IPA FE Subject A 2026 Question 8
A question IPA Japan published in 令和８年度 基本情報技術者試験 科目A 公開問題 問8, translated into English. ITPEC is based on the same exams.
In a wireless LAN, when radio waves of the same frequency transmitted from multiple terminals collide, the radio waves interfere with each other, so the signals may not be demodulated when they are received. Which of the following is used to avoid this problem?
Explanation
CSMA/CA — wireless LAN stations avoid collisions, since they cannot detect a collision while they are transmitting.
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is the access control method of IEEE 802.11 wireless LANs. A station first checks whether the channel is in use (carrier sense). When it is free, the station still waits a fixed interval plus a random backoff time before sending, so stations that were waiting for the same channel do not all start at once. The receiver returns an ACK, and a frame without an ACK is sent again. RTS/CTS can also reserve the channel in advance.
A radio cannot listen while it sends, so the wired LAN method of detecting collisions (CSMA/CD) does not work; collisions have to be avoided instead.
Answer (a)
Why not others:
- •(b) SSID — the name that identifies a wireless LAN network; it does nothing about radio interference
- •(c) Carrier aggregation — bundles several frequency bands (carriers) to raise the speed of a mobile connection such as LTE or 5G
- •(d) Tethering — shares a smartphone’s mobile Internet connection with other devices
Key rule: Wired Ethernet detects collisions (CSMA/CD); a wireless LAN avoids them with carrier sense plus a random backoff (CSMA/CA).
Networks · AI-written, may contain errors
This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.