INVITATION TO Computer Science 1 1 Chapter 7 Computer Networks, the Internet, and the World Wide Web

Objectives After studying this chapter, students will be able to: Describe and compare different network technologies, including dial-up, broadband, and wireless Explain how different kinds of networks (LAN, WAN, WLAN, WWAN) are connected, and how communications works in each Explain the importance of standards and protocols for communications among computing devices Name the layers of the protocol hierarchy, and describe the purpose of each layer Invitation to Computer Science, 6th Edition2

Objectives (continued) After studying this chapter, students will be able to: Demonstrate how protocols like the ARQ algorithm, DNS, IP,TCP, and HTTP function Explain how the existence of the Internet has led to new modes of communication, such as e-mail, resource sharing, and e-commerce Describe the highlights of the history of the Internet and the Web, and explain how the modern system came into being Invitation to Computer Science, 6th Edition3

Introduction Computer networks have had revolutionary impact Electronic commerce Worldwide communications Spread of information and data We take for granted: access to information on any subject immediate contact with people around the world streaming audio and video wired or wireless access from every device Invitation to Computer Science, 6th Edition4

Basic Networking Concepts Computer network: made up of computing devices, nodes, and interconnections Networks may be wired or wireless; communication links use various technology Wired: Dial-up Broadband Wireless: WLAN WWAN Invitation to Computer Science, 6th Edition5

Basic Networking Concepts (continued) Switched, dial-up telephone lines Analog lines Transmit digital data Modem modulates carrier wave Speeds up to 56k bps Bandwidth = capacity Invitation to Computer Science, 6th Edition6

Basic Networking Concepts (continued) Broadband: transmission rate > 256k bps Home users: Asymmetric download/upload times Digital subscriber line (DSL) Uses phone lines, but sends digital signal on different frequencies than voice Down: 5-15 Mbps, Up: 1-2 Mbps Cable modem Uses cable TV lines Down: 10-20 Mbps, Up: 1-3Mbps Invitation to Computer Science, 6th Edition8

Basic Networking Concepts (continued) Commercial/institutional users: Ethernet (1970s) Dedicated coaxial cable Operates at 10 Mbps Fast Ethernet (early 1990s) Dedicated lines (coaxial, fiber-optic, or twisted-pair) Operates at 100 Mbps Gigabit Ethernet Standard (late 1990s) From gigabit networking research project IEEE standard Operates at 1000 Mbps Invitation to Computer Science, 6th Edition9

Basic Networking Concepts (continued) Wireless data communication Radio, microwave, infrared signals to mobile computers Laptops, tablet computers, smartphones, etc. Mobile computing: deliver data regardless of location Bluetooth: Low-power, close range (30-50 feet), connect devices like wireless mice, cameras, video games Invitation to Computer Science, 6th Edition11

Basic Networking Concepts (continued) Wireless local area network (WLAN) Computers transmit wirelessly to base station with wired connection Range of 150-300 feet Terminology: Wi-Fi (Wireless Fidelity) IEEE 802.11 wireless network standards Wi-Fi hot spot: e.g., libraries, campuses, coffee shops Metropolitan Wi-Fi service: cities provide routers Invitation to Computer Science, 6th Edition12

Basic Networking Concepts (continued) Wireless wide area network (WWAN) Computers transmit wirelessly to remote base station with wired connection Cellular technology: antennas on towers miles apart Example: 4G: voice and data, transmits at 5-20 Mbps Signal may be blocked when indoors Errors with data transmission can slow performance Security: wireless signals easy to intercept Invitation to Computer Science, 6th Edition13

Basic Networking Concepts (continued) Local Area Networks (LAN) Wired connection Computers, printers, and servers in close proximity Examples: same room, office building, campus Privately owned and operated Topology: how computers connected, affects how they communicate Invitation to Computer Science, 6th Edition14

Basic Networking Concepts (continued) Bus topology Shared lines Take turns using line Ring topology Messages circulate until reach source Star topology All send to central node, which routes to destination Invitation to Computer Science, 6th Edition

Basic Networking Concepts (continued) Ethernet LAN with shared cable Bus topology Single cable over short distances Multiple cables over longer distances Repeater amplifies signal Bridge routes messages only when necessary Invitation to Computer Science, 6th Edition

