Topic 2 · Data transmission
Packets and transmission methods
How data is split into packets, how packet switching routes them, and serial, parallel, simplex, and duplex transmission.
Data travels in packets
Sending a whole file in one piece would block the network and, if anything went wrong, the whole file would need resending. Instead data is split into small, equal-sized packets.
Every packet has three parts:
- Header
- Destination IP address, originator's IP address, and the packet number.
- Payload
- The actual data being carried, a small piece of the file.
- Trailer
- Marks the end of the packet and holds error-checking data, such as a checksum.
Packet switching
Each packet makes its own way across the network. At every router the best available route is chosen, so packets from the same file can take different paths and arrive in a different order.
- 1SplitThe file is broken into numbered packets.
- 2RouteRouters send each packet along the best route at that moment.
- 3ArrivePackets reach the destination, possibly out of order.
- 4ReassemblePacket numbers put the file back in order. Missing packets are requested again.
Exam tip. Benefits: routes avoid busy or broken links, and only lost packets are resent. Drawback: packets can be lost or delayed, and reassembling takes time.
Serial and parallel
Serial transmission sends bits one after another down a single wire. Parallel transmission sends several bits at the same time down several wires.
| Serial | Parallel | |
|---|---|---|
| Wires | One | Several (often 8 or more) |
| Speed | Slower over short distances | Faster over short distances |
| Distance | Reliable over long distances | Bits can arrive out of sync (skew) over long distances |
| Cost | Cheaper cable | More expensive cable |
| Example | USB, connecting to the internet | Inside a CPU and between chips |
Simplex, half-duplex, and full-duplex
Direction matters too:
| Method | Direction | Example |
|---|---|---|
| Simplex | One direction only | Computer to printer, a radio broadcast |
| Half-duplex | Both directions, but only one at a time | Walkie-talkie |
| Full-duplex | Both directions at the same time | Phone call, broadband connection |
USB
The Universal Serial Bus (USB) is a standard serial interface for connecting devices to a computer.
- Benefits: one standard connector for many devices; devices are detected and drivers loaded automatically; it can power and charge devices; it is backwards compatible.
- Drawbacks: limited cable length (a few metres); it can be slower than some other connections; older versions transfer data slowly.
Key terms
- Packet
- A small unit of data sent across a network, with a header, payload, and trailer.
- Packet switching
- Sending packets independently, each taking the best route, then reassembling them.
- Router
- A device that directs packets towards their destination.
- Serial
- Transmitting one bit at a time down a single wire.
- Full-duplex
- Data can travel in both directions at the same time.
Check yourself
Name three items stored in a packet header.Show answer
The destination IP address, the originator's IP address, and the packet number.
Why do packets need a packet number?Show answer
Packets can take different routes and arrive out of order, so the number lets them be reassembled in the right order.
Why is serial transmission used for long distances instead of parallel?Show answer
In parallel, bits on different wires can arrive at slightly different times (skew), corrupting data. Serial sends one bit at a time, so this cannot happen, and the cable is cheaper.