CS101 Exam 1
gate
a simple circuit that controls the flow of electricity -manipulates the Flow of electricity
with one big exception, every device connected to the internet has a
unique IP address there are 2³² or 4 billion possible IP addresses
what does a 0 at the end of an IP address indicate
we are talking about not a single IP address, but rather a subnetwork
adding binary numbers
we have to make sure to engage in carrying! carry the one over to the next column (1+1=0, carry one)
index files
web servers treat any file named 'index.html' as a default file to return if no other file is specified since it is the default, we dont have to write out the entire URL
early "calculators"
were women! Katherine Johnson worked at NACA (and then NASA as a mathematician doing complex calculations by hand. It was a common job in math- heavy industries like aerospace, insurance, and the government
BCD
Binary Coded Decimal binary, but only allowing values 0-9 **advantage: is that we can encode two decimal digits in one byte...one in each nibble** 45= 4 5 = 0100 0101 more common in electronics than in computing (like displays, thermometers) more "natural" feeling numbering system
nibble
4 bits
Because of the way computer memory is built (among other things), we organize bits into groups of...
4, 8, 16, 32, 64, etc
101101 in decimal
45
HTL bold
<B>...</B>
HTML italics
<I>...</I>
HTML paragraph
<P>...</P>
hyperlinks
<a>...</a> they have two parts 1. URL to the linked page 2. the text ti display as part of the link (<a href="http://bu.edu">BU homepage</a>)
Header in HTML
<h1>Header</h1>
image tags
<img> it has attributes which are important, mainly is its location of the image file <img src= "http://...... .jpg">
bit
A binary digit, a 0 or a 1.
integrated circuit
A group of tiny transistors and electric wires built on a silicon wafer, or chip. A thin slice of silicon that contains many solid-state components.
physical characteristics of a LAN wire
-8 thin wires, twisted into oairs, packages as 4 pairs into a single cable -because of the physical characteristics of wire, theres an upper limit to the speed and distance we can send the 1s and 0s -for common LAN cabling, each pair is limited to around 250 Mbits/s, but you can combine pairs to achieve more bits
operating system
-An operating system is a collection of programs that manage peripheral devices and other resources -in the 60's, operating systems enabled time-sharing, where users share a computer by swapping jobs in and out
Converting hexadecimal to decimal
-Each position in the hex word is a power of 16 rather than a power of 2, so multiply each digit by that power of 16 3E6= 3*16² +14*16¹+ 6*16⁰= (3*256) + (14*16) + (6*1)= 768+224+6= 998
coding scheme history
-Representing characters by using codes has been around much longer than computers have been -Morse code, Baudot code (and the ITA1/2 derivatives), light signals for ships, all are early predecessors -Some coding schemes (smoke signals, flags) can encode words or phrases, making communication significantly faster
protocol
-a set of rules that describe ow devices can talk to each other -its like agreeing on a common language -protocols describe the format of the messages that are sent between devices -the messages are sent as groups of bytes; just 1s and 0s!
LAN protocols
-at the local level, there are protocols responsible for allowing devices to share a limited resource, the network connection -there might be two or two hundred protocols looking to share the network two general approaches 1. token ring(orderly) 2. ethernet (free-for-all)
token ring
-devices are attached to the LAN in a ring -a special message, called a token, is passed from one device to the next around the ring -a device can only transmit information when it has the token -the token has a timer, so that after a certain amount of time it must be passed on the token ring is very efficient, since only one device can transmit at a time -they are very rare, you'll most often find ethernet even though this is the superior system
differential Manchester encoding
-differential means that we look for transitions between one level to another ether than an absolute high or low value -Manchester indicates that we send along the extra transition in the middle of each bit -the transition in the middle of each bit provides clocking -a transition at the START OF A PHASE is a 0 -NO TRANSITIONAT THE START OF A PHASE is a 1
ethernet
-dveloped by Xerox (specifically Bob Metcalfe) in the 1970s. The protocol calls for messages of u to 1500 bytes and describes how decides decide when to send their message and when to listen -ethernet addresses are 48 bits (6 bytes) long -each message includes the recipients address, the senders address, and some additional info about the message -even though this is chaotic, it is what most LANs use
generation 4: VLSI
-late 1970's- Very Large Scale Integration -by the late 1970's, manufacturing advances allowed placing hundreds of thousands of transistors w/ circuitry on a chip -this "very large scale integration" resulted in mass-produced microprocessors and other useful IC's -since computers could be constructed by simply connecting powerful IC's and peripheral devices, they were easier to make and more affordable -with VLSI came the rise of personal computing (1975 Microsoft, 1977 Apple, 1980 IBM PC, 1984 Apple Macintosh, 1985 Windows) -1980s- object oriented programming began
Generation 1: Vacuum Tubes
-mid 1940s: vacuum tubes replaces relays. a vacuum tube is a light bulb containing a partial vacuum to speed electron flow. they could control the flow of electricity faster than relays since they had no moving parts. -1940's: hybrid computers using vacuum tubes and relays were built -COLOSSUS 1943: first "electronic computer", built by the British govt. (based on designs by Alan Turing) used to decode Nazi information during the second world war -ENIAC 1946: first publicly-acknowledged "electronic computer", built by Eckert & Mauchly (UPenn), used vacuum tubes and relays -COLOSSUS and ENIAC were not general purpose computers, they could only do one kind of computation -Von Neumann Arcitecture
networking layers
-modern networks are arranged in layers, with each layer responsible for a small portion of the overall network communication -from lowest to highest, the layers are physical, data link, network, transport, session, presentation, and application
