Computer science revision notes Flashcards

Question

secondary memory Virtual memory:

Answer 1

Examples: HDD, USB, SD, magnetic tape, floppy disk Or auxiliary storage. Relatively slow but higher capacity than primary memory. Can be written to but non-volatile. Known as persistent storage because contents are persistent, not wiped out after process is finished, or when there’s no power. Data and instructions are copied from secondary storage to main memory (RAM) before being executed by CPU. Because RAM has limited capacity & processed data needs to be stored somewhere non-volatile, processed data and instructions are moved to secondary memory to make space for new ones. Without persistent storage, computer would only be able to perform basic operations and all data re-inputted every time. Need to store processed data so we can use it again later, e.g. editing a saved file. Virtual memory: Part of secondary memory used as if it is main memory. If RAM is too full to load new program, another program/ part of another program is loaded on virtual memory to make room for the new program. If the other program is needed again, it’s swapped back out

Answer 2

Operating system: Set of software that controls computer's hardware resources and provides services for programs. Roles of an operating system: 1. Peripheral communication: OS controls hardware and provides interface for applications to communicate with peripherals 2. Memory management: Manages how memory is used by applications. OS allocates sections of memory to each program being run, keeps track of what data is in which location. If there’s too many processes for RAM, uses virtual memory to run programs simultaneously 3. Data management: Accessing and storing data. Keeping track of files and location in memory to make best use of memory available and provide reliability 4. Resource allocation: Manages resources and multitasking so tasks share CPU time, including memory and processor time 5. Security: Provides measures such as password authentication, magnetic cards, access rights to prevent unauthorised access. Log files keep track of activity

Answer 3

Binary representation Binary digit (bit): Basic unit of info, 1 or 0. One digit in a binary number. Byte: 8 bits. Kilobyte: 1024 bytes. Megabyte: 1024 kilobytes. Decimal/ denary: Positional system that uses 10 digits to represent a number. Base-10. Binary: Uses 2 digits to represent a number. Base-2 Hexadecimal: Positional system that uses 16 digits to represent a number. Base-16 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F 10110112 = base 2, therefore binary 10410 = base 10, therefore decimal 8416 = base 16, therefore hexadecimal Converting 128 64 32 16 8 4 2 1 1 0 1 1 1 0 ^Binary representation of [00]101110. Add up all the values where there’s a 1. Converting from Dec to Binary could involve algorithm: divide by 2, retain remainders until you're dividing 0 by 2. Rest of the digits are 0 if you haven’t reached 8 digits. Read result last to first to get binary number left to right. e.g. 104/2 = 52 remainder 0 52/2 = 26 remainder 0 26/2 = 13 remainder 0 13/2 = 6 remainder 1 6/2=3 remainder 0 3/2 = 1 remainder 1 1/2 = 0 remainder 1 0 (add a zero to make 8 digits) 104= 01101000

Answer 4

Constructing truth tables Example: Tania won't go to play if it is cold and raining, or if she has not done her homework. Won't play = (Cold AND Raining) OR (NOT Homework)

Answer 5

Network: Set of computers or computer systems that are interconnected and share resources and data Node: Device on network Server: Computer system or software package that provides service to other devices in network, e.g. ability to file share. Fulfils requests from client programs or computers. Client: Device that requests service from server in the same network, e.g. email software client requesting email server software to fetch new emails Hub: Connection point for all devices on network, devices connected through Ethernet cables. Data from device goes to the hub, then sent to all devices. Hub takes data and distributes it to other devices, that is sent through the server. Switch only sends to device that it needs. (security) Target device receives, others ignore Switch: Connection point but can identify which device is connected to which port, so can send data to target device Router: Device that connects network to another e.g. home network to internet. Types of networks  Local Area Network (LAN): Network devices are connected within limited geographical area, e.g. a room, a home, a school, office building etc. Data and peripherals can be shared. Usually client-server, connected with hub/ switch and Ethernet cabling  Virtual Local Area Network (VLAN): Devices or LANs from different segments in a LAN connected logically, e.g. splitting a hospital LAN into VLANs according to department so each department can only access certain data. Usually via software, not hardware  Wireless Local Area Network (WLAN): Connects nodes in limited geographical area but with no wires. Allows for mobile devices and easier connection/ removal, although less secure  Wide Area Network (WAN): Connects different computer systems or LANs from different geographical areas, can span over a city, country or the world, e.g. internet, different sites of organisations connected  Storage Area Network (SAN): Type of LAN designed to handle large data transfers. Each client is connected to a server that provides access to storage devices  Personal Area Network (PAN): Network covering individual's working area, very small space. e.g. PC, phone, tablet sharing data. Can be wired (e.g. USB) or not (e.g. Bluetooth)  Peer-to-Peer Network (P2P): Different to client-server, no centralised server. All nodes have equal privileges and responsibility for processing and sending data. All are client and server at same time, consuming and supplying resources from and to other peers Extranet: is a wide area network Internet: A WAN. Connect to it through internet service provider (ISP) for monthly fee. Not owned by single entity, no central server. All computer systems are independent to share resources. Internet provides services such as support for email, file transfer, and the WWW, which consists of webpages Extranet: External extension to a LAN- Part of a network that uses internet protocols to allow controlled access by specific users to LAN or WAN. e.g. business wants to share some data/ information with clients or partners but not all, so extends part of network (extranet) available for access but with security/ privacy measures (e.g. firewall) so outsiders cannot access secure data Virtual Private Network (VPN) Allows clients from distant locations to connect, that otherwise wouldn't be able to connect with LAN (too far for cables) or WAN (too far for signal to be picked up). e.g. if worker goes abroad but still needs access to the network. Technologies needed...  Can use hardware for public networks like the internet through tunnelling, which allows the network to send data via other network's connections as if connected to LAN  Hardware/ software requirements like internet access, VPN software, routers  Protocols to encrypt and authenticate traffic to ensure secure transfer over virtual tunnel, e.g. IPSec (Internet Protocol Security) Advantages:  Info can be accessed in remote places  No need for long-distance leased lines = lower cost  Enhanced security through encryption, can hide your IP address Disadvantages:  Needs a professional with detailed understanding of security issues and configuration to ensure sufficient security and protection  Reliability of VPN is not directly under the organisation's control, but under ISP  Not all VPN products are compatible across different vendors VPN can change working patterns, e.g. changes in costs, flexibility to employees and remote offices, global networking opportunities, less time travelling for remote users VPN vs Extranet  VPN authenticates sender before establishing tunnel  VPN access and transmission are always encrypted, whereas extranet has limited encryption  VPN users have access to everything wh

