Alohasim is a time-driven simulator of shared access to a communication channel with pure and slotted Aloha protocol written in C. The networked terminals transmit packets randomly, at any time (pure Aloha) or at time intervals multiple of the duration of a packet (Slotted Aloha). It can simulate the presence of a digipeater which retransmits the intact packets it receives on the same channel. When the digipeater repeats a packet, the terminal stations do not transmit.

Alohasim is released under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version, and is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

Table of contents

  • Operation of the simulator
  • Results of the simulations
  • Downloads
  • Screenshots

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  • Operation of the simulator

    The transmission channel is simulated by an array of integers having a number of elements equal to the duration of the simulation.   Each bit, except the most significant one, is associated with a terminal of the network; the most significant bit is reserved for the digipeater, if present.  When a terminal on the network transmits, the corresponding bit of the current time slot and of the subsequent time slots occupied by the packet are set to 1 with a logical OR, and the traffic counter is incremented.  If the simulation is of the "pure Aloha" type, the transmission of each terminal can start at any time.  The situation described is illustrated by the following figure, which represents the content of the array during a "pure Aloha" simulation:


    If the simulation operates in "Slotted aloha" mode, the start of transmission can only take place at precise moments in time, multiples of the duration of a packet.


    When the digipeater is activated, each valid packet is immediately repeated.  During the repetition the transmission of the terminals is inhibited.


    The throughput check is done by analyzing the previous time slots; if there is only one packet in the PACLEN previous time slots (being PACLEN the duration of a packet expressed in time slots), the throughput counter is incremented. To avoid incorrect counts when a station issues two valid packets in succession, valid packets are deleted from the array immediately after being detected (the situation is not shown in the previous figures).

    The simulation is repeated with increasing transmission rate.  At each step, normalized traffic and normalized throughput are recorded and printed on the screen.
    At the end of the simulation, if requested, the result can be exported in graphic form (graph of the normalized throughput as a function of normalized traffic) and/or in a CSV file.   The graphic output consists of an image file in "xbm" format (monochrome, 512x512 pixels), which can be viewed and converted to other formats using external conversion software (eg The GIMP).   The CSV file uses the semicolon ";" as a separator, and can be read by the most common spreadsheets.
    It is possible to set the following simulation parameters by changing them in the source file and recompiling it:

    The duration of the simulation, by varying the DURATION parameter;
    The length of a packet, by varying the PACLEN parameter;
    The number of terminals connected to the shared channel, by varying the BITS parameter;
    The maximum transmission rate, by varying the MAXRATE parameter.

    The higher the DURATION, PACLEN and BITS values the more accurate the simulation, but the more time and computational resources are required.

    The simulation parameters are currently set as follows:

    DURATION 524287
    PACLEN 32
    BITS 32
    MAXRATE 255

    The current settings ensure an accurate result, at the expense of simulation duration and computational resources.

    An acceptable result which requires less computing resources can be obtained by setting the parameters in the source file as follows:

    DURATION 131071
    PACLEN 16
    BITS 24
    MAXRATE 255

    and then by recompiling the source code and running the simulation.

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  • Results of the simulations

    The results of the simulations are reported below (normalized throughput as a function of normalized traffic).  The theoretical reference curves are also shown on each graph.   The theoretical curves without digipeater in the two cases "pure Aloha" and "Slotted Aloha" are those foreseen by the theory; the curves in the two cases with the digipeater are obtained from the previous ones by applying the "renormalization" procedure described in the paragraph 3.1 "An APRS network with a single isofrequency digipeater" of the document "APRS Performance and limits" published on this site.

    Pure Aloha without digipeater


    The simulation result is in excellent agreement with the theoretical curve s = G * exp (-2G), which provides for a maximum throughput of 18.4% in correspondence with normalized traffic G equal to 50% of the transmission channel capacity.

    Slotted Aloha without digipeater


    Also in this case the simulation result is in excellent agreement with the theoretical curve s = G * exp (-G), which provides for a maximum throughput equal to 36.8% in correspondence with a normalized traffic G equal to 100% of the transmission channel capacity.

    Pure Aloha with digipeater


    The simulation result confirms the reduced impact of the digipeater in the case of access to the shared channel with pure Aloha protocol, in accordance with the provisions of paragraph 3.1 "An APRS network with a single isofrequency digipeater" of the document "APRS Performance and limits" (maximum normalized throughput equal to 15.5% corresponding to a normalized traffic equal to 42.2% of the transmission channel capacity).
    The presence of the digipeater in fact involves a reduction of the overall throughput by only 3 percentage points (-3%) in correspondence with an overall traffic just below that expected for pure Aloha (-8%).

    Slotted Aloha with digipeater


    The simulation, in this case, shows that the impact of the digipeater is more significant than in the case of slotted Aloha without the repeater, resulting in a reduction of the maximum normalized throughput by 10% compared to Slotted Aloha access without digipeater.   Also the corresponding normalized traffic is strongly reduced (-30%).

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  • Downloads

    Here is the link to the source code:  alohasim.c

    You can easily compile the source code for your platform with a C compiler.  The source code is written in plain C, no extra libraries or header files are required.  Popular free compilers you can use are MinGW for Windows platforms and gcc for Linux and Mac OS X.

    And here are the links to the previous images in "xbm" format:

    Pure aloha without digipeater: alohasim-p-n.xbm

    Slotted aloha without digipeater: alohasim-s-n.xbm

    Pure aloha with digipeater: alohasim-p-y.xbm

    Slotted aloha with digipeater: alohasim-s-y.xbm

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  • Screenshots

    The main menu: starting the simulation


    End of the simulation


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