Pure Aloha (sender may start transmission during any time) has a maximum efficiency that is half that of Slotted Aloha (sender can only start transmission at synchronised time slots). Explain in detail why Pure Aloha is only able to achieve half the efficiency.
Try an answer before revealing the guidance below.
Key Concepts
- Vulnerable period
- Frame time
- Slotted ALOHA
- Pure ALOHA
- Collision
Answer Approach
- Draw or imagine one frame’s transmission interval.
- Find the time span in which another start causes a collision for pure ALOHA.
- Compare it with the slotted case.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
In pure ALOHA, another sender beginning up to one frame time before or one frame time after a frame begins can overlap it: a vulnerable period of two frame times. Slot synchronization confines starts to slot boundaries, so slotted ALOHA has a one-frame-time vulnerable period. Under the standard random-attempt model, their maximum throughputs are 1/(2e) and 1/e respectively.
Pure ALOHA maximum = 1 / (2e)Slotted ALOHA maximum = 1 / eShortcuts: K concepts · A approach · F answer · R reviewed · B bookmark · ← / → previous / next