MIS MIDTERM

More Notes

The standard URL ("Uniform Resource Locator") used by web browsers has the basic form PROTOCOL://hostname/additional-information Thus, http://www.rutgers.edu/ means "HTTP protocol, computer with hostname www.rutgers.edu, no additional information", and http://eckstein.rutgers.edu/mis means "HTTP protocol, computer with hostname eckstein.rutgers.edu, additional information 'mis'". Here, "additional information" typically specifies a particular webpage. In a URL, a numeric IP address can also be used instead of a hostname, as in http://123.57.12.92/obscure-stuff (but is often a clue that the person providing the URL is trying to disguise it, and it may not be safe to connect to). The hostnames in URL's are often "aliased" so that a single server can support multiple websites. For example, http://eckstein.rutgers.edu actually maps to the same server as http://business.rutgers.edu, and if a small firm purchases a domain name like "mylittlecompany.com", the associated website may very likely be hosted by a computer at the company from which it purchased the domain name, such as godaddy.com. As you move downwards in the protocol layer "stack", more and more "bookkeeping" data — also called "headers" — get appended around the data you are sending. This means the actual number of bits transmitted can be significantly more than you might think. For example, when a message is divided into packets, information is added to each packet so that it gets routed correctly and the packets can be reassembled in the right order. The key advantage of layering is that existing network applications can easily adapt to new kinds of hardware, and new applications can easily be created for old hardware. If we provide a correct implementation of the network layer, then we can introduce a new kind of physical layer, and all our existing applications (e-mail, WWW, etc.) should still run properly. If we have a new kind of network application, we may simply assume that we have correct implementations of the physical/network/transport layers, and focus on designing a new application-layer protocol. Even that may not be necessary if we can figure out how to adapt one of the more flexible existing application protocols (such as HTTP or SSH) to our needs.

Applications-last step of software layer

"Native" software does most of its computation on the processor of the system the user is directly interacting with, and saves its data on the secondary storage of that system. This category includes Microsoft Office, other standard PC software, and many smartphone and tablet "apps". The alternative "thin-client" approach to building software has the system close to the user concentrate on the user interface, with other processing occurring on a remote server contacted over a network. Interactive websites are the most common examples of such software: the user only needs a browser, and the main functionality of the website is implemented on the server. Originally, such software tended to have fairly simple functionality, but more recently it has become more elaborate, as in the case of services like Google Docs.

Unconfined electromagnetic waves

(radio/microwave/infrared/light) - "wireless" o Microwave links (can be directional - may be formed into a narrow beam) o Satellite links o Within-building ("wi-fi") broadcast: capacities typically 1-54 Mb/s for standard "802.11g" systems, and up to 300 Mb/s for newer "802.11n" systems. o Mobile broadband: this technology has emerged mainly to support the "smart phone" market, blending voice and data transmission, although it can also be used by laptops and tablets. The most recent version is generally called "4G LTE", although there are many variations. "4G LTE" can theoretically provide up to just under 300 Mb/s download speeds, although users are not likely to experience such speeds in practice, and uploads are slower. Note that wi-fi and mobile broadband are effectively "bus" networks within the range of a given "base station" such as a hotspot or cell-phone tower, meaning that only than only one connected device can be sending information at any given time. But since time "slices" are rotated very rapidly between connected devices, this property is not directly observable by humans. However, the more users are in the area, the less frequently any single device will be able to send or receive data, and the more slowly the network will appear to operate.

Central Processing Unit

*Processes data from the memory, using instructions stored in the same memory* -a 1940's innovation called the Von Neumann architecture, after the famous Hungarian mathematician, who came up with the idea after moving to New Jersey -Each instruction tells the computer, for example, to add two numbers stored in different "registers" inside the processor, store the value of a register into memory, or "jump" to retrieving instructions from a different location in memory. - 1970's, pioneered by Intel, was to put all the circuits implementing CPU functions on a single semiconductor "chip". -Modern processor chips tend to have multiple "cores", each of which can process an independent stream of instructions. Typical numbers of cores per processor chip currently range from 1 to 12, but the counts are likely to grow in the future.

