Serial and parallel transmission
Data can be sent between devices in two ways. In serial transmission the bits travel one after another down a single wire. In parallel transmission several bits travel at the same time along several wires side by side.
Parallel is faster over very short distances, but over longer distances the wires can suffer from skew (bits arriving out of step), so serial is more reliable and cheaper. This is why USB, which sends data serially, is used for most external connections.
Key idea
Serial = one bit at a time on one wire. Parallel = many bits at once on many wires.
Direction of transmission
- Simplex — data flows in one direction only (for example, keyboard to computer).
- Half-duplex — data flows both ways, but only one direction at a time (for example, a walkie-talkie).
- Full-duplex — data flows both ways at the same time (for example, a phone call).
Packets and packet switching
Large messages are broken into small packets. Each packet has a header (containing the destination address and packet number), the payload (the actual data) and a trailer. In packet switching the packets can travel by different routes across a network and are reassembled in the correct order when they arrive. Packet switching keeps a network efficient, because if one route is busy or fails, the remaining packets can simply be sent another way without holding up the whole message.
Remember
- USB uses serial transmission.
- Full-duplex allows simultaneous two-way communication.
- Packets may arrive out of order and are reordered using the packet number.