Serial and parallel data transmission

Serial transmission

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

• Fewer wires, so cables are cheaper and thinner.
• No skew: bits cannot arrive out of step with each other.
• Less crosstalk (interference between neighbouring wires).
• Reliable over long distances, and can run at very high clock rates.
• Used by USB, Ethernet, SATA and PCI Express.

Parallel transmission

In parallel transmission several bits (for example 8, 16 or 64) are sent at the same time, each on its own wire.

• Can move more bits per clock pulse, so it is fast over short distances.
• Used inside the computer, for example the internal buses between CPU and memory.
• Drawbacks: more wires (more expensive, bulkier); skew (bits travel at slightly different speeds and arrive at different times), which gets worse with length; crosstalk between wires. This is why long-distance and external links are now serial.

Comparing data rates

If both links use the same clock rate, a parallel link with n wires sends n bits per pulse and a serial link sends 1. Bits per second = bits per pulse x pulses per second. In practice serial links can be clocked far faster because they avoid skew.

Worked example

A parallel bus has 16 data lines and is clocked at 100 MHz, sending one bit per line per pulse. What is its data rate in Mbit/s?

1. Bits per pulse = 16
2. Pulses per second = 100 x 10⁶
3. Rate = 16 x 100 x 10⁶ = 1600 x 10⁶ bit/s

Answer: 1600 Mbit/s

Check yourself

1. What is serial transmission?
   • Sending several bits at the same time on separate wires
   • Sending data only from the CPU to memory
   • Sending bits one after another along a single channel
   • Sending data without any wires
   Show answer

   Sending bits one after another along a single channel. Serial means in a series, one at a time.

2. What is skew in parallel transmission?
   • Bits sent at the same time arriving at slightly different times
   • Data being encrypted during transfer
   • Bits being sent in reverse order on purpose
   • The clock running too slowly
   Show answer

   Bits sent at the same time arriving at slightly different times. Small differences between wires mean bits drift out of step, worse over longer cables.

3. Where is parallel transmission still typically used?
   • Long-distance network cables between buildings
   • Undersea internet cables
   • Internal buses inside the computer over short distances
   • Wireless connections to phones
   Show answer

   Internal buses inside the computer over short distances. Short distances keep skew small.

4. A serial link sends one bit per clock pulse at 2.5 GHz. What is its data rate in Gbit/s?
   Show answer

   2.5 Gbit/s. 1 bit per pulse x 2.5 x 10⁹ pulses per second = 2.5 Gbit/s.

5. An 8-line parallel link sends one bit per line per pulse at 50 MHz. What is its data rate in Mbit/s?
   Show answer

   400 Mbit/s. 8 x 50 x 10⁶ = 400 x 10⁶ bit/s = 400 Mbit/s.

6. Explain why serial transmission is used for long-distance connections.
   Show answer

   Serial uses fewer wires, so it is cheaper, and with only one data line there is no skew and less crosstalk, so data stays reliable over long distances.. Give at least two reasons: cost and skew or interference.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.