Various Appliances

*computers that help operate your car, appliances in your house, etc., secondary memory may not be necessary. Just main memory (with the program in ROM or EPROM read-only memory) and a processor are sufficient in such cases. In terms of numbers of processors produced, ARM is the most common architecture, since it dominates the embedded-system marketplace in addition to tablets and smartphones*

Bits vs Bytes

- A byte is simply 8 bits of memory or storage. This is the smallest amount of memory that current standard computer processors manipulate in a single operation. You can convert bits to bytes by dividing by 8, and convert bytes to bits by multiplying by 8. Often, a lower-case "b" is used to denote bits and an upper-case "B" to denote bytes.

Architectural Designs

-Small portable devices, mainly smartphones - Tablets -Personal computers (from laptops to desktops to "workstations") -Servers (large computers that run web sites etc.) -Server farms: large complexes of servers connected by fast networks -Supercomputers: these resemble server farms, but their multiple processor chips collaborate on complex mathematical calculations or data analysis, and their network interconnections may be event faster. Weather-forecasting simulation models, for example, run on supercomputers.

Raster vs. Bit Map

A raster or bitmap representation. The picture is represented as a rectangular grid of pixels, each of which has its own color. For example: If such an image is small enough and viewed from a sufficient distance, our eyes will perceive a smooth contour rather than an array of pixels. 2. A vector or object representation. In this case, the picture is represented as a superposition of abstract shapes such as squares, circles, or characters from specific fonts. we can think of a vector/object image as the kind of data manipulated by a "draw" program (such as embedded in Microsoft Word), while a raster format is a "paint" or "photo" image as manipulated by a program such as the Windows "Paint" accessory program or Adobe PhotoShop. Each format has its own advantages. Vector/object representations tend to require less storage and tend to look attractive when scaled to different sizes - for example, to display them on different-size screens or different kinds of printers. The popular "PDF" format for distributing documents is a sophisticated vector/object format (although it may contain embedded raster information as well). Raster representations are more appropriate for photographic or scanned information. The vast majority of the devices now used to display information (printers, screens, and projectors) are fundamentally raster-based, so displaying vector/object visual information requires an implicit conversion to a raster form; however, this conversion may sometimes occur only within the video card or printer.

Families of Processors (CPU's)

ARM: A low-power design that dominates the smartphone and tablet markets X86: The processors in PC's, Macs, and many servers.

Kinds of Networks (Cont'd)

An internet refers to a network created by interconnecting two or more smaller networks. o The idea of an internet preceded the current notion of the internet - "the" internet came into existence when almost everything got connected into one huge internet. o The "IP" network layer was specifically designed to make it easy to create internets. That is why all internets essentially merged into "the" internet that grew so quickly in the 1980's and 1990's, and conversely why IP is now the dominant network layer (see immediately below for a definition of the network layer concept). o Separate internets still exist - for example, computers with US "classified" secret information are on their own internets, completely disconnected from the main internet. Intranet - the PORTION of an organization's enterprise network that is not accessible to outside internet users Extranet - when MULTIPLE organizations connect their networks in a way that is not fully accessible from outside that set of organizations VPN - "Virtual Private Network" - an intranet or extranet that physically uses the general internet, but is encrypted in such a way that it functions like a private WAN that outsiders should not be able to eavesdrop on

Software Layer Architecture

BIOS/Firmware: The BIOS (Basic Input/Output System) handles basic operations low-level functions such as basic interactions with external devices such as USB ports. It is typically provided by the HARDWARE manufacturer and stored permanently or semi-permanently in ROM or EPROM. This kind or ROM/EPROM-resident basic software is called FIRMWARE Some embedded systems have only firmware and no other software. Operating System: which is the "base" program which controls the computer and its resources, and controls external interactions such as with networks or other "peripheral devices" such as keyboards and screens. The operating system calls on the BIOS to perform basic input/output functions. For PC's and larger systems the dominant operating systems at present are Various forms of Microsoft windows The Linux family (open source) Mac OSX (Apple only) Other flavors of the "Unix" family (from Oracle and other firms) For Tablets and smartphones, the dominant operating systems are: iOS (Apple only) Android (based on Linux) There are also some lesser-used OS's including Symbian, BlackBerry (once popular but fading), and mobile forms of Windows.

