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CAIE 9618 · Paper 1 · Topic 1.2.1

Communication
Methods

Serial · Parallel · Simplex · Half Duplex · Full Duplex · Bandwidth · Bit Rate

CSZone Cambridge International AS & A Level Computer Science 9618
Learning Objectives

By the end of this video you will be able to...

Compare serial and parallel data transmission
Distinguish between simplex, half duplex, and full duplex transmission modes
Define bandwidth and bit rate and calculate data transfer times
Describe latency and its impact on data communication
Serial vs Parallel Transmission

How Are Bits Sent Across a Link?

SERIAL TRANSMISSION
Bits sent one at a time along a single wire
Slower — 1 bit per clock cycle
No synchronisation problems
Used for: USB, Ethernet, long-distance links
PARALLEL TRANSMISSION
Multiple bits sent simultaneously on multiple wires
Faster — 8 bits per clock cycle (e.g. 8-wire bus)
Can suffer from skew at high speeds
Used for: internal computer buses (short distances)
Skew: Bits on different wires arrive at slightly different times, causing errors. This limits parallel transmission to short distances.
Direction of Transmission

Simplex, Half Duplex, Full Duplex

ModeDirectionSimultaneous?Example
Simplex One direction only N/A TV broadcast, smoke alarm sensor
Half Duplex Both directions Not at same time Walkie-talkie, CB radio
Full Duplex Both directions Yes, simultaneously Phone call, Ethernet switch
Full duplex is most efficient — both parties can send and receive at the same time, maximising throughput
Bandwidth and Bit Rate

How Fast Can We Transfer Data?

BANDWIDTH
The maximum capacity of a communication channel
Measured in bits per second (bps, Mbps, Gbps)
Determined by the physical medium
BIT RATE
The actual rate at which data is transmitted
Cannot exceed bandwidth
Affected by: noise, signal strength, distance
TRANSFER TIME FORMULA
Time (s) = File size (bits) ÷ Bit rate (bps)
Example: 50 MB file at 10 Mbps = (50×8×10⁶) ÷ 10×10⁶ = 40 seconds
Latency

Delay in Data Transmission

Latency is the time delay between data being sent and received. It is measured in milliseconds (ms).
Propagation delay — time for signal to travel through the medium
Transmission delay — time to push all bits onto the medium
Queuing delay — time waiting in router/switch buffers
Processing delay — time for a device to process the packet
High bandwidth does NOT mean low latency — they are independent. A fibre link can have high bandwidth but still have latency due to distance.
Transmission Media

Types of Physical Medium

MediumSpeedDistanceUse
Copper (UTP)Up to 10 Gbps~100 m maxLAN Ethernet cabling
Fibre opticTbps+Km+ rangeInternet backbone, long distance
Wireless (radio)Up to ~9.6 Gbps (Wi-Fi 6)VariesWLAN, mobile networks
Fibre advantages: immune to electromagnetic interference, higher bandwidth, lower signal loss over distance, more secure
Exam Practice

Cambridge-style questions

Question 1
Describe the difference between serial and parallel data transmission. Give one advantage of each.
4 marks
✓ MARK SCHEME
1 mark
Serial: bits sent one at a time on a single wire
1 mark
Parallel: multiple bits sent simultaneously on multiple wires
1 mark
Advantage of serial: no skew/synchronisation issues; reliable over long distances
1 mark
Advantage of parallel: faster data transfer (multiple bits per clock cycle)
Exam Practice
Question 2
A file of size 120 MB is to be transferred over a network with a bandwidth of 20 Mbps. Calculate the minimum time in seconds for this transfer.
3 marks
✓ MARK SCHEME
1 mark
Convert: 120 MB = 120 × 8 = 960 Mb = 960,000,000 bits
1 mark
Time = 960,000,000 ÷ 20,000,000 = 48 seconds
1 mark
Answer: 48 seconds
Common Mistakes

Don't lose easy marks

1
Confusing bandwidth with bit rate — bandwidth is the theoretical maximum capacity of a channel. Bit rate is the actual speed data is transmitted at, which may be lower due to interference or congestion.
2
Forgetting to convert MB to Mb in transfer time calculations — 1 MB = 8 Mb. The file size must be in bits, and the bit rate is in bits per second. Mixing units gives a wrong answer.
3
Saying parallel is always faster — at very high speeds, skew causes problems that make parallel unreliable. This is why modern high-speed interfaces (USB 3, PCIe) use serial, not parallel.
4
Confusing half duplex and full duplex — half duplex means only one device can transmit at a time. Full duplex means both can transmit simultaneously. Walkie-talkies need you to say "over" — that's half duplex.
Topic Summary — 1.2.1

What You Need to Know

SERIAL vs PARALLEL
Serial: 1 wire, 1 bit at a time — no skew
Parallel: many wires, many bits at once — faster
Parallel suffers skew at high speed/distance
Modern high-speed uses serial (USB, PCIe)
DIRECTION MODES
Simplex: one direction only
Half duplex: both directions, not at same time
Full duplex: both directions simultaneously
Full duplex is most efficient
BANDWIDTH & BIT RATE
Bandwidth: max capacity of channel (bps)
Bit rate: actual data rate (≤ bandwidth)
Time = file size (bits) ÷ bit rate
Latency: delay independent of bandwidth
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Next Video

1.2.2
Network Topologies
Bus · Star · Ring · Mesh · Hybrid
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