Computer Networks 5th edition by Tanenbaum chapter 4 question 4

Question

In an infinite-population slotted ALOHA system, the mean number of slots a station waits between a collision and a retransmission is $4$. Plot the delay versus throughput curve for this system.
Please help me to find the expression only.

Answer 1

In case of slotted aloha efficiency=G*e^-G

1-chances of success on first attempt in=e^-G(according to poisson distribution)

G attempts made in time slot,P=e^-2=.135 (G=2)

2 - Probability of K collisions=(1-e^-G)^k*(.135)=(.865)^k*(.135)

3-Expected number of transmissions=e^G=e^2=7.38

Comments

why G=2 is taken here ?

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because only 2 attempts are made in a slot

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i m not gettting that why 2 attempts are made in a slot.