Secondary Memory

Bigger and Slower, stores info for long period of time, and will retain info even if power is lost mix of: Hard disks: (rotating magnetized platters with mechanically actuated read/write magnets), generally storing dozens of gigabytes to several terabytes. Speed varies with access pattern. "Flash" memory: electronic memory with no moving parts, between a gigabyte and hundreds of gigabytes. This is the memory in thumb drives, smart phones, and electronic cameras. *slower than RAM but faster than hard disk memory* Flash memory has the minor disadvantage of only being able to tolerate a limit number of write cycles before it become unreliable, but the allowed number writes is quite large - typically about 100,000. With special "load leveling" techniques, flash memory will likely outlast most devices it is used in, but it remains unsuitable for certain specialized applications

The IP Network Layer

Currently, IP assigns each computer has a 32-bit "IP address" (usually split into four bytes, printed as separate number separated by periods, as in "128.6.59.202"). IP addresses have structure - for example "128.6" in the first two bytes of the address means somewhere at Rutgers (although 165.230 also means Rutgers). The remainder of the address also has structure, although there is some flexibility into how it is divided. For example, the "59" in "128.6.59.202" might designate a particular subnet (meaning roughly a building or part of a building), and the "202" might identify a particular computer on the subnet. The identifier for the individual computer is typically the last 8 bits of the IP address, but it can be a few bits more or a few bits less, controlled by something called the subnet mask. Note that most computers also have a hostname that is easier for humans to remember than a number, such as "business.rutgers.edu" or "www.amazon.com". Special computers on the network, called name servers, provide translation between the IP address and hostname identification schemes. Small organizations may not have a name server, relying on a name server from their internet service provider (ISP). Very large organizations like Rutgers typically have their own name servers. The end of the hostname is often called the domain name. For example, the domain name in "business.rutgers.edu" is "rutgers.edu". The domain name usually indicates which institution controls the computer connection, and what kind of institution it is. "Rutgers.edu", for example, means the computer is at Rutgers, which is an educational institution. Because of the explosive growth of the number of computing devices attached to the internet, 32 bits are no longer enough space for an IP address, and we will eventually have to migrate from IPv4 (32 bits) to IPv6 (128 bit-addresses). This change will not affect hostnames, but will require a massive reallocation of IP addresses. Until this conversion occurs, various workarounds for the lack of IP addresses in IPv4 suffice in the meantime: Dynamically allocating IP addresses only when computers are actively connected to the network ("DHCP" is a common way of doing this), or Grouping small sets of computers to share a single IP, a technique called network address translation or "NAT". For example, if you have a router in your house or dorm room, it is probably doing NAT translation, so that all the computers using the router effectively share a single IP address

Node

Each computer on the network data should be able to travel from any network node to any other node, although it may have to be forwarded through intermediate nodes if there is no direct connection Some nodes on the network might not be what one might immediately think of as a computer, although they contain embedded computing devices: o Network printers and scanners o Refrigerators, thermostats, etc. Some nodes on the network are specialized computers that serve primarily as connection points whose job is to make sure data gets sent to right place. Some common varieties of such nodes include: o Switches o Routers o Hubs Most of this equipment is not particularly visible to most people, although a lot of us now own wireless routers for our homes

Main Memory (RAM)- Random Access Memory

Fast memory that stores the instructions and data currently being used. (Ranges from typical computers to large scientific computers) ROM AND EPROM are similar to RAM but contain permanent to semi permanent information that cannot be readily changed