Basic Networking Concepts (continued) Ethernet LAN with switch Bus topology, still Shared cable is inside switch Wiring closet contains switch and ports Ethernet jacks in rooms connect to switch in closet Wireless base stations also connect to switch in closet Invitation to Computer Science, 6th Edition17

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Basic Networking Concepts (continued) Wide Area Networks (WANs) Wired connection Computers located at great distances Examples: across state or country Dedicated point-to-point lines Computers connect to other computers on individual lines Store-and-forward, packet-switched Packets go from node to node until reach destination Invitation to Computer Science, 6th Edition19

Basic Networking Concepts (continued) Routing of packets determined dynamically A-B-C-D or A-B-F-D or A-E-F-D or A-E-F-B-C-D Redundant paths, fault tolerance, responsive to traffic load Invitation to Computer Science, 6th Edition20

Basic Networking Concepts (continued) Internet structure Combination of LANs and WANs Connected by routers that direct message traffic Internet service provider (ISP) provides access to the Internet for private individuals and organizations ISPs exist at multiple levels: local, regional, national, international (tier-1 network or Internet backbone) Invitation to Computer Science, 6th Edition22

Basic Networking Concepts (continued) Internet growth has been astonishing: Invitation to Computer Science, 6th Edition26

Communication Protocols Protocol: a standard set of rules for communicating Standards evolve over time International agreements make Internet possible Internet Society makes standards and promotes research: www.isoc.orgwww.isoc.org Protocol hierarchy/protocol stack, TCP/IP layers of protocols physical transmission to end application rules and standards Invitation to Computer Science, 6th Edition27

Communication Protocols Physical Layer (continued) Physical layer protocols Rules for exchange of binary data across physical channel (fiber-optic, twisted-pair, wireless, etc.) How to know when a bit is present on the line How much time the bit will remain on the line Whether the bit is digital or analog in form What physical quantities represent 0 and 1 Shape of the connector between computer and transmission line Create abstract bit pipe for higher layers to use Invitation to Computer Science, 6th Edition29

Communication Protocols Data Link Layer (continued) Data Link protocols Ensure reliable transmission of bits Error detection and correction: notice failures in transmission and fix them Framing: determine which bits belong to one message Two parts: Layer 2a: Medium Access Control Layer 2b: Logical Link Control Invitation to Computer Science, 6th Edition31

Communication Protocols Data Link Layer (continued) Medium Access Control protocols Rules for communicating on shared lines Ethernet: Contention-based protocol When node wants to send a message Listen to the line and wait until it is free Begin transmitting as soon as it is free If collision results, wait a random amount of time Repeat Advantage: distributed, no master bottleneck Invitation to Computer Science, 6th Edition32

Communication Protocols Data Link Layer (continued) Logical Link Control protocols Rules for detecting and correcting errors ARQ algorithm (Automatic Repeat Request) Invitation to Computer Science, 6th Edition34 Sender: Transmit packet and wait for ACK or time out If receive ACK, go on to next packet Otherwise, repeat on current packet Sender: Transmit packet and wait for ACK or time out If receive ACK, go on to next packet Otherwise, repeat on current packet Receiver: If no error, return acknowledgement message (ACK) Otherwise, return nothing Receiver: If no error, return acknowledgement message (ACK) Otherwise, return nothing

Communication Protocols Data Link Layer (continued) Packet contains: Markers for start and end of packet (SOP and EOP) Sequence number for packet (e.g., 2 of 5) Packet data Error-checking bits Invitation to Computer Science, 6th Edition35

Communication Protocols Data Link Layer (continued) Purpose of Data Link layer Create virtual error-free message pipe Messages go in one end Come out the other correct and in the right order Invitation to Computer Science, 6th Edition36

Communication Protocols Network Layer (continued) Network layer protocols Transmit message across multiple nodes in a network Good faith transmission Requirements: Standard for addressing all network nodes Routing method for finding route from any node to any other node Internet network layer: IP (Internet Protocol) Invitation to Computer Science, 6th Edition37

Communication Protocols Network Layer (continued) Addressing Host name: human-friendly name for node IP address: unique numerical address used by computer, 141.140.1.5 Domain Name Service (DNS): map host names to IP addresses Symbolic host name goes to local DNS server If it has no record, goes to remote servers until one has the host name and retrieves the IP address Invitation to Computer Science, 6th Edition38

Communication Protocols Network Layer (continued) Routing Picking a path through network from source to destination Seek shortest/best path: fastest travel Massive network requires efficient path-seeking Networks are dynamic: nodes come online and go offline all the time. Routing must adapt quickly Invitation to Computer Science, 6th Edition39

