Invitation to Computer Science 6 th Edition Chapter 7 Computer Networks, the Internet, and the World Wide Web 1

Invitation to Computer Science 6th Edition

Chapter 7 Computer Networks, the Internet,

and the World Wide Web

1

Invitation to Computer Science, 6th Edition 2

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


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”


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


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


Figure 7.1 Two Forms of Information Representation


Figure 7.2 Modulation of a Carrier to Encode Binary Information


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



• Options for broadband communications

– Home use

• Digital subscriber line (DSL), asymmetric,

• Cable modem

– Commercial and office environment

• Ethernet

• Fast Ethernet

• Gigabit Ethernet



• 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



• 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)



• 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



• 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 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



• 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


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


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


Figure 7.4 Some Common LAN Topologies



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


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


Figure 7.5 An Ethernet LAN Implemented Using Shared Cables


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


Figure 7.6 An Ethernet LAN Implemented Using a Switch




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


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


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


Figure 7.8(a) Structure of a Typical Company Network


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


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)

• 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


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


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.

Invitation to Computer Science, Java Version, Third Edition 37


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


Figure 7.11 The Concept of a Bit Pipe

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



Figure 7.12 The Medium Access Control Protocols in Ethernet


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


Figure 7.13 A Message Packet Sent by the Data Link Protocols



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


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



• 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



• Routing– Process of selecting one specific path

• Shortest path– Path via which message can travel the fastest





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


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


Figure 7.14 Relationship between IP Addresses and Port Numbers


Application Layer

• Implements the end-user services provided by a network

• There are many application protocols


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


Figure 7.17 Behavior of the HTTP Application-Level Protocol


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


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


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


Figure 7.18 The Client-Server Model of Computing


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


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


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


Figure 7.19 A Network of Networks


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


Figure 7.20 State of Networking in the Late 1980s


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


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


Figure 7.21 Hypertext Documents


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)


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