Expanding storage

For example, if a four-byte integer is stored x bytes from the beginning of the record, then x should be a multiple of four for maximum processor efficiency. A few bytes of "filler" may be added to achieve this alignment. Relational database systems often introduce additional information called "indexes" to help them quickly locate particular rows of the table, without having to search through all the rows exhaustively. Index information is nearly always stored for a table's primary key, but one may wish to index other fields as well, so that records matching specific criteria can be located quickly. Index information also consumes space. Other objects and information may also be stored in the database file - for example, the description of the structure of the table and the properties of each field, often called metadata, require some space. Other Access objects such as forms, queries, and reports also occupy space within the database file. The file may be longer than needed to provide "room for growth" so that the database program does not have to request extra file space from the operating system for each individual row added to a table or each new object introduced into the database. For example, Access 2013 requests 336 KB from the operating system just to create a completely empty database file. Sometimes, as in Access, the space taken up by deleted records may not be immediately reclaimed and returned to the operating system. Instead you need to "compress" the database to reclaim this space (this is a different meaning of "compression" from the description of audiovisual data below).

Kinds of Networks

LAN - a "Local Area Network" on the level of a single building, part of a building, or office WAN - "Wide Area Network", a somewhat vague term for a network that covers a "nonlocal" geographic area, that is, something larger than a LAN. An example would be the network that connects Rutgers' various campuses. Enterprise network - refers to the totality of an organization's networks, both its LANs and its WAN(s) together

Layering

Layering is the critical principle in the modern design of networks. The idea is to divide the network hardware and software into layers or tiers that each has a well-defined function. The lowest layers are those that perform the most basic tasks, while the higher layers use the lower ones to perform more complicated tasks. This layering is conceptually similar to the standard layering of software into BIOS, operating system, and application programs; it also similar to layered structure of many other aspects of computing system design.

The Physical Layer

Link speed is usually measured in bits per second (b/s), with the usual prefixes K (kilo/1,000), M (mega/1,000,000), G (giga/1,000,000,000), and so forth. Note that for network speeds, it is customary to use the decimal form of these prefixes, not the binary ones. Unless otherwise specified, you should assume that all data transmission rates use decimal prefixes, and are measured in bits per second rather than bytes per second. Wires (usually copper) these can be used in many ways. o Unshielded twisted-pair (UTP) wires, such as Regular telephone wiring; internally, these cables consist of two (or sometimes four) copper wires braided together. Ethernet cables (also called "CAT 5", "CAT 5e", or "CAT 6"), which are commonly used at the following speeds: 10 Mb/s, 100 Mb/s, and 1Gb/s (in configurations often called 10BASE-T, 100BASE-T, and 1000BASE-T, respectively). These cables resemble telephone cables, but are larger. -- 4 -- o Coaxial cables, often abbreviated "co-ax", like those used for cable TV. These have higher theoretical capacity but are harder to work with (they are stiff and relatively difficult to connect together). In this type of cable, one of the two conductors (the "metallic shield" in the picture below) forms a ring around the inner conductor, which carries the main signal. This configuration limits electromagnetic interference with, and unwanted radio emissions from, the central core conductor. For conventional metal wires, there is a trade-off between distance and the number of bits per second that may be transmitted. You can transmit at high data rates, or over long distances, but not both. This trade-off arises from the basic electrical resistance properties of metal wires. The terms of the trade-off may be improved by using higher quality wiring and shielding, along with better transmission and receiving circuitry, but it cannot be fundamentally eliminated

Compression

Lossless, meaning that the recovered data stream exactly matches the original data stream "Lossy", meaning that the recovered data stream does not exactly match the original data stream, but is (in principle) a close enough approximation to be useful. Cell phone and satellite phone audio are good examples of lossy compression: the speaker can generally be understood, but may sound somewhat distorted. Lossless compression must be used for some kinds of information, such as downloaded computer software, where absolute accuracy is critical. Some compression algorithms, such as MP3, may be either lossy or lossless, depending on their configuration parameters **CDS dont have compression**

Common Network Protocols

Most networks we currently encounter use "TCP/IP", a shorthand for the combination of the IP network layer and the TCP transport layer. In general, several higher-level protocols may run simultaneously on top of the same lower-level protocol. For example, some network applications transmit simple "bookkeeping" messages using the relatively simple UDP protocol instead of TCP. UPD and TCP messages can coexist in the same IP network.

