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Invitation to Computer Science
6th Edition
Chapter 7
Computer Networks, the Internet,
and the World Wide Web
1
Objectives
In this chapter, you will learn about:
• Basic networking concepts
– Communication links
– Computer Networks
• Communication protocols
• Network services and benefits
• The Internet and the World Wide Web
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Introduction
• Networking
– Might foster growth of democracy and global
understanding
– Can equalize access to information and eliminate the
concept of “information haves” and “information havenots”
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Basic Networking Concepts
• Computer network
– Set of independent computer systems connected by
telecommunication links
• Individual computers on the network
– Referred to as nodes, hosts, or end systems
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Communication Links
• In early days of networking:
– Most common way to transmit data was via
switched, dial-up telephone lines
– Transmission rate: 56,000 bps (56 Kbps)
• Voice-oriented dial-up telephone network
– Originally a totally analog medium
• Modem
– Modulates, or alters, a standard analog signal called
a carrier so that it encodes binary information
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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)
• Broadband
– Has rapidly been replacing modems and analog
phone lines for data communications
– Transmission rate: Exceeding 256,000 bps (256
Kbps)
– kilobit (kbit)
210
– megabit (Mbit)
220
– gigabit (Gbit)
230
– terabit (Tbit)
240
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Communication Links (continued)
• Options for broadband communications
– Home use
• Digital subscriber line (DSL), asymmetric,
• Cable modem
– Commercial and office environment
• Ethernet
• Fast Ethernet
• Gigabit Ethernet
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Communication Links (continued)
• Digital subscriber line
– Provided by either your local telephone company or
someone certified to act as their intermediary
• Asymmetric digital subscriber
– Does not have the same transmission speed in the
download direction as in the upload direction
• Cable modem
– Makes use of links that deliver cable TV signals into
your home
– Offered by cable TV providers
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Communication Links (continued)
• Ethernet (commercial and office)
– Designed to operate at 10 Mbps using coaxial cable
• Fast Ethernet
– Transmits at 100 Mbps across coaxial cable, fiberoptic cable, or regular twisted-pair copper wire
• Gigabit networking
– Transmission lines that support speeds in excess of
1 billion bits per second (Gbps)
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Communication Links (continued)
• Ten-gigabit Ethernet standard
– Version of Ethernet with data rate of 10 billion bits
per second
– Adopted by IEEE in 2003
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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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Communication Links (continued)
• Wireless local access network
– User transmits from his or her computer to a local
wireless base station
• Wi-Fi (Wireless Fidelity)
– Used to connect a computer to the Internet when it is
within range of a wireless base station
• Metropolitan Wireless Local Access Network
– Routers provide convenient, low cost wireless
Internet access to all residents
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Communication Links (continued)
• Bluetooth
– Often used to support communication between
wireless computer peripherals
• Wireless wide area access network
– Computer transmits messages to a remote base
station provided by a telecommunications company
• 3G
– Offers voice services as well as data communication
at rates of 0.5 to 2.4 Mbps
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Computer Networks
• Local area network (LAN)
– Connects hardware devices such as computers,
printers, and storage devices that are all 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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Computer Networks
• Bus topology
– All nodes are connected to a single shared
communication line
• Ring topology
– Connects the network nodes in a circular fashion
• Star topology
– A single central node that is connected to all other
sites
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Figure 7.4 Some Common
LAN Topologies
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Computer Networks (continued)
• Ethernet
– Most widely used LAN technology
– Uses the bus topology
– Two ways to construct an Ethernet LAN
• Shared cable
• Switch: The most widely used technology
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Computer Networks (continued)
• Shared cable
– A wire is strung around and through a building
– Users tap into the cable at its nearest point using a
transceiver
• Repeater
– Device that simply amplifies and forwards a signal
• Bridge (switch)
– “Smarter” device that has knowledge about the
nodes located on each separate network
– It examines every message to see if it should be
forwarded
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Figure 7.5 An Ethernet
LAN Implemented Using
Shared Cables
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Local Area Networks (continued)
