Consider an Ethernet segment with a transmission speed of 10^8 bits/sec and a…

Key idea: the frame transmission time must be at least the round-trip propagation delay so that a sender can detect a collision.

• One-way propagation delay = segment length ÷ propagation speed = 500 m ÷ (2 × 10^8 m/s) = 2.5 × 10^-6 s.

• Round-trip propagation delay = 2 × one-way delay = 2 × 2.5 × 10^-6 s = 5 × 10^-6 s.

• Minimum frame size (bits) = transmission speed × round-trip delay = 1 × 10^8 bits/s × 5 × 10^-6 s = 500 bits.

• Therefore the minimum frame size required for collision detection is 500 bits.

Note: Practical Ethernet standards often choose a slightly larger minimum frame size (for example 512 bits / 64 bytes in some legacy specifications) to allow for additional margins and protocol overhead; the calculated 500 bits is the theoretical minimum based solely on propagation and bit-rate.