# Q1A · Pure versus Slotted ALOHA

Subject: Computer Networks

Exam: M2

Reader: https://deep-learning-study-room.vercel.app/cn/#m2/mocks/mock-c/q1a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/m2/mocks/mock-c--q1a/

Q1(a)+Q1(b) compulsory; choose any two of Q2/Q3/Q4 · 10 marks attempted

### Question. Compare Pure ALOHA and Slotted ALOHA in terms of start time, vulnerable period and maximum throughput. [2 practice marks]

#### Answer

| Feature | Pure | Slotted |
| --- | --- | --- |
| Start time | Any time | Slot boundary only |
| Synchronization | No common slots required | Required |
| Vulnerable period | 2T | T |
| Maximum S | 1/(2e) ≈ 0.184 | 1/e ≈ 0.368 |
| G at maximum | 0.5 | 1 |

#### Practical example

If the slot duration is 1 ms and a frame becomes ready at 2.4 ms, the next permitted start is 3 ms. Another station choosing the same slot can still collide.