Data Transmission
Cambridge O Level Computer Science 2210 Topic 2 revision chapter covering types and methods of data transmission (why data is broken into packets, the structure of a packet as header, payload and trailer with the destination address, packet number and originator's address in the header, the process of packet switching including routers choosing routes, packets arriving out of order and being reordered on arrival, serial and parallel transmission, simplex, half-duplex and full-duplex transmission with the advantages and disadvantages of each, and the universal serial bus interface with its benefits and drawbacks), methods of error detection (why errors occur during transmission through interference causing data loss, data gain and data change, parity checks using odd and even parity including the parity byte and parity block check, checksums, echo checks, check digits used to detect data-entry errors including international standard book numbers and bar codes, and automatic repeat query using positive and negative acknowledgements, timeout and retransmission), and encryption (the need for and purpose of encryption when transmitting data, and how data is encrypted using symmetric encryption with a shared secret key and asymmetric encryption with public and private keys).Show moreShow less
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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. 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.
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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 2.2.4
- Understand the need for and purpose of encryption when transmitting data. 2.3.1
- Understand how data is encrypted using symmetric and asymmetric encryption, including the use of public and private keys in asymmetric encryption. 2.3.2
Key terms in Data Transmission
- Packet
- A packet is a small unit of data that a larger block of data is broken down into for transmission; it consists of a packet header, a payload and a trailer, and carries only part of the original data.
- Payload
- The payload is the part of a packet that holds the data being transmitted; it carries only part of the original data and does not contain the addressing information.
- Packet Header
- The packet header is the part of a packet that carries the control information for its journey: the destination address, the packet number and the originator's address.
- Serial Transmission
- Serial transmission is a method of data transmission in which bits are sent one after another along a single data line, one bit at a time.
- Full-Duplex
- Full-duplex transmission is data transmission in both directions at the same time, so both devices can send and receive simultaneously.
- Parallel Transmission
- Parallel transmission is a method of data transmission in which several bits are sent at the same time, each along its own separate data line.
- USB
- USB, the universal serial bus, is a standard interface used to connect devices to a computer and transmit data between them using serial transmission.
- Half-Duplex
- Half-duplex transmission is data transmission in both directions, but only one direction at a time and never both simultaneously.
- Transmission Method Selection
- Transmission method selection is the reasoned choice of serial or parallel transmission together with simplex, half-duplex or full-duplex direction, made by matching distance, speed, cost and the direction data must travel to the demands stated in a scenario.
- Packet Switching
- Packet switching is a method of data transmission in which data is broken into packets that are routed independently across a network, may take different routes, may arrive out of order, and are reordered at the receiver once the last packet has arrived.
- Automatic Repeat Query
- Automatic repeat query (ARQ) is a method of establishing that data is received without error, using an error-detection method together with positive and negative acknowledgements, a timeout and retransmission of data that is not acknowledged.
- Simplex
- Simplex transmission is data transmission in one direction only, from sender to receiver, with no return channel. A keyboard sending keystrokes to a computer is the standard example; half-duplex and full-duplex add a return direction.
- Router
- A router is a network device that reads the destination address in a packet's header and controls the route the packet takes, forwarding it onward towards its destination.
- Symmetric Encryption
- Symmetric encryption uses the same secret key to encrypt and to decrypt the data, so both parties must hold that key and it must be exchanged securely beforehand.
- Check Digit
- A check digit is an extra digit calculated from the other digits of an identifier and stored with them; the system recalculates it whenever the identifier is entered or scanned and compares the two values to detect data-entry errors.
- Checksum
- A checksum is a value calculated from a block of data using an agreed calculation; it is sent with the data, recalculated by the receiver, and a difference between the two values shows that an error occurred during transmission.
- Parity Check
- A parity check is an error-detection method in which an extra parity bit is added to a group of bits so that the total number of 1 bits is even (even parity) or odd (odd parity); the receiver recounts the 1 bits and a mismatch shows that an error has occurred.
- Parity Block Check
- A parity block check is an error detection method in which a block of bytes is given both a parity bit per byte and an extra parity byte across the columns, so that a single changed bit is located by the row and the column that both fail.