hexadecimal
-much more common than octal or BCD, or even binary at this point -there are 16 symbols rather than 2 (for binary), 8 (for octal) or 10 (for decimal) 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F -in hex, each digit represents 4 bits (a nibble) -hex values are often denoted as 0x4A (or any hex value)
bar codes
-numbers are encoded by groups of four bars -the longer abs are used for scanner alignment -limited data can be encoded
other ways to work with data and numbers aside from binary
-octal -BCD -hexadecimal
analog to digital conversion (A/DC)
-the basic idea is that we take a sample of an analog signal at regular intervals -the sample is usually an electrical value, measured in voltage -once the sample is made, the recorded value is converted to a binary number for storage
Local Area Network (LAN)
-these are generally the derives around you -they are local devices that are connected directly together and can talk to each other
octal
-uses just 8 symbols, 0-7 -began use because early computers had word sizes of 12, 24, or 36 **octal can be derived from binary by grouping the bits into groups of 3 ** -UNIX and Linux and OS/X use octal, sometimes it is used to code special characters in programing
Decimal 0 in binary
0000
Decimal 1 in binary
0001
two kinds of compression
1. lossless 2. lossy
Decimal 9 in binary
1001
word
16 bits, 2 bytes
Generation 0: Mechanical Computers
1642: Pascal built a mechanical calculating machine which used mechanical gears, a hand crank, dials and knobs. other similar machines followed. 1805: Jacquard's loom, the first programmable device (it used punch cards, which represented the pattern and were fed into the loom. This allowed for the mass production of tapestries mid 1800's: Babbages "analytical engine", its design expands on mechanical calculators and was programmable via punch cards like Jacquard's loom. His design was beyond the technology of his day, but he described the general layout of modern computers 1930's: computers built with relays. MARK I followed the basic design of Baggage, limited capabilities by modern standards: could store only 72 numbers, required 1/10 sec to add, 6 sec to multiply but was still much faster than previous technology
why do we have binary
Because switches only have two states, on and off, we use a numbering system that has only two values, 0 and 1 (zero and one) every decimal can be represented in binary form -All the numbers, text, graphics, and so on in your computer are stored and manipulated in binary, then converted for display to something we humans can read
storing colors
We need some way to map colors onto numeric equivalents which we can...you guessed it... convert to binary and store on the computer common color storing scheme is RGB
digital signal
a digital signal "jumps" between levels, it is not continuous the computer's world is digital
the basic components of digital electronics, including computers, is
a simple switch just like a light switch, an electronic switch has two states: on and off switches are combined into circuits to do useful things
lossy compression
accepts some loss of quality in return for a smaller file removes the "unnecessary" parts of the original file to reduce its size
analog signal
an analog signal is one that varies continuously (like a sine graph) the world around us is analog
Base 10 (decimal)
everything is understood in terms of powers of 10 10 symbols, 0-9 10⁰=1 10¹=10 10²=100 10³=1000 and so on
Generations of computing technology
generation 0 (1642-1945, mechanical devices [gears, relays]) generation 1 (1945-1954, vacuum tubes) generation 2 (1954-1963, transistors) generation 3 (1963-1973, integrated circuits) generation 4 (1973-1985, very large scale integration) generation 5 (1985-?, parallel processing and networking)
long word
greater than 32 bits
ethernet frame
in a Lan, the messages are passing back and forth are called frames te frames are just addresses, data, and a bit of extra info preamble→to address→from address→data (aka body)→error check
we have many numbering schemes, but
in the end everything is stored and manipulated by the computer as just ones and zeros.
use of protocols
organizing bits from sender to make it into a meaningful message for the receiver
DSL
relatively old technology, but still prevalent in some parts of the US -it uses the Plain Old Telephone System to transmit data alongside voice information -fibers are used to separate voice from data -it is slower because it is limited to old telephone wires -the device used to attach a DSL line is a modulator/demodulator (modem) -the system uses audio to transmit data -low tone for 1, high tone for 0
impedance
similar to resistance, but varies according to the frequency of the signal, thus limiting the overall speed
Going from decimal to binary
subtract each power of 2 156 to binary--> 2⁷=128 156-128= 28 1xxxxxxx 2⁴=16 28-16=12 1001xxxx 2³=8 12-8=4 10011100
resistance
the electricity flowing through the wire meets resistance in the copper, and the signal is diminished. this limits DISTANCE, since the longer the wire, the more diminished the signal
what IS the ethernet?
the low level, point-to-point protocol for laptops, printers, desktops, video decides, and just about anything else you can think of -it is a WIRED MEDIUM
in computing we do everything in powers of _______ because of switches
two
what uses ethernet
almost all network devices!
the vast majority of programs you use every day are
FOSS web servers, apps, operating systems, data bases
Decimal 8 in binary
1000
byte
8 bits
FOSS
Free/ Open source software
which protocol does the internet use
packet switching
Storing Characters
when we type a document, we may feel as though we are storing letters of the alphabet, but the computer can only store 0s and 1s. because of this, we need some way to map a binary number to a letter so the computer can store and work with it. this is the job of a coding scheme
Because HTML is a markup language that defines the structure of text, it's quite usable for
writing books eBooks are a good example, they are mini websites in themselves, they are packaged in a ZIP file
lossless compression
you can exactly recreate the original version from the compressed file
why is the separation of structure and content so useful?
