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.
to estimate the number of terminals in backlog. It computes an optimum
new frame length.
- 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.
Hear Frits Schoute, inventor of Dynamic Frame Length ALOHA
.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
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
- a successful transmission
- a collision