Network

Multiple computers connected by links

The Physical layer (Cont'd)

Optical fiber (carries light pulses): essentially, special strands of glass that carry pulses of light. o Fiber-optic technology began to be commercialized in the 1970's, and became increasingly prevalent in the 1980's and 1990's. o Much better than wire for combining high data rates and long distances. Links can have capacities in the many Tb/s o More difficult to work with than either twisted-pair wires or coaxial cables. In particular, it's relatively hard to "splice" two of them together (however, this property also makes optical fibers more secure than metal wires)

Link

Something that connects two computers

Sound

Sound consists of rapid fluctuations in air pressure: Digital audio simply records the air pressure at closely spaced time intervals called samples. The number of bits per sample specifies how accurately each air pressure reading is made. If viewed closely enough, one might perceive the difference between the original sound "waveform" and the digitally recorded version: If the samples are closely spaced enough and the air pressure measurements accurate enough, we obtain a reasonably accurate reproduction of the recorded sound (although an animal with more sensitive high-frequency hearing, such as a dog or bat, might be able to perceive a difference where a human could not). A typical sampling rate, for example used in CD recording, is 44,100 samples per second, or about twice the rate of the highest frequency oscillation audible to most humans. Many sound recordings also have multiple channels, meaning they consist of several parallel airpressure measurements. The most typical multi-channel approach is stereo, which consists of two channels, one channel for the left ear and one for the right; theater and home-theater soundtracks may have more channels

The TCP transport layer

TCP specifies up to 64K (binary K) conceptual ports for each computer on the network. Each port is identified by a whole number between 0 and 65,535; many ports tend to be dedicated to a particular part of the application layer. For example, port 80 is typically reserved for web page interactions. For each possible port, there may be one or more sessions, that is, logical connections to another computer. For example a web server whose pages are being viewed by different ten users (anywhere in the world) would have at least ten sessions open on TCP port 80, one for each user. It is also possible to have several sessions on the same port connected to the same remote computer. For example, you could have two independent web browser windows or tabs connected from your PC to the same website; each of these windows would represent a different session for TCP port 80 on the server. Each session is initiated by one computer sending a request to another. If the second computer agrees, the session is opened, and then data may then flow in either direction within the session. For each session, there may be a sequence of messages in each direction. TCP is a packet switched protocol, meaning that messages are cut up into small "packets" that might take different paths through the network and are reassembled in the correct order at the destination. For example, if you send an e-mail from New Jersey to Los Angeles, part of it could be routed through Chicago and Denver, and another part through Atlanta and Phoenix; at the destination in Los Angeles, the message is reassembled correctly, even if the packets arrived out of order. By contrast, traditional telephone networks are "circuit switched" - all the data in any given telephone call use the same route through the network, and remain in order throughout transmission. The size of TCP packets varies with the application and the network hardware in use, but a typical size is on the order of 1KB.

Unix

The Unix family of operating systems dates back to the early 1970's and was originally developed at AT&T (in New Jersey!) but could not be marketed commercially under the terms of the regulated telephone monopoly that existed then. Mac OS X (but not earlier Mac operating systems) and other modern server operating systems are based on the Unix design. The Linux open-source operating systems are also member of this family, and share the same basic design philosophy, although some components were rewritten in "open source" form. The Android operating system for smartphones and tablets is derived from Linux. Apple's iOS derives indirectly from Unix as well. Windows is the only widely used modern operating system family that does not derives in some way from the original Unix design. Current versions of Windows derive from Windows NT, which bears was developed by a team that Microsoft acquired largely from the now-defunct minicomputer maker Digital Equipment Corporation (DEC), based in the Boston suburbs.

