On an Ethernet network using CSMA/CD, when a device has a frame ready to send, what is the first action taken by the CSMA/CD logic before transmitting?

Introduction / Context:
This question tests your understanding of the Carrier Sense Multiple Access with Collision Detection method used in traditional shared Ethernet networks. Although many modern networks are switched and full duplex, CSMA/CD remains an important concept for exams and for understanding how Ethernet evolved.

Given Data / Assumptions:
- The Ethernet network is operating in half-duplex mode and uses CSMA/CD.
- A device on the network has a frame ready to transmit.
- The question asks what the CSMA/CD logic does first before sending the frame.

Concept / Approach:
CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. Carrier sense means that each device listens to the medium to see whether a signal (a carrier) is already present. Multiple access means many devices share the same medium. Collision detection means that if two devices transmit at the same time and a collision occurs, they detect it and back off before retrying. The first step in this process is to listen to the medium to determine whether it is currently idle. Only when the medium is idle is the device allowed to transmit.

Step-by-Step Solution:
Step 1: Recall that carrier sense requires a device to listen for existing traffic on the cable before transmitting.Step 2: When the device has a frame to send, it first checks whether the medium is busy or idle.Step 3: If the medium is busy, the device waits until it becomes idle before attempting transmission.Step 4: Once the medium is idle, the device transmits and continues to monitor the medium to detect any collision.Step 5: If a collision is detected, a jamming signal is sent and the device performs a backoff algorithm before retrying.

Verification / Alternative check:
Ethernet standards and textbooks describe CSMA/CD in this sequence: listen (carrier sense), transmit if idle, detect collisions, send jamming, and back off. This is quite different from token-based approaches such as Token Ring, where a token is passed between devices. Observing how hubs and early shared Ethernet behaved in simulations or labs aligns with this order of operations.

Why Other Options Are Wrong:
Option A describes token passing, which is used in Token Ring and other technologies, not in Ethernet CSMA/CD. Option B mentions a jamming signal, but this is sent only after a collision is detected, not before transmission. Option C refers to discovering whether the destination is on the same subnet, which is not the first step in CSMA/CD media access control. Option E suggests immediate transmission without checking the medium, which would lead to constant collisions and is the opposite of carrier sense.

Common Pitfalls:
Some learners confuse CSMA/CD with token-based access methods and expect a token to be present. Others focus on collision detection and jamming but forget that listening to the medium is the first step. Also, many students learn CSMA/CD historically without realizing that full-duplex switched Ethernet no longer uses collision detection, which can lead to confusion when comparing old and new environments. Remember the name itself: Carrier Sense comes first.

Final Answer:
When a device is ready to send a frame on a CSMA/CD Ethernet, it first listens to the Ethernet to see if the medium is busy or idle before attempting to transmit.