A method has two parts: bits + direction
Every transmission method has two parts. Bits axis: serial sends bits one at a time over a single wire (e.g. USB); parallel sends multiple bits at once over multiple wires (e.g. the internal data bus) but suffers skew over long distances. Direction axis: simplex = one direction only; half-duplex = both directions, one at a time (walkie-talkie); full-duplex = both at once (telephone). Combine one from each, e.g. serial half-duplex.
A packet: header, payload and trailer
Before transmission, data is split into small, equal-sized packets, each with three parts. Header: destination (receiver) address, packet number, and originator (sender) address — usually IP addresses; the packet number lets the receiver reassemble packets that arrive out of order. Payload: the actual data carried. Trailer: an end-of-packet marker plus an error-checking method to detect corruption.
Packet switching: route then reorder
Packet switching sends the message as independent packets: (1) the data is split into packets; (2) a router controls the route each packet takes; (3) each packet can take a different route; (4) packets may arrive out of order; (5) once the last packet arrives they are reordered using the packet numbers. A missing or corrupted packet is re-sent on its own. Examiners note the two marks most often dropped are different routes and reordering at the end.
Drawn from real examiner reports.
Vague serial vs parallel definitions
Define by mechanism, not consequence. Serial = bits sent one at a time over a single wire/channel; parallel = multiple bits sent at once over multiple wires. Answers like "serial is slower" or "parallel sends more data" describe a result and score zero. Always state the number of bits and the number of wires.
Nov 2023 examiner report: serial/parallel descriptions were a strong area, but vague answers that omit the one-wire / many-wire mechanism miss the defining mark.
Giving only one part of the method
A transmission method needs the serial/parallel component and the simplex/half-/full-duplex component; "serial data transmission" alone is incomplete. The common wrong answer is serial simplex when data actually flows both ways. Decide from the scenario: both send and receive at once = full-duplex; take turns = half-duplex; one way only = simplex.
Nov 2023 examiner report, Q6: candidates gave "serial data transmission" instead of "serial simplex"; the most common incorrect duplex answer was serial simplex.
Incomplete packet-switching description
When describing packet switching, many candidates say only that data is split into packets and stop. Two mark points are routinely dropped: each packet can take a different route (chosen by routers) and packets are reordered at the destination once the last packet arrives. Include both for a full technical description.
June 2023 examiner report: few candidates gave a full description of packet switching — the missed points were packets taking a different path and the reordering of packets at the end.
Being vague about USB ("a USB")
Asked for a benefit of USB, weak answers just say "a USB" without naming a property. Name a specific benefit: the connector only fits one way; plug-and-play with drivers loaded automatically; a widely supported, backward-compatible standard; it can also carry power as well as data.
Nov 2023 examiner report: USB answers were too vague such as "USB" — responses must be specific, e.g. name the benefit.
Half-duplex vs full-duplex
Both half-duplex and full-duplex carry data in both directions — the difference is timing. Half-duplex allows only one direction at a time (devices take turns, like a walkie-talkie); full-duplex allows both directions at the same time (like a telephone call). Do not use simplex for either — simplex is strictly one-way.
Header vs payload vs trailer
Keep the packet parts straight. The header holds the destination and sender addresses plus the packet number; the payload is the actual data; the trailer marks the end of the packet and carries the error check. A common slip is naming the payload as the header, or listing the trailer error check as a header item.
Answer with a two-axis grid
For "state the method of data transmission", give two parts: the bits axis (serial = one bit, one wire; parallel = many bits, many wires) and the direction axis (simplex, half- or full-duplex). Write both, e.g. "serial full-duplex".
Recite the packet-switching chain
For "describe packet switching", recite the fixed chain: split into packets -> a router routes each -> packets take different routes -> arrive out of order -> reordered by packet number. The different-routes and reorder-at-the-end steps are the most-missed marks.
Pick the duplex from the scenario
Choose the direction axis by questioning the scenario. Does the receiver ever send back? If never, it is simplex. If it does, can both send at the same instant? If yes, full-duplex; if they must take turns, half-duplex.
When data is sent between devices or across a network, it is rarely sent as one continuous stream. It is broken into packets, sent using a particular method (serial or parallel; simplex, half-duplex or full-duplex), and — across the internet — moved by packet switching. The Universal Serial Bus (USB) is the everyday interface that puts these ideas into practice.
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