Chapter 12 - Ship

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12
-
1

Chapter 12
-

Computer Networks


12.1 Introduction


Computer network
-

computers connected for the purpose
of exchanging information.


Give the potential to equalize access to information and
eliminate the concept of “information haves” and
“informati
on have nots”.


Geography would no longer play a role in accessing
knowledge and ideas.




12.2 Basic Concepts in Computer Networking


Computer network

(second definition) is a set of
independent computer systems connected by
telecommunication links that t
ogether are called the
interconnection network
.



Individual computers in the network referred to as
nodes

or
hosts
.


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12.2.1 Communication Links


Telecommunication links vary in physical characteristics.

Switched dial
-
up phone lines can form a circu
it


Phone lines are analog, varying continuously in
voltage.

Computer produces digital information

Figure 12.1


Modem

modulates or alters a standard analog signal called
a
carrier

so that encodes binary information


Figure 12.2
: modem modulating the ampl
itude of a carrier
wave to encode the binary signal.


Telephone circuits have high error rates and low capacity,
or
bandwidth
.... 56,000 BPS seems max.


Higher rates achieved using dedicated rather than switched
lines.
Figure 12.3


Twisted
-
pair

copper wi
re: inexpensive, short distance

Coaxial cable
: higher rate, less errors

Fiber
-
optic cable
: Highest rate, transmits signals
using reflected light waves


Wireless communication

using microwave signals
facilitates
mobile computing


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12.2.2 Local Area Ne
tworks (
LAN
s)


LAN connects hardware devices such as computers,
printers, and mass storage devices that are all in close
physical proximity.


The owner of the computers is also the owner of the means
of communication.... a private network.


Most widely use
d model is called the
Ethernet
.


Developed in mid 70’s at Xerox PARC in Palo Alto.

10 Million bits per second

Fast Ethernet transmits at 100 Mbps


Two types of Ethernet LAN


Shared cable

(Figure 12.4), single wire stretched to
each machine.

Users tap into
cable using a
transceiver

Separate cables connected via a hardware device
called a

bridge
.


Hub

(Figure 12.5) configuration, a node joins the
network by connecting to the hub.


Rules describing for sending and receiving messages
between network nodes are c
alled
communications
protocols.



Only nodes using the same protocol can talk directly to
each other

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Both types of Ethernets use the same protocols.


Ethernet protocols use a
contention
-
based

transmission technique. Figure 12.6


If the line is not b
usy, then the node transmits the
message immediately by
broadcasting

it across the
cable.... goes to each node on the net.


Address field

on front of message identifies
destination


If the line is busy, then the node wishing to send
continuously monitors t
he line status and sends when
it becomes idle.


If two nodes send at the same time, then a
collision

occurs.


Responsibility of for network operations is shared
equally by all nodes... network is
distributed
.



12.2.3 Wide Area Networks (
WAN
s)


Connects d
evices that are physically distant.


Must purchase telecommunication services from an
external provider.


Typically connections are
point
-
to
-
point

that directly
connect two machines and are not shared. Figure 12.7

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WANs usually use a
store
-
and
-
forwa
rd, packet
-
switching

communications protocol.


Unit of transmission in a WAN is a
packet

-

not a
message.


Packet is fixed in size, message is made up of packets.


Destination address is appended to the packet.


Figure 12.8


Packet sent to adjacent node, a
cknowledgment
message (ACK) sent back


Repeat process until reaching the destination
node.


If the is an error, then ACK is not sent and after a
period of time, sender resends.


Routing

is one of the most important parts of a WAN
protocol


Routing algorith
ms and
shortest path

Problems with a node failure

Address can indicate overall route.


A
router

is a special device that transmits messages
between two distinct networks that use different
protocols. Figure 12.9

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12.3 Network Services


One of the m
ost important services of networking is
resource sharing
,

physical such as printer or disk

logical such as a data file or piece of software.


Print server, file server, compute server

Client
-
server computing

is where some network
nodes provide services and

the remaining nodes are
users (clients) of those services.

Figure 12.10


Information sharing

via a WAN


Electronic data interchange (
EDI
) is computer
-
to
-
computer communications.


Information utility

is something like an online
library


Networks support gr
oup efforts in producing a shared
document:
Groupware


Electronic Mail


Bulletin boards

and news groups, chat rooms


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Electronic commerce

(EC) where computers and
networking are used to support paperless exchange of
goods, information, and services i
n the commercial sector.


ATM machines

Automatic paycheck deposit

Use of the world wide web


This brings out the issue of network security

Who people really are
-

authentication

Security of information during transmission
-

encryption

Hardware
-
software fa
ilure
-

fault tolerance



12.4 A Brief History of the Internet and the World Wide
Web


The global Internet and its most popular component, the
World Wide Web


12.4.1 The Internet


Mid 1960’s at MIT


1966 Roberts to Advanced Research Project Agency
(ARP
A) to develop ARPANET


1969 first computer
-
to
-
computer message sent


Lots of proprietary networks developed DECNet,
SNA

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In 1970’s Robert Kahn at ARPA developed the concept of
internetworking.

different WANS connected by gateways.

common addressing
scheme and protocols at
gateway
s

Figure 12.11, network of networks.


Cerf and Kahn

addressing

global, hierarchical addressing scheme that
uniquely identifies a computer user located
anywhere in the world

dahast@ark.ship.edu
-

converted to 32
-
bit number

Lo
cal
-
to
-
global hierarchical addressing scheme is
called the Domain Name System


Protocols

TCP/IP: Transmission Control Protocol / Internet
Protocol

Rules and procedures networks would use for
message formats, routing, and error control


FTP and Telnet pac
kages


Development in mid 80’s of NSFNet, a transcontinental
network called a
backbone

network.
Figure 12.12



The term
Internet

was adopted in 1995,

NSFNet closed up shop and
ISP
s were born.

See figure 12.13 for number of computers on internet


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12.4.2 The World Wide Web


Berners
-
Lee at CERN in 1989

Hypertext

documents called
pages

which can contain
links

to other documents.


A hypertext link refers to a
URL

-

Uniform Resource
Locator: protocol://internet address/page


Web browsers

-

Mosaic in 1
993


In addition to establishing connections between
machines via TCP/IP, procedures were required for
identifying the page being requested and returning the
page to the user.


HTTP

-

Hypertext Transfer Protocols

Figure 12.15


What is the Web today?


Labor
atory Experience 20