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

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
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3. 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”
Twitter revolution
Students
Physicians medical records, test results specialists
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4. 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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5. 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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6. Figure 7.1 Two Forms of Information Representation
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7. Figure 7.2 Modulation of a Carrier to Encode Binary Information
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8. 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
Home users
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9. 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
Home users
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10. 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
Home users
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11. 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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12. 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 an
Image at Different Transmission Speeds
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13. 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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14. 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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15. 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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16. 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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17. Computer Networks Bus topology Ring topology Star 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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18. Figure 7.4 Some Common LAN Topologies
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19. Invitation to Computer Science, 6th Edition

20. 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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21. 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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22. Figure 7.5 An Ethernet LAN Implemented Using Shared Cables
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23. 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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24. Figure 7.6 An Ethernet LAN Implemented Using a Switch
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27. 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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28. 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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29. 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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30. Figure 7.8(a) Structure of a Typical Company Network
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31. 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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32. Figure 7.8(b) Structure of a Network Using an ISP
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33. Figure 7.8(c) Hierarchy of Internet Service Providers
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34. 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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35. Communication Protocols
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
1. To have meaningful communications, we need a set of procedures that specifies how the exchanges will take place. Talk on the telephone. Telephone etiquette.
conventional requirements
2. Millions of machines, exchange messages correctly and efficiently, one reason, they have all agreed to use the same protocols to govern that exchange.
3. Managed by the governments or international agency like the united Nations. Operated by the Internet society. (foundations, governmental agencies, educational institutions, companies)
4. Subcommittees establishes and enforces network protocol standards.
5. The protocols are set up as a multilayered hierarchy, with each layer addressing one aspect of the overall communications task. This hierarchy is also referred to as TCP/IP. Two important protocols
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36. Figure 7.10 The Five-Layer TCP/IP Internet Protocol Hierarchy
Wifi , 3G, physical layer
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37. 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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38. Physical Layer Physical layer protocols Goal of the physical layer
Govern the exchange of binary digits across a physical communication channel
Goal of the physical layer
To create a “bit pipe” between two computers
How we know when a bit is present on the line
How much time the bit will remain on the line
Whether the bit is in the form of a digital or an analog signal
What physical quantities are used to represent a binary 0 and a binary 1
The shape of the connector between the computer and the transmission line
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39. Figure 7.11 The Concept of a Bit Pipe
Moderm, dsl , mobile phone with wireless data capabitlites.
No concerned about the voltage levels, wavelengths or radio frequencies.
Send a bit and receive a bit
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40. Data Link Layer Error detection and correction problem Framing 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
The link is not an error-free channel,
Due to interference or weather or any number of other factors.
Not exact copy of the bits that went in
Want to receive complete messages, not raw streams of bits, which bits in the incoming stream belong together.
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41. Figure 7.12 The Medium Access Control Protocols in Ethernet
A collision occurs, stop sending, wait a random amount of time
Exact same random waiting period
One reason the ethernet protocal is so popular is that control is distributed.
Centralized manner takes a good deal of time. Send requests to master, reply the decision. Fault tolerant
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42. 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
Arq automatic repeat request.
Adds some additional information to form a packet, sequence number uniquely identifying each packet. Adds some error-checking bits, whether corrupted during transimission
Finally, adds a start of packet (SOP) and end of packet (EOP) delimiter to determine exactly where the packet begins and ends.
SOP and EOP will not be the same as the content of the message in the packet
ACK acknowledgement message
Disassemble
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43. Figure 7.13 A Message Packet Sent by the Data Link Protocols
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44. Invitation to Computer Science, 6th Edition

45. 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
Great majority of nodes are not directly connected.
It is often said that the network layer is the glue that holds the entire network together.
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46. 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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47. 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
Local name server
Checks to see if it has data record containing a specific IP address
The DNS is a massive database, distributed over literally thousands of machines.
Symbolic name, forward to a computer called a local name server.
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48. Network Layer (continued)
Routing
Process of selecting one specific path
Shortest path
Path via which message can travel the fastest
As the number of network nodes increases, the solution time for these “brute force” algorithm grows exponentially. For tiniest network.
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52. 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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53. Transport Layer UDP (User Datagram Protocol)
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
TCP uses the same ARQ algorithm described in our discussion of the data link level.
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54. Figure 7.14 Relationship between IP Addresses and Port Numbers
With these four protocol layers in place, we now have a complete end to end delivery service.
The only thing left to specify is the content of those messages, what does a program want to say to another program.
What types of applications do we want to give to our network users and exactly how do we implement them.
Figure 7.14 Relationship between IP Addresses and Port Numbers
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55. Application Layer
Implements the end-user services provided by a network
There are many application protocols
The appearance of exciting new and critically important applications (often called killer apps) has fueled the rapid growth of networking and the Internet
in the 1970s, chat room in 1980s, the web and e-commerce int 1990s
And social networking in the twenty-first century.
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56. 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
Use the HTTP protocol as a general model for how application layer services.
A single web page is identified by a symbolic string called a uniform resource locator
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57. Figure 7.17 Behavior of the HTTP Application-Level Protocol
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58. 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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59. Network Services and Benefits
Electronic 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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60. 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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61. Figure 7.18 The Client-Server Model of Computing
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62. 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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63. 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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64. 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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65. Figure 7.19 A Network of Networks
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66. 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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67. Figure 7.20 State of Networking in the Late 1980s
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68. The World Wide Web Tim Berners-Lee Hypertext Uniform Resource Locator
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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69. The World Wide Web (continued)
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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70. Figure 7.21 Hypertext Documents
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71. Summary Computer network Options for transmitting data on a 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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72. Summary (continued) The Internet TCP/IP World Wide Web
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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73. 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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