Transcript OSI model

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.
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。