Transcript CH06

Chapter 6
Computer
Networks
OBJECTIVES
After reading this chapter, the reader should
be able to:
Understand the rationale for the existence of networks.
Distinguish between the three types of networks: LANs, MANs,
and WANs.
Understand the OSI model and TCP/IP.
List different connecting devices and the OSI layers in which
each device operates.
Understand client-server models.
6.1
NETWORKS,
LARGE AND SMALL
Network

Computer network –
A combination of computers connected
through transmission media.
 LAN
(Local Area Network)
 MAN (Metropolitan Area Network)
 WAN (Wide Area Network)

Internetwork – (e.g. Internet)
Networks can be connected using
connecting device.
Model and Protocol
Model –
the specification set by a standards
organization as a guideline for designing
networks.
 Protocol –
a set of rules that controls the interaction of
different devices in a network/internetwork.

6.2
OSI MODEL
Note:
The OSI (Open Systems Interconnection) model
is a theoretical model that shows
how any two different systems
can communicate with each other.

OSI Model –
is a framework of 7 layers
that gives network designers an idea of the
functionality of each separate but related layer.
Figure 6-1
The OSI model

control information
is added to the data in the form
of headers or trailers.

the header or trailer
is dropped in each layer.
Flow of data in the OSI model
Figure 6-2
Functions of the Layers
Physical
1.
•
transmit a bit stream over a physical medium.
Data-Link
2.
•
•
Organizes bits into logical units called frames.
Node-to-node delivery
Network
3.
•
Source-to-destination delivery of a packet.
Transport
4.
•
Source-to-destination delivery of the entire message.
Session
5.
•
Establishes, maintains, and synchronizes
the dialog between communicating systems.
Presentation
6.
•
Deals with the fact that different systems use
different coding methods.
Application
7.
•
Enables the user to access the network.
OSI Layers 實例：
7
Application
Layer
6
Presentation
Layer
5
Session
Layer
4
Transport
Layer
3
Network
Layer
快遞公司：決定使用那個機場、經由哪條路徑傳遞
2
Data Link
Layer
機場管理局：班次﹑泊位﹑進場/出場時間間隔﹑收發貨物
1
Physical
Layer
飛機：運送包裹
產品行銷：策略、合同、邀訪、樣品
秘書：將型錄、報價單、合約翻譯成各國語言的文件格式
一筆生意：寄型錄、寄報價單、寄合約、…
資料打包寄送：將資料拆開、分裝、編號成幾個信封﹐再進行郵寄
6.3
CATEGORIES
OF
NETWORKS
Categories of networks
Figure 6-3
LAN (Local Area Network)

Allow resource sharing between computers.
 Computers
 Peripheral
devices
 Transmission medium (e.g. cable)

3 types of topology
 Bus
topology
 Star topology
 Ring topology
LANs

Figure 6-4
Hub –
a
device that facilitates connection
 LAN acts logically like a bus.

Star –
the dominant topology today.
MAN
Figure 6-5
MAN (Metropolitan Area Network)

Uses services provided by a network service
provider. (Tel. Company)
 Individual
users’ computers
 Organizations’ LANs

Many Tel. Company provide a popular MAN
service called SMDS (Switched Multimegabit Data Services)
 Prior
to SMDS's arrival in 1995,
the only way to connect LANs was through
a dedicated private line.
WAN
Figure 6-6
WAN (Wide Area Network)

The connection of individual computers or LANs
over a large area (country, world).

User using a telephone line to connect to an ISP
(Internet Service Provider) is using a WAN.
 Negotiates


fee
ISP
Tel. company
6.4
CONNECTING
DEVICES
Figure 6-7
Connecting devices
Repeater (L1)


Regenerates the signal.
Extends the physical length of a network.
Figure 6-8
Note:
Repeaters operate at the first layer of
the OSI model.
Bridge/Switch

Bridge
 A traffic
controller
 Divide a long bus into smaller segments
so that each segment is independent trafficwise.
 Regenerate data

Switch
 A sophisticated
bridge with multiple interfaces.
 A station that needs to send a frame sends it directly to
the switch.
Bridge (L1-L2)
Figure 6-9
Note:
Bridges operate at the first two
layers of the OSI model.
Figure 6-10
Switch
Router
Connect LANs, MANs, and WANs.
 Routes a packet based on the logical address
(network layer) of the packet.
 Connect two independent networks: LAN-WAN,
LAN-MAN, WAN-WAN…
V.S.
 Bridge

 filters
a frame based on the physical address (datalink layer) of the frame.
 Connect two segments of a LAN or two LANs
belonging to the same organization.
Routers (L1-L3) in an internet
Figure 6-11
Note:
Routers operate at the first three
layers of the OSI model.
Gateway (L1-L7)
A protocol converter.
 Understands the protocols used by each
connected network and is able to translate
from one to another.

Figure 6-12
Connecting devices and the OSI model
internetworking
IP address
Mac address
packet
frame
networking

ISP (Internet Service Provider)
a company that provides access to the Internet. For a monthly fee,
the service provider gives you a software package, username,
password and access phone number. Equipped with a modem, you
can then log on to the Internet and browse the World Wide Web, and
send and receive e-mail.

