178)
Reference:
Discovering Computers 2003/2004
Course Technology, Thomson Learning
Chapter 9
1
Network
(Chp 1) p.4 -6
A network is a collection of computers
and devices (also known as the node or
the host)
connected by communication channel or
media (such as network cable, fiber optic
cable, etc.)
Communicating signals include computer
instructions or data.
Networks can interconnect with other
networks.
2
Network: Purposes
HW 1
Sharing of Data: e.g. sharing data on the
centralized file server:


Access to the most updated version by different
users.
More intensive care on back-up and data protection
on file server.
Sharing of Hardware: e.g., network printer by
different users.
Sharing of Software: e.g., DBMS is installed on
database server, but on not all computers.
Communications among users
Internet Sharing: use of one Internet
connection by many computers.
3
Overview to Inter-Network
Communications
Back to S#29
199.95.72.2
199.95.72.1
199.95.72.3
Gateway
server
…
128.11.7.3
…
Hub & Switch
Router
…
201.95.72.1
…
Gateway
server
…
128.10.7.2
201.95.72.2
…
…
Different starting number in IP
4
for different networks
(Computer) Server
(Chp 9) p.173 - 178
A server is a computer installed with
particular software serves its specific
function(s) to other client computers on the
network.
Dedicated servers perform specific tasks:
e.g.



Our school is a client/server LAN. You have
to logon to get access to the file server for
the common storage.
A database server stores and provides
access to a database.
A web server stores and delivers webpages
(web document)
5
Server Performance
Efficiency of Hardware



Number of CPUs and their speed
The size of RAM
Disk capacity and its speed
Amount of services required


It is important not to overload a server.
E.g., A server as both file server (for file sharing)
and authentication server (for user login) may be
overloaded.
6
Network Models
(Chp 9) p.172 -173
Client/Server model:

where one or more
computers act as a server
computers in network to
serve requests from other
client computers
Peer-to-peer model

Where computers
communicates with each
other as equal
workstations for sharing
of data or hardware.
7
Advantages of Client/Server model
Higher security by centralized security on server(s).
Centralized backup is easier and faster.
Better control of network resources by dividing user
groups.
Disadvantages of Client/Server model
professional administration on server management is
required.
Dedicated server requires more powerful hardware,
such as hard disk, RAM and CPU.
More costly server version of system software is
required.
8
Advantages of peer-to-peer model
It is easier to set up.
Less powerful hardware, such as hard disk, RAM and
CPU, is needed.
Less costly standalone version of system software is
required.
Disadvantages of peer-to-peer model
Lower level of security among different workstation.
Difficulty without centralized backup.
Difficulty to manage users among different
workstations.
9
Local Area Network (LAN)
(Chp 4) p.88-90
A local area network (LAN) is a group of
computers and associated devices
typically within a small geographic area
(for example, within an office building).
that share/use one common
communication line i.e. one type of
network cable.
10
MAN vs WAN
For more than one type of communication
line:
E.g., network cable within the office but
telephone lines between the main office
server and the branch office server.
Either one of the followings:


Metropolitan Area Network (MAN) connects two
or more LAN within a city.
Wide Area Network (WAN) is a network that
covers a large geographic area
11
Internet as the largest public WAN
Satellite: wireless
communication
Fig 9.15
Branch
Phone
line
Branch
Main
Office
Serve
r
Internation
al Co.
12
Summary of Network Devices
(1) Network devices
1) Sending and receiving devices: send or
receive transmission of signal
E.g., computers, servers, network
printer, cellular phones
Request to access
common storage drive
Files on the common
storage drive
13
(2) Network devices
2) Communication media: is
the channel where the
signal is transmitted
14
(2-a) Communication Media/Channel
that can be wired media, e.g.:


For digital signal:
 network cable in LAN,
For analogue signal
 TV cable or cable TV wire,
 fiber optic/ phone line
that can be wireless media



Radio waves
e.g. by Bluetooth technology
WLAN: Wireless LAN
Through waves: by
using transceiver
and receiver in the
communication
15
(2-b) Wireless communications
Advantages:


Flexible in placing network devices
Flexible in adding and removing workstations.
Disadvantages:


Speed problem: Comparatively slow in
transmission
Security problem: Data are more easily
intercepted.
16
(2-c) Media: Speed of transmission
The higher the bandwidth, the faster the
transmission speed
(influenced by bandwidth) can be
classified into:


Baseband: that transmit one signal at a time,
e.g. network cable in LAN and phone line;
Broadband: that can transmit multiple
signals simultaneously, e.g., TV cable
More from Supplementary notes
17
(3) Network devices
3) Communication devices: are used to
convert signals so that they are suitable
for the communication media in sending,
or for the computer when receiving.
(Recall: Modem)
18
(3-a) Communication Devices
(Chp 1) p.15
Network Interface card (also
called as LAN card or LAN
adapter) is a card that
resides in the computer in
LAN
Control data flow in/out to
/from computers for
communication between
connected computers.
19
(3-b) Communication Devices
Modem: convert signals for transmission over the
associated communication media.
20
(3-c) Examples of modem
depending on the transmission media:




Dial-up modem: converts signals for standard
telephone line and computer.
Cable modem: convert signals for TV cable and
computer.
DSL modem: converts signals for DSL and
computer.
ISDN modem: converts signals for ISDN and
computer.
21
(4-a) Connecting Devices
(Chp 1) p.19 - 21
Hub:
is a cable concentrator
that joins the network
cables
so as to be the
collection point where
signals coming from/to
computers and
devices in network.
22
(4-b) Connecting Devices
Switch:
is also a cable concentrator
But that can determine
where signal is transmitted
within a LAN
Therefore, the use of
switch can improve the
transmission efficiency.
Note: The functions of hub
and switch can be
combined in a device that
is called a switching hub or
switch directly.
Switch
23
(5) Inter-network devices
5) Inter-network communication devices:
are used for the communication
between networks
24
(5-a) Inter-network
connecting devices
Bridge: is a
device connecting
LANs
it determines if
the signal is to be
within the same
LAN, or not.
Note: It can only
connect LANS
that are
compatible (of
hardware and
software).
25
(5-b) Inter-network
connecting devices – router (1)
Router: is also a
device connecting
LANs that are
compatible.
but it determines
which network
point the signal
should be
forwarded in the
most efficient path
by a routing table
(like a road map).
26
(5-b) Inter-network
connecting devices – router (2)
Therefore, router is more efficient for internetwork communication than bridge is.
E.g., used for connecting to Internet.
27
(5-c) (Inter-)network
connecting devices
Gateway: is a
computer server with
proper software
installed,
connecting to another
incompatible networks
(e.g., different
operating systems),
unlike bridge and
router.
Network
Gateway
Gateway
server
28
Further uses
(1) as a gateway server connecting with other
incompatible network, it can also be
(2) a proxy server: acts as an intermediary with
the other network (such as Internet)
(3) a firewall server: with software installed to
restrict outsiders from accessing data and
information on a network.
29
p.135
Proxy server
an intermediary with the other network (such as
Internet) so that the enterprise can ensure


administrative control: e.g., filtering unwanted
websites requested from the company’s network.
and caching (e.g., caching for a frequently required
web page without loading from a specific computer
of the network. Problem: not updated information)
Internet
Internet
Service
Provider
Network.
Proxy server
30
p.135
Firewall server
Filtering
access-in (that
differs from
filtering
program that
filter the access
outside
Firewall
Also known as a firewall system
with the firewall software installed on a gateway server to
screen outsider’s requests (This software differs from
anti-virus software that detects and erase virus.) .
examines each network packet to determine whether to
forward it toward its designated computer on network.
can prevent unauthorized access to data and information
on a network
Network
31
An Example: connecting to Internet
In general, a server
installs software that
serves its specific
functions on the
network.
Company’s
Network
Internet
Internet
Service
Provider
Gateway server
Proxy server
Firewall server
32
HW 3
LAN topologies
(Chp 4) p.91-93
refer to the arrangement of the devices (e.g.,
computers, printers) in a network.
3 commonly used network topologies:



Bus network
Ring network
Star network
33
In a Bus network,
each computer or device is connected to a
main cable


The main link is also called the backbone or the
bus link. If the main cable fails, the entire
network fails.
Highest Reliability: If one workstation fails to
operate, all the rest can still communicate with
each other.
34
In a Star Network,
All the computers and devices connect to a
network device (such as a switch)
Reliability
If the network device goes down, the entire
network will suffer.
 If only one workstation goes
down, the network
still operates.

35
In a Ring network,
the connection forms a closed ring with all
computers and devices in the network

Less reliable than
 the star and bus;
 One node’s breakdown
affect the others.
Usually for peer-to-peer LAN
where there is no server-client
relationship
36
Set-up & Changes
Star
Very flexible to add
or remove nodes
Easy to set up and
maintain
Need more cabling and
hence more cabling
problem.
The central device may
hinder network
performance, especially
with many nodes.
Bus
Relatively flexible to
add or remove
nodes (than Ring)
Relatively difficult to set up
the main cable.
Ring
Least flexible to add
or remove nodes
Relatively simple to set up
37
IP (Internet Protocol) Address
Recall: IP address is a set of numbers that
uniquely identifies each computer on Internet /
Inter-networking as well as network.
More:



It consists of 4 groups of numbers(from 0 to 255),
each separated by a dot: 199.95.72.10
In general, there is the first portion identifying the
network, and then the second portion identifying the
specific computer in that network
The first portion can include the 1st to 3rd groups of
the number, depending on the size of the network.
(Example on Slide 2)
38
Global vs Local IP address
Global IP address : also known as a static IP
address or registered IP address, is a permanent
address assigned on a network/Internet
However, it is impossible to have every computer
connected to network/Internet to have a static IP
address.
Local IP address: also known as dynamic IP
address, is a temporary address assigned by server
(so that that address can be re-used by other
computer when it is not used by the original
computer.) E.g.,


Local IP addresses of ISP’s customers are assigned
by the ISP’s server.
Each assigned IP address can be changed by the
ISP’s server.
39
Network Performance
Performance of a network refers to the
efficiency in data transmission for the tasks
carried out.
Factors:


Bandwidth: refers to the speed of transmission for
the media. (on S18) More from Supplementary notes
Throughput: the amount of data successfully
transmitted as being affected by the connecting
devices, such as hubs,
switch, etc.
40