Layering-protocol stack

The bottom level is the physical layer, which denotes the physical links and their operation (wire, fiber, wi-fi, etc.). By analogy to a PC, the physical layer constitutes the hardware and BIOS of the network. The Network layer (the most common version is "IP", standing for "internet protocol"): handles the identification of particular computers on the network, and describes the structure of the network. Transport layer (the most common version is "TCP", although "UDP" is also commonly used for short "housekeeping" messages). This layer manages the movement of data from one node to another. Together, the network and transport layer may be considered to be the "operating system" of the network. Application layer: specifies different protocols that use the transport layer to interchange information for various particular uses. HTTP (hypertext transfer protocol, which you see at the front of most web URL's) is one of the most common application-layer protocols. Other common application protocols are SMTP for e-mail and SSH for secure text terminal connections and file transfers. These protocols constitute the "apps" of the network.

Mobile Devices and Cloud Applications

The thin-client approach is conducive to developing "cloud" application in which data are stored on a central server and appear identical to the user no matter what device they are using. This approach is particularly attractive for mobile devices, which may have limited memory and processing power. Continuing the same activity when moving from one device to another is called ROAMING. Thin-client, roaming-enabled cloud applications are not limited by the memory capacity of their local device; HOWEVER, they are limited by the memory available on the server, which is shared with other users. Google Docs, Google Calendar, and Gmail (using the standard web interface) are simple examples of cloud applications. A downside of cloud applications is that the company providing the server has ultimate control of the data being processed by the application, which may present privacy or security issues. Having your organization operate its own "private cloud" servers can combine the best of both worlds; Microsoft Outlook (a mail, calendar, and contact-management program) and SharePoint (a file-sharing system) provide such capabilities for large organizations. Office 365, on the other hand is a more traditional "software as a service" (SaaS) version of Microsoft office in which data are stored on Microsoft's servers. As mobile devices become ubiquitous, employees of many organizations seek to mix personal and business software and data on mobile devices, most often a personally-owned device of a kind they prefer. This trend is called "BYOD" (Bring Your Own Device). Relatively open organizations like universities tolerate such mixing, except for certain highly sensitive data. Many other organizations do not because they cannot control the non-work-related software on their employees' devices, and are worried about leakage or loss of sensitive information. BYOD policies are currently a fairly contentious issue in the IT world

Common Application-Layer Protocols

There are many different application protocols that utilize TCP/IP. All may be used simultaneously on the same TCP/IP network. Some common TCP/IP-based application protocols include: TELNET (from the original ARPANET and one of the oldest protocols) - run a terminal session (a back-and-forth text interaction between a person and a computer - basically a networked version the "command prompt" in Windows or a "terminal window" on Macs) FTP (also old) - move files back and forth between computers (still in some use when security isn't an issue) SSH - ("secure shell") encrypted terminal sessions and file transfers. This accomplishes the same basic tasks as TELNET and FTP, but is far more secure. Other protocols may be "tunneled" through SSH to make them secure. HTTP and HTTPS - hypertext transmission. These application protocols were developed to support the World Wide Web, and rapidly evolved into a way of projecting a wide range of graphical user interfaces across the internet. All browser- and web-based network applications rely on HTTP and HTTPS. The "S" in HTTPS means secure/encrypted; otherwise HTTPS is identical to HTTP. HTTP/HTTPS is often a much easier and more secure way to do arbitrary things on a distant user's screen than making that user run customized software (however, that approach has experienced a resurgence with the development of "apps" for smart phones and tablets). SMB, NFS - file system sharing. Makes collections of files on a distant computer's disk look like they're on a disk connected to your own computer (with the right software, you can also exploit SSH for such purposes). SMB is Microsoft's version, and NFS is the Unix/Linux version. SMTP - sending e-mail to and between mail servers (computers that can route e-mail). SMTP is a "push" protocol: the computer requesting the connection sends the e-mail messages; when your computer sends an e-mail, it typically uses SMTP (unless you are using a webmail site). POP3, IMAP - retrieving mail from e-mail servers. These are "pull" protocols: the computer requesting the connection receives the e-mail messages (if there are any to be delivered) o The standard transmission pattern for e-mail is as follows: the message is "pushed" from computer to computer using SMTP until it reaches the recipient's mail server. From there, it is usually "pulled" to the recipient's mail client program through POP3 or IMAP. The final movement from recipient's server to recipient may also happen through HTTP/HTTPS if the recipient uses a webbased client program such as gmail (or any other "webmail" site). A few mail systems, such as Microsoft Exchange or BlackBerries, may use a "push" protocol to move mail from the recipient's mail server to their PC or mobile device. And many, many, more application protocols ... Typically, each protocol uses a single TCP port (or perhaps a few). For example, HTTP usually uses port 80, and SSH usually uses port 22.

