Communication Methods

This lesson covers how data is transmitted across networks, including serial vs parallel transmission, simplex/half-duplex/full-duplex modes, synchronous vs asynchronous transmission, bandwidth, and latency. These concepts are part of the OCR H446 specification section 1.3.

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Serial vs Parallel Transmission

Serial Transmission

In serial transmission, data bits are sent one after another along a single communication channel (wire).

Sender: [1][0][1][1][0][1][0][0] →→→ single wire →→→ Receiver

Feature	Detail
Speed	Generally slower than parallel (for short distances)
Distance	Suitable for long distances — less interference
Cost	Lower — fewer wires needed
Skew	No skew issues (only one wire)
Examples	USB, SATA, Ethernet, HDMI

Parallel Transmission

In parallel transmission, multiple bits are sent simultaneously along multiple wires.

Sender: [1] →→→ wire 1 →→→ Receiver
        [0] →→→ wire 2 →→→
        [1] →→→ wire 3 →→→
        [1] →→→ wire 4 →→→
        ... (8 wires for 8 bits at once)

Feature	Detail
Speed	Faster over short distances (multiple bits at once)
Distance	Unsuitable for long distances — timing issues
Cost	Higher — more wires needed
Skew	Bits may arrive at different times (data skew) due to slight differences in wire length or signal propagation
Examples	Internal computer buses, old printer cables (parallel port)

Why Serial Has Replaced Parallel for External Communication

Despite parallel sending more bits at once, serial has become dominant for external connections because:

• At high speeds, data skew in parallel cables becomes a serious problem — bits arrive at different times.
• Crosstalk (electromagnetic interference between adjacent wires) worsens with more wires.
• Serial protocols like USB 3.0 and SATA achieve very high speeds using advanced encoding and a single channel.

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Transmission Direction: Simplex, Half-Duplex, Full-Duplex

Mode	Description	Data Direction	Example
Simplex	Data flows in one direction only	A -> B (never B -> A)	TV broadcast, keyboard to computer
Half-duplex	Data flows in both directions, but not at the same time	A -> B or B -> A (one at a time)	Walkie-talkies, CB radio
Full-duplex	Data flows in both directions simultaneously	A <-> B (at the same time)	Telephone call, modern Ethernet

Comparison

Feature	Simplex	Half-Duplex	Full-Duplex
Direction	One-way	Two-way (alternating)	Two-way (simultaneous)
Bandwidth usage	Full in one direction	Shared between directions	Full in both directions
Complexity	Simplest	Moderate	Most complex
Cost	Lowest	Moderate	Highest

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Synchronous vs Asynchronous Transmission

Synchronous Transmission

In synchronous transmission, data is sent in a continuous stream, with the sender and receiver synchronised by a shared clock signal.

Feature	Detail
Clock	A shared clock signal keeps sender and receiver in sync
Data format	Continuous blocks of data without start/stop bits
Efficiency	High — no overhead per character
Speed	Faster for large data transfers
Example	SPI bus, Ethernet, high-speed data links

Asynchronous Transmission

In asynchronous transmission, data is sent character by character, with start and stop bits framing each character. There is no shared clock.