References only, not question text. Open Official paper ↗ where Cambridge publishes the paper,
or see the official Cambridge past-papers page; for other papers, ask your
school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Error detection (parity, checksum, echo check, check digit, ARQ) · Core · Paper 1 style · 5 marks
(a)Give two reasons why errors can occur when data is transmitted.[2]
(b)Name three methods that can be used to detect errors after data has been transmitted.[3]
Show answer
(a) Electrical interference; data loss, gain or change caused by problems in the connection (e.g. a damaged cable or weak wireless signal).
1 mark each for any two of: interference (electrical/electromagnetic); data loss; data gain; data change/corruption; poor connection or damaged cable; weak/obstructed wireless signal; packet collisions.
1 mark each for any three of: parity check (byte or block); checksum; echo check; ARQ.
Q2Error detection (parity, checksum, echo check, check digit, ARQ) · Core · Paper 1 style · 4 marks
(a)A system uses even parity. The parity bit is the left-most bit. Write the complete 8-bit byte for each of these 7-bit data values:(i) 1011001(ii) 0110100(iii) 1111111[3]
(b)Explain what is meant by even parity.[1]
Show answer
(a) (i) 01011001 (ii) 10110100 (iii) 11111111
1 mark each. (i) four 1s → parity 0; (ii) three 1s → parity 1; (iii) seven 1s → parity 1.
(b) The total number of 1s in the byte, including the parity bit, must be even.
B1.
Q3Error detection (parity, checksum, echo check, check digit, ARQ) · Core · Paper 1 style · 4 marks
(a)Describe how an echo check works.[3]
(b)Give one drawback of an echo check.[1]
Show answer
(a) The data is sent to the receiver; the receiver sends a copy of the data back to the sender; the sender compares the returned copy with the original; if they are different an error has occurred and the data is sent again.
1 mark each for any three of: data sent; receiver returns the data to the sender; sender compares the two copies; if different, error detected and data resent.
(b) It is not known whether the error happened on the way to the receiver or on the way back; it doubles the amount of data transmitted.
B1 cannot tell in which direction the error occurred / doubles transmission time or traffic.
Q4Error detection (parity, checksum, echo check, check digit, ARQ) · Core · Paper 1 style · 6 marks
(a)State what is meant by a check digit.[2]
(b)Give two examples of where check digits are used.[2]
(c)Name two kinds of data-entry error that a check digit can detect.[2]
Show answer
(a) An extra digit added to the end of a number (e.g. a barcode), calculated from the other digits, used to check the number has been entered correctly.
1 mark: final/extra digit calculated from the other digits. 1 mark: used to detect errors when the number is entered or scanned.
(b) ISBNs on books; product barcodes (also bank account numbers, national ID numbers).
1 mark each for any two sensible examples.
(c) An incorrect digit; two digits transposed (also a digit missed out or an extra digit).
1 mark each for any two of: wrong digit; transposition; omitted digit; extra digit; phonetic errors (e.g. 13 for 30).
Q5Error detection (parity, checksum, echo check, check digit, ARQ) · Standard · Paper 1 style · 5 marks
(a)A school's network uses odd parity. These four bytes are received:Byte 1: 10110011Byte 2: 01101100Byte 3: 11100000Byte 4: 00011101Identify which bytes have been received with an error. Give a reason.[3]
(b)Byte 3 passed the parity check but was still incorrect. Explain how this is possible.[2]
Show answer
(a) Byte 2 (four 1s) and Byte 4 (four 1s) – each has an even number of 1s, so it fails the odd parity check.
1 mark: byte 2; 1 mark: byte 4; 1 mark: reason – an even number of 1s when odd parity is used (bytes 1 and 3 each have five or three 1s, which is odd).
(b) If two bits (an even number of bits) are changed during transmission, the number of 1s stays odd, so the parity check cannot detect the error.
1 mark: two/an even number of bits changed. 1 mark: parity is unchanged so the error is not detected.