Closed vs Open Source

Traditional, "closed-source", software is written by paid programmers. It may be licensed to users for money or distributed freely, but in either case its source code is the property of the developing company and general cannot be viewed by others. Open-source software is relatively new development in which the source code may be freely viewed by anybody, and reused either without restriction or subject to certain conditions. Sometimes open-source software is developed by groups of collaborating volunteers. Linux is an example of such a project, while Android, which is based on Linux, is an example of open-source software largely developed and distributed by a for-profit company (Google)

Video

Video consists of a series of images, called frames, shown in rapid succession. If the frames follow quickly enough upon one another, our eye perceives smooth motion. 10 frames per second is the minimum required for an illusion of smooth motion, and typical values are 24, 30, or 60 frames per secon

Using decimal or binary style

Whether the binary or decimal style is used depends in a somewhat arbitrary way on the context. *Computer RAM memory*, for example, is usually measured in the binary system. When you buy a 512 MB RAM chip for a computer, you get 512 binary megabytes, or about 536.87 million bytes (512 MB × 1024 KB/MB × 1024 B/KB). Your computer BIOS and operating system should report the RAM in the chip as 512 MB. *Disk drives, however, are typically marketed using the decimal system*. When you buy an 80 GB hard drive, however, you might well get 80 decimal gigabytes, so when the drive is formatted, your computer's operating system might state its size as 74.5 binary GB (80 × 109 B ÷ (10243 B/GB). You haven't lost 5.5 GB; instead, the size was measured using two different systems. Furthermore, the operating system might show an even smaller size because the infrastructure of the file system the operating system places on the disk also consumes some space, and only the remaining space can be allocated for user files

Virtualization

allows several operating systems, or multiple instances of the same operating system, to run simultaneously on one physical computer. A simple example is using Parallels or VMWare Fusion to run Windows on a Mac, "on top of" and simultaneously with the host operating system, which has ultimate control of the system and in this case would be MacOS. In this case, Windows would be called a guest operating system

"Last mile" technologies

means of moving data between the backbone fiber links of the internet and individual dwellings and small businesses. Large organizations typically lease high-capacity fiber connections to internet backbones, but for individual dwellings and smaller firms, this option is typically too expensive. Typical last-mile options include: o By phone and modem; once common but is now essentially extinct. o Cable modem - signals carried over the same coaxial cable that distributes TV signals. Connection speeds are usually 0.5-7.0 MB/s, but capacity must be shared with other users nearby, so performance will degrade during busy periods. Cable companies can improve link performance by running fiber-optic links to neighborhood "drop" points that serve relatively few customers. o DSL - signals carried over regular phone lines, but not at audible frequencies. Connection speeds are usually 0.5-1.0 Mb/s, but occasionally faster. Typically, DSL only works if you are within 10,000 feet (just under two miles) of wire from a telephone switching center, but does not have the same capacity sharing issues as cable modem technology. o Direct fiber connection (example: Verizon FiOS) - run optical fiber all the way to individual houses or small business. This is the most straightforward technology, and becoming increasingly common. Typical download speeds are 5-20 Mb/s and typical upload speeds are 1-5 Mb/s, but faster rates are now being marketed. To stimulate local economic activity, some cities have built their own similar systems organized as a municipal utility.

Streaming

mitigate the issue of long download times by allowing you to start watching a movie before it has finished downloading; thus, the download time is "overlaid" with viewing and one may be able to start viewing a movie within a few seconds of beginning the download. However, a data rate well above 1 Mb/s would still be required for tolerable streaming in the example above: we need to be able to download the movie at least as fast as it can be viewed, and probably significantly faster to allow for network congestion.