ITPEC FE Morning April 2021 Question 31

Wait for an idle medium, and after a collision wait a random backoff time — that is how a node on a CSMA/CD LAN transmits.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection), the method of classic Ethernet, has no transmission right. Each node listens to the shared medium and sends only when it is idle. Two nodes may still start at once; they detect the collision, stop, and each waits a random time before retrying, so they are unlikely to collide again.

Answer (c)

Why not others:

• •(a) — a special frame circulating around a ring, held by the node allowed to send, is token ring
• •(b) — a transmission right passed along a logical order of nodes is token bus, a token-passing method on a bus
• •(d) — sending only in an assigned time slot is TDMA (time division multiple access)

Key rule: Token methods avoid collisions by granting a right to send; CSMA/CD lets any node send and recovers from collisions with a random backoff.

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