Data transmission

Worth about 12 of the 75 marks on a typical Paper 1. Packet switching, parity and encryption come up most.

1. Packets and their structure

Data is broken down into small units called packets before it is sent across a network.

Part	What it contains
Packet header	The destination address, the packet number and the originator’s (sender’s) address
Payload	The actual data being sent
Trailer	Marks the end of the packet and holds error-checking data, such as a checksum

2. Packet switching

1. The data is broken down into packets.
2. Each packet can take a different route across the network.
3. Routers control the route each packet takes, choosing the best path at that moment.
4. Packets may arrive out of order.
5. When the last packet arrives, the packets are reordered using their packet numbers.

3. Serial and parallel transmission

	Serial	Parallel
How	One bit at a time along a single wire	Several bits at the same time along several wires
Speed	Slower over short distances	Faster transmission over short distances
Distance	Reliable over long distances: bits arrive in order	Bits can arrive out of sync (skew) over long distances
Cost	Cheaper: fewer wires	More expensive: more wires
Used for	USB, long-distance networks	Inside a computer, e.g. internal buses

4. Simplex, half-duplex and full-duplex

Method	Direction	Example
Simplex	One direction only	A computer sending data to a monitor; a radio broadcast
Half-duplex	Both directions, but only one way at a time	Walkie-talkies
Full-duplex	Both directions at the same time	A phone call; broadband internet

Methods combine: for example, serial full-duplex sends one bit at a time, in both directions at once.

5. USB

Universal serial bus (USB) is a standard interface that sends data using serial transmission.

Benefits	Drawbacks
Devices are detected automatically and the right driver is loaded	Maximum cable length is short (a few metres)
Industry standard: works with almost any computer	Older USB versions transfer data slowly
Can supply power to devices, e.g. charging a phone	Different connector types (USB-A, USB-C, micro-USB) can need adapters
Connectors only fit one way (USB-C fits either way)

6. Why errors happen

Data can be corrupted during transmission by interference, for example electrical noise or a weak signal. Bits can be lost (data loss), extra bits can appear (data gain), or bits can flip from 0 to 1 or 1 to 0 (data change). So the receiver needs a way to check the data.

7. Parity checks

• The sender and receiver agree on even or odd parity.
• The sender sets a parity bit (here, the leftmost bit) so the number of 1s in each byte is even (or odd).
• The receiver counts the 1s in each byte. If the count does not match the agreed parity, an error is detected.
• A parity check cannot spot an even number of flipped bits, or say which bit is wrong.

Parity block check

Several bytes are sent together with a parity byte. Parity is checked for every row (byte) and every column. The row and column that both fail point to the exact bit that is wrong, so it can be corrected.

8. Checksum

1. The sender calculates a value (the checksum) from the data using an agreed algorithm.
2. The checksum is sent with the data.
3. The receiver recalculates the checksum from the data it received.
4. If the two values do not match, an error is detected and the data is sent again.

9. Echo check

The receiver sends a copy of the data back to the sender, which compares it with the original. If they are different, the data is sent again.

Drawback: if the copy is different, the sender cannot tell whether the error happened on the way there or on the way back.

10. Check digits

A check digit is an extra digit calculated from the other digits of a number. It detects errors in data entry, such as a digit typed wrongly or two digits swapped. It is used in ISBNs and barcodes.

ISBN-13 example: 978-0-306-40615-?. Multiply the digits alternately by 1 and 3 and add them: 9 + 21 + 8 + 0 + 3 + 0 + 6 + 12 + 0 + 18 + 1 + 15 = 93. Take the remainder after dividing by 10 (3), then subtract from 10: the check digit is 7. When the ISBN is scanned or typed, the calculation is repeated; if the result differs from the check digit, the entry is wrong.

11. Automatic repeat query (ARQ)

1. The receiver checks each packet for errors, for example with a checksum.
2. If it is correct, the receiver sends a positive acknowledgement. If not, it sends a negative acknowledgement, asking for the packet again.
3. The sender starts a timeout when it sends a packet. If no acknowledgement arrives before the timeout ends, it sends the packet again.
4. This repeats until the packet is received correctly.

12. Encryption

Encryption scrambles data (plaintext) into ciphertext using a key. It does not stop data being intercepted or stolen; it makes the data meaningless to anyone without the key.

	Symmetric	Asymmetric
Keys	One key, used to both encrypt and decrypt	A pair: a public key to encrypt, and a private key to decrypt
How it works	The same key must be shared with the receiver	The receiver shares their public key with anyone; only their private key can decrypt the message
Weakness	If the key is intercepted while being shared, the data can be read	Slower to encrypt and decrypt

13. Examiner tips

1. Packet header = destination address, packet number, originator’s address. Name all three.
2. Routers choose the route each packet takes; it is not decided by the sender.
3. Say what parallel is faster at: “faster transmission of data over short distances”, not just “faster”.
4. Skew is a parallel problem, not a serial one.
5. Describe parity fully: a parity bit is added to each byte, and the receiver counts the 1s to check the agreed parity.
6. Encryption does not stop data being stolen; it makes stolen data meaningless without the key.
7. A firewall does not protect data in transit. Encryption does.
8. Use technical terms: payload, acknowledgement, timeout, public key, private key.
9. Choose a method for the scenario: long distance → serial; both ways at once → full-duplex.
10. Check digits are for data entry, not transmission errors.

14. Practice questions

15. Topic test: predict your grade