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IGCSE Computer Science
As humans, we process ........data, but a computer cannot process this type of data. For a computer to be able to process data it needs to be converted to ............data. As humans, we mostly use a ............ number system; this is a base ........ number system. Computers use a .............number system; this is a base ..........number system. Complete the sentence by using these words: denary, 2, 10, binary, analogue, digital - ANS analogue digital denary 10 binary 2 Explain why binary is used to store data in a computer system. - ANS Computer consist of transistors / logic circuits ... that can only store/process data in two states / as high-low / on-off / 1 and 0 Convert 1001 1011 to denary - ANS 128+16+8+2+1= 155 Convert 85 to binary - ANS 1010101 How many bits is in nibble and byte? - ANS 1nibble=4 bits 1 byte=8 bits How many bytes in a kilobyte? - ANS 1024 bytes
Explain the differences between the binary system and the denary sytem. - ANS -The Binary number system is base 2 -The Denary number system is base 10 -The Binary number system uses 0 and 1 values -The Denary number system uses values from 0 to 9 -The Binary number system has units / placeholders / column headings that increase by the power of 2 -The Denary number system has units / placeholders / column headings that increase by the power of 10 -Binary has more digits for the same value OR Denary has fewer digits for the same value What is hexadecimal? - ANS Base 16 number system Convert the hexadecimal number B5 into binary. - ANS 1011 0101 Convert the binary number 1111 0110 into hexadecimal. - ANS F What is the advantage of using hexadecimal numbers? - ANS -They are easier to read and understands -They make it easier to spot mistakes -They are shorter / More can fit on the screen / 16 bits can be represented by 4 hex digits -It is easy to convert between Hex and Binary. -Faster than binary for entering numbers What is a MAC address? - ANS A Media Access Control (MAC) address is a number that uniquely identifies a device on the internet. --> (first part manufacturer ID) (second part serial number)
e.g. Durations of note The volume of the note Tempo Type of instrument Pitch of the note Modulations What are 2 types of file compression? - ANS Lossy and Lossless What is lossy compression? - ANS Compression in which some of the information is permanently eliminated What is lossless compression? - ANS Making the size of a file smaller without losing any of the information in the file. How image file like jpeg reduce the file size? - ANS JPEG is an image file that used lossy compression to reduce the size of the file. It reduces the range of colour used/colour depth/bits per pixel and reduces the number of pixels / removes unnecessary data. How mp3 reduce the file size? - ANS The mp3 file should use a lossy compressed file. It compresses by discarding any unnecessary sounds and sounds that hum ANS can't hear by reducing the rate of the amplitude of the sounds. How mp4 reduce the file size? - ANS The mp4 file is suitable to use lossy compressed files. It compresses by reducing resolution and colour palette that is indistinguishable, so each pixel requires fewer bits and removes the detail that is unnecessary, like sound that hum ANS can't hear.
How text file reduce the file size? - ANS The text file is suitable to use as a lossless compressed file because if you remove any word it can have an impact. A compression algorithm is used to find and index repeated words or patterns (or sections of words) within the data. The indexed words/patterns are then replaced with numerical values. The index will need to be stored with the data to allow decompression with no loss of data. What is the advantage of using lossy compression over lossless compression in photo images? - ANS - Lossy will reduce the file size more than lossless -The redundant data can be removed from the files -Images can still be of similar quality -There is no requirement for the files to be exactly the same as the original -Photos can be sent quicker / faster to upload / faster to download The file Michele is sending contains the source code for a large computer program. Identify which type of compression would be most suitable for Michele to use and explain why? - ANS Lossless because the file can be restored/decompressed to the exact same state it was before compression and because it contains a text, with the use of lossy compression it can ruin and remove the code and cannot run the code properly. What are the 2 methods of transmission? - ANS Serial transmission - sends data one bit at a time over a single channel. (use a single wire) Parallel data transmission - when several bits are sent at a time over multiple channels. What are the 3 directions of transmission? - ANS Simplex transmission - allows for data to be sent in only one direction.(unidirectional)(serial) e.g. a microphone
How checksum detected the error? - ANS The sender computers use the block of data, and a predefined mathematical algorithm, to calculate a checksum value. The sender sends a checksum with data. The receiver receives data and the checksum is recalculated. The two checksum values are compared by the receiving computer. Error detected if checksums are different. Mark scheme:
- Calculation performed on data (to get the checksum) • Checksum sent with data
- Checksum recalculated after transmission • Comparison made between checksum before and checksum after transmission
