Communication Methods

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.

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Serial vs Parallel Transmission

Serial Transmission

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

Parallel Transmission

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)

Data Skew

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.

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Direction of Transmission

Simplex

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.

Half-Duplex

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.

Full-Duplex

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.

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Comparison Table

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

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Synchronous vs Asynchronous Transmission

Synchronous Transmission

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)

Asynchronous Transmission

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]

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Error Detection Methods

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.

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Summary

• Serial: one bit at a time, one channel, good for long distances, no skew.
• Parallel: multiple bits at a time, multiple channels, short distances, skew issues.
• Simplex: one direction only. Half-duplex: both directions, alternating. Full-duplex: both directions simultaneously.
• Synchronous: clock-synchronised, high speed. Asynchronous: start/stop bits, lower speed.
• Error detection: parity, checksum, check digit.