CAIE 9618 · Paper 3 · Topic 3.2.3

Wireless
Networking

Wi-Fi · CSMA/CA · WEP/WPA · Bluetooth · RFID · 4G/5G

CSZone Cambridge International AS & A Level Computer Science 9618

Wi-Fi & CSMA/CA

Carrier Sense Multiple Access / Collision Avoidance

Wi-Fi is a wireless LAN technology (IEEE 802.11). Since all devices share the same frequency, CSMA/CA is used to avoid collisions — wireless devices cannot detect collisions while transmitting (unlike CSMA/CD on Ethernet).

CSMA/CA PROCESS

1. Listen — device checks if channel is free (carrier sense)

2. Wait — if busy, wait a random backoff time before checking again

3. Transmit — if free, send RTS (Request To Send) frame

4. CTS — access point sends Clear To Send

5. Send — device transmits data after receiving CTS

6. ACK — acknowledgement confirms receipt

WHY NOT CSMA/CD?

Wireless devices can't detect collisions while transmitting (the transmitted signal drowns out any incoming collision signal). CSMA/CA avoids collisions before they happen using RTS/CTS.

HIDDEN NODE PROBLEM

Two devices may be in range of the access point but out of range of each other — both hear channel as free, causing collision at AP. RTS/CTS solves this.

Wireless Security

WEP, WPA, WPA2, WPA3

Protocol	Full Name	Encryption	Security Level
WEP	Wired Equivalent Privacy	RC4 (40/128-bit)	Broken — cracked in minutes. Not used.
WPA	Wi-Fi Protected Access	TKIP (128-bit)	Improved over WEP but has vulnerabilities
WPA2	Wi-Fi Protected Access 2	AES-CCMP (128-bit)	Currently widely used and secure
WPA3	Wi-Fi Protected Access 3	AES-256 + SAE	Latest standard — strongest protection

ADDITIONAL SECURITY MEASURES

• SSID hiding — don't broadcast network name
• MAC address filtering — allow only known devices

• Strong unique password on router
• Use HTTPS for all web traffic (end-to-end encryption)

Bluetooth & RFID

Short-Range Wireless Technologies

Bluetooth

IEEE 802.15.1 standard, 2.4 GHz ISM band

Range ≈10m (Class 2), up to 100m (Class 1)

Uses frequency hopping (1600 hops/sec) to avoid interference

Forms a piconet (up to 8 devices, 1 master + 7 slaves)

Used for: headphones, keyboards, mice, fitness trackers

Bluetooth LE — low energy for IoT sensors

RFID

RADIO FREQUENCY IDENTIFICATION

Reader emits radio waves — powers passive RFID tags inductively

Tag stores and transmits a unique ID back to reader

Passive tags: no battery, powered by reader field, cheap

Active tags: battery-powered, longer range, more expensive

Uses: contactless payment, stock tracking, passport chips, animal microchips, toll collection

Exam Practice

Cambridge-style questions

Question 1

Explain why CSMA/CA is used in wireless networks rather than CSMA/CD. Include in your answer an explanation of how CSMA/CA avoids collisions. [4]

1

CSMA/CD (Collision Detection) cannot be used in wireless networks because a device cannot hear a collision while it is transmitting — its own signal overwhelms any incoming signals.

1

CSMA/CA (Collision Avoidance) prevents collisions before they occur. A device listens to check the channel is clear before transmitting.

1

If busy, the device waits a random backoff period, then checks again — the random delay reduces the chance two devices will retry at exactly the same moment.

1

RTS/CTS frames can be used: the sender sends a Request To Send to the access point, which responds with Clear To Send, reserving the channel so other devices know not to transmit.

Common Mistakes

Don't lose easy marks

1

Writing "CSMA/CA detects collisions" — it does NOT detect collisions; it avoids them. The key distinction is prevention (before) vs detection (after). "CD" = detect, "CA" = avoid.

2

Saying "WEP is secure enough" — WEP has been broken since 2001 and should never be used. The expected answer is WPA2 (AES) as the minimum, WPA3 as current best. Any suggestion that WEP is acceptable loses marks.

3

Confusing Bluetooth and Wi-Fi — both use 2.4 GHz but they're different technologies for different purposes. Wi-Fi (802.11) = LAN, high data rate, larger range. Bluetooth (802.15) = personal area network (PAN), lower power, shorter range. Don't describe one as an improvement of the other.

Topic Summary — 3.2.3

What You Need to Know

CSMA/CA

Listen → wait random backoff if busy → send RTS → receive CTS → transmit → receive ACK. Used instead of CSMA/CD because wireless can't detect collisions during transmission.

WIRELESS SECURITY

WEP broken · WPA improved · WPA2 (AES) current standard · WPA3 latest. Also: SSID hiding, MAC filtering, strong passwords.

BLUETOOTH & RFID

Bluetooth — 2.4GHz, piconet, frequency hopping, PAN connections (headphones, IoT).
RFID — reader powers passive tags inductively; stores unique ID; used for contactless payment, stock tracking, passports.

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3.3.1

Processors

RISC vs CISC · Pipelining · Parallel Processing · GPU

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