Data travels in packets
A file is not sent as one continuous stream. It is broken into packets, each sent separately and reassembled at the far end. That arrangement seems roundabout and is the reason networks work at all.
Packets from many different transfers can share the same cable, so one large download does not block everything else. A packet lost or corrupted can be resent on its own rather than restarting the whole file. And packets can take different routes, so a broken link is routed around rather than stopping the transfer.
| Part of a packet | Contains |
|---|---|
| Header | sender address, receiver address, packet number, total packets |
| Payload | the actual data, typically around 64 KB |
| Trailer | an end marker and an error-checking value |
Why the packet number matters
Packets may arrive out of order, because different packets can take different routes and some routes are slower. The header carries the packet number and the total, so the receiver can reassemble them correctly and knows when none are missing. Without that number the data would arrive scrambled, which is why it is part of every header.
Serial and parallel
Serial transmission sends one bit at a time along a single wire. Parallel sends several bits simultaneously along separate wires — eight wires carry eight bits at once.
Parallel is faster over a very short distance, which is why it is used inside a computer between the processor and memory. Over any real distance it fails, because the bits travel at very slightly different speeds down different wires and arrive out of step — a problem called skew. Parallel cables also suffer crosstalk, where the signal in one wire interferes with its neighbours.
Serial avoids both, so every long-distance and external connection is serial: USB, Ethernet, HDMI and the entire internet.
Set Parallel and imagine the wires stretched over a hundred metres. The eight bits set off together and arrive at slightly different times — that is skew, and it is why long-distance links are always serial.
Simplex, half duplex and full duplex
Separately from how many wires there are, transmission has a direction, and there are three possibilities.
Simplex travels one way only, ever — a keyboard to a computer, or a computer to a printer with no status reporting. Half duplex travels both ways but only one at a time, like a walkie-talkie where one person must stop before the other can speak. Full duplex travels both ways simultaneously, like a telephone call or a broadband connection downloading while uploading.
The two classifications are independent
Serial or parallel describes the wires; simplex, half or full duplex describes the direction. A link can be serial full duplex, or parallel half duplex, or any other combination. Exam questions ask for both, and answering only one of the two loses half the marks.
USB
The Universal Serial Bus is the standard the syllabus names, and its advantages are worth knowing as a list.
It is a universal standard, so one connector serves many devices. The connector fits only one way round, so it cannot be inserted incorrectly. Devices are detected automatically and the correct driver loaded — plug and play. It supplies power as well as data, so small devices need no separate supply. And later versions are backward compatible with earlier ones.
Its limits: the cable is restricted to a few metres, and older versions are slow by modern standards, though USB 3 and later are fast enough for external storage.
Before you leave this chapter
- Data is sent in packets: header with addresses and packet number, payload, trailer with error check.
- Packets allow sharing, resending one packet alone, and routing around failures.
- Serial is one bit at a time on one wire; parallel is several at once on several wires.
- Parallel suffers skew and crosstalk over distance, so long links are always serial.
- Simplex one way, half duplex one way at a time, full duplex both at once — independent of serial or parallel.
Choosing a transmission method
Exam questions describe a situation and ask which method suits it. The answer is decided by three questions, asked in order, and each one narrows the choice.
How far does the data travel? Beyond a few centimetres, serial is the only sensible option. Does data need to flow both ways, and simultaneously? That settles simplex, half duplex or full duplex. And how much does reliability matter relative to speed — which decides how much error checking is worth its overhead.
| Situation | Method | Why |
|---|---|---|
| CPU to RAM inside a machine | parallel | distance is tiny, so speed wins |
| Computer to external drive | serial full duplex | a metre or more, reading and writing |
| Keyboard to computer | serial simplex | one direction only |
| Two-way radio | serial half duplex | one party transmits at a time |
| Broadband connection | serial full duplex | upload and download together |
| Across the internet | serial, packet switched | long distance, shared infrastructure |
Answer both classifications
A question asking for "the method of transmission" usually expects two answers: serial or parallel, and simplex, half duplex or full duplex. They are independent, so a full answer names one from each list. Giving only "serial" typically scores half the available marks.