Answer 6

Network communication layers An application goes through different layers to send data between systems. Data packets go through different layers with different protocols before being reassembled at the other application. Open Systems Interconnection (OSI) model: Model that defines layers of network interaction. Each layer deals with one aspect/ abstraction of network communication 1. Physical- e.g. cabling system components 2. Data link- e.g. Network Interface Card (NIC) 3. Network- Routing 4. Transport- Transmission- error detection 5. Session- Retransmission of data if not received by device 6. Presentation- Encryption and decryption of message for security 7. Application- The end-receiver application, e.g. e-mail

Answer 7

Data packet: Unit of data for transmission with a format, it is part of a message made into a single package. Contains address and data. Packet switching: Network communication method: routers direct data packets transmitted along different paths through networks to the best next step closer to destination address (where there’s not too much traffic, or no down devices blocking transmission). Eventually the packet reaches the router that knows the address of the destination device. Packets assemble there Protocol: Sets of rules to facilitate a process being carried out correctly. In case of data transfer, protocols are rules that ensure data is transferred correctly between systems. A protocol recognised as the standard for a certain type of transfer is called standard protocol.

Answer 8

Necessity of protocols  Data integrity: Ensures data is not changed or corrupted during transmission  Flow control: Controlling flow/load of resources due to limited bandwidth. Transport layer uses protocols, e.g. to request sending application to slow down data flow rate  Deadlock prevention: Prevents situation where two or more competing programs or actions are sharing a resource and preventing each other from using it  Congestion management: Prevents requests on network resources exceeding capacity  Error checking

Answer 9

Speed of data transmission Bandwidth: Amount of data that can be transmitted over a certain period of time, i.e. theoretical rate of data transfer or speed. Measured in bps or kbps Throughput: Actual transfer rate of data Bottleneck: Created where there are slower segments in a network due to different factors below. Throughput becomes only as fast as the slowest link or path Data is often compressed (encoding data using fewer bits than the original) to take up less bandwidth and reduce transmission time. Receiver has a program to decompress the file

Answer 10

ransmission media  Wireless: e.g. microwave/ radio signals, satellites, infrared (short distance). Compared to metal cabling and fibre optics, is least reliable and slowest but cheapest  Metal conductor: e.g. copper cable, UTP cable, Coaxial cable. Faster, more reliable and expensive than wireless but less than fibre optic.  Fibre optics: Fine optical fibres carrying beams of light as signals. Fastest and most secure but also most expensive. Wireless networking Wireless devices communicate with no cables, e.g. with radio signals. Cell phones, tablets,

Answer 11

Modem: Allows you to connect to internet  Wireless router: Same as router but also has Wireless Access Point (WAP) to connect to existing wired network and allow wireless data transfer between device and network  Wireless repeater: Receives signal from router or access point and rebroadcasts it  Dynamic Host Configuration Protocol (DHCP): Allows server to automatically assign IP address to client device.  Wireless Application Protocol (WAP): Provides protocols for accessing info over a mobile wireless network, used in mobile devices to access internet, emails etc.  Firewall: Controls what data comes in and out of network

Answer 12

Encryption: Altering a message into form not understandable to unauthorised person. Only person with the correct key can decode the message and read it. Symmetric-key encryption: Or single/secret key. Device has a key to encrypt packets, the receiving device has the same key to decrypt. E.g. Data Encryption Standard (DES) Public-key encryption: Or asymmetric key. Has a public key for encryption and a private one for decryption, both mathematically linked. Secure Socket Layer (SSL) and Transport Layer Security (TLS) encryption protocols used on internet  MAC addresses: Medium Access Control. Every device in a network (their NIC) has a unique MAC address of 12 characters. Network permits access only to specified, trusted devices based on their MAC address. MAC address of an untrusted is device blocked so it cannot connect to the network.  User ID: Use password to access device and/or password to access web interface used to set up wireless routers or access points  Firewall: Either software installed on each device, or router hardware firewall that protects from hackers accessing devices through wireless connection. Filters incoming traffic and can block some messages coming through, as well as control/ limit user's own access to the internet. Extent of firewall use depends on an organisation's policy, e.g. one organisation may only allow communication between users and outside the network via email but not accessing websites etc.