Transcript Slide - Computer Science

Chapter 7
Computer Networks, the
Internet, and the World Wide
Web
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
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
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
Communication Links

Switched, dial-up telephone line

A circuit is temporarily established between
the caller and callee
• Circuit lasts for the duration of the call.


Analog medium
Requires modem at both ends to transmit
information produced by a computer
• Computer produces digital information
Figure 7.1 Two Forms of Information Representation
Figure 7.2
Modulation of a Carrier to Encode Binary Information
Communication Links
(continued)

Dial-up phone links



Initial transmission rates of 300 bps
Later in 1980s increased to 1,200-9,600 bps
Transmission rate now: 56,000 bps (56 Kbps)
• Too slow for web pages, MP3, streaming video


Still used to access networks in remote areas.
Broadband


Transmission rate: Exceeding 256,000 bps
(256 Kbps)
Is “always on”, so do not have to wait for
connection
Communication Links
(continued)

Options for broadband communications


Broadband refers to transmission rates over 256K
bps.
Home use
• Digital subscriber line (DSL)
• Cable modem

Commercial and office environment
• Ethernet
• Fast Ethernet
• Gigabit Ethernet
Communication Links (continued)
 Digital



DSL is a “permanently on” connection
Transmits digital signals at different
frequencies, so no interference with voice.
Line is often asymmetric with download &
upload bandwidths.
 Cable



subscriber line (DSL)
modem
Uses links that deliver TV signals to homes.
Speeds roughly comparable to DSL.
Signal is “always on”.
Communication Links (continued)
 Commercial

and office environment
Ethernet – mid 1970’s
• Most common with 10Mbps
• Often available in dorms and new homes

Fast Ethernet developed in
• 100 Mbps – 1990’s
• Coaxial cable, fiber-optic cable, or regular
twisted-pair copper wire.

Gigabit Ethernet project
• In 1998, achieved 1000 Mbps
• In 2003 achieved 10 billion bps

Transmit entire contents of 1,700 books, each 300
pages long in a single second
Figure 7.3
Transmission Time of an Image at Different Transmission
Speeds
Gigabit Networks Applications
 Transmitting
real-time video images
without flicker or delay
 Exchanging 3D medical images
 Transmitting weather satellite data
 Supporting researchers working on
Human Genome project.
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
Wireless data communication

Wireless local access network





Transmits few hundred feet
Connected to standard wired network
Found in home, coffee shops, offices
Wi-Fi (Wireless Fidility) connects wireless computer
to internet within 150-300 ft of access point (called
hot spots).
Wireless wide-area access network



Computer transmits message to remote base
station, which may be miles away
Base station typically a large antenna on top of a
tower.
Some are only line-of-sight, so not more than 10-50
miles apart.
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
Figure 7.4
Some Common
LAN
Topologies
Common LAN Topologies
 Buses


If two or more nodes use the link at the same
time, messages collide and are unreadable
Nodes must take turns using net.
 Rings

Messages circulate in either clockwise or
counterclockwise direction until at destination
 Star

Central node routes information directly to any
other node.
Local Area Networks (continued)

Ethernet





Most widely used LAN technology
Uses the bus topology
Node places message including destination
address on bus.
This message is received by all other nodes
All nodes check address to see if message is
for them.
• Nodes who are not addressed discard message
Local Area Networks (continued)

Two ways to construct an Ethernet LAN



Shared cable
Hubs: The most widely used technology
Shared cable features



Wire is strung through the building.
Users connect into cable at nearest point.
Repeater amplifies and forwards signal
• If two Ethernets are connected using a repeater, they
function as a single network.

Bridge (or switch) joining two networks has
knowledge about nodes on each
• It forwards messages to other network only if needed.
Figure 7.5
An Ethernet LAN Implemented
Using Shared Cables
Hubs for LANs
 In
this construction of a LAN, a box called
a hub is used in place of shared cable
 Hubs are boxes with a number of ports.
 Hubs are placed in a wiring closet.