Q6Error detection (parity, checksum, echo check, check digit, ARQ) · Standard · Paper 1 style · 7 marks
(a)Describe how a checksum is used to detect errors in transmitted data.[4]
(b)A device calculates its checksum as the total of the data values MOD 256. It sends these values: 156, 201, 77, 98, 240. Calculate the checksum. Show your working.[2]
(c)The receiver gets 156, 201, 77, 89, 240 and the checksum 4. Explain whether the error will be detected.[1]
Show answer
(a) Before sending, a value (checksum) is calculated from the data using an agreed algorithm; the checksum is sent with the data; the receiver recalculates the checksum from the data it receives using the same algorithm; if the two checksums are different an error has occurred and the data is requested again.
1 mark each for: checksum calculated from the block of data before sending; transmitted with the data; recalculated by the receiver using the same algorithm; compared – if they do not match, an error is detected / retransmission requested.
(c) Yes – the receiver's total is 763, and 763 MOD 256 = 251, which does not match 4.
B1 detected because recalculated checksum 251 ≠ 4.
Q7Error detection (parity, checksum, echo check, check digit, ARQ) · Standard · Paper 1 style · 4 marks
(a)An online banking app in Islamabad uses an automatic repeat query (ARQ). Describe how ARQ ensures that data arrives correctly.[4]
Show answer
(a) The receiver checks the data using an error-detection method; if correct it sends a positive acknowledgement; if an error is found it sends a negative acknowledgement and the sender resends the data; the sender starts a timer (timeout) when it sends, and if no acknowledgement arrives before the timeout it resends automatically.
1 mark each for any four of: receiver uses an error check (e.g. parity/checksum); positive acknowledgement sent if no error; negative acknowledgement if error; sender resends on negative acknowledgement; timeout starts when data sent; data resent if no acknowledgement before timeout; repeats until received correctly (or a set number of tries).
Q8Error detection (parity, checksum, echo check, check digit, ARQ) · Standard · Paper 1 style · 5 marks
(a)An ISBN-13 check digit is calculated like this:1. Starting from the left, multiply the 12 digits alternately by 1 and 3.2. Add up the results.3. Find the remainder when the total is divided by 10.4. If the remainder is 0, the check digit is 0; otherwise the check digit is 10 − remainder.Calculate the check digit for the book number 978969350712. Show your working.[3]
(b)State when the check digit is calculated and when it is checked.[2]
(b) It is calculated when the code is created/printed; it is recalculated and compared each time the number is entered or scanned.
1 mark: calculated from the other digits when the number is first created. 1 mark: recalculated on input and compared with the given check digit.
Q9Error detection (parity, checksum, echo check, check digit, ARQ) · Challenge · Paper 1 style · 5 marks
(a)Six bytes of data were sent using even parity, with a parity block. The left-most bit of each byte is its parity bit, and the final row is the parity byte (it gives even parity for each column). The data received is:Byte 1: 0 1 0 1 1 0 0 1Byte 2: 0 0 1 1 0 1 1 0Byte 3: 0 1 1 1 0 0 0 1Byte 4: 1 0 0 0 1 1 1 1Byte 5: 0 1 1 0 0 1 1 0Byte 6: 0 0 1 0 1 1 0 1Parity byte: 1 1 0 1 1 1 1 0One bit has been changed during transmission. Identify the byte and the bit position (1 = left-most) of the error, and explain how you found it.[4]
(b)State one advantage of a parity block over a single parity bit per byte.[1]
Show answer
(a) Byte 4, bit position 6. Byte 4 has five 1s (odd) so the error is in that row; column 6 contains an odd number of 1s (including the parity byte) so the error is in that column; the bit where they cross is wrong (it should be 0).
1 mark: byte 4. 1 mark: bit position 6. 1 mark: row with odd number of 1s identified. 1 mark: column with odd number of 1s identified / intersection gives the bit, which is then flipped to correct it.
(b) It can locate exactly which bit is wrong, so the error can be corrected without resending the data.
B1 identifies the position of the error / can correct a single-bit error.
