Pure ALOHA Vulnerable Time vs Frame Transmission Time

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Nov 4, 2026

In pure ALOHA, a transmitted frame is vulnerable to collision during a time window that starts when another station begins transmitting up to the instant your frame finishes. This vulnerable time is twice the frame transmission time, i.e., $2T$ where $T$ is the frame transmission time. Therefore, for the question:

“In pure ALOHA, the vulnerable time is ______ the frame transmission time.”

the correct option is: (ii) two times.

Key intuition:

In pure ALOHA, frames can start at any time (not aligned to slots).
If another frame begins within your transmission period (and even slightly before it ends), overlap occurs → collision is possible.
That overlap possibility spans a total duration of $2T$ (from $-T$ to $+T$ around your frame’s start/end window).

Mermaid overlap visualization:

Keywords: pure ALOHA , vulnerable time , frame transmission time , collision

Why pure ALOHA has vulnerable time = $2T$

1
Step 1
Let $T$ be the time to transmit one full frame.
2
Step 2
Your frame occupies the interval from $0$ to $T$ .
3
Step 3
If another frame starts at time $t=-T$ , it will still overlap with your frame for the interval $0$ to $T$ .
4
Step 4
If another frame starts at time $t=+T$ , it overlaps at the boundary moment; thus collisions are possible up to the end of the $2T$ window depending on overlap definition in the model.
5
Step 5
Therefore, collision vulnerability spans from $t=-T$ to $t=+T$ , totaling $2T$ .

Vulnerable time window in ALOHA (pure vs slotted)

Pure ALOHA vulnerable time is $2T$ (random start times).

Quick check: mapping to the MCQ

Pro Tip

When you see “vulnerable time” in ALOHA MCQs: pure ALOHA → $2T$ , slotted ALOHA → $T$ .

Common pitfall

Don’t confuse probability of successful transmission with vulnerable time. Vulnerable time is a time-window overlap argument; success probability is derived from it under assumptions about traffic (often Poisson arrivals).

Collision vulnerability window for one pure ALOHA frame

Collision can start

t = -T

Another station begins up to $T$ before your frame starts, causing possible overlap."

Your frame starts

t = 0

Your frame runs from $0$ to $T$ ."

Your frame ends

t = T

Other frames that started before/near this time can still collide."

Collision window ends

t = 2T

The last time a new frame can start while still overlapping your vulnerable interval is $2T$ ."

Knowledge Check

Question 1 of 4

Q1Single choice

In pure ALOHA, the vulnerable time is ______ the frame transmission time.

the same as

two times

three times

None of the above

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