Ethernet/Collision Doubt

Question

A and C are the only two stations on an Ethernet with data rate is 75 Mbps, the distance between station A and C is 4 km, and the propagation speed is 2 x 10⁸ m/s. Station A start sending a long
frame at time t₁ = 1μs ; station C starts sending a long frame at time t₂ = 4.5 μs. The size of the frame is long enough to guarantee the detection of a collision by both stations. Find the time in microseconds when station A hears the collision and when station C hears the collision .
A)15.5, 20
B)21, 24.5 
C)24.5, 29
D)24.5 ,21

Also Please Tell at what distance from A or C the collision had occurred.

Comments

Answer : Option D, Collision occurs at 2350m from A

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Thanx for answer can you explain

Answer 1

'A' starts at 1 unit time. (Not zero, keep in mind). {Stall of 1 unit}

Propogation Time = 20 units (do the calculation)

By the time 'B' starts at 4.5 units, 'A' has already travelled 3.5 units [4.5 - 1.0 ('A' started at 1 unit)]

Remaining time is (20 - 3.5) = 16.5 .

Now, Collision happens at -> 1.0 (Stall) + 3.5 + 16.5/2  = 12.75 units.

'A' detects at -> 1.0 + 2 * (3.5 + 16.5/2) = 24.5 units.

'B' detects at -> 4.5 (B's Start Time) + 2 * 16.5/2 = 21 units

Collision occurs at (From 'A') -> (3.5 + 16.5/2) * (2 * 10⁸) metres = 2350 metres

1 UNIT = 10^-6 Seconds