ALOHA
An analysis of the protocol
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What is this?
The plot above shows the throughput, or mean success probability of a Markov chain modeling the ALOHA communication protocol, computed using three different methods:
- Gauss-Seidel
- Chain simulation
- Direct method
The following sections present a basic overview of the experiment and the theoretical framework behind it.
ALOHA
This section provides a basic overview or the ALOHA protocol.What it is?
ALOHAnet was a pioneering computer networking system developed at the University of Hawaii, which became operational in June, 1971 and provided the first public demonstration of a wireless packed data network. ALOHA is the protocol designed for the ALOHAnet to use.
How it works?
The first version of the protocol (called Pure ALOHA) was quite simple:
- If you have data to send, send it.
- If, while you are transmitting data, you receive any data from another station, there has been a message collision. All transmitting stations will need to try resending "later".
An improvement to the original ALOHA protocol was "Slotted ALOHA", which introduced discrete timeslots. A station can send only at the beggining of a timesolot, and thus collisions are reduced. The experiment used this slotted version of ALOHA.
How do I read the plot?
In ALOHA, each node has a probability σ of sendind a message. If said message collides, the node resends it with probability τ. These probabilities are the x & y axis of the plot. The z axis is the throughput, i.e. the rate of successful message delivery over a communication channel.
Three layers of data are present on the plot (you can trigger them by clicking the corresponding buttons). The first layer is the throughput computed using the Gauss-Seidel method. The second layer is an approximation obtained by a simulation of the chain. Finally, the third layer are the exact results obtained by resolving the Markov chain's equation system directly.