Data Transmission (Years 9–11) — Computer Science

How data is broken into packets and sent along wires as serial or parallel, simplex, half-duplex or full-duplex traffic.

When you send a photo to a friend it does not travel as one giant blob. The computer slices it into hundreds of small packets, stamps each with an address, and fires them off to weave their own way across the network — sometimes by different routes — before they are reassembled at the far end. Understanding how those bits travel, and along how many wires, is the heart of data transmission.

Why this matters: Every download, video call and USB stick you plug in uses these ideas. Cambridge expects you to describe packet structure, compare serial with parallel, and match simplex, half-duplex and full-duplex to real devices.

A packet: a header (addresses and number), the payload (data), and a trailer (error check).

Packets and packet switching

Large messages are divided into equal-sized packets. Each packet carries three parts:

Header — the sender and receiver IP addresses, the packet number (so they can be reordered), and how many packets make up the message.
Payload — the actual chunk of data being carried.
Trailer — an error-checking value (such as a checksum) and a marker showing the packet has ended.

In packet switching each packet finds its own route across the network, so they may arrive out of order or by different paths. The packet numbers let the receiving device reassemble them correctly, and any missing packet is requested again.

Serial vs parallel

Serial sends bits one after another down a single wire; parallel sends several bits at once on several wires.

	Serial	Parallel
Wires	One data line	Several data lines (e.g. 8)
Speed over distance	Reliable over long distances	Faster over short distances
Cost	Cheaper (fewer wires)	More expensive
Skew / interference	No skew; less interference	Bits can arrive out of step (skew)
Used by	USB, network cables	Internal computer buses

Direction: simplex, half-duplex, full-duplex

Simplex — one direction only (e.g. a keyboard to the computer, or a TV broadcast).
Half-duplex — both directions, but only one at a time (e.g. a walkie-talkie).
Full-duplex — both directions at the same time (e.g. a phone call or broadband).

USB

The Universal Serial Bus is a serial connection. Advantages: it is fitted the same shape everywhere, it is fast, it can charge/power the device, and it is fitted automatically (plug-and-play). Drawbacks: the standard cable length is limited (a few metres), and older USB standards are slower than some rivals.

Worked example (Describe, 2 marks): A photo is sent over the internet. Describe how packet switching delivers it.
The photo is split into numbered packets (1 mark). Each packet travels independently, possibly by different routes, and they are reassembled in order using their packet numbers at the destination (1 mark).

Common mistake: Muddling half-duplex with full-duplex. A walkie-talkie is half-duplex (press to talk, release to listen); a phone call is full-duplex (both speak at once). Also, do not call parallel "always faster" — over long distances serial wins because parallel suffers from skew.

Memory hook: Simplex = single direction, duplex = double. Half takes turns, full does both at full pelt.