Chapters 2 & 3: Human Interface and Networking
Chapters 2 & 3: Human Interface and Networking 01:198:170
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Date Created: 02/21/16
Chapter 2: Exploring the Human-Computer Interface A FEW USEFUL CONCEPTS Feedback Computer = assistant o Always give the user feedback Feedback: any indication that either the computer is still working or has completed the request o User Interface (UI) o Editing changes – highlighting, shading o Time-consumption operation – rainbow spinner, hourglass, radial bar spinner Progress bar – a meter that is shown when time can be predicted o When an operation is processing a series of inputs, the “completion count” gives the tally of the completed instances, or equivalently, the number remaining o Always expect feedback and notice it Consistent Interface Example: MS Word and MS PowerPoint have similar icons and menus Fundamental Similarities File and Edit menu File: New, Open, Close, Save, Save As, Page Setup, Print, Exit, and others Edit: Cut, Copy, Paste, Undo, Redo, Select All, Find, Replace, and others Reasons o Same coding o Helps you learn the other applications o Main reason: certain operations are fundamental to processing information Clicking and Blazing When we install a new app: o “Clicking around” justified by the consistent interface o “Blazing away” = “going boldly where you’ve never gone before” justified because running software can’t break the computer New Instance New command under the File menu – it creates a “blank” instance of the kind of information or files the application creates All information is grouped into types Any specific piece of information-an image, a month, or a document-is an instance of its type PERFECT REPRODUCTION Digital information An Exact Duplicate By representing information as a sequence of 0’s and 1’s, we can make another copy simply by duplication the sequence, Furthermore, because it is digital, we can directly check to make sure the two sequences are identical, verifying that no mistakes were made; computer systems make such checks continually Perfect reproduction Is NOT a property of analog information Analog information comes from or is stored on a continuously variable medium Perfect reproduction Copying Copy/Paste/Edit Advantages o Generally faster o All instances have the exact same form o No tiny mistakes C/P reproduces the content and other characteristics of the source value, minimizing formatting mistakes Find and Replace All In F/R editing operations, we give the source content to Find in the document, and the target content to Replace it with. Find/Replace All is an “applied in all cases” version of Copy/Paste Save Typing with a Placeholder While writing your paper, adopt a placeholder for someone’s name. It doesn’t matter what the placeholder is. Any short sequence that is easy to type works as long as you can remember it and it is not used anywhere else in your paper. Whenever you refer to the name, just use the placeholder Using a placeholder is faster because you don’t spend time searching for an earlier instance where you typed Placeholder Technique The clever placeholder technique is used to “hide” part of the text that shouldn’t be tampered with when using F/RA Three-step process o Hide correct items o Edit as needed o Restore correct items WHAT WE SEE AND WHAT WE THINK Metaphors Metaphor: an icon or image or a concept used as a representative of or symbolic of a computation When designers create a technology, they use metaphors to help users know how to operate their devices without reading a manual The Desktop Alto (the first personal computer) was created a the Xerox Palo Alto Research Center o The designers introduced a graphical user interface (GUI) to replace the prevailing command line interface (CLI), which only a techie could love. The CLI’s a metaphor-as the name suggests-is a rather militaristic one of a sequence of commands o The PARC designers were building a personal computer for an office worker, so they adopted the desktop metaphor: What the users see on the screen is a virtual desktop o Overlapping windows = papers on a desktop o Mouse invented by Douglas Engelbart in 1967 – let users reposition their focus on the screen by pointing Macintosh was introduced in 1984 – the first successful personal computer with a graphical user interface o Showcased the mouse for painting and drawing applications A year and a half later, Microsoft introduced their Windows operating system, adding even more icons and more desktop-motivated features The Touch Metaphor Computers went mobile which caused a problem for the mouse o Stylus is one soltion o Changed to Touch metaphor Swiping, Tap, Double Tap, Drag, Pinch/Pull Fingers, Two- Finger Scroll, Flick Relationship Between Metaphors The Touch Metaphor With the touch metaphor, there is no change to the display Navigation motion: In the touch metaphor you sweep your finger left to go later in the list; using the scroll bars of the desktop metaphor you pull the slider right to and go later in the list. The direction of motion is the opposite between the touch and the desktop metaphors The navigation action is different in the two metaphors because how you think about what you see-how the metaphor represents the screen content-is different o Touch Metaphor: “have your hands on the content.” You push it around to locate your point of interest o Desktop Metaphor: “slide a window over the content.” You’re not touching it. o They are different actions because they support different metaphors Summary of Metaphors Created to simplify our use of the devices Desktop metaphor = classic Touch metaphor = serving users in places where file cabinets and wastebaskets have no place They will co-exist, use