- Error detected if checksums are different How ARQ detected the error? - ANS ARQ uses acknowledgement and time out. The sender transmits a block of data. The sender waits for an acknowledgement. If no response/acknowledgement or any errors occur(negative acknowledgement is used), the request(with data) is repeated till the data is sent correctly or the request time out. Find the check digit for this identification number -multiply each digit by its digit position -add up the results of the multiplication -divided the answers by 11 -if the remainder is 0, the identification number and check digit are valid(correct) 5 0 2 4 1 __ - ANS (56)+(05)+(24)+(43)+(2*1) = 30+0+8+12+2 = 52 (52+3= 55/11 = 5 remainder 0). Therefore, the check digit is 3. What is USB? - ANS Universal Serial Bus. USB uses serial transmission for sending data externally. (USB is off-line storage)
Leonard has a new laser printer to print letters for his business. Leonard connects his printer to his computer using the USB port. Give 3 benefits of using the USB port to connect the printer to the computer. - ANS -It is a universal standard − It can't be inserted the wrong way around − Supports different transmission speeds − Automatically detects if correct driver installed What is integrated circuits? - ANS Integrated Circuits (IC) uses parallel data transmission for sending data internally.(also RAM) Due to this, parallel transmission is best suited for situations where data is to be sent over a very small distance. What is advantage of serial transmission? - ANS -Cheaper wiring because uses only a single wire -Less likely to have interference(error) -More efficient over long distances. -Sent a bit at a time, therefore less chance of data being skewed/out of synchronization What is advantage of parallel transmission? - ANS -Faster rate of transmission than serial transmission -Great for a short distance Give an example of online security threats to a computer system. - ANS • Hacking
- Cracking
- Virus
- Denial of service
Some antivirus can detect,use an on screen keyboard,look out for the clues that your keyboard is monitored What is wardriving? - ANS The act of locating and using wireless internet connections illegally , usually from a moving vehicle, using a laptop or smartphone. Use WEP encryption, use of complex passwords before accessing the internet,use of firewalls to prevent outsiders gaining access, How to keep data secure? - ANS -making regular backups of files (backup copies should be stored in fireproof safes or in another building) -Using antivirus software to protecting yourself against viruses -Use a strong password (minimum of 8 letters,use uppercase and lowercase letter and special characters also need to be use) -Don't download software from unknown sources -Quality staff training, so less risk to have someone hack/data lose. -Use correct shutdown and start up procedures What is biometrics? - ANS Biometric security makes use of unique physical characteristics and features to identify people when they are using a computer system. (scan of fingerprint,retina scan,facial recognition,voice recognition) To have a good and security passwords what you should include? - ANS -Long password, atleast 8 characters -Including the uppercase and lowercase letters,also special characters -Don't use the same passwords for everything because if you get hack, the hacker can get everything. What is a firewall? - ANS A part of comptuer security systems designed to prevent unauthorised access to or from your computer or private network.
--> Acts as a filter for incoming and outgoing data. --> Firewall can be either hardware and software --> Uses rules and criteria, set by the user, to help protect a network or system from unauthorised access. True or False Firewalls can monitor incoming and outgoing traffic. - ANS True True or False Firewalls can be used to block access to certain websites - ANS True True or False Firewalls will automatically stop all malicious traffic - ANS False True or False Firewalls allow a user to set rules for network traffic - ANS True How Firewalls Work? - ANS 1.Examining the traffic between the user's computer and a public network checking what has been requested and received. 2.The checks are to ensure that traffic (data) meets a given set of criteria (rules). 3.Any traffic (data) that doesn't meet the criteria is filtered and will block the traffic and sends signal to warn user What is a proxy server? - ANS Computer that acts as an intermediary between a web browser and the Internet. Why it is important to have a proxy server? - ANS -Keep your identity (IP Address) secret, accessing the internet via the proxy identity. -By using the cache,they can speed up access to a website by storing the pages after the first visit.
What is encryption? - ANS The process of encoding a message so it can be read only by the sender the intended receipt. (Plain text is convert to cipher text) Describe how the data is encrypted. - ANS − Encryption key is used − Encryption algorithm is used − Encryption key / algorithm is applied to plain text − ... to convert it into cypher text − Same key is used to encrypt and decrypt the text What is decryption? - ANS Converting ciphertext back into the plaintext What is cipher text? - ANS The encrypted message Decode this cypher text message. (Right shift of 7) Agbbm Pmubq - ANS Hello World Convert these words to cypher text. (Left shift of 7) Computer Science - ANS Vmilozgu Rvwgyvg What is plain text? - ANS The data / message before it is encrypted What are 2 types of encryption? - ANS Symmetric and asymmetric What is symmetric encryption? - ANS Symmetric encryption uses the same secret key to encrypt and decrypt a message/data.
