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AQA 7517 · Paper 2 · 4.9.1

Serial & Parallel
Transmission

Baud rate · Bandwidth · Latency · Simplex/Duplex · Section 4.9

WHAT YOU'LL LEARN
Serial vs parallel · Baud rate · Bandwidth · Latency · Simplex, half-duplex, full-duplex
AQA SPEC LINK
4.9.1 — Data transmission: serial, parallel, baud rate, bandwidth, latency
Serial Transmission

Serial Data Transmission

Serial transmission sends bits one at a time along a single wire/channel. The bits of each byte are sent sequentially.
Simpler, cheaper — only one wire/channel needed
No crosstalk issues — bits can't interfere with each other
Used for: USB, Ethernet, SATA, PCIe — all serial despite appearing to be fast
Good for long distances — signal integrity maintained over cables
Parallel Transmission

Parallel Data Transmission

Parallel transmission sends multiple bits simultaneously across multiple wires. An 8-bit bus sends all 8 bits of a byte at once.
Faster theoretical throughput — 8 bits sent per clock cycle vs 1
Problem: skew — bits arrive at slightly different times over long distances
Problem: crosstalk — adjacent wires interfere at high frequencies
Only practical for short distances — used inside computers (CPU bus, memory bus)
Modern high-speed external interfaces (USB 3, PCIe) use serial because serial can clock faster without skew
Baud Rate

Baud Rate & Bit Rate

Baud rate = number of signal changes per second (symbols per second)
Bit rate = number of bits transmitted per second
If each symbol carries 1 bit: bit rate = baud rate
If each symbol carries 4 bits (e.g. QAM): bit rate = 4 × baud rate
Example: 4800 baud with 2 bits per symbol → bit rate = 9600 bps. ADSL modems use complex modulation to achieve Mbps over phone lines with relatively low baud rates.
Bandwidth & Latency

Bandwidth & Latency

BANDWIDTH
The maximum amount of data that can be transmitted per unit time. Measured in bps, Kbps, Mbps, Gbps. Higher bandwidth = more data/second.
LATENCY
The delay between sending data and it being received. Measured in milliseconds (ms). Affects interactive applications (gaming, video calls) more than throughput-heavy ones (file downloads).
Analogy: Bandwidth = width of a motorway (how many cars/second). Latency = how long it takes to get from A to B.
Direction of Transmission

Simplex, Half & Full Duplex

SIMPLEX
Data flows in ONE direction only. Examples: TV broadcast, keyboard to computer, smoke detector sensor.
HALF-DUPLEX
Data flows in BOTH directions but NOT simultaneously. One party transmits at a time. Examples: Walkie-talkie, some WiFi, HTTP/1.x.
FULL-DUPLEX
Data flows in BOTH directions simultaneously. Examples: Phone call, Ethernet, HTTP/2, modern WiFi.
Synchronous vs Asynchronous

Synchronous & Asynchronous

SYNCHRONOUS
Sender and receiver share a clock signal. Bits sent at regular intervals. Efficient — no start/stop bits needed. Used for: SPI, I2C, Ethernet (with Manchester encoding).
ASYNCHRONOUS
No shared clock. Each byte framed with start bit (0) and stop bit (1). Receiver knows when a byte begins. Used for: UART (serial ports), keyboard input, older modems.
Calculations

Bandwidth Calculations

Time to transfer = File size ÷ Bandwidth

Example: 500 MB file over 100 Mbps connection
500 MB = 500 × 8 Mb = 4000 Mb
Time = 4000 Mb ÷ 100 Mbps = 40 seconds
Remember: 1 byte = 8 bits. Convert carefully! MB/s ≠ Mb/s. ISPs often quote Mb/s (megabits per second) not MB/s (megabytes per second).
AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style
(a) Explain the difference between serial and parallel data transmission. [2]
(b) Give ONE reason why parallel transmission is not used for long-distance data transfer. [1]
(c) A network has a bandwidth of 50 Mbps. Calculate the time to transfer a 250 MB file. Show your working. [3]
(d) Explain the difference between latency and bandwidth. [2]
[8 marks]
2 marks
(a) Serial: bits sent one at a time on a single wire [1]. Parallel: multiple bits sent simultaneously on multiple wires [1]
1 mark
(b) Skew — bits sent at the same time arrive at different times over long distances, causing corruption / crosstalk between wires at high frequencies
3 marks
(c) 250 MB = 250 × 8 = 2000 Mb [1]. Time = 2000 ÷ 50 [1] = 40 seconds [1]
2 marks
(d) Bandwidth is the maximum data transfer rate (capacity) [1]; latency is the delay between sending and receiving data [1]. High bandwidth doesn't mean low latency and vice versa.
Summary

Key Points to Remember

Serial — 1 bit at a time; long distances; no skew; used externally (USB, Ethernet)
Parallel — multiple bits at once; skew at long range; inside computers (buses)
Baud rate = symbols/sec; bit rate = bits/sec; bit rate ≥ baud rate
Bandwidth = capacity (Mbps); Latency = delay (ms)
Simplex: one way; Half-duplex: both ways alternating; Full-duplex: both ways simultaneously
🎉 Lesson complete — move to the quiz!