Transcript Chapter 7 Powerpoints

Chapter 7: Computer
Networks, the Internet, and
the World Wide Web
Invitation to Computer Science,
C++ Version, Third Edition
 2004 Course Technology
Additions by Paul Durand, Kent State Univ, 2005
Objectives
In this chapter, you will learn about:

Basic networking concepts

Communication protocols

Network services and benefits

A brief history of the Internet and the World Wide
Web
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Introduction

Computer network

Computers connected together

Purpose: exchanging resources and information

Just about any kind of information can be sent

Examples: television and radio signals, voice,
graphics, handwriting, photographs, movies
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Basic Networking Concepts


Computer network

Set of independent computer systems connected
by telecommunication links

Purpose: sharing information and resources
Nodes, hosts, or end systems

Individual computers on a network
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Communication Links

Switched, dial-up telephone
line

A circuit is temporarily
established between the
caller and callee

Analog medium

Requires modem at both
ends to transmit information
produced by a computer

Computer produces digital
information
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Diagram by PJD, 2005
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Diagram by PJD, 2005
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Figure 7.1 Two Forms of Information Representation
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Figure 7.2
Modulation of a Carrier to Encode Binary Information
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Communication Links (continued)

Dial-up phone links


Transmission rate: 56,000 bps (56 Kbps)
Broadband

Transmission rate: exceeding 128,000 bps (128
Kbps)
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Communication Links (continued)

Options for broadband communications


Home use

Digital subscriber line (DSL)
Broadband Tutorial

Cable modem
CableModem Tutorial
Commercial and office environment

Ethernet

Fast Ethernet

Gigabit Ethernet
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Ethernet Tutorial
Links added by PJD, 2005
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Figure 7.3
Transmission Time of an Image at Different Transmission Speeds
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Communication Links (continued)

Wireless data communication

Uses radio, microwave, and infrared signals

Enables “mobile computing”

Types of wireless data communication

Wireless local access network

Wireless wide-area access network
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Local Area Networks

Local area network (LAN)

Connects hardware devices that are in close
proximity

The owner of the devices is also the owner of the
means of communications

Common wired LAN topologies

Bus

Ring

Star
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Figure 7.4
Some Common
LAN Topologies
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Local Area Networks (continued)

Ethernet

Most widely used LAN technology

Uses the bus topology

Two ways to construct an Ethernet LAN

Shared cable

Hubs: the most widely used technology
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KSU
12/1/2004
A bridge is also called a switch.
- It is 'smarter' than a repeater.
- It is aware of which nodes are on which side
of the lan.
- It determines whether a message should be
passed on to another network
Figure 7.5: An Ethernet
LAN Implemented
Using Shared Cables
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Comment by PJD, 2005
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Figure 7.6
An Ethernet LAN Implemented Using a Hub
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Wide Area Networks

Wide area networks (WANs)

Connect devices that are across town, across the
country, or across the ocean

Users must purchase telecommunications
services from an external provider

Dedicated point-to-point lines

Most use a store-and-forward, packet-switched
technology to deliver messages
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•UOT = Packet
- approx 1000 bytes
•Non-fixed routing
•Fault Tolerant
Text by PJD, 2005
Figure 7.7
Typical Structure of a Wide Area Network
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Overall Structure of the Internet

All real-world networks, including the Internet,
are a mix of LANs and WANs

Example: a company or a college

One or more LANs connecting its local computers

Individual LANs interconnected into a wide-area
“company network”
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KSU
12/1/2004
A router connects various networks.
It differs from a bridge. a bridge connects
two identical types of networks.
A router can transfer messages between
Figure
two distinct networks, each using
a totally7.8(a)
different communication
technique.
Structure
of a Typical Company
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Network
Comment by PJD, 2005
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Overall Structure of the Internet
(continued)


Internet Service Provider (ISP)

A wide-area network

Provides a pathway from a specific network to
other networks, or from an individual to other
networks
ISPs are hierarchical

Interconnect to each other in multiple layers to
provide greater geographical coverage
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Figure 7.8(b)
Structure of a Network Using an ISP
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Figure 7.8(c)
Hierarchy of Internet Service
Providers
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Overall Structure of the Internet
(continued)

Internet

A huge interconnected “network of networks”

Includes nodes, LANs, WANs, bridges, routers,
and multiple levels of ISPs

Early 2003

170 million nodes (hosts)

Hundreds of thousands of separate networks
located in over 225 countries
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Communication Protocols
The internet is operated by the Internet Society, a non-profit, nongovernmental, professional society.
This group establishes and enforces network protocol standards.