1.The sender uses an encryption key to encrypt their message from plain text to cipher text. 2.Now the message is encrypted. 3.If the user wants to decrypt the message, the sender needs to send a copy of the key to allow the receiver to decrypt the message. What is asymmetric encryption? - ANS In asymmetric encryption there are 2 keys using which is private key and public key. A public key can be given to anyone. Anyone can then use this key to encrypt a message. However, the public key cannot decrypt a message - only private key can do that. So long as the private key is never given out, messages will stay safely encrypted. What is SSL used for? - ANS SSL (Secure socket layer) allows data to be sent and received securely across a network. SSL uses authentication and encryption to create a widely used online security protocol. How you can tell that the web browser has SSL? - ANS You can tell the web browser has SSL by : -https before the website URL -Padlock in the browser address -Browser address bar may turn green How the web browser receives SSL certificates and ensures its security. - ANS 1.The web browser attempts to connect to a web site which is secured by SSL 2.The web browser will send a request to the websites web server to request access 3.The web server sends the web browser a copy of its SSL certificate
What is TLS? - ANS TLS is similar to SSL but in a better version; more security. What are the two types of protocols in tls uses - ANS Handshake and Record protocal. What is TLS Handshake Protocol? - ANS The TLS Handshake is used to establish encryption and trust between the server and the client.
- TLS layer carries out authentication of server and client. What is TLS Record Protocol? - ANS This part of the protocol allows for the communication between the two parties, in other words, it contains the data that is being transferred between the two parties. What is a DDoS attack? - ANS Denial of service attacks (DOS attack) occured when legitimate user requests flood the webserver and become overloaded by an attacker who sends out 1000+ of requests. Mark scheme: -A large number of requests are sent to the network / server all at once -Designed to flood a network / server with useless traffic / requests -The network / server will come to a halt / stop trying to deal with all the traffic / requests -Prevents users from gaining access to a website / server What happend if a web server get a DDoS attack? - ANS Web server crashes and runs slow Give three different way that data can be accidentally damaged. - ANS -Human error e.g deleting / overwriting -Physical Damage Power faliure / surge -Hardware faliure
-Software crashing What is ISP? - ANS ISP (Internet Service Provider) - Company that provides individual's access to the Internet and other services such as web hosting and emails. The examples of ISP (in Thailand): TrueMove , TOT, AIS. Describe the role of an Internet Service Provider (ISP). (6 marks question) - ANS - Provide access to the internet / dial up / broadband - Usually charge a monthly fee
- Monitor usage
- Give users an IP address
- Determine bandwidth
- Provide security services
- Provide web hosting facilities
- Provide access to Email / Mailbox
- Provides online data storage What is a web browser? - ANS A software application used to locate, retrieve and display content on the www(World Wide Web), e.g. webpages,videos and other files. What is http? - ANS HyperText Transfer Protocol: language used to communicate between web browsers and servers What is https? - ANS Hypertext Transfer Protocol Secure and it me ANS the website uses SSL/TLS (i.e. much more secure than http).
A .................. is a program that allows a user to view ............................ An .............. is a company that provides a connection to access the .................. The main .............. that governs the transmission of data using the Internet is HTTP. The ........... is provided by the network, and given to each device on the network. Complete the sentence by using these words: internet, ISP, browser, IP address, protocol, webpages - ANS browser webpages Internet Service Provider (ISP) Internet protocol IP addres What is a URL? - ANS Uniform Resource Locator(URL) is the address that is used in the web browser to request a resource (web page) from the www. What are the 3 components of URL? - ANS Protocal,Domain and Webpage/File name State the part of the URL: https://winyou2549.wixsite.com/website-1/computer-science-note - ANS Protocol: https:// Domain: winyou2549.wixsite.com File name: (/website-1)/computer-science-note What is a cookie? (4 mark questions) - ANS a piece of data stored in a user's web browser. Cookies stored in text files allow a website to detect whether a viewer has viewed specific web pages.
- Data/files
- Stored in a text file
- Downloaded to a user's computer when a website is visited // web server sends to a web browser
- Stored on a user's computer
- Stored by a browser
- Detected by the website when it is visited again Give an example of uses of cookies? - ANS • To store personal information/data
- To store login details
- To save items in an online shopping basket
- To track/save internet surfing habits // to track website traffic
- To carry out targeted advertising
- To store payment details
- To customise a webpage // to store user preferences
- Store progress in online games/quizzes Explain why the music company uses cookies. - ANS − To store items that a customer has added to an online shopping basket − To store a customer's credit card details − To store log-in details − To track what product a customer browses // Track music preferences − Targeted advertising // making recommendations − Personalises/customises the experience − Shows who are new and returning customers − To speed up/allow single click purchases − Improves the experience