Q10Error detection (parity, checksum, echo check, check digit, ARQ) · Challenge · Paper 1 style · 6 marks
(a)A librarian types the ISBN-13 9789693507126 as 9789693570126. Using the method where the 13 digits are multiplied alternately by 1 and 3 and the total must be divisible by 10 for the number to be valid, show that the error is detected.[3]
(b)Name the type of error the librarian made.[1]
(c)Explain why a check digit cannot detect every typing error.[2]
Show answer
(a) Correct number total = 150 (divisible by 10). Typed number total = 136, which is not divisible by 10, so the number is rejected.
M1 multiplies each digit of the typed number alternately by 1 and 3; A1 total 136; A1 136 is not divisible by 10 so error detected (accept showing the correct ISBN totals 150).
(b) Transposition error (two digits swapped).
B1.
(c) Two or more errors could cancel each other out, so the total still gives the same remainder and the incorrect number appears valid.
1 mark: some combinations of errors give the same (valid) remainder. 1 mark: so an incorrect number would pass the check.
Q11Error detection (parity, checksum, echo check, check digit, ARQ) · Challenge · Paper 1 style · 5 marks
(a)A company gives each product a 5-digit code followed by a check digit. The check digit is calculated as follows:1. Multiply the five digits from left to right by 6, 5, 4, 3 and 2.2. Add the results.3. Find the remainder when the total is divided by 11.4. The check digit is 11 − remainder (if this gives 11 the check digit is 0; if it gives 10 the check digit is X).Calculate the check digit for the code 40731.[3]
(b)Calculate the check digit for the code 52918.[2]
Q12Error detection (parity, checksum, echo check, check digit, ARQ) · Challenge · Paper 1 style · 6 marks
(a)A satellite sends large blocks of scientific data to a ground station in Pakistan. It takes the signal a long time to travel. Compare the suitability of an echo check and a checksum for this link.[4]
(b)Explain why a parity check on each byte would be less reliable than a checksum for this data.[2]
Show answer
(a) An echo check doubles the transmission (all the data must be sent back), which would be very slow over a long-delay link; the error could also occur on the return journey. A checksum adds only a small value to each block, so little extra data is sent; the ground station can check each block itself and request only the faulty blocks again. So a checksum (with ARQ) is more suitable.
1 mark each for any four of: echo check sends all data back – doubles traffic/time; long delay makes echo check slow; error may occur on return so ambiguous; checksum adds little extra data; receiver can check without returning data; only faulty blocks resent; conclusion that checksum is more suitable.
(b) A parity check cannot detect an even number of bit errors in a byte, and with noisy signals several bits may change; a checksum based on the whole block is more likely to detect such errors.
1 mark: parity misses errors where an even number of bits change. 1 mark: checksum covers the whole block / more likely to detect multiple errors.
Q13Transmission methods (serial/parallel, simplex/duplex, USB) · Core · Paper 1 style · 4 marks
(a)Describe serial data transmission.[2]
(b)Describe parallel data transmission.[2]
Show answer
(a) Data is sent one bit at a time along a single wire/channel.
1 mark: one bit at a time. 1 mark: over a single wire/channel.
(b) Several bits (e.g. a byte) are sent at the same time along several wires/channels.
1 mark: several bits sent simultaneously. 1 mark: over multiple wires/channels.
Q14Transmission methods (serial/parallel, simplex/duplex, USB) · Core · Paper 1 style · 5 marks
(a)Identify whether each of these uses simplex, half-duplex or full-duplex transmission:(i) a radio station broadcasting to car radios(ii) a walkie-talkie used by security guards, where only one person can talk at a time(iii) a video call between Karachi and Toronto[3]
(b)Define half-duplex transmission.[2]
Show answer
(a) (i) simplex (ii) half-duplex (iii) full-duplex
1 mark each.
(b) Data can travel in both directions, but only in one direction at a time.
1 mark: both directions. 1 mark: not at the same time.
Q15Transmission methods (serial/parallel, simplex/duplex, USB) · Core · Paper 1 style · 4 marks
(a)Describe what happens when a new device is first plugged into a USB port.[3]
(b)State what USB stands for.[1]
Show answer
(a) The computer detects that a device has been connected; the device is identified; the correct device driver is loaded (or the user is asked to install one); the device can then be used.