• Switch
– Located in a room called a wiring closet
– Contains a number of ports, with a wire leading from
each port to an Ethernet interface
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Figure 7.6 An Ethernet LAN Implemented Using a Switch
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Computer Networks
• Wide area network (WAN)
– Connects devices that are not in close proximity but
rather 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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Computer Networks
• Packet
– Information block with a fixed maximum size that is
transmitted through the network as a single unit
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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Figure 7.8(a) Structure of a Typical Company Network
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Overall Structure of the Internet
• Individual networks are interconnected via a device
called a router
• Internet Service Provider (ISP)
– A wide-area network
– Provides a pathway from a specific network to other
networks, or from an individual’s computer 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)
• International ISP
– Also called tier-1 network or an Internet backbone
– Provides global coverage
• Internet
– Huge interconnected “network of networks” that
includes nodes, LANs, WANs, bridges, routers, and
multiple levels of ISPs
– Early 2011
• 818 million nodes
• Hundreds of thousands of separate networks located in
over 230 countries
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Communication Protocols
• Protocol
– Mutually agreed upon set of rules, conventions, and
agreements for the efficient and orderly exchange of
information
• Internet Society
– Nonprofit, nongovernmental, professional society
composed of more than 100 worldwide organizations
• Internet protocol hierarchy
– Also called a protocol stack
– Has five layers
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Figure 7.10 The Five-Layer TCP/IP Internet Protocol
Hierarchy
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Why need layers?
• The layered concept of networking was developed
to accommodate changes in technology. Each
layer of a specific network model may be
responsible for a different function of the network.
Each layer will pass information up and down to the
next subsequent layer as data is processed.
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Physical Layer
• Physical layer protocols
– Govern the exchange of binary digits across a
physical communication channel
• Goal of the physical layer
– To create a “bit pipe” between two computers
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Figure 7.11 The Concept of a Bit Pipe
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Data Link Layer
• Error detection and correction problem
– How do we detect when errors occur, and how do
we correct them?
• Framing problem
– Identifying the start and end of a message
• Data link protocols (layer 2a, layer 2b)
– Address and solve error handling and framing
• Medium access control protocols (layer 2a)
– Determine how to arbitrate ownership of a shared
line when multiple nodes want to send messages at
the same time
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Figure 7.12 The Medium
Access Control Protocols in
Ethernet
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Data Link Layer (continued)
• Collision
– Two or more messages transmitted at exactly the
same time
– Common occurrence in contention-based networks
like Ethernet
• Layer 2b logical link control protocols
– Ensure that message traveling across this channel
from source to destination arrives correctly
• ARQ algorithm
– Basis for all data link control protocols in current use
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Figure 7.13 A Message Packet Sent by the Data Link Protocols
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Network Layer
• Network layer protocols
– Deliver message from the site where it was created
to its ultimate destination
• Responsibilities of the network layer
– 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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Network Layer (continued)
• Nodes identify each other using a 32-bit IP
address
• Domain Name System (DNS)
– Converts from a symbolic host name such as
macalester.edu to its 32-bit IP address 141.140.1.5
• Local name server
– Checks to see if it has data record containing a
specific IP address
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Network Layer (continued)
• Routing
– Process of selecting one specific path
• Shortest path
– Path via which message can travel the fastest
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Transport Layer
• Provides a high-quality, error-free, orderpreserving, end-to-end delivery service
• Transport layer protocols
– Assigns port numbers to programs
– Remembers which program goes with which port
• Well-known port numbers
– Used by all important applications on the Internet
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Transport Layer
•
TCP (Transport Control Protocol)
– Primary transport protocol on the Internet
– Requires the source and destination programs to initially
establish a connection
– Use port number
• UDP (User Datagram Protocol)
– Not a reliable protocol