them daily, being largely unaware of them, and how extensively they determine our thinking and behavior Chapter 3: The Basics of Networking COMPARING COMMUNICATION TYPES General Communication Synchronous communication: requires that both the sender and the receiver are active at the same time o Example: a telephone conversation, FaceTime Asynchronous communication: the sending and receiving occur at different times o Examples: postcards, text messages, email Broadcast communication: involves a single sender and many receivers o Examples: radio, television Multicast: a type of transmission of information from one sender to many receivers o Example: magazine Point-to-point communication: a type of transmission of information from one sender to one receiver o Examples: telephone, text-messages The Internet’s Communication Properties Fundamental feature of the Internet: it provides a general communication “fabric” linking all computers connected to it o Computers and the network become a single medium that can be applied in many ways to produce alternatives to established forms of communication Internet = a universal communications medium o More effective with each additional computer The Client/Server Structure Client/server interaction: A Brief Encounter Web Server: “client” any situation where one computer, the client, gets services from another computer, the server o Only a single service request and response are involved o Brief relationship, Short Many Brief Relationships Advantage: the server can handle many clients at a time o Very efficient because the server is busy with you only for as long as it takes to perform your single request Getting More Connected Software has built on the Internet to implement the many forms of communication we use Appearing to Stay Connected Problem for sites that want to “stay connected” with users Two Solutions Cookies: small files stored on the client computer by the server, and returned to the server with each page request. The file contains enough data from the server, such as a unique identifier, that it can connect you to earlier interactions URL Parameters: information added by a client to a URL when it connects to the server Both techniques allow your client to send enough data to the server for it to figure out which of its recent interactions was yours, and to give you the illusion of having stayed connected Client Side/Server Side Who does the work? Developers have many reasons to prefer computing on one side or the other. Reducing traffic between the two computers might be a reason to shift work to the client, for example THE MEDIUM OF THE MESSAGE The Name Game of the Computer Addresses IP Addresses Internet Protocol address: a sequence of four numbers separated by dots o Each computer connected to the internet is give a unique IP address IP Packet A computer communicates with another computer by sending an IP Packet to its IP address. Payload: Routing and Switching When an IP packet arrives at a switch, the switch reads the destination IP address, decides which of the routers that it’s connected to will take the packet closer to its destination, and forwards the packet on. Hop: The transition from one router to the next Many Paths All routers and switches are connected to several others. They can send packets to any of their neighbor routers If a router’s neighbor is not responding, the router simply uses some other neighbor IP packets headed to the same place can take different routes to their destination Trace Route Because 2 packets can take different routes to 1 destination, networking engineers record the routs packets take Trace Route: Following Protocol Transmission Control Protocol/Internet Protocol (TCP/IP) TCP/IP Postcard Analogy Sending any amount of information, including a whole novel, is possible by breaking it into a sequence of small fixed-size units An IP packet (like the postcard) has space for one unit of information, a destination and return IP address, and a sequence number. IP Packets are filled in order and assigned sequence numbers. The packets are sent over the Internet one at a time, independently, using whatever route is available. At the destination, they are reordered by sequence number to assemble the information Packets Are Independent Because each packet can take a different route, congestion and service interruptions do not delay transmissions Engineers made TCP/IP packets independent TCP/IP Protocol = robust o It continues to work under adverse conditions It’s okay to kill packets for congestion relief or other reasons, because when they don’t show up at the destination quickly, the recipient server will request a resend. Also, because packets take different routes, they can arrive out of order. o Discovered during packet assembly the system to recover from unusual circumstances Far and Near: WAN and LAN Wide area networks (WANs): Local area network (LAN): o Main technology for LANs: Ethernet Appropriate for connecting all the computers in a lab or building Ethernet, the Setup Channel: Engineers “tap” the channel to connect a computer allowing it to send a signal. All computers connected to the channel can detect the signal, including the sender. o Channel supports broadcast communication Ethernet, Party Analogy Friends and stories Ethernet, the Protocol Ethernet communication works like the party protocol When a computer is sending signals on the channel, as when someone is telling a story, all of the computers listen to it A pause indicates the end of the transmission when no computer is sending signals and the channel is quiet Ethernet scheme = completely decentralized and requires no schedule or plan o Each computer listens to the channel and if it’s quiet, it’s free o The computer