Probabilistic analysis of CSMA/CA 802.11e

Modelling contention in IEEE 802.11 (802.11e) standard – applet

This applet allows to model, analyse and predict IEEE 802.11 EDCA behaviour by means of parameters of wireless stations. The user chooses parameters of stations competing for gaining the access to the wireless medium. The applet displays a variety of information, in particular the probabilities of winning the contention and the probability of collision.

The applet has a number of functions and types of display, see the following description:

Technical information

The applet has been tested in Java (including Java plugin in Mozilla Firefox browser) 1.6 (1.6.0_23).
The standalone desktop application can be downloaded from here. This version allows also saving and loading user's configurations for the stations.

Basic information

The EDCA method is a distributed method. In practice it means that every station willing to utilize variable waiting period must support 802.11e standard and its QoS implementation.
The applet represents a single contention instance in the case when no collision appeared within the previous contention.
Of course, one can mimick any contention scenario by setting proper AIFSs and CWs. Stations not suppporting variable waiting period can be modelled this way as well.

Matrix of stations

Integers placed at particular positions in the matrix represent number of stations sharing the respective combination of AIFS and CW. Letters K, E, O, I are located at positions corresponding to typical setting of stations as recommended by 802.11e. See the Legend.
The numbers along the two axes represent the respective time slots /TS/.

AIFS is the fixed part of the waiting period.
CW determines the range from which the variable waiting period of a station is randomly generated.
Example: when AIFS = 7 TS and CW = 3 TS, then the possible waiting period is 7, 8, 9 or 10 time slots.

Placing mouse cursor over a specific field in the matrix shows the probability Pwin /in regime Current state/ or both Pwin and Pcoll /in regime After adding a station/.

Show

Serves for choosing whether Pwin or PColl shall be displayed via colour.
(When Pcoll is chosen in the Current state regime, then every field shows the same colour which reflects the probability of collision.)

Calculation mode

Current state: Serves to analyze model of the network consisting of the present stations. The probability of winning is computed for each station. The probability that a collision occurs is computed as well. The result is displayed in the table entitled Detail.
After adding a station: The applet shows virtual probabilities — “What would Pwin/Pcoll be in case a new station would be added with such parameters?”

Pointer mode

Edit: Left mouse button adds a station to the Matrix of stations. The right click removes one.
Show details: In regime After adding a station, an independent network model is hidden under (and computed at) every field of the Matrix of stations.
Left mouse button pushed at a field in the Matrix of stations fills the table Detail with parameters of such model.

Detail

Shows AIFS, CW, number of stations, and winning probabilities Pwin in percent for every station present in the network model.
The collision probability Pcoll is shown just below the table.

Options

Settings: Opens a dialog, where the size of the Matrix of stations can be determined, as well as the parameter aCWmin, which influences the distribution of access categories in the Matrix of stations /see Legend/. In case of the standalone application, two buttons Save and Load are active here serving to manupulate the network configurations.
Reset: Clears all user defined stations. The size of the matrix does not change.
Update + Real Time: In case of a high number of stations, and in particular in the regime After adding a station the computation can take a long time. After leaving Real Time unchecked the computation and display is performed only after pushing the Update button.

Gamma

Allows emphasizing certain part of the probability colormap.

Choose colormap

Allows choosing three standard colormaps for presenting the probabilities: Jet, Hot and Grey.