Serial Transmission
In serial transmission, bits are sent one after another along a single wire or channel. This is the most common method for long-distance communication.
- One bit at a time is transmitted
- Uses a single channel/wire
- Slower throughput per cycle, but reliable over long distances
- Examples: USB, Ethernet, SATA, internet data
Parallel Transmission
In parallel transmission, multiple bits are sent simultaneously along multiple wires. Faster in theory, but has significant limitations.
- Multiple bits transmitted at once (e.g. 8 bits along 8 wires)
- Faster throughput over short distances
- Problem: skew — bits on different wires can arrive at different times, causing errors
- More expensive (more wires)
- Only practical over very short distances
- Example: internal motherboard buses (older), printer ports
| Feature | Serial | Parallel |
|---|
| Bits sent at once | 1 | Multiple (e.g. 8) |
| Wires/channels | 1 | Multiple |
| Distance | Long | Short only |
| Skew problem | No | Yes |
| Cost | Lower | Higher |
Simplex, Half-Duplex and Full-Duplex
These terms describe the direction of data flow in a communication channel.
Simplex
Data travels in one direction only. The sender can never receive and the receiver can never send.
Example: A TV broadcast, a keyboard sending data to a computer
Half-Duplex
Data can travel in both directions, but not at the same time. One end must wait for the other to finish before transmitting.
Example: Walkie-talkies, older CB radio, some Wi-Fi channels
Full-Duplex
Data can travel in both directions simultaneously. Both ends can send and receive at the same time.
Example: A telephone call, modern Ethernet, mobile phone conversations
Exam tip: Cambridge often asks you to identify which transmission method is used in a given scenario. Remember: serial is used for long distances; parallel for short. Walkie-talkies = half-duplex; phone calls = full-duplex; TV broadcast = simplex.
⚠️ Common Mistakes
- Thinking parallel is always faster in practice — skew and distance limitations mean serial is preferred for most real applications
- Confusing half-duplex with simplex — half-duplex goes both ways (just not simultaneously)
- Saying phone calls are half-duplex — modern phone calls are full-duplex (you can talk and listen at the same time)