you have the ability to separate documents based on headers, body, etc and change just those portions -you can bold just certain parts of the text, make a table of contents, etc
the NOT gate
"the inverter", the simplest logic gate output is the inverse of the input. The Boolean expression for the NOT gate is Y = A' A Y 0 1 1 0
TRACEROUTE
(orTRACERTonWindows)shows the name or IP address of each router between you and the destination, and the ping time between them and your machine:
Packet Switching
- a method of transmitting data in which the data packet is transmitted as one entity irrespective of the whole message - ex: routers read the destination address in a packet header and decide the best route to reach the destination quickly -a circuit is established for only the amount of time that it takes to send a single message -Once the message is sent, the circuit is dropped -Long messages are broken up into many smaller messages
impact of the microchip
- lower cost due to mass productionn - faster operation speed due to the close proximity of circuits on chips -simpler design constriction of computers using prepackaged components
here are three IP subnets that are special in that they DO NOT pass through to the Internet -they are just for local networks
-192.168.0.0 (64k addresses) -172.16.0.0 to 172.31.0.0 (1M addresses) -10.0.0.0 (16M addresses) These private networks allow us to build large IP networks that are not directly connected to the Internet
wireless LANs
-All those wireless access points you see at Best Buy use a protocol with the mellifluous name '802.11' -802.11 operates in a similar fashion to Ethernet; each wireless device has a unique address, and messages are sent point-to-point from one device to another -The access point operates as a kind of switch, receiving messages and resending them
thick client
-As desktop computers started becoming more powerful and less expensive in the 80s and 90s, the trend was to move work from the mainframe to the client -Since the client is doing most of the work, we call it "thick" (no, I don't know why) -The client requests data from the backend, works on it,and sends the results back for storage -This trend started prior to the introduction of what we know as the modern PC, and the PC accelerated it
router hierarchy
-At the local level, a router might know about a very small number of subnets -At the next level up, the router will know how to deliver to subnets part from those few subnets -At the very top level of the Internet, each route includes millions of addresses -At the top of the internet router tree, high-speed special optical fibers are used to connect the routers together
the universal gate
-Boston University Slideshow Title Goes Here The NAND gate, which we've see earlier is known as a 'universal' gate because NAND can be combined to create a circuit that mimics every other logical operation -Because it is easier to manufacture gates that are uniform, most microcircuitry is designed almost exclusively as a collection of NAND gates
the DEFAULT route
-Each router has an extra port that is used for a "default" route -The default route is used when the router doesn't know how to deliver a message in the hopes that the next router might know where to send it
web architecture
-Early web sites were built as thin client...alloftheworkwasdoneat the web server, and just the things that were to be displayed were sent back to the browser -this evolved into a more balances model as web-oriented programming language like java script were developed -today, much of the work is done in the browser, with the server passing code to the browser, with the server passing code to the browser to execute locally
bluetooth
-It really was named after a Viking (Harald Bluetooth, a 10th-century Danish king) -Bluetooth was intended to replace physical wires to connect things like a mouse to a computer -These days it's used to connect all sorts of very local devices -Just like Ethernet, Bluetooth is a protocol, or set of rules, on how to send messages between two devices -It has its own set of addresses which are 48 bits (6 bytes) and are unique -Usually Bluetooth is used just as a wire replacement and isn't intended for forming complete networks -it is slower than ethernet
is thin or thick client better?
-Over the past few decades we've gone from thin to thick, back to thin, to thin again, and these days we're somewhere in between -what it really depends on is the application is trying to do, and building the correct system to solve the problem
whats behind the green screen?
-Usually there's some sort of mainframe computer doing the work-a mainframe is just a large, industrial-strength, fast, fabulously expensive computer -Applications fall into many categories, but you'll see them in use in the airline industry, banking, insurance, and so on -Many of these applications are relatively old-15 to 30 years-but still quite usable; many are written in COBOL (remember the Y2K bug?) -Companies with main frame systems have very large investments in their computers and want to hang on to them for as long as possible
problems with synchronized cloud
-What if a file is changed simultaneously in two places?Which is correct? -what happens if the file is deleted? is it deleted everywhere -if you aren't paying for a service, you are the product (there aint no such thing as a free lunch)
XML records tell us
-about something, or what we will use to describe the thing (like a template you fill in the blanks for)
other image atrributes
-align="right" (or left, center) - width or height="100px"
caching
-finding and sending messages for just one address is a lot of work that the computer doesn't want to have to do more than once, so once you find it once, your computer stores the address in a table so next time itll be faster
Richard Stallman
-harvard, MIT -in 1989, he published the first version of the GNU General Public License, which says that the recipient and user of the software is free to use, modify, and redistribute and otherwise own the software provided that they realize modifications back into the community -he sees program as an expression of free speech -free as in free puppy, not free beer -because of his understanding of free, software licensed under the GPL is open code and available for all to see, but may also be bought and distributed -copyleft
thin client
-historically most computer applications have been thin client -A simple display and keyboard, called a "dumb terminal" connect over a network to a more powerful computer and is used to type in data and make requests -a remote computer does all the work, the terminal just does the display -used in a bank, a green screen (Cheap terminals often use monochrome, green phosphor, and so these are called "green screens" )
state
-in request-response architectures, connections to the server are usually stateless -each click on a webpage is a unique request -the server doesn't know what you were just doing -techniques like cookies help address this problem
chips to microchips
-initially, circuits were built by wiring together individual transistors, but this did not lend itself to mass production. it also meant that even simple circuits consisting of tens or hundreds of transistors were quite large (to allow space for human hands).
