Invitation to Computer Science 6th Edition
Chapter 7 Computer Networks, the Internet,
and the World Wide Web
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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 have-nots”
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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 7.2 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, fiber-
optic 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
Figure 7.3 Transmission Time of anImage 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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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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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(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
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
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, order- preserving, 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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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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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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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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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
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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