Which Access Method Listens Before Transmitting?
In media-access control (MAC), “listens before transmitting” corresponds to carrier sensing—the node checks whether the shared channel appears idle/busy before it sends. The access method that explicitly uses this “listen” behavior is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), which senses the medium before transmitting.
A good way to map the options:
Answer: (ii) CSMA/CA.
Key concepts you’ll use:
- carrier sensing
- collision avoidance
- random access
- token
CSMA/CD vs CSMA/CA (Carrier Sense) — quick intuition
Compare the four access methods (conceptually)
CSMA/CA is designed around the idea that you listen first. In contrast:
- Slotted-ALOHA uses time slots and probabilistic transmission; it does not rely on sensing the channel before sending.
- ALOHA transmits based on randomness; it does not listen before transmitting.
- Token passing uses permission (token) rather than sensing before each transmission.
Which access method “listens” before transmitting?
Qualitative comparison (carrier sensing vs no sensing).
How to answer “Which one listens before transmitting?”
- 1Step 1
Interpret “listens before transmitting” as using carrier sensing (checking the channel state).
- 2Step 2
Choose the method explicitly based on “sense” before sending—this is CSMA/CA.
- 3Step 3
Remove Slotted-ALOHA and ALOHA because they rely on timing/probability, not sensing; remove token passing because permission comes from token ownership, not listening.
- 4Step 4
The correct choice is (ii) CSMA/CA.
Pro Tip
When you see “CSMA,” read it as “Carrier Sense Multiple Access.” “Sense” is the listening step.
Common trap
Don’t confuse “collision avoidance” with “listening.” In CSMA/CA, listening is the carrier-sensing part; collision avoidance is the additional strategy used after sensing.
Decision Logic During Transmission
Carrier sensing (listen)
1. Channel assessmentCSMA/CA checks whether the medium appears idle before transmitting."
No carrier sensing
2. Permission / scheduleALOHA / slotted-ALOHA transmit based on probability or time slots; token passing depends on token ownership."
Send or back off
3. Transmit decisionCSMA/CA may back off to avoid collisions; others transmit according to their rule."
Quick FAQ
Knowledge Check
Which access method listens (carrier senses) before transmitting?