Each node in Ethernet LAN is connected to a
port
 Hubs
connects these ports using a shared
cable inside hub
Figure 7.6
An Ethernet LAN Implemented Using a Hub
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, packetswitched technology to deliver messages
Figure 7.7
Typical Structure of a Wide Area Network
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

Routers are used to connect networks of both
similar and dissimilar types.
• A router can connect an LAN to a WAN.
• A bridge can only connect two networks of
identical type.
Figure 7.8(a)
Structure of a Typical Company Network
Overall Structure of the Internet
(continued)

Internet Service Provider (ISP)




Normally a business
Provides a pathway from a specific network to other
networks, or from an individual’s computer to other
networks
This access occurs through a WAN owned by the ISP
ISPs are hierarchical


Interconnect to each other in multiple layers to
provide greater geographical coverage
A regional or national ISP may connect to an
international IPS called a tier-1 network or Internet
backbone.
T1 & T3 Lines

A dedicated phone connection supporting data
rates of 1.544 Mbits per second.



Consists of 24 individual channels, each of which
supports 64Kbits per second.
Each 64Kbit/second channel can be configured to
carry voice or data traffic.
T-1 lines are a popular leased line option for
businesses connecting to the Internet and for
Internet Service Providers (ISPs) connecting to
the Internet backbone.
 The Internet backbone itself consists of faster
T-3 connections.
 Reference: Webopedia
Figure 7.8(b)
Structure of a Network Using an ISP
Figure 7.8(c)
Hierarchy of Internet Service
Providers
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 2005
• 317 million nodes (hosts)
• Hundreds of thousands of separate networks
located in over 225 countries
Communication Protocols

A protocol

A mutually agreed upon set of rules, conventions, and
agreements for the efficient and orderly exchange of
information

Internet is operated by the Internet Society, a
nonprofit society consisting of over 100
worldwide organizations, foundations,
businesses, etc.
 TCP/IP




The Internet protocol hierarchy
Governs the operation of the Internet
Named after 2 of the most successful protocols
Consists of five layers (see next slide)
Figure 7.10
The Five-Layer TCP/IP Internet Protocol Hierarchy
Physical Layer

Protocols govern the exchange of binary
digits across a physical communication
channel
 Specify such things as





What signal is used to indicate a bit on the line
How long will “bit on the line” signal last
Will signal be digital or analog
What voltage levels are used to represent 0 & 1
Goal: Create a bit pipe between two
computers
Data Link Layer

Protocols carry out


Error detection and correction
Framing
• Which bits in incoming stream belong together
• Identifying start and end of message

Creates an error-free message pipe
 Composed of two stages

Layer 2a: Medium access control

Layer 2b: Logical link control
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
Layer 2a
Medium access
control protocols
Automatic Repeat Request (ARQ) Algorithm
Part of Logical Link Protocols - layer 2b
Assures message travels from A to B correctly
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
Stop and Wait ARQ
Sender
Sends the packet,
then waits to hear
from receiver.
Sends the
next packet
Receiver
Sends
acknowledgement
Sends negative
acknowledgement
Resends the
packet again
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.
Sources of Errors and Prevention
Source of Error
What causes it
How to prevent it
More important
mostly on analog
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)
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)
– If the same  No
errors in transmission
– If different  Error(s)
in transmission
Error Detection Techniques
 Parity
checks
 Longitudinal Redundancy Checking (LRC)
 Polynomial checking


Checksum
Cyclic Redundancy Check (CRC)
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
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
receiver
01101011
parity
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
Using LRC for Error Detection
Example:
Send the message “DATA” using ODD parity and LRC
Parity bit
Letter ASCII
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
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:
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%)
Network Layer

Delivers a message from the site where it
was created to its ultimate destination

Critical responsibilities


Create a universal addressing scheme for all
network nodes
Deliver messages between any two nodes in
the network
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
Universal addressing scheme used

IP (Internet Protocol) layer

Example:
• One author’s host name is macalester.edu
• The IP address for host is (in decimal)
141.140.1.5
• Corresponds to following 32 bit address
10001101 10001100 0000000 100000101
• Clearly, the symbolic name is easier to remember.
Network Layer (cont.)

Domain Name System (DNS)




It is the job of the DNS to convert symbolic names to
a sequence of 32 binary bits
DNS is a massive distributed database
If local name server does not recognize host name, it
is forwarded to a remote name servers until one
locates its name.
Routing Algorithms



There are normally multiple routes a message can
take to its destination.
A routing algorithm called “Dijkstra’s shortest path”
algorithm is used, based on time required to send a
message along various paths.
For large networks with 107 nodes or more, algorithm
may require 1014 or more calculations.
Network Layer (cont)
 Keeping
these routing tables up to date is
an enormous task.


