1.2.1 Communication Methods — Cambridge 9618

Serial vs Parallel Communication

Data can be transmitted between devices in two fundamental ways:

Serial Communication

In serial communication, bits are sent one after another along a single wire (or channel), one bit at a time.

More reliable over long distances (no timing skew between bits)
Cheaper to implement (fewer wires)
Used in USB, SATA, Ethernet, internet connections

Parallel Communication

In parallel communication, multiple bits are sent simultaneously along multiple wires, one bit per wire per clock cycle.

Theoretically faster for short distances (more data per cycle)
Over longer distances: timing skew causes errors (bits arrive at different times)
More expensive (more wires/connectors)
Historically used inside computers (system buses, printer ports)

Property	Serial	Parallel
Wires used	1 (or few)	Many (e.g. 8 for 1 byte at once)
Speed (short distances)	Slower	Faster (more bits per cycle)
Reliability (long distances)	More reliable	Less reliable (clock skew)
Cost	Cheaper	More expensive
Examples	USB, Ethernet, SATA, internet	Old printer ports, internal buses

Synchronous vs Asynchronous Data Transmission

Synchronous Transmission

In synchronous transmission, sender and receiver share a common clock signal. Data is transmitted at fixed, regular intervals.

No start/stop bits needed — more efficient, faster
Both devices must be synchronised (same clock)
Used for high-speed data transfer between devices (e.g. bus communications, HDMI)

Asynchronous Transmission

In asynchronous transmission, there is no shared clock. Instead, each character/byte is framed with a start bit and one or more stop bits so the receiver knows when data begins and ends.

Start bit (logic 0) signals start of data
Stop bit(s) (logic 1) signal end of character
Less efficient (extra bits per character), but simpler and suitable for irregular data
Used in serial communication to keyboards, mice, older modems

Simplex, Half-Duplex and Full-Duplex

Mode	Direction	Example
Simplex	One direction only — transmitter sends, receiver receives; cannot reverse	TV broadcast, radio, keyboard input
Half-duplex	Both directions, but only one direction at a time	Walkie-talkie, older Ethernet hubs
Full-duplex	Both directions simultaneously	Telephone call, modern Ethernet, Wi-Fi

Bandwidth and Baud Rate

Bandwidth

Bandwidth is the maximum amount of data that can be transmitted per second, measured in bits per second (bps), Mbps or Gbps.

Baud Rate

Baud rate is the number of signal changes (symbols) per second. If each symbol encodes one bit, baud rate = bit rate. If each symbol encodes multiple bits (e.g. QAM), bit rate = baud rate × bits per symbol.

Latency

Latency is the time delay between a signal being sent and received. Low latency is critical for real-time applications (video calls, online gaming).

Exam tip: Cambridge exam questions commonly ask for the difference between serial and parallel, or synchronous and asynchronous. Know that serial is more reliable over long distances (no skew). For synchronous vs asynchronous: asynchronous uses start/stop bits, synchronous uses a shared clock. For duplex modes: simplex (one-way), half-duplex (one at a time), full-duplex (simultaneous both ways).

⚠️ Common Mistakes

Thinking parallel is always faster than serial — over long distances, parallel is slower due to skew
Confusing baud rate with bit rate — they are only equal if each symbol carries one bit
Saying asynchronous means "no timing" — asynchronous still has timing within each character; the start/stop bits provide synchronisation
Confusing half-duplex with simplex — half-duplex can go both ways but not simultaneously

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Worksheet — 1.2.1 Communication Methods

8 questions · instantly marked · Cambridge 9618 standard

Q1State two differences between serial and parallel data transmission.[4]

✅ Mark scheme

Mark scheme

Any two differences (2 marks each): Serial sends one bit at a time on one wire; parallel sends multiple bits simultaneously on multiple wires [2]; Serial more reliable over long distances (no clock skew); parallel more reliable over short distances only [2]; Serial cheaper to implement; parallel more expensive [2].

Q2Explain what is meant by 'clock skew' in parallel communication and why it causes problems.[3]

✅ Mark scheme

Mark scheme

Clock skew is the difference in arrival times of bits sent simultaneously on parallel wires [1]; because wires have slightly different lengths/properties, bits arrive at slightly different times [1]; the receiver may read bits at the wrong time, causing data corruption — the problem worsens over longer distances [1].

Q3Distinguish between synchronous and asynchronous data transmission.[4]

✅ Mark scheme

Mark scheme

Synchronous: sender and receiver share a common clock signal; data sent at fixed regular intervals; no start/stop bits needed — more efficient [2]; Asynchronous: no shared clock; each character framed with start bit (0) and stop bit(s) (1) to synchronise the receiver; used for irregular data transmission [2].

Q4Explain the role of start bits and stop bits in asynchronous communication.[3]

✅ Mark scheme

Mark scheme

The start bit (logic 0) signals the beginning of a character to the receiver, allowing it to begin sampling [1]; the stop bit(s) (logic 1) signal the end of the character [1]; this framing allows synchronisation of sender and receiver without a shared clock signal [1].

Q5State the difference between simplex, half-duplex and full-duplex communication. Give one example of each.[6]

✅ Mark scheme

Mark scheme

Simplex: data flows in one direction only [1]; example: TV/radio broadcast, keyboard input [1]; Half-duplex: data flows in both directions but only one direction at a time [1]; example: walkie-talkie / CB radio [1]; Full-duplex: data flows in both directions simultaneously [1]; example: telephone call / modern Ethernet switch [1].

Q6Explain why USB connections use serial rather than parallel communication.[2]

✅ Mark scheme

Mark scheme

USB cables connect devices over longer distances where parallel would suffer from clock skew causing errors [1]; serial is more reliable and cheaper to implement (fewer wires in the cable), and modern serial can achieve very high speeds [1].

Q7Define bandwidth and latency. Explain why both are important for a video streaming service.[4]

✅ Mark scheme

Mark scheme

Bandwidth: the maximum amount of data that can be transmitted per second [1]; Latency: the time delay between data being sent and received [1]; For video streaming: high bandwidth is needed to transmit large video data at sufficient quality without buffering [1]; low latency ensures the video starts quickly and keeps up with playback — particularly important for live streaming [1].

Q8A baud rate of 9,600 baud is used, with each symbol encoding 2 bits. Calculate the bit rate (data transfer rate) in bps.[2]

✅ Mark scheme

Mark scheme

Bit rate = baud rate × bits per symbol [1]; = 9,600 × 2 = 19,200 bps [1].

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Term	Definition

Serial vs Parallel Communication

Serial Communication

Parallel Communication

Synchronous vs Asynchronous Data Transmission

Synchronous Transmission

Asynchronous Transmission

Simplex, Half-Duplex and Full-Duplex

Bandwidth and Baud Rate

Bandwidth

Baud Rate

Latency

Worksheet — 1.2.1 Communication Methods

Mini Test — 1.2.1 Communication Methods