LAN to WAN
-local network is primarily Ethernet, which ties together all of your local devices such as printers, laptops, media and so on -Anything outside your local area goes through a router to the IP network and the Internet -Long-haul routes, such as North America to Africa, are typically made on undersea fiber cables or satellite shots
HTML elements
-text is marked up in HTML coding to indicate what is a header, what is a list, and so on -HTML is display independent -the HTML language includes a large variety of structural tags that web developers use to lay out web pages. the tags are often used for marking up the text and for the overall layout of the page
whats behind thick client
-the client is doing most of the work, so really all we need on the other side is data, usually stored in a database or a set of files -computers on the back end tend to be highly optimized and data centric -The back end is still expensive...those Oracle licenses can be several hundred thousand dollars per year per CPU (and a machine might have dozens of CPUs
client-server summary
-the internet is primarily a client-server architecture -the client requests information, and the servers send it back -we sometimes also call this a request-response architecture -web servers are just applications that listen for requests and return a response
switches
-the most basic tool for controlling the flow of electricity is a switch -a switch can be flipped to connect or disconnect two wires, thus regulating the flow of electricity between them -example: a light switch on a wall serves as an intermediary between the power line entering your home and the outlet that operates a lighting fixture -if the switch is turned on, then the wires that link the outlet to the power line are connected, and the lighting fixture receives electricity
manufacturing ICs
-the production of integrated circuits is one of the most complex engineering processes in the world -Here since a silicon chip is fragile, the chip is encased in plastic for protection ■ metal pins are inserted on both sides of the packaging, facilitating easy connections to other microchips
what "free" means
-there is a bug debate around what "free" means -stewart brand→information wants to be free. it has value, but it also wants to be free
apache
-this project grew out of a commercial web server offered by Netscape in the late 1990s -a small group of enthusiasts periodically published fixes to the buggy wed service code, and at some point this created a new version of the server with all the big fixes applied -a big fix=a 'patch', once, apache (a-patchy-server) -the project grew rapidly and is today responsible for most of the software that runs the web -its free as in free beer -its very robust due to its open source nature, and is also very secure -its easy to maintain, and does not require windows
truth table
-used to describe to output of a logical circuit -It lists the inputs, the operation, and the result
Structure vs. Content
-when we write a document, it of course has content, but it also embodies some form elements
disadvantages of packet switching
1. Breaking up messages into small pieces means that they have to be reassembled at the receiving end 2. The small message pieces don't necessarily take the same path through the network, which means they might arrive out of order 3. It's possible that some message pieces might never make it to the destination
circuit switching
1. In a circuit-switched network, a connection (called a circuit) is made between two devices 2. The circuit is said to be "nailed up" in reference to old telegraph wires 3. The circuit remains up until the connection is no longer needed, even if no messages are being sent
transistors as switches
1. PMOS, positively doped, so that the switch is "closed" when there is no current on the control 2. NMOS- is negatively doped, so that the switch is "open" when there is no current, but "closes" when there is current
advantages of packet switching
1. The circuit is not occupied if no messages are sent, so other devices can use it 2. If part of the network is congested, different circuit can be established to go around that portion
Advantages of Circuit Switching
1. We can analyze the circuit for noise and adjust the speed of the data being sent 2. Messages arrive in the same order that they are sent 3. the circuit is always available
ways to connect devices after leaving the LAN
1. circuit switching (emulating the old telephone network) 2. packet switching
document structure
1. entire document starts and ends with <html>...</html>, which marks it as an HTML doc 2. the first part of the document contains METADATA, the TITLE of the page, keywords for use in search, authors name, and s on. this is called the HEAD and is surrounded by <head>...</head> tags 3. after the head comes the BODY, which is the main content of the page and is surrounded by <body>...</body> 4. after the body comes the FOOTER, it is typically placed at the bottom of the page and is surrounded by <footer>...</footer>
two ways to implement cloud
1. store files on the cloud and just move the items to the Clint as they are worked on or streamed. save changes bak to the cloud (how chrome books work) 2. store files on the cloud, but keep client machines in synch with all changes, files reside both on the cloud and on the client
two styles of computing
1. thick client 2. thin client all about where the work is being done
saving and viewing a web page
1. you can use just about any text editor to write HTML code 2. save the file with the .html extension 3. in a browser, click the File/ open...menu to open your file 4. the browsers read the file and displays it as a web page
advances in switching technology have defined the generations of computers
1930s- electromagnetic relays as switches whose on/off positions were controlled by the voltage to a magnet 1940s- vacuum tubes replaced relays, which were faster (since no moving parts) but tended to overheat and burn out frequently 1948-the transistor was developed by Bardeen, Brattain, and Shockley
DNS
Domain Name Service -automatically translates website names into IP addresses for us -DNS servers are set up as a hierarchical tree just like routers -at the top of the tree is a server for the root domain, called "." -all domain names end in a dot www.bu.edu. but we omit it because it is in everything - Every time you access a site, www.amazon.com your local computer looks in a local cache (a table in memory) to see if it knows the IP address -If it doesn't, your computer asks its local DNS server, which looks in its cache -If the local DNS server doesn't know the address, it will ask the "." server if it knows which other server might know the answer -The "." server is quite busy, so it will probably suggest asking the .com server - The .com server will point us to the "A" server (a for amazon)
HTML tables
HTML has a rich set of tags to create tables, which are logically organized into rows (prettier ones use CSS rules) table syntax looks like: <table> <tr> <th>column 1 head</th> <th>column 2 head</th> </tr> <tr> <td> Col 1 data </td> <td>col 2 data </td> </tr> </table> each table can take a width parameter, expressedin percentage of the page or div or row width, or an absolute size table 80% of the page is expressed as <table width =80%> data item 20% of row width: <td width=20%> another table attribute is align, which can be left, center, or right. a row 80% wide and aligned right : <row width="80%" align="right"> it helps to write HTML tables just as you want them to appear
What does HTML stand for?