1 mark each for any three of: computer detects the device automatically; device identified; driver loaded automatically; if no driver is found, the user is prompted to install it; device ready to use.
(b) Universal Serial Bus.
B1.
Q16Transmission methods (serial/parallel, simplex/duplex, USB) · Core · Paper 1 style · 4 marks
(a)State the type of transmission (simplex, half-duplex or full-duplex) used in each case, and give a reason:(i) a computer sending data to a printer that never sends anything back(ii) an online multiplayer game where players send and receive moves all the time[4]
Show answer
(a) (i) Simplex – data only travels from the computer to the printer. (ii) Full-duplex – data needs to travel in both directions at the same time.
Each part: 1 mark type, 1 mark reason. (Accept half-duplex for (i) if the student says modern printers report status back.)
Q17Transmission methods (serial/parallel, simplex/duplex, USB) · Standard · Paper 1 style · 5 marks
(a)Explain why serial transmission is used for long-distance connections even though parallel transmission sends more bits at once.[4]
(b)Give one situation where parallel transmission is suitable.[1]
Show answer
(a) In parallel transmission the bits travel along separate wires and can arrive at slightly different times (skewing), which becomes worse over long distances and corrupts the data; serial transmission uses one wire so bits arrive in order with no skew; it is also cheaper because fewer wires are needed and suffers less interference (crosstalk).
1 mark each for any four of: parallel bits can become skewed / arrive out of sync; skew increases with distance; serial has no skew – bits arrive in order; serial needs fewer wires so cheaper; less crosstalk/interference with one wire; serial more reliable over distance.
(b) Over very short distances, such as inside the computer between the CPU and memory (internal buses).
B1 internal buses / within an integrated circuit / very short connection.
Q18Transmission methods (serial/parallel, simplex/duplex, USB) · Standard · Paper 1 style · 6 marks
(a)A student connects a keyboard, a printer and a phone charger cable to her computer using USB. Give three benefits of using USB.[3]
(b)Give two drawbacks of using USB.[2]
(c)State whether USB uses serial or parallel transmission.[1]
Show answer
(a) Devices are detected automatically and the driver is loaded automatically; the connector only fits one way/is a standard used by many devices; it can supply power to the device; backward compatible.
1 mark each for any three of: automatically detected; driver automatically installed/loaded; industry standard / many devices supported; can power or charge devices; fast transfer rate; backward compatible; connector cannot be inserted incorrectly (USB-C reversible).
(b) Maximum cable length is short (about 5 m); older USB versions may be slow; not all devices are supported by older versions.
1 mark each for any two of: limited cable length; slower than some other connections; older versions may not support newer devices or speeds; limited power supplied.
(c) Serial.
B1.
Q19Transmission methods (serial/parallel, simplex/duplex, USB) · Standard · Paper 1 style · 4 marks
(a)Explain what is meant by skewing in data transmission.[2]
(b)Explain why skewing can cause errors in the received data.[2]
Show answer
(a) In parallel transmission, bits sent at the same time on different wires arrive at slightly different times, so they are no longer synchronised.
1 mark: bits arrive at different times / out of synchronisation. 1 mark: occurs in parallel transmission because wires have slightly different properties/lengths.
(b) The receiver may read a bit belonging to the next (or previous) byte, so the byte is put together incorrectly.
1 mark: bits mixed between bytes. 1 mark: data reconstructed incorrectly/corrupted.
Q20Transmission methods (serial/parallel, simplex/duplex, USB) · Standard · Paper 1 style · 4 marks
(a)A rescue team in the northern areas uses handheld radios on a single channel. Explain why this system is half-duplex and state one problem this could cause.[3]
(b)State one advantage of full-duplex transmission for a telephone call.[1]
Show answer
(a) Messages can travel both ways between the radios but only one person can transmit at a time (others must listen); problem: if two people transmit at once, the messages collide and neither is received, or urgent information is delayed until the channel is free.
1 mark: data in both directions. 1 mark: only one direction at a time. 1 mark: suitable problem.
(b) Both people can speak and hear at the same time, so the conversation is natural with no waiting.
