These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2021 · Paper 1 · Networks & Data Transmission
Benedict has a computer that is assigned an Internet Protocol (IP) address. The IP address is: 198.167.214.0 The IP address is represented as denary values. (a) Convert the denary values 167 and 214 from the IP address to 8-bit binary. (b) Benedict's computer is also assigned a Media Access Control (MAC) address. (i) Identify one similarity between an IP address and a MAC address. (ii) Identify two differences between an IP address and a MAC address.
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(a) 167 = 10100111; 214 = 11010110.
(b)(i) Both are addresses used to identify a device/computer on a network.
(b)(ii) Difference 1: an IP address is a logical address that can change (assigned dynamically); a MAC address is a physical address permanently burned into the NIC and does not change. Difference 2: an IP address is 32 bits (IPv4) and used at the network layer for routing; a MAC address is 48 bits and used at the data link layer for local delivery.
167: 128+32+4+2+1=167 -> 10100111. 214: 128+64+16+4+2=214 -> 11010110.
May/June 2021 · Paper 1 · Internet Security
Two internet risks are phishing and pharming. Describe what is meant by phishing and pharming.
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Phishing: an attempt to obtain sensitive information (e.g. passwords, bank/credit card details) by pretending to be a trustworthy organisation, often through fraudulent emails, texts or websites that lure the user to enter details.
Pharming: redirecting a user from a genuine website to a fake website without their knowledge (e.g. by poisoning the DNS cache) so that their details are captured when they log in.
Phishing = fake communication tricking user into giving data; pharming = silently redirecting to fake site.
May/June 2021 · Paper 1 · Multimedia & The Internet
A music company has a website that allows users to stream music. The music is stored in sound files. (a) The sound files are compressed using lossless compression. (i) Describe how the sound files are compressed using lossless compression. (ii) State one reason why the music company would compress the sound files using lossless, rather than lossy, compression. (iii) Give one benefit, to the user, of the music company compressing the sound files. (iv) Give one drawback of the music company using lossless, rather than lossy, compression for the sound files. (b) Describe how the web pages for the website are requested and displayed on a user's computer. (c) The web server that hosts the website suffers a denial of service (DoS) attack. Explain why this will prevent users from accessing the website.
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(a)(i) Lossless compression reduces the file size without losing any information, so the original file can be reconstructed exactly; techniques include run-length encoding and Huffman coding. (ii) Lossless keeps the original sound quality / no loss of data, which is important for music. (iii) Benefit to user: the files download / stream faster and use less storage. (iv) Drawback: lossless files are larger than lossy files, so they need more storage and more bandwidth.
(b) The user types a URL or clicks a link; the browser sends an HTTP request to the web server; the server sends the web page files (HTML, CSS, images, etc.) back; the browser receives them and renders/display them on screen. …
The full model answer is in the app.
May/June 2021 · Paper 1 · Data Transmission & Error Detection
Four 7-bit binary values are transmitted from one computer to another. A parity bit is added to each binary value creating 8-bit binary values. All the binary values are transmitted and received correctly. (a) Identify whether each 8-bit binary value has been sent using odd or even parity by writing odd or even: 01100100; 10010001; 00000011; 10110010. (b) An error may not be detected when using a parity check. Identify why an error may not be detected. (c) The data is sent using parallel half-duplex data transmission. (i) Describe how data is sent using parallel half-duplex data transmission. (ii) State two drawbacks of using parallel data transmission.
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(a) 01100100 -> odd parity (3 ones). 10010001 -> odd parity (3 ones). 00000011 -> even parity (2 ones). 10110010 -> even parity (4 ones).
(b) Parity does not detect an error when an even number of bits are changed (e.g. two bits flip), because the total number of 1s stays the same, so the parity remains correct.
(c)(i) In parallel transmission several bits are sent at the same time over separate wires/channels; half-duplex means data can be sent in either direction but only one direction at a time. (ii) Two drawbacks: parallel needs many wires so the cable is thicker, heavier and more expensive; crosstalk/skew can occur between the wires so bits may not arrive at exactly the same time.
Odd parity total ones odd; even parity total ones even. 01100100=3(odd),10010001=3(odd),00000011=2(even),10110010=4(even).