Hypertext Markup Language
outside of the LAN, addresses are in the ________format
IP dotted decimal format, has a 32 bit binary equivalent
borrowing images
If you find an image that you like on the web, And you have permission to use it, you can just right-click on the image itself on the web site and copy the URL of the image. Then just use that URL in the <img> tag on your own page
popular FOSS tools
Libre office, GIMP, linux we often equate open source with free, but that isnt always the case. open source SIMPLY MEANS THAT THE CODE FOR THE SOFTWARE IS PUBLICLY AVAILABLE FOR INSPECTION AND MODIFICATION
Moores law
Moore's Law describes the remarkable evolution of manufacturing technology -Moore noted that the number of transistors that can fit on a microchip doubles every 12 to 18 months -this pattern has held true for the past 30 years -industry analysts predict that it will continue to hold for the near future
HTML is a _____ language
STRUCTURAL, not visual this means that a page might look different depending on which browser is being used
cookies
The most common way to fix statelessness in web transactions is to use a cookie -A cookie is a small file stored on your computer that contains information about you and the site you are on -Each site creates and maintains its own cookie -the cookie, might, for example, contain your user ID -The cookie is sent along with every click you make on the site -When the server gets the cookie, it opens it up and extracts information it needs, such as your user ID -The server then looks up things in its data base about you and what you are doing, for example the last page you visted, your ID, your name -these allow you to look like youre logged in
XOR gate
The output of this gate is logic 1 if an odd number of its inputs are logic 1; otherwise, the output of this gate is logic 0. Y=A⊕B or The Boolean expression for the XOR gate is Y = (A · B') + (A' · B) A B Y 0 0 0 0 1 1 1 0 1 1 1 0
carrying truth table
X Y Carry 0 0 0 0 1 0 1 0 0 1 1 1
a truth table for addition
X Y X+Y 0 0 0 0 1 1 1 0 1 1 1 0
electricity
a flow of electrons, the negatively charged particles in atoms, through a medium -good conductors of electricity like gold and Copper allow for the flow of electrons with little resistance -other elements, mainly non-metals, are poor conductors
synching cloud files
a small program on your computer that watches files both on your computer and on the cloud. when one changes, the new version is copied to any place that needs it. If you change it on your computer, the changed file is copied to the cloud server
transistors
a transistor is a solid piece of metal attached to a wire that serves as a switch by alternatively conducting or resisting electricity. transistors allowed for the development of smaller, faster machines at a lower cost the first transistors were made of germanium and gold, but modern transistors are constructed from silicon (a semiconductor)through a process known as doping, impurities are added to a slab of silicon, causing the metal to act as an electrical switch
NAND gate
an AND gate followed by a NOT gate (THE OPPOSITE OF AN AND, any except both) the universal logic gate The Boolean expression for the NAND gate is Y = (A · B)' A B Y 0 0 1 0 1 1 1 0 1 1 1 0
the <a> tag
an attribute, which is additional information about the tag href is an attribute, which means hypertext reference
client side programs
an example is javascript, it runs in the browser this is how interaction or live features can exist
semiconductors
are metals that can be manipulated to be either good or bad conductors of electricity
the goal of markup languages is to
be ablate control the "look", or structure, of an element without messing with the content
microchip
circuitry (transistors + connections) is layered onto a single wafer of silicon, known as a microchip since every component is integrated onto the same microchip, these circuits became known as integrated circuits
well known port numbers
common programs like web servers, email servers, and so on use standard port numbers -Since they are standard, we don't need to type them in on things like URLs...