B1.
Q21Transmission methods (serial/parallel, simplex/duplex, USB) · Challenge · Paper 1 style · 6 marks
(a)For each situation, recommend a transmission method using one option from each pair (serial or parallel, and simplex, half-duplex or full-duplex). Justify each choice.(i) A temperature sensor in a remote field sends readings 2 km to a farm computer; the computer never sends data back.(ii) Data is moved between the CPU and memory inside a laptop.(iii) Two banks' servers exchange transaction data continuously over the internet.[6]
Show answer
(a) (i) Serial simplex – long distance so serial avoids skew; data only flows one way. (ii) Parallel half-duplex – very short distance so skew is not a problem and parallel is fast; data goes both ways but on a bus only one direction at a time. (iii) Serial full-duplex – long distance; both servers need to send and receive at the same time.
Each part: 1 mark for the correct combination, 1 mark for justification.
Q22Transmission methods (serial/parallel, simplex/duplex, USB) · Challenge · Paper 1 style · 4 marks
(a)Parallel transmission sends 8 bits at once, yet USB, a serial method, is used for most external devices. Explain why.[4]
Show answer
(a) Serial cables need fewer wires so they are cheaper, thinner and more flexible; there is no skewing, so serial connections can be run at very high clock rates; parallel cables suffer crosstalk/interference between wires; modern serial links can therefore be faster in practice over cable lengths of a few metres.
1 mark each for any four of: fewer wires – cheaper/thinner cable; no skew; higher frequencies possible with serial; parallel suffers crosstalk; parallel limited to very short lengths; serial more reliable over metres; standard connector.
Q23Transmission methods (serial/parallel, simplex/duplex, USB) · Challenge · Paper 1 style · 5 marks
(a)A factory must connect a control room to a machine 200 m away. The control room sends commands while the machine sends status updates at the same time. An engineer suggests a parallel half-duplex cable. Evaluate this suggestion and recommend a better method.[5]
Show answer
(a) Parallel is unsuitable: over 200 m the bits would become skewed and the cable would be expensive (many wires) and prone to crosstalk. Half-duplex is unsuitable because commands and status updates need to travel at the same time, so one would have to wait. Recommendation: serial full-duplex transmission, which avoids skew over the distance, uses cheaper cable and allows simultaneous two-way communication.
1 mark each for any five of: parallel suffers skew over 200 m; many wires – costly; crosstalk/interference; half-duplex cannot send both ways at once; delays in commands/status; recommends serial; recommends full-duplex; justification of recommendation.
Q24Symmetric and asymmetric encryption · Core · Paper 1 style · 5 marks
(a)State the purpose of encryption.[1]
(b)Define the terms plaintext and ciphertext.[2]
(c)Name the two types of encryption.[2]
Show answer
(a) To make data meaningless/unreadable to anyone who intercepts it without the key.
B1. (Not: to stop data being intercepted.)
(b) Plaintext is the original readable data before encryption; ciphertext is the scrambled data produced by encryption.
1 mark each.
(c) Symmetric and asymmetric.
1 mark each.
Q25Symmetric and asymmetric encryption · Standard · Paper 1 style · 5 marks
(a)Describe how symmetric encryption is used to send a message securely.[3]
(b)Explain the main weakness of symmetric encryption.[2]
Show answer
(a) The sender uses a key and an encryption algorithm to turn the plaintext into ciphertext; the ciphertext is sent; the receiver uses the same key to decrypt the ciphertext back into plaintext.
1 mark each: plaintext encrypted using a key/algorithm; ciphertext transmitted; same key used to decrypt.
(b) The key must be shared with the receiver, and if it is intercepted while being sent, anyone with the key can decrypt the messages.
1 mark: same key must be sent/shared with the receiver. 1 mark: if intercepted the data can be decrypted.
Q26Symmetric and asymmetric encryption · Standard · Paper 1 style · 5 marks
(a)Describe how asymmetric encryption allows Usman to send a confidential document to his accountant.[4]
(b)Explain why it does not matter if a hacker intercepts the public key.[1]
Show answer
(a) The accountant has a public key and a matching private key; the accountant's public key is shared openly/sent to Usman; Usman encrypts the document with the accountant's public key; the ciphertext is sent; only the accountant's private key can decrypt it, and the private key is never shared.