A protocol
IAB – Internet Architecture Board
IETF – Internet Engineering Task Force


A mutually agreed upon set of rules, conventions,
and agreements for the efficient and orderly
exchange of information
TCP/IP

The Internet protocol hierarchy

Governs the operation of the Internet

Five layers
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Comment by PJD, 2005
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Internet Maps

http://www.cybergeography.org/atlas/more_isp_maps.html

http://www.isoc.org/internet/infrastructure/maps.shtml
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Figure 7.10
The Five-Layer TCP/IP Internet Protocol Hierarchy
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Physical Layer

Protocols govern the
exchange of binary digits
across a physical
communication channel

Goal: create a “bit pipe”
between two computers
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Figure added by PJD, 2005
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Data Link KSU
Layer
12/1/2004
Medium access control
- How to arbitrate ownership of a shared line?

Protocols carry out
- Central master control node?

Error handling

Framing
- Contention based approach?
- no central control
(identify
start
and end
message)
- equal
access
forofall

Creates an error-free “message pipe”

Composed of two services

Layer 2a: medium access control

Layer 2b: logical link control
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Data Link Layer (continued)

Medium access control protocols


Determine how to arbitrate ownership of a shared
line when multiple nodes want to send at the
same time
Logical link control protocols

Ensure that a message traveling across a channel
from source to destination arrives correctly
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Layer 2a
Medium access
control protocols
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Automatic Repeat Request (ARQ) Algorithm
Part of Logical Link Protocols - layer 2b
Assures message travels from A to B correctly
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Automatic Repeat Request (ARQ)


Process of requesting that a data transmission be
resent
Main ARQ protocols

Stop and Wait ARQ (A half duplex technique)



Sender sends a message and waits for acknowledgment,
then sends the next message
Receiver receives the message and sends an
acknowledgement, then waits for the next message
Continuous ARQ (A full duplex technique)


Sender continues sending packets without waiting for the
receiver to acknowledge
Receiver continues receiving messages without
acknowledging them right away
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Stop and Wait ARQ
Sender
Sends the packet,
then waits to hear
from receiver.
Receiver
Sends
acknowledgement
Sends the
next packet
Sends negative
acknowledgement
Resends the
packet again
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Continuous ARQ
Sender sends packets
continuously without
waiting for receiver to
acknowledge
Notice that
acknowledgments now
identify the packet
being acknowledged.
Receiver sends back
a NAK for a specific
packet to be resent.
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Sources of Errors and Prevention
Source of Error
What causes it
How to prevent it
More important
Line Outages
Faulty equipment, Storms,
Accidents (circuit fails)
White Noise (hiss)
(Gaussian Noise)
Movement of electrons (thermal
energy)
Increase signal strength
(increase SNR)
Impulse Noise
Sudden increases in electricity
(e.g., lightning, power surges)
Shield or move the wires
Cross-talk
Multiplexer guard bands are too
small or wires too close together
Increase the guard bands, or
move or shield the wires
Echo
Poor connections (causing signal to
be reflected back to the source)
Fix the connections, or
tune equipment
Attenuation
Gradual decrease in signal over
distance (weakening of a signal)
Intermodulation
Noise
Signals from several circuits
combine
Use repeaters or
amplifiers
Move or shield the wires
Jitter
Analog signals change (small
changes in amp., freq., and phase)
Tune equipment
Harmonic
Distortion
Amplifier changes phase (does not
correctly amplify its input signal)
Tune equipment
(Spikes)
mostly on analog
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Error Detection
Sender calculates an
Error Detection Value
(EDV) and transmits
it along with data
Receiver recalculates
EDV and checks it
against the received EDV
Mathematical
calculations
Mathematical
calculations
?
=
Data to be
transmitted
EDV
Larger the size, better
error detection (but
lower efficiency)
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– If the same  No
errors in transmission
– If different  Error(s)
in transmission
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Error Detection Techniques



Parity checks
Longitudinal Redundancy Checking (LRC)
Polynomial checking


Checksum
Cyclic Redundancy Check (CRC)
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Parity Checking


One of the oldest and simplest
A single bit added to each character



Receiving end recalculates parity bit


Even parity: number of 1’s remains even
Odd parity: number of 1’s remains odd
If one bit has been transmitted in error the received parity
bit will differ from the recalculated one
Simple, but doesn’t catch all errors


If two (or an even number of) bits have been transmitted in
error at the same time, the parity check appears to be
correct
Detects about 50% of errors
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Examples of Using Parity
To be sent: Letter V in 7-bit ASCII: 0110101
EVEN parity
sender
01101010
number of all
transmitted 1’s
remains EVEN
ODD parity
receiver
parity
sender
number of all transmitted
1’s remains ODD
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receiver
01101011
parity
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LRC - Longitudinal Redundancy Checking

Adds an additional character (instead of a bit)



Block Check Character (BCC) to each block of data
Determined like parity but, but counting longitudinally
through the message (as well as vertically)
Calculations are based on the 1st bit, 2nd bit, etc. (of all
characters) in the block