- Transmission Error
- A transmission error is a change to data between sending and receiving, caused by interference, a long or damaged cable, a loose connector or lost packets, which makes the received data differ from the data that was sent.
- Asymmetric Encryption
- Asymmetric encryption uses two mathematically related keys: a public key that may be shared and is used to encrypt data for its owner, and a private key that is kept secret by its owner and is used to decrypt that data.
- Private Key
- A private key is the key of an asymmetric key pair that must be kept secret by its owner and is never transmitted; it is used to decrypt data that was encrypted with the matching public key.
- Echo Check
- An echo check is an error-detection method in which the receiver sends a copy of the received data back to the sender, who compares it with the original; a difference shows that an error occurred.
- Encryption
- Encryption is the process of scrambling readable plaintext into unreadable ciphertext using an encryption key, so that data intercepted during transmission is very difficult to understand without the required key.
- Public Key
- A public key is the key of an asymmetric key pair that may be shared with anyone; it is used to encrypt data intended for the key pair's owner and cannot decrypt that data.
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.
Frequently asked questions
What is the difference between serial and parallel transmission?
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 separate data line. Parallel can move more bits per unit of time over a short link, but over a longer distance the separate lines lose their timing alignment and interfere with one another, which is why long-distance links, such as USB, almost always use serial transmission instead.
What is the difference between simplex, half-duplex and full-duplex transmission?
These describe which directions data may travel and whether both directions can be active at once. Simplex transmission sends data in one direction only, such as a keyboard sending keystrokes to a computer. Half-duplex allows data both ways, but not at the same time. Full-duplex allows data to travel in both directions at the same time, so both devices can send and receive simultaneously. The two words that decide every question here are direction and simultaneously.
Why is data broken down into packets before it is transmitted across a network?
Splitting data into packets means many users can share the same network instead of one transfer occupying the link from start to finish, means each packet can be routed independently by a router, and means a single damaged packet can be dealt with on its own rather than resending the whole file. Each packet carries a header with the destination address, packet number and originator's address, a payload, and a trailer, so the receiver can reorder and rebuild the original data once every packet has arrived.
What is the difference between a parity check and a checksum?
A parity check adds one extra parity bit to a group of bits so the total number of 1 bits is even or odd, as agreed in advance; the receiver recounts the 1 bits, and a mismatch shows an error. A checksum is a value calculated from a whole block of data using an agreed calculation; the sender transmits it with the data, the receiver repeats the calculation, and a difference between the two values shows the data changed during transmission. Neither method proves the data is completely correct if the values still match.
How does automatic repeat query (ARQ) get corrupted data resent?
ARQ uses an error-detection method, such as a parity check or checksum, on the data the receiver gets. If the data passes the check, the receiver sends back a positive acknowledgement. If it fails, the receiver sends a negative acknowledgement and the sender retransmits the data. If no acknowledgement arrives at all before the sender's timeout expires, the sender assumes the data did not arrive and retransmits anyway. This repeats until the data is acknowledged or the permitted number of attempts is used up.
What is the difference between symmetric and asymmetric encryption?
In symmetric encryption, the same secret key encrypts and decrypts the data, so both parties must hold that key and it must be exchanged securely beforehand. In asymmetric encryption, two mathematically related keys are used: a public key, which may be shared with anyone and encrypts data for its owner, and a private key, kept secret by its owner and used to decrypt it. Because the public key that encrypts a message cannot decrypt it, asymmetric encryption avoids the problem of exchanging a secret key securely.
Does encryption stop data being intercepted while it is transmitted?
No. Encryption does not prevent interception itself, and it does not stop data being deleted or corrupted on the way — its purpose is confidentiality. Encryption scrambles readable plaintext into unreadable ciphertext using a key, so if an interceptor does capture the transmission, what they obtain is ciphertext that should be very difficult to understand without the required key. An exam answer should state that encryption protects the content, not the transmission path itself.
Syllabus reference and sources
Written against: Cambridge O Level Computer Science (2210) 2026–2028 Syllabus (Subject Content, Topic 2: Data Transmission).
Written by: Academiq Edu Instructor Panel
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