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This lesson covers the fundamental ways in which data can be transmitted between devices. At A-Level you must understand the differences between serial and parallel transmission, simplex, half-duplex and full-duplex modes, and the practical trade-offs that determine which method is used in different situations.
In serial transmission, data is sent one bit at a time over a single communication channel (wire or signal path).
Sender ───[bit 0][bit 1][bit 2][bit 3]───→ Receiver
(one bit at a time, single channel)
| Feature | Detail |
|---|---|
| Bits per cycle | 1 |
| Number of channels | 1 |
| Cable cost | Low (fewer wires) |
| Distance | Suitable for long distances |
| Synchronisation | Simpler — no skew problems |
| Examples | USB, SATA, Ethernet, RS-232 |
In parallel transmission, multiple bits are sent simultaneously over multiple channels (wires).
Sender ───[bit 0]───→ Receiver
───[bit 1]───→
───[bit 2]───→
───[bit 3]───→
(multiple bits at a time, multiple channels)
| Feature | Detail |
|---|---|
| Bits per cycle | Multiple (e.g. 8 or 32) |
| Number of channels | Multiple (one per bit) |
| Cable cost | Higher (more wires) |
| Distance | Short distances only |
| Synchronisation | Harder — data skew can occur at higher speeds or longer distances |
| Examples | Internal CPU buses, older printer cables (IEEE 1284), PCI (legacy) |
When data travels in parallel over multiple wires, small differences in wire length, impedance, or interference can cause bits to arrive at slightly different times. This is called data skew (or timing skew). At high speeds or long distances, skew becomes so severe that the receiver cannot reliably reconstruct the data.
This is the primary reason why most modern external interfaces (USB, SATA, Ethernet) use serial transmission — they avoid skew entirely by sending one bit at a time at very high frequencies.
Exam Tip: A common exam question asks: "Explain why serial transmission is preferred over parallel for long-distance communication." The key points are: (1) parallel suffers from data skew over long distances, (2) serial uses fewer wires so cables are cheaper, (3) modern serial interfaces compensate with very high clock speeds, achieving high throughput despite sending one bit at a time.
Data flows in one direction only. The sender can only send; the receiver can only receive.
Sender ────────→ Receiver
Examples: Television broadcast, keyboard to computer, a fire alarm sensor sending data to a control panel.
Data can flow in both directions, but only one direction at a time. Devices must take turns.
Device A ←──────→ Device B (one direction at a time)
Examples: Walkie-talkies, some Wi-Fi protocols (early 802.11), CB radio.
Data can flow in both directions simultaneously. Both devices can send and receive at the same time.
Device A ←══════→ Device B (both directions at once)
Examples: Telephone conversations, modern Ethernet (using separate wire pairs for send and receive), USB.
| Mode | Direction | Simultaneous? | Example |
|---|---|---|---|
| Simplex | One way | N/A | TV broadcast |
| Half-duplex | Both ways | No — take turns | Walkie-talkie |
| Full-duplex | Both ways | Yes | Telephone |
Data is sent in a continuous stream synchronised to a shared clock signal. The sender and receiver use the clock to agree on when each bit starts and ends.
| Feature | Detail |
|---|---|
| Clock | Shared or embedded in the signal |
| Overhead | Low (no start/stop bits per byte) |
| Speed | Higher throughput |
| Use | High-speed links (e.g. Ethernet, SPI, internal buses) |
Data is sent in discrete chunks (typically bytes), each preceded by a start bit and followed by one or more stop bits. There is no shared clock — the receiver uses the start bit to synchronise for each chunk.
| Feature | Detail |
|---|---|
| Clock | No shared clock — start/stop bits used |
| Overhead | Higher (extra bits per byte) |
| Speed | Lower throughput |
| Use | Slower, intermittent communication (e.g. RS-232 serial, keyboard input) |
Asynchronous byte frame:
[Start bit] [D0] [D1] [D2] [D3] [D4] [D5] [D6] [D7] [Parity] [Stop bit]
When data is transmitted, errors can occur due to noise or interference. Common error detection methods include:
| Method | How It Works |
|---|---|
| Parity bit | An extra bit is added to make the total number of 1s either even (even parity) or odd (odd parity). Detects single-bit errors only. |
| Checksum | The data bytes are summed; the result is sent alongside the data. The receiver recalculates and compares. |
| Check digit | A calculated digit appended to numeric data (e.g. ISBN check digit). |
Exam Tip: Parity can detect an odd number of bit errors but will miss an even number of errors (because the parity will still appear correct). This is a key limitation you should mention in exam answers.