1st bit of BCC  number of 1’s in the 1st bit of characters
2nd bit of BCC number of 1’s in the 2ndt bit of characters
Major improvement over parity checking


98% error detection rate for burst errors ( > 10 bits)
Less capable of detecting single bit errors
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Using LRC for Error Detection
Example:
Send the message “DATA” using ODD parity and LRC
Letter
ASCII
Parity bit
D 10001001
A 10000011
T 10101000
A 10000011
BCC 1 1 0 1 1 1 1 1
Note that the BCC’s parity bit
is also determined by parity
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Polynomial Checking



Adds 1 or more characters to the end of message
(based on a mathematical algorithm)
Two types: Checksum and CRC
Checksum





Calculated by adding decimal values of each character in
the message,
Dividing the total by 255. and
Saving the remainder (1 byte value) and using it as the
checksum
95% effective
Cyclic Redundancy Check (CRC)

Computed by calculating the remainder to a division
problem:
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Cyclic Redundancy Check (CRC)
P/G=Q+R/G
Quotient
Message
(whole
(treated as
number)
one long
binary
A fixed number
number)
(determines the
length of the R)
Example:
P = 58
G=8
Q=7
R =2
Remainder:
–added to the
message as EDV)
–could be 8 bits, 16
bits, 24 bits, or 32
bits long
– Most powerful and most common
– Detects 100% of errors (if number of errors <= size of R)
–Otherwise: CRC-16 (99.998%) and CRC-32 (99.9999%)
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Network Layer

Delivers a message from the site where it was
created to its ultimate destination

Critical responsibilities

Creating a universal addressing scheme for all
network nodes

Delivering messages between any two nodes in
the network
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Network Layer (continued)

Provides a true “network delivery service”


Messages are delivered between any two nodes
in the network, regardless of where they are
located
IP (Internet Protocol) layer

Network layer in the Internet
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Ethernet (IEEE 802.3) Frame
Used by Virtual LANs; (if no
vLAN, the field is omitted
If used, first 2 bytes is set
to: 24,832 (8100H)
Repeating
pattern of 1’s
and 0’s
(1010101010)
(number of bytes
in the message
field)
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Used to hold sequence number,
ACK/NAK, etc., (1 or 2 bytes)
00
01
10
11
Used to exchange
control info (e.g.,
type of network layer
protocol used)
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Transport Layer

Provides a high-quality, error-free, order
preserving end-to-end delivery service

TCP (Transport Control Protocol)

Primary transport protocol on the Internet

Requires the source and destination programs to
initially establish a connection
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Figure 7.15
Logical View of a TCP Connection
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Application Layer

Implements the end-user services provided by a
network

There are many application protocols, including:

HTTP

SMTP

POP3

IMAP

FTP
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Figure 7.16
Some Popular Application Protocols on the Internet
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Application Layer (continued)

Uniform Resource Locator (URL)

A symbolic string that identifies a Web page

Form
protocol://host address/page

The most common Web page format is hypertext
information

Accessed using the HTTP protocol
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Network Services and Benefits

Services offered by computer networks

Electronic mail (email)

Bulletin boards

News groups

Chat rooms

Resource sharing

Physical resources

Logical resources
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Network Services and Benefits
(continued)

Services offered by computer networks

Client-server computing

Information sharing

Information utility

Electronic commerce (e-commerce)
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A Brief History of the Internet and
the World Wide Web:
The Internet

August 1962: first proposal for building a
computer network


Made by J. C. R. Licklider of MIT
ARPANET

Built by the Advanced Research Projects Agency
(ARPA) in the 1960s

Grew quickly during the early 1970s
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The Internet (continued)

NSFNet: A national network built by the National
Science Foundation (NSF)

October 24, 1995: Formal acceptance of the
term “Internet”

Internet service providers start offering Internet
access once provided by the ARPANET and
NSFNet
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Figure 7.20
State of Networking in the Late 1980s
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The World Wide Web

Development completed in May 1991

Designed and built by Tim Berners-Lee

Components

Hypertext


A collection of documents interconnected by
pointers called links
URL (Uniform Resource Locator)

The worldwide identification of a Web page located
on a specific host computer
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Figure 7.21
Hypertext Documents
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Summary of Level 3

Virtual environment



Created by system software
Easy to use and easy to understand
Provides services such as:





Resource management
Security
Access control
Efficient resource use
Operating systems continue to evolve
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Summary

Computer network: a set of independent
computer systems connected by
telecommunication links

Options for transmitting data on a network: dialup telephone lines, DSL, cable modem,
Ethernet, Fast Ethernet

Types of networks: local area network (LAN) and
wide area network (WAN)
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Summary

The Internet is a huge interconnected "network
of networks"

TCP/IP is the Internet protocol hierarchy,
composed of five layers: physical, data link,
network, transport, and application

The World Wide Web is an information system
based on the concept of hypertext
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