CITS3002 Computer Networks  
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Slotted ALOHA (1972)

The slotted Aloha mechanism makes an obvious improvement to improve expected throughput:

  • the satellite generates a timing pulse.
  • users then only transmit when they get this pulse.

This quantization of time reduces the vulnerability period by half (now to a single packet transmission time).
Hence :

S = G * e -G

Maximum throughput is now achieved when G = 1,

giving S = 1 / e

or a best possible utilization of 36.79%.
Note that small increases in G dramatically reduce performance.

slotted


CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs and WLANs), p5, 20th March 2024.