physical layer

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Oct 24, 2013 (4 years and 14 days ago)

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©
Brooks/Cole, 2003

Chapter 6

Computer

Networks

©
Brooks/Cole, 2003

Understand the rationale for the existence of networks.

Distinguish between the three types of networks: LANs,
MANs, and WANs.

After reading this chapter, the reader should

be able to:

O
BJECTIVES

List different connecting devices and the OSI layers in
which each device operates.

Understand client
-
server models.


Understand the OSI model and TCP/IP.

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Brooks/Cole, 2003

NETWORKS,

LARGE AND SMALL

6.1

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Brooks/Cole, 2003

Computer networks


A
computer network

is a combination
of systems connected through
transmission media.


Local area network (
LAN
)


Metropolitan (
都市的
) area network (
MAN
)


Wide area network (
WAN
)

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Brooks/Cole, 2003

Model and protocol


A
model

is the specification set by a
standards organization as a
guideline
for designing networks.


A
protocol

is a set of
rules

that
controls the interaction of different
devices in a network or an
internetwork.

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Brooks/Cole, 2003

OSI MODEL

6.2

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Brooks/Cole, 2003

The Open Systems Interconnection
(OSI) model

is a theoretical model that shows
how any two different systems can
communicate

with each other.

Note:

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Brooks/Cole, 2003

Figure 6
-
1

The OSI model

Figure 6
-
2

Flow of data in the OSI model

Header

Header

Header

Trailer

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Brooks/Cole, 2003

Seven layers of OSI model


Physical layer


Data
-
link layer


Network layer


Transport layer


Session layer


Presentation layer


Application layer


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Brooks/Cole, 2003

Physical layer


The physical layer is responsible for
transmitting a
bit stream

over a
physical medium.


It encodes and decodes bits into
groups of bits
.


It then transforms a stream of bits into
a
signal
.

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Brooks/Cole, 2003

Data
-
link layer


The data
-
link layer organizes bits into
logical units

called
frames
.


The data
-
link layer is responsible only
for
node
-
to
-
node delivery

of the frame.


The data
-
link layer is often
responsible for
error handling
between two adjacent stations
.

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Brooks/Cole, 2003

Network layer


The network layer is responsible for
delivery of a

packet

between the
original source and final destination.


Using
logical addresses (IP addresses)

instead of physical addresses.


Example of IP address


140.122.76.121 (4 Bytes)

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Brooks/Cole, 2003

Transport layer


The transport layer is responsible for
source
-
to
-
destination delivery of the
entire
message
.


The transport layer is responsible for
breaking the
entire message

into
several
packets

and delivery them to the network
layer.


The transport layer is responsible for
ensuring that the whole message is
transmitted.


If packets arrive
out of order
, they must be
reorganized.

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Brooks/Cole, 2003

Session layer


The session layer is designed to control the
dialog

between users.


The
synchronization (
同步的
) points

divides
a long 浥ssage into s浡ller ones and
ensure that each section is received and
acknowledged by the receiver.


Most network implementations

today

do
not

use a separate session layer, their services
are usually included in the

application

layer.

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Brooks/Cole, 2003

Presentation layer


The presentation layer is concerned with
the syntax and semantics of the
information
exchanged

between two systems.


It deals with the fact that different systems
use
different coding methods
.


Compress (
壓縮
) and⁤ecompress (
解壓縮
) data


Encrypt (
加密
) and decrypt

解密
) data


Most implementations do

not

use a
presentation layer
today,

their services are
usually included in other layer.

©
Brooks/Cole, 2003

Application layer


The application layer enables the

user

to access the network.


It defines
common applications

that
can be implemented to make the job
of the user
simpler
.


Will be discussed later…

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Brooks/Cole, 2003

CATEGORIES

OF

NETWORKS

6.3

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Brooks/Cole, 2003

Figure 6
-
3

Categories of networks

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Brooks/Cole, 2003

Figure 6
-
4

LANs

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Brooks/Cole, 2003

Local area network


Bus topology



When a station sends a frame to another
computer,

all
computer receiver the frame
and check its destination address.


Star topology


Hub (all, like a bus) v.s. switch (one)


Ring topology


When a computer needs to send a frame to
another computer, it sends it to its
neighbor
.

©
Brooks/Cole, 2003

Figure 6
-
5

MAN

Figure 6
-
6

WAN

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Brooks/Cole, 2003

Wide area network


WAN is the connection of individual
computers or LANs over a
large area
.


A person using a telephone line to
connect to an ISP is using a WAN.


ISP: internet service provider

©
Brooks/Cole, 2003

CONNECTING

DEVICES

6.4

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Brooks/Cole, 2003

Figure 6
-
7

Connecting devices

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Brooks/Cole, 2003

Figure 6
-
8

Repeater


A repeater is an electronic device and operate
only in the
physical layer

of the
OSI
model.