routers
connects IP networks together we have lots of smaller routers in order to do this job each router is responsible for a small number of IP addresses -the addresses are divided into groups called sub-networks (or just sub-nets) -Each router is connected to one or more subnets, one on each of its ports -When a message comes in one port, the router looks at the subnet number of the TO address and decides which port it will use to deliver it -The router keeps track of its routes by using a little table of ports and subnets and does a lookup each time a message comes in -Your laptop or desktop connects (via either Ethernet or 802.11 wireless) to a local router
HTML tag format
each HTML tag has a beginning and an end -the / indicates the end of the tag -For every tag, there should be a matching end tag -tags can be nested, you can italicize things within a paragraph
ports
each application has a unique address -Each computer uses 64k unique port numbers,from 0 to 65535. each application is assigned one of these numbers The message. going to or coming from the network carries both the sender's and receiver's port number, just like the "to" and "from" addresses we've seen earlier -This lets us use one network connection to service multiple applications (called "multiplexing")
modern computers are powered by
electricity they use electrical signals to store and manipulate information -the components of a computer require electrical power to carry out their assigned task (lights for computer screen, hard drive motor, main memory and CPU use electrical signals to manipulate the data
low level routers are typically connected together by
ethernet
website names and IP addresses are just conveniences for humans
everything is actually just in binary
client-server
ex-->a local client, like your browser or your email or your Spotify program talking to a remote server -request-response architecture; the client makes a request and the server sends a response
to avoid interference with other devices on the unlicensed portion of the radio spectrum, wireless LANs in 802.11 use
frequency hopping
amperes
gauge electron flow
gratis v. libre
gratis→free with responsibility (Stallman and the free software movements view of how "free" should be understood" libre→totally free
404 error
if the file doesnt exist n the server in that directory, the server returns a file called 404.html, the server code which means the couldn't find the file
local v. remote images
images coming from web servers have an http at the start of their location if the image is stored on the same computer as the web server, you only need to include the directory and the file name"
OR gate
implements the Boolean OR function where the output is logic 1 when any input to the OR gate is logic 1. The output is true if any of the inputs are true The Boolean expression for the OR gate is Y = A + B A B Y 0 0 0 0 1 1 1 0 1 1 1 1
routers are arranged...
in a tree at each level the routers know how to deliver to larger and larger numbers of subnets
frequency hopping
invented by Heddy Lamar in the 1940s as a way for allied ships to communicate securely -The basics: Transmit on one channel for a short period of time (a few milliseconds), then switch to another channel, then another... -The transmitters and receivers use the same hopping pattern and stay in sync -The result is that it's statistically unlikely for two devices to be using the same channel at the same time, and even if you do, it is just for a little bit so you dont catch anything too important
display independent
it is up to the browser to read and interpret the markup and display the element -ex--browsers display headers in different way
the whole point of the web is to
link pages together
the web server...
listens fo a request in the form of a URL, uses the URL to locate a file in its directory structure, may run code inside the file to create a dynamic HTML via a database, and returns the HTML file to the browser/client
whois
lots of info about who owns a domain name or an IP address (like contact info for admin and registrant, important dates for the page, etc)
nearly everything you see on the web is
marked up in HTML
voltage
measures physical force behind the flow of electrons
internet operating systems
on the server side, the major operating systems are Linux/Unix and Windows apples OS/X has about 0.1% market share (not much)
the cloud
on the server side, there are racks and racks of physical hard disks. files are redundantly stored across several physical disks to prevent data los if one disk fails When you upload a file to the cloud, it is indexed, tagged for your account, and copied across those redundant disks
because HTML is not a visual language...
over the past several years, a large number of 'extra' tags and modifiers have been introduced that deal with the visual aspects of web page
a server side programming example
popular server side language--PHP -server side means that the code is run on the server and produces HTML to send back to the client -PHP lets you add dynamic info to the page -server side pages are often connected to a databade -the database holds info tat is queried, and the result is placed on the HTML page -CNN does this, the news stories are in a database and places in the HTML page by a program
wireless LANs use
radio
and then the client...
recipes a file from the web server, interprets the HTML code inside it, may run additional programs inside the file like Javascript, and displays HTML according to its own internal riles (how the text is displayed)
copyleft
requires that new software which contains portions of code licensed under GPL must also be GPLd its entirety you may sell, distribute, modify, and basically do whatever you want to with the software, however, you must provide the source code to all your product if any of it uses GPL licensed code -many companies make money by selling services around open source products
how are connections from device to device manages
routers
servers
servers run applications that listen to ports and respond to requests -A web server, for example, receives a request for a page, assembles the page, and sends it to the client -A file server receives a request for a file, retrieves it,and sends it to the client -a single machine might host several different features
Ethernet LANs
since ethernet uses wires, the physical size of the network is limited -there is also a practical limit to the number of ports a switch can have, though it is possible to connect switches together -we normally consider ethernet to be a protocol for LANs -it is relatively fast
