CSMA/CD Flow Diagram

              +---------------------+
              |  Station ready to    |
              |  transmit a frame    |
              +---------------------+
                        |
                        v
              +---------------------+
         +--->|  Sense the channel   |
         |    +---------------------+
         |              |
         |         Channel busy?
         |         /          \
         |       Yes            No
         |        |              |
         |  (wait, keep          v
         |   sensing)   +----------------------+
         |              | Begin transmitting    |
         |              | the frame              |
         |              +----------------------+
         |                        |
         |                        v
         |              +----------------------+
         |              | Continue listening    |
         |              | WHILE transmitting     |
         |              +----------------------+
         |                        |
         |              Collision detected?
         |               /              \
         |             Yes               No
         |              |                 |
         |              v                 v
         |    +-------------------+  +-------------------+
         |    | Send jam signal;   |  | Finish transmitting|
         |    | abort transmission |  | frame successfully |
         |    +-------------------+  +-------------------+
         |              |
         |              v
         |    +-------------------+
         +----| Wait RANDOM backoff|
              | time, then retry   |
              +-------------------+

Explanation: A station first senses the channel ("listen before talk"). If busy, it waits; if idle, it begins transmitting while continuing to listen for a collision (unlike ALOHA, which doesn't sense at all). If a collision is detected mid-transmission, it immediately stops, sends a jam signal to ensure all colliding parties notice, and backs off for a random time before retrying — saving bandwidth by aborting early instead of completing a doomed transmission.

Comparison: CSMA/CD vs ALOHA

Aspect	ALOHA	CSMA/CD
Channel sensing before transmit	None — transmits blindly whenever ready	Senses channel first; only transmits if idle
Collision detection during transmission	No — only inferred later via missing ACK	Yes — detects collision while still transmitting and aborts early
Vulnerable time	2 x Tfr (pure) or 1 x Tfr (slotted)	Much smaller — limited to the propagation-delay window before carrier sense takes effect
Maximum throughput	~18.4% (pure) / ~36.8% (slotted)	Significantly higher — CSMA/CD on a well-designed LAN can approach 80–90%+ under moderate load
Wasted transmission time on collision	Full frame time wasted (no early detection)	Only a fraction of frame time wasted, since transmission is aborted as soon as collision is detected

In short, CSMA/CD's "listen before and during transmission" strategy is a substantial improvement over ALOHA's completely uncoordinated transmission approach, both reducing collision frequency and limiting the cost of the collisions that do occur.