Data Transmission
Core Revision Module
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Interactive revision notes with exam tips and worked examples for this chapter.
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A summary of this Computer Science chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.
What is Data Transmission about?
Every idea in Topic 2 sits somewhere on one journey. Data has to travel; it is divided into packets so that many users can share the same network; routers choose a route for each packet independently; the method of transmission (how the bits travel and in which directions) is chosen to suit the situation; interference on the way can corrupt the data; an error-detection method reveals that corruption; automatic repeat query asks for the damaged data again; and encryption keeps the contents private from anyone who intercepts them on the way. Learn the chapter as that single chain and every individual method has an obvious place to hang.
Topic 2 belongs to Paper 1: Computer Systems. Paper 1 is a written paper of 1 hour 45 minutes carrying 75 marks and is worth 50% of the qualification. It consists of short-answer and structured questions set on Topics 1–6 of the subject content, all questions are compulsory, and calculators are not permitted. It is externally assessed. Every candidate takes this paper and Paper 2; both assess assessment objectives AO1, AO2 and AO3 across the full A* to G grade range, and there is no Core or Extended version of either.
Before data is transmitted across a network it is broken down into packets. Each packet carries a part of the original data, not the whole of it, together with information about where it is going and where it belongs in the sequence. Splitting the data this way means many users can share the same network instead of one transfer occupying the link from start to finish; it means each packet can be routed independently; and it means a single damaged part can be dealt with on its own. At the far end the receiving system reassembles the parts into the original data.
A packet of data contains a packet header, a payload and a trailer. The packet header includes the destination address, the packet number and the originator's address. The payload is the part of the data being transmitted that this packet is carrying. The trailer is the information at the end of the packet; depending on the protocol in use it may contain error-checking information and a marker showing that the end of the packet has been reached.
Packet switching is a method of transmitting data in which the data is broken down into packets and each packet is sent across the network independently. A router controls the route a packet takes, so each packet could take a different route from the same sender to the same receiver. Because the routes can differ in length and congestion, packets may arrive out of order. Once the last packet has arrived, the packets are reordered using the packet numbers in their headers, and the original data is reconstructed.
Serial and parallel answer one question: how many bits travel at once, and on how many data lines? In serial transmission, bits are sent one after another along a single data line. In parallel transmission, several bits are sent at the same time, each along its own data line. Neither is simply “better”: parallel moves more bits per unit of time over a short link, but over distance the separate lines lose their alignment and interfere with one another, which is why long links are almost always serial.
Key ideas to remember
- The one-line spine of Chapter 2: divide → label → route → corrupt → detect → resend → reorder → rebuild, with encryption wrapped around the contents the whole way.
- Four sentences that defuse most of the eight: the header carries the address; every packet is routed independently; detection is not correction; and encryption hides meaning, not the data itself.
- The single most useful row in Matrix 6: encryption says “no” to every problem except confidentiality, and error detection plus ARQ says “no” to confidentiality. They are not alternatives — they solve different problems and a well-designed system uses both.
What you need to be able to do
- Understand that data is broken down into packets to be transmitted — and be able to say why that is done rather than simply that it happens. 0478 2.1.1(a)
- Describe the structure of a packet — the packet header, the payload and the trailer, and the three things the header contains: the destination address, the packet number and the originator's address. 0478 2.1.1(b)
- Describe the process of packet switching — in correct order, including the router controlling the route a packet takes, packets possibly taking different routes, packets arriving out of order, and packets being reordered once the last one has arrived. 0478 2.1.1(c)
- Describe how data is transmitted using different methods — serial, parallel, simplex, half-duplex and full-duplex, including the advantages and disadvantages of each. 0478 2.1.2(a)
- Explain the suitability of each method for a given scenario — choose a method for a described situation and justify the choice from the features of that situation. 0478 2.1.2(b)
- Understand the universal serial bus (USB) interface and explain how it is used to transmit data, including the benefits and drawbacks of the interface. 0478 2.1.3
- Understand the need to check for errors after data transmission and how those errors can occur — interference causing data loss, data gain and data change. 0478 2.2.1
- Describe the processes involved in parity check (odd and even), checksum and echo check for detecting errors in data after transmission, including the parity byte and parity block check. 0478 2.2.2
- Describe how a check digit is used to detect errors in data entry, and identify examples of where check digits are used, including international standard book numbers (ISBN) and bar codes. 0478 2.2.3
- Describe how an automatic repeat query (ARQ) can be used to establish that data is received without error, including positive and negative acknowledgements and timeout. 0478 2.2.4
- Understand the need for and purpose of encryption when transmitting data. 0478 2.3.1
- Understand how data is encrypted using symmetric and asymmetric encryption, including the use of public and private keys in asymmetric encryption. 0478 2.3.2
Common mistakes to avoid
- Trap this fixes. Under even parity, a group of bits containing no 1 bits at all already satisfies even parity, because 0 is even — so the parity bit is 0, not 1.
