Data Transmission · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 4% of your exam marks.
Serial vs parallel and simplex/half-duplex/full-duplex appear as definition or comparison questions.
The first wired-transmission choice is how many bits travel at once.
In serial transmission, bits travel one at a time along a single wire (or one pair of wires). The bits arrive in the order they were sent.
Used by: USB, Ethernet, the internet itself, SATA disk interfaces, HDMI.
In parallel transmission, several bits travel simultaneously along several wires. With 8 wires you can send a whole byte at once. A clock signal usually keeps the wires synchronised.
Used by: the buses inside a computer (between the CPU and RAM, for example), old printer cables (LPT), older external hard-drive interfaces (IDE).
Parallel only works while every wire delivers its bit at the same instant. Over long distances, the wires drift slightly out of step (one wire is microscopically longer, or has higher resistance, or is more noisy), so the bits arrive at slightly different times. This effect is called skew, and it corrupts the data because the receiver can no longer tell which bits belong to which group.
Skew gets worse as distance grows. That is why short, internal connections inside a computer can use parallel happily, but long external links almost always use serial.
| Feature | Serial | Parallel |
|---|---|---|
| Bits per moment | 1 | Many (one per wire) |
| Number of wires | Few (often 1 or 2) | Many (8, 16, 32 or more) |
| Cable cost | Cheaper | More expensive |
| Speed over short distances | Slower (one bit at a time) | Faster (multiple bits at a time) |
| Reliability over long distances | Much better | Poor (skew, interference) |
| Typical use | USB, Ethernet, internet, modern SSDs | Internal computer buses, legacy printer / drive cables |
A common surprise: USB 3.0 (serial) runs at 5 Gbps, far faster than the old parallel printer port (about 8 Mbps). Modern serial links use very high clock speeds and avoid the skew problem, so they outrun every classic parallel interface. Parallel is not "always faster than serial"; it only wins over very short distances where skew is negligible.