S4 CIT

Computer Network

In the past… Computers are standalone machine. What can we do if we want to…

share a file with someone? print a file but the printer is connected

to another computer?

Computer Network It is a system in which two or more

computers are connected so that data communications resources sharing (File sharing,

hardware sharing e.g. printer)

among the computers become possible.

Types of Network LAN (Local Area Network)

links computer and other peripheral devices over short distances.

For examples:• network at home• network in school campuses and offices

a LAN with an internal e-mail system, file server or message board is called Intranet.

Types of Network WAN (Wide Area Network)

covers a large geographic area using a communication channel such as dedicated telephone lines and radio waves.

a WAN consists of two or more LANs. The Internet is the world’s largest WAN.

Network Connection Protocol Peer to Peer

Every computer is connected directly to every other computer.

Each computer can use information from, and provide information to each other.

There is no server. Security is weak. Example is WinMX.

Network Connection Protocol

Client / Server Individual computers (clients or workstations) are

connected to a central computer (server). Server computer maintains the network user

accounts. Clients can access programs or files stored on the

server. Clients request for services whereas servers

provide services. Since the server can give different levels of rights

( 權力 ) to different users, so security is high. Examples: School network, Playing on-line games

Hardware Requirementfor a Network - NIC

NIC (Network Interface Card) also called LAN Card an expansion board added to the

computer’s motherboard key hardware component for

connecting the computer to a network (wired or wireless)

Hardware Requirementfor a Network - NIC

NIC can be connected to the computer by means of: PCI Bus USB Bus PCMCIA Bus (In Notebook Computer)

Hardware Requirementfor a Network –

Transmission Medium Data and signal need to be

transmitted over a network through a medium ( 媒體 )

It can be a cable (wired) or through microwave (wireless)

Transmission Medium –Twisted-pair Cable

Sometimes called the UTP Cable (Unshielded Twisted Pair).

Looks similar to telephone cable The most commonly used types of

twisted pair wire is Cat5 cable. The connector of this kind of cable is

called RJ45.

Transmission Medium – Coaxial Cable

Transmit at a higher speed when compared with twisted-pair cables

Can transmit signal over long distances so they can be laid underground or underwater.

Transmission Medium – Optical Fibre

Bundles of smooth, hair-thin strands of glass

Transmit data with high efficiency in terms of light beams

Can transmit signal over long distances

Largely replaced coaxial cables nowadays.

Transmission Medium – Microwave

Electromagnetic waves which can wirelessly carry data through the atmosphere

It travels in straight line from one microwave relay station( 轉發站 ) to another.

Hardware Requirementfor a Network –

Connecting Devices

Hub provides a central connection point for

cables in a network slower than switch

Hardware Requirementfor a Network –

Connecting Devices

Switch advanced type of hubs faster than hubs

Hardware Requirementfor a Network –

Connecting Devices

Router connect multiple networks connect LANs to form a WAN connect a LAN to the Internet

Wireless Network Usually a Wireless LAN (WLAN) Use microwave (Radio Frequency, RF) to

transmit and receive data over air Have same features as a LAN, but without

the limitations of a cable. Access Point (AP)is required in a WLAN. It is a device that transports data between

a wireless LAN and a wired LAN. wireless NICs are also required in each

workstation.

Wireless Network

Wireless NIC

Access Point

Wireless PCMCIA Card

WirelessUSB Adapter

Advantages of usingWireless network

Save the cost in laying network cables

No cluttering of cables More flexible when adding

workstations Notebook computer can access

network at anywhere under the coverage area.

MODEM

MODEM – Modulator and Demodulator Modulate

Convert a digital signal into analogue signal.

Demodulate Convert an analogue signal into digital

signal.

MODEM

Analogue Signal ( 模擬信息 ) Can be transmitted along telephone

lines and UTP

Digital Signal ( 數碼信息 ) ie: 01011010 Computer can only process digital

signal

Using MODEMto connect to the Internet

Traditionally, Use telephone lines Speed 56kbps

Nowadays Use telephone lines & UTP Cables Called ADSL MODEM Speed 1.5Mbps, 3Mbps, 6Mbps

Network Topology It is the logical layout or the way in

which the computers and other devices are connected.

Types: Ring Star Bus

Ring Topology

All devices are connected on a circular path

data flows in one direction. When a computer sends data, the data

travels to each computer on the ring until it reaches its destination.

Star Topology

All devices on the network connect to a central computer (usually a server)

All data transferred from one computer to another passes through the server.

Bus Topology

All computers and other devices are connected to a single central cable.

It is flexible because computer can be attached to or

detached from the network without disturbing the rest of the network.

Comparison ofdifferent Topologies - Ring

Ring Topology Advantage:

• Easy and cheap to setup

Disadvantage:• Data transmission in one direction.• Failure of a single device means all devices

after cannot function.

Comparison ofdifferent Topologies - Star

Star Topology Advantage:

• One device failed will notaffect the other devices

Disadvantage:• If the server breaks down, the whole

network fails.

Comparison ofdifferent Topologies - Bus

Bus Topology Advantage:

• Failure of one devicewill not affect otherdevices.

Disadvantage:• Failure of the cable means the whole

network is broken down.

Practical Network Topology

more than one topology is used in a network.

For example, in a school network, a bus-star topology is used with a high

bandwidth bus as the backbone (usually a optical fibre).

the backbone connects a number of star networks such as a LAN inside the computer room.

Network Operating System

Windows 98/ME, Windows 2000/XP, Linux, etc include the software necessary to establish communication with a network server.

Old operating system like DOS do not have these network features.

Brief Introduction to OS

The most important program that runs on a computer.

Every general-purpose computer must have an OS to run other programs.

OS perform basic tasks recognizing input from the keyboard sending output to the display screen keeping track of files and directories on the

disk controlling peripheral devices such as disk

drives and printers.

Network Server Software multi-user like a traffic cop

different programs and users running at the same time do not interfere with each other.

controls file access from the server’s hard disk manages the print queue

responsible for security ensuring that unauthorized users do not access the

system. tracks user data such as user IDs and passwords

Examples include Windows 2000 Server Edition, Windows 2003 Server Edition

Network Client Software Gathers login information Handles drive mapping Directs printouts to the network

printer Examples include Windows 98/ME,

Windows 2000/XP Home or Professional Edition

Practical Consideration

The server and the client software components do not necessarily have to match.

A Windows NT computer in library and some Windows XP computer in classroom can login to the school’s Windows 2000 Server.

Common Services on Network - File server

A high speed computer that stores the programs and data files shared by users.

Most file servers also store user accounts and the network information.

Data in a file server are important and must be back up regularly.

Common Services on Network - Mail server

It provides e-mail services to users. Each user has a mailbox which stores

the incoming mails and copies of sent mails.

Note the differences of Yahoo! Mail and Outlook Express

Common Services on Network - Print server

It manages one or more printers on a network.

It allows a shared printer to be accessed by authorized users.

Sometimes, a Print Server may not be a computer.

In most case, it is connected to the main server in the LAN.

But a workstation can also act as a Print Server.

Common Services on Network - Web server

A web server stores the contents of web pages.

It delivers web pages according to the requests from other users on the Internet.

Web pages are delivered through a program called web server program. Eg: Microsoft IIS, Apache

Common Services on Network - Proxy server

It improves the efficiency of Internet access by temporarily storing copies of web pages visited by previous users of the same LAN.

It also filters web pages to prevent users from accessing certain sites.