- Trap. “So that it transmits faster” on its own is not creditworthy — the same total amount of data still has to travel.
- Trap. Do not put any of these in the payload. The payload holds part of the data being transmitted and nothing else.
- Trap. Out-of-order arrival is normal, not a fault. It does not by itself trigger a retransmission — only a missing or damaged packet does that.
- Trap. “Because serial is faster” is wrong. Parallel moves more bits per unit of time over a short link; the problem over distance is skew, interference and cost.
- Trap. Every one of the three definitions turns on direction and, for the last two, on at the same time. Leave out “at the same time” and half-duplex and full-duplex become indistinguishable.
- Trap. Do not answer this by describing what a USB flash drive stores. The question is about the interface.
- Trap. Count the 1 bits, never the 0 bits. And remember zero is an even number, so an all-zero group already satisfies even parity.
- Trap. The receiver must use the same calculation; that is what makes the comparison meaningful.
- Trap. The comparison happens at the sender, not the receiver.
- Trap. A check digit shows only that a number is internally consistent with its own calculation. It does not prove the identifier corresponds to a real book or a real product.
- Trap. Name the error-detection method. An answer that says only “ARQ checks the data” has described half the mechanism.
- Trap. The key is never sent alongside the ciphertext — that would defeat the whole purpose.
Examiner tips
- Read the verbs, not just the nouns. Topic 2 is dominated by describe and explain. A list of nouns — “header, payload, trailer” — answers identify, not describe. A described process needs the steps in the right order, and an explained choice needs the reason tied back to the scenario you were given.
- The sentence shape that always works for a scenario answer: “Use [method], because the scenario says [feature of the scenario], and [method] [property that matches that feature].” For example: “Use serial transmission, because the cable runs 40 metres to the sensor, and serial sends bits one after another on a single line, so there is no skew between separate wires over that distance.” The middle two clauses — the feature of the scenario, and the property that matches it — carry the substance of the answer.
- The convention used throughout this chapter. Seven data bits are used, and the parity bit is placed in the left-hand (most significant) position to make an eight-bit group. That position is a convention, not a rule — a question may place the parity bit at the right-hand end instead. What never changes is the method: count the 1 bits in the whole group, including the parity bit, and check the total against the agreed parity. Always state which convention you are using if a question does not.
- The acknowledgement can be lost too. If the data arrives safely but the positive acknowledgement is lost on the way back, the sender's timeout expires and it retransmits data the receiver already has. That is not a flaw in the design — it is the price of a mechanism that only ever sees one side of the conversation. A real protocol handles the duplicate using the sequence numbers already carried in each frame's header.
- The three sentences that must never appear in your answer. “The public key decrypts the message” — it does not, that is the private key's job. “The recipient sends their private key to the sender” — the private key is never transmitted to anybody, ever. “The two keys are both secret” — the public key is deliberately published. If you can keep those three straight, you have the whole of 2.3.2's asymmetric half.
- How to use this table. Cover the last two columns and read only the myth. Say the correction out loud, then check. Any row where your spoken answer was vaguer than the printed exam-safe sentence is a row to come back to — vagueness, not ignorance, is what usually costs the mark.
- How to use a failed tick. Do not simply reread the section. Go to that section's pause and recall box, attempt it cold, and only then read. A statement you have just failed to produce is the single most valuable thing on this page — it tells you exactly where the next twenty minutes should go.
- Two rules that make the schedule work. First, always attempt before you look — producing a wrong answer and correcting it builds far more durable memory than reading a right one. Second, if a session takes noticeably longer than the time shown, that is data, not failure: it tells you which competency to put in the next session.
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