Data changes regularly, so they have to be
recalculated frequently.
When a network node or sections of network
fails, must route around the failed nodes.
 Network



Layer also handles other things
Network management
Broadcasting
Locating mobile nodes
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
Uses ARQ algorithm on sending & receiving
messages.
Messages sent in packets and have to be
reassembled in order at destination
Standard applications (e.g., POP3 or IMAP for email)
have fixed port numbers for all computers.
Figure 7.15
Logical View of a TCP Connection
Application Layer

Implements the end-user services provided
by a network

There are many application protocols

HTTP

SMTP

POP3

IMAP

FTP
Figure 7.16
Some Popular Application Protocols on the Internet
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
• Example:
http://www.macalester.edu/about/history.html
Network Services and Benefits

Services offered by computer networks

Electronic mail (email)
• Send message when you want & read at receiver’s
convenience
• From U.S., arrives anywhere in world in about a
minute.

Bulletin boards
Internet Services (cont)
 Services
offered by c. networks (cont)

News groups

Chat rooms

Resource sharing
• Physical resources
• Logical resources
Network Services and Benefits
(continued)

Services offered by computer networks

Client-server computing

Information sharing (databases)

Information utility
• reference documents online

Electronic commerce (e-commerce)
A Brief History of the Internet and
the World Wide Web: The Internet

August 1962: First proposal for building a
computer network


Written by J. C. R. Licklider of MIT
ARPANET



Built by the Advanced Research Projects Agency
(ARPA) in Dept of Defense in the late1960s
Packet switching viewed as more secure than phone
lines.
First two nodes were at Stanford Research Institute
(SRI) and UCLA, in 1969
• Univ. of Calif. at Santa Barbara & Univ. Utah added in 1969

Grew quickly during the early 1970s
The Internet (continued)

ARPANET (cont)

Electronic Email developed in 1972

Huge success of ARPANET lead to creation of
other research and commercial networks
 In 1973, Kahn at ARPA and Cerf at Stanford
started work to allow different networks to
communicate



A common address scheme
The TCP/IP protocols that provided a common
language
Additional “Killer Applications” in early 1980’s


Telnet – Log on remotely to another computer
FTP – File Transfer Protocol
The Internet (continued)

Most people with Internet access in early 1980’s
were physicist, engineers, & computer scientist


ARPANET connected only 235 computers in 1982,
one of which was KSU Mathematical Science Dept.
NSFNet: A national “network of networks” built
by the National Science Foundation (NSF) in
1984


Goal was to bring internet services to all academic
and professional groups.
Included universities, libraries, museums, medical
centers, etc.
The Internet (continued)

Many countries quickly followed lead of NSFNet
and connected their professional groups to the
internet.
 October 24, 1995: Formal acceptance of the
term Internet by the U.S. Govt.
 Internet service providers start offering Internet
access once provided by the ARPANET and
NSFNet
Figure 7.20
State of Networking in the Late 1980s
The World Wide Web

Development completed in May 1991



Designed and built by Tim Berners-Lee at CERN
(European High Energy Physics Lab) in Geneva
Switzerland.
Purpose was to allow scientists to more easily
exchange information such as research articles,
experimental data
Would be a big improvement of earlier methods
of email messaging and FTP.
Figure 7.21
Hypertext Documents
The World Wide Web (cont)

Components
 Hypertext
• A collection of documents interconnected by
pointers called links

Hypertext links called a URL (Uniform
Resource Locator)
• The worldwide identification of a Web page located
on a specific host computer

In 1993 CERN announced their web technology
would be freely available.

Realized impact it could have on research throughout
world.
The World Wide Web (cont)

A graphic browser (MOSIAC) was also released
in late 1993



Allowed public to make use of new technology
Forerunner of Netscape Navigator (1994) and
Microsoft Internet Explorer (1995)
The WWW became the killer app of the 21
century
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
Summary

Computer network: A set of independent
computer systems connected by
telecommunication links

Options for transmitting data on a network:
Dial-up telephone lines, DSL, cable modem,
Ethernet, Fast Ethernet

Types of networks: Local area network (LAN)
and wide area network (WAN)
Summary (continued)

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