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Figure 7.14 Relationship between IP Addresses and Port Numbers
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Application Layer
• Implements the end-user services provided by
a network
• There are many application protocols
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Application Layer
• Application layer protocols
– Rules for implementing end-user services provided
by a network
• Uniform Resource Locator (URL)
– protocol://host address/page
• HTTP request message
– Sent on the TCP connection from the client to the
server, specifying the name of a Web page
• HTTP response message
– Returned from the server to the client along the
same TCP connection
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Figure 7.17 Behavior of the HTTP Application-Level Protocol
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Browse http://www.gsu.edu
• 1. Convert domain name to IP address by using
DNS protocol
• 2. Establish TCP connection at 80 port
• 3. Send HTTP Get Meesage
• 4. Get HTTP Reponse Message
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Network Services and Benefits
• Electronic mail (e-mail)
– Single most popular application of networks for the
last 30 years
• Bulletin boards
– Shared public file where anyone can post messages
and everyone is free to read the postings of others
• Social networks
– Systems that create communities of users who share
common interests
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Network Services and Benefits
(continued)
• Resource sharing
– Ability to share physical and logical resources
• Client-server computing
– Some nodes provide services, while the remaining
nodes are users of those services
• Information sharing
– A network is an excellent way to access scientific,
medical, legal, and commercial data files
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Figure 7.18 The Client-Server Model of Computing
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Network Services and Benefits
(continued)
• Information utility (data warehouse)
– Nodes contain massive amounts of information that
can be electronically searched for specific facts
• Collaborative software (groupware)
– Facilitates the efforts of individuals connected by a
network and working on a single shared project
• Electronic commerce (e-commerce)
– Use of computers and networking to support the
paperless exchange of goods, information, and
services
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A Brief History of the Internet and the
World Wide Web
• ARPA
– Small research office of the Department of Defense
charged with developing technology that could be of
use to the U.S. military
• ARPANET
– Formally demonstrated to scientific community at an
international conference in 1972
• Internetworking
– Any WAN is free to do whatever it wants internally
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A Brief History of the Internet and the
World Wide Web (continued)
• Robert Kahn and his colleagues needed to create:
– A standardized way for a node in one WAN to
identify a node located in a different WAN
– A universally recognized message format for
exchanging information across WAN boundaries
• Telnet
– Allows users to log on remotely to another computer
and use it as though it were their own local machine
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Figure 7.19 A Network of Networks
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A Brief History of the Internet and the
World Wide Web (continued)
• FTP (file transfer protocol)
– Provides a way to move files around the network
quickly and easily
• NSFNet
– Used TCP/IP technology identical to the ARPANET
– Interconnected six NSF supercomputer centers with
dozens of new regional networks set up by the NSF
• Internet service providers
– Offered Internet access once provided by
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
• Tim Berners-Lee
– Researcher at CERN
– First developed the idea for a hypertext-based
information distribution system in 1989
• Hypertext
– Collection of documents interconnected by pointers,
called links
• Uniform Resource Locator
– Worldwide identification of a Web page located on a
specific host computer on the Internet
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The World Wide Web (continued)
• World Wide Web
– Completed and made available to all researchers at
CERN in May 1991
• Mosaic
– Web browser developed in late 1993 and made
available to the general public
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Figure 7.21 Hypertext Documents
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Summary
• Computer network
– 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)
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Summary (continued)
• The Internet
– Huge interconnected "network of networks"
• TCP/IP
– Internet protocol hierarchy, composed of five layers:
physical, data link, network, transport, and
application
• World Wide Web
– An information system based on the concept of
hypertext
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Test 4
• Chapter 6, 7, 9; time: Apr 24, 3pm-4:15pm
• 15 questions:
–
–
–
–
–
–
Assembly language programming
Transmission rate
Routing
Communication protocol
Java programming
Software development life cycle
• Question types
– single answer choice, short answer, programming
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