Download WHAT IS A NETWORK?

WHAT IS A NETWORK? a group of >= 2 computers/devices linked together.
BENEFITS
Information sharing(分享資料)-e.g.database, data and files
Hardware sharing (分享硬件) -e.g. printer
Software sharing (分享軟件)-a license purchased to install a copy for many users
Data Communication(數據溝通)
Collaborative environment(共同作業) – work together in group project online
Cut costs(減成本)
2 TYPES
LAN (local area network) (區域網絡)– limited geographical area
WAN(wide area network) (廣域網絡)- large geographical area
DATA COMMUNICATON-data transmission from one place to another.
Requirements:
A sending device (a computer)
A communication hardware(modem, cable modem)
a network transmission hardware ( NIC , Hub, Bridge, Gateway, Router)
A communication media (twisted pair, coaxial or fiber optic cable)
radio, microwaves, satellites signals)
A receiving device(a computer)
Communication software (email, icq)
USES OF COMMUNICATION TECHNOLOGIES – Voice Mail, Fax, Email,
Instant Messaging, Chat rooms, newsgroups,telephony, videoconferencing,
collaboration, groupware, Global Position System
Data Communication
Internet
Digital signals
Analog signals
Sending
computer
Digital signals
Analog signals
modem
modem
ISP
Telephone lines
ISP
Receiving
computer
Telephone lines
Digital signals (數碼 訊息) (e.g. bits 101010) are used by computers.
Analog signals (類比 訊息) (electrical wave) are used by telephone line.
A modem converts digital signals from the sending computer into analog
signals to be used by telephone line.(modulate)
Data travels through telephone lines to sender’s ISP server.
Data are divided into packets (細訊息郵包) which are then routed (路由)
through Internet to the ISP server of the receiver. Data are then reassembled(重
新整合).
Another modem converts analog signals back into digital signals to be used by
the receiving computer. (demodulate)
Communication Media
Physical Media(有線)
Wireless Media(無線)
Twisted Pair cable(雙絞線)
•Cheap,For telephone system
•2 insulated copper wires twisted
around one another to reduce noise.
•Prone to interference
Coaxial cable(同軸電纜 )
•More expensive. For long distance
•For cable TV/broadcast industry
•Cable heavily shielded, less prone
to interference
•Fiber optic cable(光纖線)
Disadvantages:expensive, fragile
hard to install and modify
Advantages:greater bandwidth,
faster, smaller, lighter, less prone to
noise,better security.
Radio signals
Broadcast radio/cellular radio
Travel in straight line
Used in mobile communication
Slow, susceptible to noise
Provide portability and flexibility
Microwave signals
•Electromagnetic waves
•Travel in straight line from one
station to another.Thus, stations are
placed high
Satellites(人造衛星)
•Rotates around the earth.
Receives transmission,Amplify it
and sends it back to earth.
Infrared(IR)(紅外線)
Bandwidth頻寬–width of communication channels. The higher the
bandwidth, the more data/information/signals the channel can transmit.
Broadband寬頻 media can transmit multiple signals simultaneously.
Network hardware
Communication hardware
Network transmission hardware
Network Interface cards (網絡介面卡)
Modem解調器
•each computer on network must have a NIC
•Convert digital signals from
•Send/receive data between PCs in LAN
computer to analog signals used
HUB(集線器)
by telephone lines or vice versa. •A device acts as a center point.
•has many ports for connecting to computers
•forwards out data arriving from these
Cable Modem
computers in one or more directions.
Bridge(橋接器)
•Uses coaxial cable to send and
A computer connecting LANs using same
receive data.
protocols. E.g. TCP/IP
•Used for cable TV.
Gateway(通訊閘)
•Greater bandwidth
A combination of hardware & software linking 2
•Connect directly to computer different type networks with different protocols.
Router(路由器)
or to a set-up box used with
Data send is divided into small packets
television.
It determines the fastest paths to send data
packets to the destination.
Redirect the traffic over alternate paths when
network fails.
ARCHITECTURE OF NETWORKS
Client/Server(主從式網絡)
A high speed , high storage host computer (server) manages network
resources.Servers can be printer server, database server, file server,
web server, mail server, FTP server or network server. Other
computers (clients) rely on the server for resources.
workstations(Client)
workstations(Client)
(server)
workstations(Client)
workstations(Client)
Peer-to-Peer(對等式網絡)
All computers are equal. Less than 10 computers connected.Each
computer has its own resources to be shared by others. Easier to setup and
manage.
workstations(Client)
Used by small offices.
workstations(Client)
workstations(Client)
Network Topology(網絡的鋪排)
terminals
Star
a central computer(server) connects to terminals.
server
Messages from one terminal must pass through
the central computer to get to another terminal.
terminals
terminals
Advantages of Star Network
terminals
efficient centralized control. (有效中央控制)
add/remove terminals from networking without affecting it.
Disadvantages of Star Network
If the central computer fails, the whole network breaks.
Bottleneck /jam(樽頸擠塞) if too many computers.
Ring (圈狀網絡)
All computers are connected in a circle.
token
A
D
Has no central computer.
B
C
One special signal called token is passing around the circle freely.
沒有記載訊息叫“代幣”無時無刻圍繞網絡行走
The computer which pick up the free token can send data by writing
message on it and send it out to the network again.
任何電腦如果想傳送信息給另一電腦, 先從網絡中取下 空“代幣”,
寫下信息和對方的地址,然後再傳送出去網絡
If one of other computers find out the token is for it, it will get the
token and read the message.Then it pass back the token to the
sending computer. It then free the token again. (如果其他電腦發現
“代幣”是給他的,便取下它和讀取其內容,再把空“代幣”傳送回
去傳送電腦,由他刪除內容)
Advantages- signals transmits in one direction and at high speed.
訊息高速單方向傳送
Disadvantages -If one computer fails, the whole network break.
Bus (線狀網絡)
All computers are connected to a single cable(backbone)
and share it. 所有電腦連接去和分享一條網絡中輸線
Only one computer allowed to occupy the cable at any one time.
任何時間,只有一部電腦可佔有中輸線傳送訊息
Thus, if two nodes try to send message at the same time, a collision occurs
Then the nodes must try to resend.
如同時一部以上的電腦想傳送訊息,訊息便會互撞擊.電腦們便要重新鬥
快傳送
Advantages
computers can be attached/detached without affecting network
電腦隨時插入網絡或可從網絡取走而不影響網絡運作
relatively inexpensive 比較平宜
easy to install and maintain 容易安裝和保養
Disadvantages -Speed is low,because many collisions happen
速度慢,因有大多訊息互撞,電腦要重新鬥快傳送
INTRANETS(內 聯網)
•An internal network that uses Internet technologies and makes company
resources accessible to employees and facilitate working in groups.
•Use TCP/IP technologies
•Has a web server
FIREWALLS(防火牆)
A security system of hardware/softwares that
•restricts access to specific authorized users
•prevent unauthorized access (非法存取) to data on the Intranet.
•Screen all incoming and outgoing messages.
•Check domain name/IP address of the message.
•Install a proxy server (代理服務器) outside the Intranet that controls
which communications pass into the company’s network.