1 mark each for any four of: two keys – public and private, a matching pair; accountant sends public key to Usman; document encrypted with accountant's public key; ciphertext sent; decrypted with the accountant's private key; private key kept secret/never transmitted.
(b) The public key can only encrypt data; data encrypted with it can only be decrypted using the private key, which the hacker does not have.
B1.
Q27Symmetric and asymmetric encryption · Challenge · Paper 1 style · 6 marks
(a)An online shop in Lahore receives customers' delivery addresses over the internet. Compare symmetric and asymmetric encryption for this purpose.[4]
(b)Even with encryption, data can still be intercepted. Explain why encryption still protects the customer.[2]
Show answer
(a) Symmetric uses one key for encryption and decryption, so the shop would have to send the key to every customer securely, which is risky; asymmetric uses a public key (shared with customers) for encryption and a private key (kept by the shop) for decryption, so no secret key is ever transmitted; therefore asymmetric is more secure for many unknown customers; symmetric is faster, so in practice asymmetric can be used to exchange a symmetric key safely.
1 mark each for any four of: symmetric – same key both ends; key distribution problem/interception risk; asymmetric – public and private key pair; public key shared, private key secret; only shop can decrypt; asymmetric more suitable for many customers; symmetric is faster / used after key exchange.
(b) Intercepted data is ciphertext, which is meaningless without the decryption key, so the hacker cannot understand or use the address.
1 mark: encryption does not stop interception. 1 mark: intercepted data cannot be understood without the key.
Q28Packets and packet switching · Core · Paper 1 style · 7 marks
(a)Data sent over the internet is split into packets. Name the three main parts of a packet.[3]
(b)State three items of data stored in the packet header.[3]
(c)State one item stored in the trailer.[1]
Show answer
(a) Packet header, payload, trailer.
1 mark each.
(b) Destination IP address, source (originator's) IP address, packet number.
1 mark each for any three of: destination IP address; source IP address; packet number/sequence number; (accept total number of packets).
(c) An error check such as a checksum or CRC (or a marker showing the end of the packet).
B1.
Q29Packets and packet switching · Standard · Paper 1 style · 5 marks
(a)Describe how packet switching is used to send an email with a large attachment from Quetta to Paris.[5]
Show answer
(a) The data is broken into packets; each packet is given a header with the destination and source IP addresses and a packet number; each packet may take a different route; routers read the destination address and send each packet to the next router on the best available route; packets may arrive out of order; at the destination they are reordered using the packet numbers to rebuild the email; missing packets are requested again.
1 mark each for any five of: data split into packets; header added with addresses and packet number; packets travel independently / may take different routes; routers decide the route for each packet; packets may arrive in a different order; reassembled using packet numbers; missing/corrupt packets requested again.
Q30Packets and packet switching · Challenge · Paper 1 style · 6 marks
(a)A file is split into 6 packets. They arrive at the destination in the order 2, 1, 5, 3, 6 and packet 4 does not arrive.Explain why the packets arrived in a different order and how the receiving computer deals with both problems.[4]
(b)Give two benefits of packet switching compared with sending all the data along one fixed route.[2]
Show answer
(a) Each packet can take a different route through the network, depending on congestion, so some arrive sooner than others; the receiver uses the packet numbers in the headers to put them in order 1–6; it notices packet 4 is missing (by the packet numbers/total) and sends a request for it to be resent (or waits for a timeout); the file is reassembled when all packets have arrived.
1 mark each for: different routes/traffic cause different arrival times; packet numbers used to reorder; missing packet identified from packet numbers; request for retransmission of packet 4 / timeout then resend.
(b) If one route is congested or broken, packets can be sent another way; only lost/corrupted packets need resending, not the whole file; the network capacity is shared efficiently.
1 mark each for any two of: avoids busy/broken routes; more reliable; only faulty packets resent; efficient use of network; no need for a dedicated line.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