HTML history
sir Tim Berners Lee was working at CERN and wanted a way o share his research documents with his coworkers in the early 1980s -in 1990 he wrote a memo to describe a simple system for sharing, and then he wrote a client and a server that used HTML -the idea of hypertext wasnt new, but Berners-Lee was the first to create a networked, sharable, and simple system -HTML is based n SGML, the Standard Generalized Markup Language designed for documents, and SGML is based on XML, the extensible Markup Language
re-duplication
store ONE copy, but also store a reference to that copy in the 10 million accounts that think they've stored their own personal copy of a song or something of that sort
SGML, XML, and HTML separate the _________ of a document from its _______
structure content
NOR gate
the combination of an OR gate followed by a NOT gate. the output is logic 0 wen any of the inputs is logic 1 ONLY IF NONE OF THEM ARE 1 The Boolean expression for the OR gate is Y = (A + B)' A B Y 0 0 1 0 1 0 1 0 0 1 1 0
SGML and HTML are markup languages that let the author identify
the form elements
AND gate
the output is logic 1 only when all inputs to the AND gate are logic 1. The Boolean expression for the AND gate is y=AB A B Y 0 0 0 0 1 0 1 0 0 1 1 1
github
the premier place for FOSS projects git→a tool to track changes to software, bugs, documentation, and to provide collaboration among teams 4,000 new projects/day
files on the server side
the server examples the URL (universal resource locator). and picks it apart to determine which file to process -files on a web server are stored in folders/directories just like on your computer
what does the server do
the web servers simply listens for a network request from a client (like a web browser) -when it gets a request, it returns an HTML page to the client -it might do some additional processing on the page before it is returned -if a page has several items on it, like graphics or ads, the client will request them one at. time from the server
802.11 Flavors
there are different flavors of wireless,802.11 a/b/n/g, etc most of the difference is in connection speed they are for the most part compatible with each other
client-server problems
there are many programs on the computer that use the network, but there's only one network connection coming out of the back of your computer, and that it has just one, unique address
popular web servers
there are several 'brands' of web server on the internet. -IIS from Microsoft is the biggest 'paid' solution -the number one web server on the internet is Apache
Header tags
there are several header tags available to create hierarchical structure in the text <h1> (the top level> <h2> . . . <h6> (the lowest level)
gates to integrated circuits
transistors are connected to form basic logic gates, which are then combined to build more advanced circuitry
circuits
transistors can be combined to form a circuit, which controls the flow of electricity in order to produce a particular behavior
the open source community
volunteers work together to make new products, they make and maintain programs when someone has an idea for a project, they'll announce it on software forums and ask volunteers to make it happen most developers work on FOSS just for the fun of it
QR codes
-Created for the Japanese auto industry - A2-dimensional grid encodes many more characters than a 1-dimensional barcode -this 33x33 grid can encode up to 50 characters
mp3 (file)
-MP3 comes from the Motion Picture Experts Group (MPEG) Audio Layer 3 -The original sounds are basically lost-we can't exactly reproduce them. for most applications, though, the quality is good enough -they can be encoded with high or low sample rates -MP3 flattens the highs and lows of the original (techno-speak:it reduces the dynamic range)
connecting ethernet devices
-Normally Ethernet devices don't plug directly into each other instead, we use a switch connect two or more ethernet devices together. the cables plug into a connecter called a port -when a switch receives a message, it looks a the recipients address and determines which port to deliver the message to
Von Neumann Architecture
-Stored-program computers -has three parts, memory, CPU, and I/O -memory stores both data and programs -Central Processing Unit (CPU) executes by loading program instructions from memory and executing them in sequence -Input/Output devices allow for interaction with the user -virtually all machines follow this architecture -programming was difficult and tedious because each machine had its own machine language, 0's & 1's corresponding to the settings of physical components. But, this was made easier because in 1950's, assembly languages replaced 0's & 1's with mnemonic names
lossless compression types
-The compression program scans the file looking for patterns, stores the patterns in a dictionary, and replaces relevant patterns with dictionary indexes -a formula to recreate it, stores a code -trees
MPEG
-a former lossy video compression -strives to remove as much information as possible from the original video stream -gets rid of things that are the same between images -looks at the color information in adjacent pixels and smooths them, averaging color values for similar colors
transistors
-a transistor is a piece of silicon whose conductivity can be turned on and off using an electric current -solid state, no moving parts and not easily breakable -they performed the same switching function of vacuum tubes, but were smaller, faster, more reliable, and cheaper to mass produce -some historians claim the transistor was the most important invention of the 20th century
encoding
-bascally, a scheme for sending 1s and 0s -we use the transition between voltages on the wire (high to low or low to high) to indicate/signal a 1 or a 0 -if we used absolute levels (high and low), a cable that was plugged in upside down would result in incorrect transmission
ASCII (American Standard Code for Information Interchange)
-each 7-bit ASCII code maps onto one character -there are letters of the alphabet, and control characters -stores values in 7 bits -this means it ca store 2⁷ (128) characters, which is a problem because we have more characters than that in the many languages of the world
problems with encoding
-each computer has its own internal clock, and this is not synchronized with other computers clocks -reading a bit Is done by timing, so if the sending and receiving clocks drift apart, eventually the timing will get out of synch and we will start missing bits (this is called clock skew)
why do we need compression?
