JPL's Wireless Communication Reference Website Chapter: Data Networks Section: Random Access, ALOHA

Dynamic Frame Length ALOHA

A terminal with a new packet for transmission is allowed to pick any slot in the next frame for transmission. If its transmission is not successful, it will know so because it will not get an acknowledgement from the base station at the end of the frame. In such case it selects at random a slot in the following frame for a retransmissions. The base station can dynamically adapt the number of slots in each frame to ensure stability of the protocol.

The base station monitors the number of collisions, successful transmissions and idle slots in a frame. It uses

• this observation
• an estimate of the arrival rate of new messages, and
• a model of the channel capture probabilities

Figure: Example of Dynamic Frame Length ALOHA. Observing that the last frame contained two collisions (slots in red), one successful transmission (shown in green) and one idle slot (shown in blue), the base station enlarges the frame length to 6 slots to accommodate the expected retransmissions.

Audio

• (.au:) How and why he developed DFL
  Frits Schoute worked on the development of Trunked Private Mobile Radio at Philips in Hilversum. ALOHA was to be used for the call set-up channel. Concerning the stability problem in ALOHA, he did two major things:
  • Compute the average time before instability effects begin to show up, as it depends on the Poisson arrival rate of new packets, lambda.
  • Invent Dynamic Frame Length ALOHA
• (.au:) How DFL works

Exercise

• a successful transmission
• a collision

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