transmits unless some other computers starts at the same moment. When that happens, both computers back off for a brief (random) amount of time and then try again Connecting Your Computer to the Internet Connection via an Internet service provider (ISP) Connection provided by a campus or enterprise network Connections by ISP Sell connections to the Internet o Examples: phone/cable companies How it works o ISP Company gives you a modem for your house o Modems convert the bits a computer outputs into a form that is compatible with the carrier o These signals are sent to the carrier’s business where they are converted into a form suitable for the server that connects to the Internet via the Internet gateway o The digital subscriber line and cable are two very common service providers Enterprise Network Connections (LAN) Connect as a user of a larger networked organization as a school, business, or governmental unit The organization’s system administrators connect the computers to form a LAN or interconnected LANs using Ethernet Intranet: With either method, ISP or LAN, you usually send and receiver information across the Internet transparently-that is, without knowledge or caring which method is used Wireless Networks Wireless networking: Router: Domains and the DNS Domain names Hierarchy is Handy Domain Name System (DNS): the hierarchical structure we use to name computers Benefit: easy to remember computer names Peers: Domains at the same level in the hierarchy are peers of each other Top-Level Domains Domain Name Systems was first set up in 1985, first recognized 7 top- level domain names o Top-level domains: com, edu, gov, int, mil, net, and org Added a set of mnemonic two-letter country domains o Examples: ca (Canada), uk (United Kingdom), de (Germany), es (Spain) A Problem People use hierarchical domain names. Computers use IP addresses How does the DNS server get the list? o There is no master list of all domains on the Internet. None o Your computer asks its DNS for the IP address, but it’s not in the list - Yikes Authoritative Name Server Authoritative name server: Every domain on the Internet has an ANS Start at the Top This is a solution that will always work: move along the domain name in the URL (right to left) asking the authoritative name server at each level what the IP address is for the ANS in the next level down. When you get to the last ANS, presto, it knows the answer. It works, but we need to know the ANS machines for the top-level domains. Root name servers Root Name Servers These 13 computers scattered around the world are the authoritative name servers for all of the top-level domains (TLDs) Where does your DNS server get the IP addresses of the 13 root name servers? o The IP addresses are preloaded when the machine boots How does your DNS choose which one to ask? o They are all the same so it doesn’t matter; ask the closest one Where do the root name servres get the list of IP addresses they keep of the TLDs? o They are just loaded when the machine boots, too. If a new TLD is added, they are all reloaded Caching Lots of shortcuts Caching: o Helps DNS servers a lot Redundancy There are 13 root name servers to help share the load, but also to make sure that some machines will always be running Redundancy: DNS Summary Internet Assigned Names Authority (IANA) takes care of the root name servers To add a new computer to the Internet: the administrator of the domain puts its name and address on “the list” of the authoritative name server for that domain THE WORLD WIDE WEB Web Servers World Wide Web (WWW) Requesting a Web Page URL (Universal Resource Locator) o Protocol o Server computer’s domain name o Page’s pathname Web browsers and Web servers both “speak” HTTP The Internet and the Web To refer to a Web server, you must give its name exactly, because your computer will ask the DNS server for the Web server’s IP address using that name Web administrators try to save users from mistakes Describing a Web Page Pages are stored as a description of how they should appear on the screen. When the Web browser receives the description (source) file, it then creates the Web page image that it displays. Two advantages to storing & sending the source rather than the image itself o A description file usually requires less information o The browser can adapt the source image to your computer more easily than a literal pixel-by-pixel description Example: the description is easier to shrink or expand in response to changes in the size of the browser window, than is the image itself Web servers don’t have to be named www o It’s just what people usually do, it is just a tradition FILE STRUCTURE Folder (aka directory): a named collection of files or other folders or both Directory Hierarchy File structure Directory hierarchy Hierarchy o Down or lower in the hierarchy means into subfolders; that is, toward the leaves o Up or higher in the hierarchy means into enclosing folders; that is, toward the root Learning the “directionality” of hierarchical references makes navigating the Web simpler The path in a URL tells the server computer how to navigate to the requested file in the server’s directory hierarchy For huge Web-services sites like Twitter and Tumblr, the servers use different techniques to provide the content you want, but for most sites, the path portion of a URL navigates the server’s directory hierarchy to your desired page Organizing the Folder Why build a hierarchy at all? Why not dump all the files into one huge folder and save typing? o Most people build hierarchies to organize their own thinking and work. Because directories cost nothing, there is no reason not to use them, and it is highly recommended
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