Part I: Hardware and communication The following are a list of definitions for most things in the first section of the syllabus for CG1(Computing Theory). To test yourself on these items, you should attempt the past paper questions in the "Exams, Exercises Quizes" page when revising. A bit is a single binary 1 or 0. A byte is 8 bits (e.g. 11110001) A word is the maximum number of bits a CPU can handle in a single instruction. E.g. 64 bits for a Sony Playstation 3. A register is a small amount of memory on the CPU which holds data before and after processing. The address bus is a pathway (or wire) which carries information on where data is to be read or stored. A control bus is a pathway (or wire) which carries read/write instructions between the control unit and other components. A data bus is a pathway on a circuit (although it could be a wire) which carries data between components like the CPU and memory. The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it. The arithmetic logic unit (ALU) is a digital circuit that performs arithmetic and logical operations. RAM is short for Random Access Memory. This is used as fast, short-term storage, where data can be stored before and after processing by the CPU. This is known as volatile storage, as when the computer is turned off, the content of the RAM is cleared. RAM is considerably faster than a hard drive, as it contains no moving parts. ROM is short for Read Only Memory. This is a permanent storage media, usually used to hold start-up instructions for a computer on its motherboard. It is usually small in size compared to RAM, but is non-volatile, keeping its contents when the computer is powered off. RAM cache is a section of memory for holding data temporarily. It is quicker to access data in cache rather than RAM, and it may store most recent data used or most frequently used data Disc cache memory is a section of memory for holding data temporarily that has been read from disc. The reason this is exists is because it is quicker to access data in cache rather than read the disc again as memory is a faster media.
When talking about hard disks, a block is the smallest unit of storage that can be transferred between backing store and main store in one read/write operation. Secondary storage is defined as any media that can be used as a backup for a computer s main means of storage. This could be (smallest first) floppy disk, CD-ROM, DVD, USB Drive, Blu-Ray disk, Tapes. For each of these, ensure you are confident you can talk about their relative speed, durability and cost. The following common secondary storage devices are ranked from the fastest to slowest with tape drive the cheapest per byte cost. Flash memory stick External hard disk CD/DVD drive Tape drive Sometimes, it may be desirable to connect a portable storage device to a network. Portable storage could be an ipod, a USB memory stick or a flash card. The advantages of doing so are that the user is then able to take their files and data away from the physical boundary of their network, and use them at home (for instance). The disadvantage of this is that the user won t get any updates that are made to those files on the network. An interface is a piece of hardware that enables communication between two devices (e.g. a printer and a computer). Data can exist in either digital or analogue form. Digital data is expressed as 1s and 0s, consisting of discrete bit patterns whereas analogue data is expressed as a continuously varying sine wave. Often, it s necessary to convert between the two. For instance, a music CD stores its tracks as digital bits. These are then converted to an analogue signal by the hardware in the CD player, which is then sent to the speakers for the listener. Another example is the automatic climate control where the temperature is analogue (a continuous varying signal) and it must be converted to digital for the climate control unit to process. A different example is the idea of using a MODEM (Modulator/Demodulator) to convert a computer s digital signal into an analogue signal for transmission over the analogue telephone system, then back to digital when received by a computer at the other end. When inputting data into a computer, there are a number of different input methods you should know about: - Speech recognition: The user speaks instructions or dictates text into a microphone, and the computer processes them. The problem with this is that it doesn t work in a noisy environment, and the computer is prone to misinterpret the instructions it receives. Touchscreen: By tapping (often with a stylus) on visual icons on a screen, data can be inputted. This is becoming more common in handheld devices. Handwriting recognition: Often linked with a touchscreen, but with similar weaknesses to speech recognition the system often requires a lot of training.
Optical Scanning: Typed documents can be scanned into the computer as images, then special software attempts to read the text, and convert it into a word processor document for electronic editing. This is used regularly in industry. Before two devices can communicate (E.g. a printer and a PC), they need to undertake a procedure called handshaking. This is used to ascertain the status of the device before communication. In other words, handshake is the exchange of signals between devices to establish their readiness to communicate. The handshaking between a computer and a printer works like this: the computer "asks" the printer if it is on line and ready to receive data. The steps involved in this handshake process are: - 1. Computer asks printer is it is ready to print 2. Printer "replies" that it is ready or not ready 3. Computer sends data for printing 4. Computer stops sending data when buffer is full 5. Process is repeated until all data is printed There are several ways to make a wireless network connection. The most common would be by using a wireless access point (a piece of hardware that plugs into a switch or router), and a wireless network card installed in a computer. Alternatively, it is possible for two computers to connect to one another via wireless cards. Network Protocols (always expand the acronyms when answering exam questions!) Protocol - Protocol is a standard set of rules to ensure the proper transfer of information. SMTP - Simple Mail Transfer Protocol is an Internet standard protocol for transmitting email across Internet. SMTP is a delivery protocol only. POP3 - Post Office Protocol version 3 is the current version of Internet standard protocol for email users to receiving emails from their email providers. POP3 supports simple download-and-delete requirements for access to remote mailboxes. POP3 is used for most webmail services such as Gmail and Yahoo. HTTP Hypertext Transfer Protocol is an Internet standard defining a set of rules regarding how messages and other data should be formatted and exchanged between servers and browsers. It is the most widely used protocol cross the world wide web, can deliver virtually all files and other data on the World Wide Web. These data could be HTML files, image files, movies, or anything else. FTP File Transfer Protocol is a networking protocol for transferring files from one computer to another over a network, such as the Internet.
VoIP Voice over Internet Protocol is an Internet protocol that uses category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls by sending voice data over the Internet. Skype uses VoIP. 802.11x a transmission protocol for wireless local area networks (WLANs), where x can be a, b, g, n and ac to denote the different generations of the protocol. Network Topology Bus Network uses a common backbone to connect all devices. A single cable functions as a shared communication medium that devices attach to. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. Advantage : computers can connect and disconnect without affecting the other machines. Not many cabling needed in comparison. It s easy to set up. Good for small to medium sized networks. Disadvantage: shared bandwidth. If two machines are exchanging a large of amount data, then the others have to wait. This problem becomes worse as the network grows. Not secure as all communications are broadcast. When the bus or main cable fails, all communication is down. Difficult to extend. Star Network: each computer is linked to a central server (hub, switch or router) on a dedicated line. Advantage: not sharing bandwidth and if one link is down, it does not affect other computers communication. Easier to extend the network. Adding more devices will not slow down the whole network. Better security with data sent directly to destination workstation. Disadvantage: more cabling is needed thus more expensive. Also if the central hub is down, then the whole network is broken.
Ring Network: every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction Advantage: it is simple to set up. With a token ring network, there are no collisions. Disadvantage: if one of the links is broken then computers on one side of the broken link can't communicate with those on the other side. Can be difficult to add new stations. Good for small office or home network. Ring Topology