Serial and parallel transmission
Serial transmission sends data one bit at a time along a single channel (one wire, one fibre or one radio channel).
Parallel transmission sends several bits at the same time, each along its own wire, for example 8 bits at once on 8 wires.
| Serial | Parallel | |
|---|---|---|
| Wires | One (plus ground) | One per bit |
| Distance | Long distances | Short distances only |
| Cost | Cheaper cable and connectors | More wires, more expensive |
| Problems | Fewer | Skew and crosstalk |
| Examples | USB, SATA, PCI Express, Ethernet, fibre | Internal buses on a chip and motherboard |
Skew: over a long cable the bits sent together arrive at slightly different times, because each wire is a little different. The receiver can read a mix of old and new bits, so the cable must be short or the transfer slowed down.
Crosstalk: signals in neighbouring wires interfere with each other.
Parallel looks faster because it sends more bits at once, but skew limits its clock speed. Serial links can run at very high speeds without skew, which is why modern connections such as USB and PCI Express are serial (PCI Express gets extra speed by using several serial lanes side by side).