-hard drives have limited space -networks are not always very fast
History of computer colors
-in older computers with color monitors, three electron guns were used to excite phosphors coated on the inside of the screen. -The phosphors on the screen were applied as red, green, and blue dots in a triad (or stripe in Trinitrons) -Circuits controlled the 'strength' of each of the guns when they were pointed at a triad -Varying levels of red, green, and blue can reproduce just about any color we need on the screen
optical fiber on a LAN
-increasingly used to connect LAN devices -fiber uses light rather than electricity to send and receive bits -each strand is made of glass and has the thickness of human hair -while there are limits to the length and speedoftrnsmission, fiber is hundreds of times faster than copper cable
Generation 2: Transistors
-mid 1950's- transistors begin to replace vacuum tubes -prompted by the space race -transistors were cheaper to produce than switches, so as prices dropped, computers became commercialized -with computer commercialization, high level languages popped up to make programming more natural -a computer industry and business grew (IBM in 1960's)
Generation 3: Integrated Circuits
-mid 1960's: integrated circuits (IC) were produced (a way to package transistors and circuitry on a silicon chip.) this advance was made possible by miniaturization & improved manufacturing and allowed for the mass production of circuitry -1971-Intel marketed the first microprocessor, the 4004, a chip with all the circuitry for a calculator -1960's saw the rise of operating systems -as computers became affordable to small businesses, specialized programming languages were developed (pascal and C)
images and JPEG
-pretty similar to video compression -a lossy method is first run to reduce the number of similar adjacent pixels and to flatter out the highs and lows of the image -a lossless compression is then run on the
digital-to-analog converter (D/AC)
-puts the binary back into what we understand -each stored number is read at a regular interval and a circuit outputs the equivalent voltage -An electronic filter is used to smooth out the jumps from number to number
text compression
-really easy to compress -The idea is similar to other schemes... look for patterns and give common ones a special code -trees (Rather than store a word or group of words, we just store directions to find them (designated by L-LL-RL-RR-etc), plus the tree itself)
video compression
-samples taken of colors -typically compressed in a lossy fashion
computers can do limited things, including
-store and retrieve numbers in memory -add numbers together -shift numbers left and right all other things are a special case of those three functions, however...for example, a computer subtracts by adding by combining switches into circuits, we can do pretty much any mathematical calculation we might need
lossy compression in music
-there are highs and lows that human ears cannot hear, so there is no need for those to be included -when two sounds are played at the same time, we can only really hear the louder one, so we can drop the quieter one -some sounds are heard better than others, so we can de-emphasize them -the result of this is an MP3 file with less information, and thus it is smaller
generation 5: parallelism and networks
-there is no new switching technologies, but there has been a change in computer usage. -parallell processing has become widespread. multi-core processors provide simple parallelism, can spread jobs across cores. High end machines like web servers can have multiple CPUs. -most computers today are networked. (the Internet traces its roots to the 1969 ARPANet, but it was mainly used by government & universities until the late 80s/early 90s) -the web was invented in 1989 to allow physics researchers to share data -almost nothing has changes in 50 years of computing; the main thing is that the devices are faster
Decimal 2 in binary
0010
Decimal 3 in binary
0011
Decimal 4 in binary
0100
Decimal 5 in binary
0101
Decimal 6 in binary
0110
Decimal 7 in binary
0111
coverting hexadecimal to binary
1. Separate each hex digit (3C1D→ 3 C 1 D) 2. convert each digit to decimal (3→3, C→12, 1→1, D→13) 3. convert decimal to binary (0011 1100 0001 1101)
converting decimal to hex
1. Successively divide by 16 (since hex is base 16) 2. Record the remainders
converting binary to hexadecimal
1. break the binary digit into 4-bit parts (11010100→ 1101 0100) 2. convert each nibble to decimal (1101→13, 0100→4) 3. write the hex equivalent (13→D, 4→4, D4)
two things to remember about the network
1. every device on a network has some sort of address, each address is unique on the network 2. every connection on a network is only between two devices (point to point)
basic rules of ethernet
1. listen to make sure no one else is using the wire. if busy, defer the transmission and try again 2. once the coast is clear, start transmitting 3. while transmitting, listen for collsisions 4. a collision happens when two devices are transmitting at the same time, cancelling each other out. if you detect a collision, stop transmitting for a Radom amount of time, then retry 5. try for a number of times (usually 16) then give up * there is no real order here, it is possible for all devices to try to transmit at the same time
physical limits of a LAN wire
1. resistance 2. impedance
two ways to connect devices together into a LAN
1. wires 2. wireless both have roughly similar protocols, for the most part wired LANs are the older technology and wireless are the current method
problems with ASCII
ASCII can only store 128 characters, but many non-English alphabets contain many more symbols than that (like Chinese, character based languages...)
CD audio`
To create a standard CD, the A/D converter samples the music at the rate of 44,100 times per second -each sample is 16 bits (2 bytes) long -two sample streams in a stereo recording -1,411,200 bits stored per second -a typical song is 32Mb long
Unicode
Unicode was developed as a 2-byte (16-bit) solution that can handle many more characters, unlike ASCII -it can hold 2¹⁶ different values, or 65,636 -the first 128 characters of unicode are ASCII values
generation 0: relays
an electromagnetic relay is a switch which can be opened or closed via electric current they were used extensively in early telephones
solving clock skew
an extra transition is inserted into the middle of each bit to provide a precise clock signal to the receiving end -this is known as differential Manchester encoding
Analog-to-Digital Conversion
converting a photo, song, any other thing like that to binary -typically an electronic chip will do the conversion; it takes an analog signal in on one side and outputs the digital result on the other
object oriented programming
designing a program by discovering objects, their properties, and their relationships -represented a new approach to program design which views a program as a collection of interacting software objects that model real-world entities
RGB
stands for red, blue, green, the color storing scheme -RGB values are 24 bits wide, with 8 bits for each of the colors -Since 8 bits can hold 256 different values, each color can be assigned a number from 0 to 255 -that means that there are 256³ combinations, or 16,777,216 different colors -RGB is widely used when working with web pages and HTML/CSS -We represent the colors as three 2-digit hex values -0xEE9572 (in HTML we'd use #EE9572) is "LightSalmon2", but this is just convenience for humans. in reality, these are stored as binary
number bases
the number of different digits or combination of digits and letters that a system of counting uses to represent numbers The base is just a notation we use to encode numeric values -binary and decimal are both number bases
Base 2 (Binary)
there are only 2 symbols (0 and 1) and we use powers of 2 instead of powers of 10 2⁰=1 2¹=2 2²=4 2³=8 2⁴=16 2⁵=32 2⁶=64 2⁷=128
Unicode variants
these are labeled by the width (number of bits) they use -UTF-8; 8 bit characters -UTF-16, 16 bit characters -UTF-32, 32 bit characters