A repeater can
regenerate

the signal and
send

it to the rest of the network.

©
Brooks/Cole, 2003

Repeaters operate at the first layer
(physical layer) of the

OSI model.

Note:

©
Brooks/Cole, 2003

Figure 6
-
9

Bridge

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Brooks/Cole, 2003

Bridges


In bus topology, a bridge is a traffic
controller.


It can divide a long bus into smaller
segments so that each
segment

is
independent

trafficwise.


The bridge uses a
table
to decide if the
frame needs to be forwarded to another
segment.


With a bridge,
two or more pairs

of stations
can
communicate at the same time
.


©
Brooks/Cole, 2003

Bridges operate at the first two
layers

(physical layer and data
-
link layer)
of the OSI model.


Note:

©
Brooks/Cole, 2003

Figure 6
-
10

Switch

one kind of dynamic bridge

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Brooks/Cole, 2003

Figure 6
-
11

Routers in an internet

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Brooks/Cole, 2003

Routers operate at the first three
layers

(physical, data
-
link, and network
layer)

of the OSI model.

Note:

©
Brooks/Cole, 2003

Routers


Whereas a

bridge

filters a frame based
on the
physical
address of the frame, a
router

routes a packet based on the
logical

address of the packet.


Whereas a
bridge

may connect
two
segments of a LAN
, a
router

can connect
two independent networks
.

©
Brooks/Cole, 2003

Gateways


A
gateway

is a connecting device that
acts as a protocol converter.


A gateway is usually a computer

installed
with the necessary software.


Today the term
gateway

is used
interchangeably with the term

router
. The
distinction between the two terms is
disappearing.

©
Brooks/Cole, 2003

Figure 6
-
12

Connecting devices and the OSI model

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Brooks/Cole, 2003

THE INTERNET

AND

TCP/IP

6.5

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Brooks/Cole, 2003

Protocols


TCP
: Transmission control protocol


UDP
: User datagram protocol



IP
: Internet protocol


FTP
: File transfer protocol


SMTP
: Simple mail transfer protocol


POP:

Post office protocol


HTTP
: Hypertext transfer protocol


Figure 6
-
13

TCP/IP and OSI model

©
Brooks/Cole, 2003

Figure 6
-
14

IP addresses in dotted
-
decimal notation

Network layer

©
Brooks/Cole, 2003

Figure 6
-
15

Client
-
server model

Application layer

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Brooks/Cole, 2003

Client
-
server model


Client:

an application program running on
a local machine


Server:

an application program running
on a remote machine


A client requests a service from a server.


The server program is always running,
and the client program runs only when
needed.

©
Brooks/Cole, 2003

Figure 6
-
16

FTP

FTP

--

Client
-
server model

©
Brooks/Cole, 2003

FTP
--

Client
-
server model


FTP was designed to resolve two problems:


Different coding systems in use


One machine may use ASCII, and other may use
Unicode


Different file formats in use

©
Brooks/Cole, 2003

Figure 6
-
17

SMTP/POP

SMTP
--

Client
-
server model

©
Brooks/Cole, 2003

Figure 6
-
18

Email address

violet@ice.ntnu.edu.tw

©
Brooks/Cole, 2003

Figure 6
-
19

TELNET
--

a general client
-
server model


Local login v.s. remote login


TELNET is a general client
-
server program on
the Internet that allow remote login

©
Brooks/Cole, 2003

Figure 6
-
20

URL

http://www.ice.ntnu.edu.tw/~violet

ftp://140.122.77.121

HTTP


URL: Uniform resource locator


A special kind of addressing using by HTTP

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WWW


WWW: World wide web


Hypertext: home page


Browser


Document Types


Static documents


Dynamic documents


Active documents


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Figure 6
-
21

Browser

©
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Figure 6
-
22

Categories of Web documents


Static documents


HTML: Hypertext Markup Language


Dynamic documents


CGI: Common Gateway Interface (Perl)


Active documents


Java language

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Brooks/Cole, 2003

Key terms


Active document


Application layer


Bridge


Browser


Bus topology


Client


Client
-
server model


Computer network


Connecting devices


Data
-
link layer


Dotted
-
decimal notation


Dynamic document


Email


HTML


HTTP


Internet


Internet address


Internetwork


Internet protocol (IP)


IP address


LAN


Lical login


MAN


Model


Network layer


©
Brooks/Cole, 2003


Node


Node
-
to
-
node delivery


OSI


Physical address


Physical layer


Presentation layer


Protocol


Remote login


Repeater


Ring topology


Router


Segment


Server


Session layer


SMTP


Source
-
to
-
destination delivery


Star topology


Static document


Switch


Synchronization point


TELNET


Topology


Trailer


TCP


TCP/IP


Transport layer


URL


UDP


Web


WAN


WWW