JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random Access, ALOHA

Dynamic Frame Length ALOHA

A Dynamic Frame Length ALOHA protocol uses frames of n time slots, where n is a parameter that is dynamically chosen by the base station.

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

to estimate the number of terminals in backlog. It computes an optimum new frame length.


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

Hear Frits Schoute, inventor of Dynamic Frame Length ALOHA discussing

Exercise

Consider a vulnerability circle model for signal capture. Find the probability distribution for the number of unsuccessful new packet transmissions in a slot given the slot was observed as



JPL's Wireless Communication Reference Website 1993, 1995.