ALOHA - exercises


This lesson consists of four exercises. The purpose of the exercises is to study the performance of the communication network which uses the ALOHA protocol. The simulation of ALOHA scheme is performed by the applet described in lesson 3.

The exercises are the following:

  1. A study of influence of the retransmission delay window (CWmax) length on the pure ALOHA scheme parameters.
    A simulation of the ALOHA access method for three lengths of the retransmission window, i.e. CWmax=10, 40, and 80 time units.
  2. Comparison of three versions of the pure ALOHA protocol implemented in the applet.
    A simulatoin of three versions of the pure ALOHA scheme implemented in the applet (Aloha 1/2/3). The retransmission window length (CWmax) is constant and equals 40 time units.
  3. Comparison of the pure ALOHA scheme and the slotted ALOHA scheme performance.
    A simulation of both schemes for the same length of the retransmission window (CWmax= 40 time units).
  4. Comparison of the performance of the slotted Aloha scheme for constant length retransmission window and exponentially rising length of this window.

All simulations should use the same applets which were used in lesson 3. The only difference is that now the applets run with a random initial number in all generators. In lesson 3 the generator initial values were the same for each program run.

Helpful hints:

Before each simulation

  1. Set the parameters of the system in window Settings (protocol type: Aloha 1/2/3 and type of the delay window: constant/exponential).
  2. Set the time of the simulation in the number of the time units (e.g. 1 million of time units).

In order to find the system performance the simulation has to be repeated several times for different values of the traffic intensity. The following steps have to be done for each simulation:

  1. Restart the system (Restart button).
  2. Set the traffic intensity for each user (Traffic intensity window).
  3. Start the simulation (button ).
  4. Read the results of the simulation from the Simulation results window.

The simulation enables us to find the system characteristics (average delay, throughput, loss ratio, etc.) versus traffic offered to the channel. The traffic offered to the channel (window Simulation results: Total traffic) is typically greater than the traffic declared in window System info: Declared traffic. The traffic offered to the channel consists of new packets and retransmitted packets.

The results of the simulations should be written to the table shown below.

Table 4.1 The examplary table for simulation experiments results

Traffic intensity / user Declared traffic Simulation results
Traffic Average delay Throughput Loss ratio
Total New Retransmitted
1 2 3 4 5 6 7 8
... ... ... ... ... ... ... ...

The 1st column contains the traffic intensity generated by a single user. We assume that each user generates the same traffic intensity. The number written in this column is set by a student. The other columns contain results of the simulations.
The 2nd column contains the value of the total traffic declared in the system. It accumulates the traffic generated by each user of the system. The same value can be found in System info window.
Columns 3rd to 8th contain results of the simulation program run found in Simulation results window.