Communication Protocols Transport Layer (continued) Transport layer protocols Application-to-application, reliable packet delivery Port number: unique identifier for program Invitation to Computer Science, 6th Edition40

Communication Protocols Transport Layer (continued) Application types have standard port numbers Web server: port 80 Domain Name Service: port 42 SMTP, sending e-mail: port 25 TCP (Transport Control Protocol) Ensures no errors Establishes ordered delivery of packets Another version of ARQ algorithm Virtual direct, quality connection between programs Invitation to Computer Science, 6th Edition41

Communication Protocols Application Layer (continued) Application layer protocols Handle formatted data transmitted between application programs Invitation to Computer Science, 6th Edition43

Communication Protocols Application Layer (continued) Hypertext Transfer Protocol (HTTP) Web page/service identified by unique URL (Uniform Resource Locator) protocol://host name/page Multiple protocols: http, mailto, news, ftp Web browser uses TCP to send formatted messages to Web server, and vice versa TCP uses network layer (IP), data link layer, and physical layer Invitation to Computer Science, 6th Edition44

Communication Protocols Application Layer (continued) Process: http://hostname/page Browser reads protocol, extracts host name (and requests IP address from DNS server) Sends a connect message to port 80 on that machine After connection established, sends Get message with page information Server responds with message containing page contents, size, and indicates connection closes at end of message Invitation to Computer Science, 6th Edition45

Network Services and Benefits Interpersonal Communications Electronic mail (e-mail) Send message to be read at recipients convenience Fast, multimedia, broadcast medium Bulletin board system (BBS) Public forum for shared communications Evolved into Internet forums, chat rooms Instant messaging and texting Social networking Online social groups designed for rich interaction Invitation to Computer Science, 6th Edition47

Network Services and Benefits (continued) Resource sharing Print server serves all computers on a LAN File server provides storage to all users Client/server computing Some nodes provide services, other nodes use those services Distributed databases and data warehouses Massive data stored in various sites online Groupware or wiki Support collaborative knowledge/data construction Invitation to Computer Science, 6th Edition48

Network Services and Benefits (continued) Electronic commerce (e-commerce) Early applications Automatic paycheck deposit ATMs Checkout scanners and inventory systems Current applications Online stores for everything Electronic bill payment Online payment systems (Paypal) Future? Invitation to Computer Science, 6th Edition50

A Brief History of the Internet and the World Wide Web A Brief History of the Internet, 1997, by some of the founders Early years Lickliders Galactic Network, 1962 ARPA-funded ARPANET, 1966 E-mail, 1972 Many networks (e.g., HEPNet, DECNet) 1970s/80s Internetworking: standards for communication Gateway: device for translating between networks Invitation to Computer Science, 6th Edition51

A Brief History of the Internet and the World Wide Web (continued) Middle years TCP/IP established standard Telnet, FTP (File Transfer Protocol) NSFNet, broadens access (1984) ARPANET only open to ARPA grant recipients NSFNet open to universities, government agencies, libraries, museum, schools Networks begin to connect, late 1980s ARPANET ceases to be as separate network NSFNet turns over to private providers, 1995 Invitation to Computer Science, 6th Edition53

A Brief History of the Internet and the World Wide Web (continued) World Wide Web High-energy physicist at CERN: Tim Berners-Lee Wanted user-friendly information and data exchange Hypertext: documents containing links to other documents Web protocols made public; rapid expansion Invitation to Computer Science, 6th Edition55

Summary Computing devices can communicate through various wired and wireless media Computer networks vary in size and form, including LANs, WANs, WLANs, and WWANs LANs are configured differently from WANs, and use different communication methods The Internet is a WAN of WANs Protocols are necessary to standardize communications across different media and among different computers Invitation to Computer Science, 6th Edition57

Summary (continued) The protocol hierarchy breaks down network communications into different layers of abstraction physical, data link, network, transport, and application Protocols like the ARQ algorithm and TCP/IP provide rules for the transfer of information The Internet has permitted new kinds of connections among people: e-mail, e-commerce, resource sharing The Internet and Web grew from ARPAnet and NSFNet as new network applications developed Invitation to Computer Science, 6th Edition58

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INVITATION TO Computer Science 1 1 Chapter 7 Computer Networks, the Internet, and the World Wide Web