ICP （Internet Content Provider）
網際網路內容提供者。指的是在網際網路上提供各種服務內容的廠商。
如Yahoo雅虎、AOL美國線上等，都是屬於ICP的一種。任何人只要
有網路的空間放置網頁，都可以成為ICP。而ICP的收入大都來自廣
告收入。

Web portal
A Web site or service that offers a broad array of resources and
services, such as e-mail, forums, search engines, and on-line
shopping malls.

ASP (Application Service Provider)
ASP 業者向軟體公司購買應用程式，然後將此程式放在 ASP 業者的
機房內，然後供企業或個人以存取網路的方式連接到機房來使用應用
程式，並依據使用方式與規定向客戶收取費用。
6.5
THE INTERNET
AND
TCP/IP
Internet

internetwork (internet)
 A network
of networks
 Connect individual LANs, MANs, and WANs.

Internet – the most famous one
TCP/IP and OSI model
TCP/IP

Transmission Control Protocol/Internet Protocol

A suite of protocols that officially control the Internet.

Physical and data-link layers:
Does not define any specific protocol.
Network layer:
Supports the Internet Protocol (IP)
Transport layer:
Defines Transmission Control Protocol (TCP) and User Datagram
Protocol (UDP)
Application layer:
Equivalent to the combined session, presentation, and application layers



TCP/IP Network layer


Supports the Internet Protocol (IP)
Every computer connected to the Internet is identified
by a unique international address – IP address. (32bits)
Figure 6-14
IP addresses in dotted-decimal notation
TCP/IP Transport layer

Defines Transmission Control Protocol (TCP) and User
Datagram Protocol (UDP)

TCP is a reliable transport protocol. It divides a
message into a sequence of segments that are
numbered sequentially.
 If
one segment is lost, it is sent again.
 If a segment is received out of order, it is ordered with
the help of the sequence numbering mechanism.
TCP/IP Application layer

Equivalent to the combined session,
presentation, and application layers of OSI
model.

Communication on the Internet uses the
client-server model.
– an AP running on a local machine.
 Server – an AP running on a remote machine.
 Client
Client-server model
FTP


File Transfer Protocol
The standard protocol on the Internet for
transferring a file from one machine to another.




SMTP(Simple Mail Transfer Protocol)
A user machine that is turned off will not receive email.
POP(Post Office protocol)
UA(user Agent)- a user interface to facilitates these
transactions
Email address


Local part – user mailbox
Domain name – the computer that serves as the SMTP server
TELNET



TErminaL NETwork
A general client-server program on the Internet allows
remote login.
Enables the establishment of a connection from a local
system to a remote system in such a way that the local
terminal appears to be a terminal at the remote system.
HTTP
HyperText Transfer Protocol
 the underlying protocol used by the World
Wide Web.
 use a special kind of addressing called
URL(Uniform Resource Locator).

URL
Method – the client-server program used
for transferring the documents.
 Host – the computer where the information
is located.
 Port – the port number of server.
 Path – the path of the file where the
information is located.

WWW




World Wide Web
Hypertext –
a document containing special text, words, and
phrases that can create a link to other documents.
Page –
a document of hypertext available on the web.
Home page
Browser
Coordinates all activities.
Display the document.
A client AP that retrieves the document.
Categories of Web documents
Document types:

Static documents
 Have
fixed contents
 HTML

Dynamic documents
 Programs
residing at the server site.
 Use CGI(Common Gateway Interface) technology
includes Perl and HTML to handle the document
creation and interpretation.

Active documents
 The
browser requests the transfer of the program.
After transfer, it is run at the browser site.
 Java

台灣網路資訊中心（TWNIC）
國家級財團法人機構，以非營利為目的，是由
交通部電信總局及中華民國電腦學會共同捐助
設立，宗旨是以超然中立及互助共享網路資源
之精神，提供IP address分配、網域名稱
(Domain Name)註冊等服務。

網路蟑螂
 1999年底，登記使用business.com的權利，以750
萬美金的天價在拍賣網站電子海灣(eBay.com)賣出。
 china.com、ibm.com
 apple.com
 whitehouse.com
即時傳訊 (Instant Messenger)

它支援在 Internet 上聊天、發送消息和文件等，另更有語音
聊天、網路攝影機等功能
1.
ICQ
全球最早出現的即時傳訊軟體，在1998年6月被AOL以2.87億美元所收購
2.
AIM﹙AOL Instant Messenger﹚
由AOL所開發出來、類似ICQ的即時傳訊軟體
3.
QQ
中國騰訊公司產品，佔中國95%使用人口（1億500 萬）
4.
MSN即時通
由微軟所開發出來的即時傳訊軟體
5.
Yahoo!奇摩即時通
由全球入口網站領導品牌Yahoo!所開發的即時傳訊軟體
6.
YamQQ
蕃薯藤 +QQ