Table Of Contents
Table Of Contents

Node Mobility

Overview

In order to simulate ad-hoc wireless networks, it is important to model the motion of mobile network nodes. Received signal strength, signal interference, and channel occupancy depend on the distances between nodes. The selected mobility models can significantly influence the results of the simulation (e.g. via packet loss rates).

A mobility model describes position and orientation over time in a 3D Euclidean coordinate system. Its main purpose is to provide position, velocity and acceleration, and also angular position, angular velocity, and angular acceleration data as three-dimensional quantities at the current simulation time.

In INET, a mobility model is most often an OMNeT++ simple module implementing the motion as a C++ algorithm. Although most models have a few common parameters (e.g. for initial positioning), they always come with their own set of parameters. Some models support geographic positioning to ease the configuration of map based scenarios.

Mobility models be single or group mobility models. Single mobility models describe the motion of entities independent of each other. Group mobility models provide such a motion where group members are dependent on each other.

Mobility models can also be categorized as trace-based, deterministic, stochastic, and combining models.

Using Mobility Models

In order for a mobility model to actually have an effect on the motion of a network node, the mobility model needs to be included as a submodule in the compound module of the network node. By default, a transceiver antenna within a network node uses the same mobility model as the node itself, but that is completely optional. For example, it is possible to model a vehicle facing forward while moving on a road that contains multiple transceiver antennas at different relative locations with different orientations.

The Scene

Many mobility models allow the user to define a cubic volume that the node can not leave. The volume is configured by setting the constraintAreaX, constraintAreaY, constraintAreaZ, constraintAreaWidth, constraintAreaHeight and constraintAreaDepth parameters.

If the initFromDisplayString parameter, the initial position is taken from the display string. Otherwise, the position can be given in the initialX, initialY and initialZ parameters. If neither of these parameters are given, a random initial position is choosen within the contraint area.

When the node reaches the boundary of the constraint area, the mobility component has to prevent the node to exit. Many mobility models offer the following policies:

  • reflect of the wall
  • reappear at the opposite edge (torus area)
  • placed at a randomly chosen position of the area
  • stop the simulation with an error

Built-In Mobility Models

List of Mobility Models

The following, potentially list contains the mobility models available in INET. Nearly all of these models als single mobility models; group mobility can be implemented e.g. with combining other mobility models.

Stationary

Stationary models only define position (and orientation), but no motion.

Deterministic

Deterministic mobility models use non-random mathematical models for describing motion.

  • LinearMobility moves linearly with a constant speed or constant acceleration.
  • CircleMobility moves around a circle parallel to the XY plane with constant speed.
  • RectangleMobility moves around a rectangular area parallel to the XY plane with constant speed.
  • TractorMobility moves similarly to a tractor on a field with a number of rows.
  • VehicleMobility moves similarly to a vehicle along a path especially turning around corners.
  • TurtleMobility moves according to an XML script written in a simple yet expressive LOGO-like programming language.
  • FacingMobility orients towards the position of another mobility model.

Trace-Based

Trace-based mobility models replay recorded motion as observed in real life.

  • BonnMotionMobility replays trace files of the BonnMotion scenario generator.
  • Ns2MotionMobility replays files of the CMU’s scenario generator used in ns2.
  • AnsimMobility replays XML trace files of the ANSim (Ad-Hoc Network Simulation) tool.

Stochastic

Stochastic or random mobility models use mathematical models involving random numbers.

Combining

Combining mobility models are not mobility models per se, but instead, they allow more complex motions to be formed from simpler ones via superposition and other ways.

  • SuperpositioningMobility model combines several other mobility models by summing them up. It allows creating group mobility by sharing a mobility model in each group member, separating initial positioning from positioning during the simulation, and separating positioning from orientation.
  • AttachedMobility models a mobility that is attached to another one at a given offset. Position, velocity and acceleration are all affected by the respective quantites and also the orientation of the referenced mobility.

More Information on Some Mobility Models

TractorMobility

Moves a tractor through a field with a certain amount of rows. The following figure illustrates the movement of the tractor when the rowCount parameter is 2. The trajectory follows the segments in 1,2,3,4,5,6,7,8,1,2,3... order. The area is configured by the x1, y1, x2, y2 parameters.

../_images/tractormobility.png

RandomWaypointMobility

In the Random Waypoint mobility model the nodes move in line segments. For each line segment, a random destination position (distributed uniformly over the scene) and a random speed is chosen. You can define a speed as a variate from which a new value will be drawn for each line segment; it is customary to specify it as uniform(minSpeed, maxSpeed). When the node reaches the target position, it waits for the time waitTime which can also be defined as a variate. After this time the the algorithm calculates a new random position, etc.

GaussMarkovMobility

The Gauss-Markov model contains a tuning parameter that control the randomness in the movement of the node. Let the magnitude and direction of speed of the node at the \(n\)th time step be \(s_n\) and \(d_n\). The next speed and direction are computed as

\[s_{n+1} = \alpha s_n + (1 - \alpha) \bar{s} + \sqrt{(1-\alpha^2)} s_{x_n}\]
\[d_{n+1} = \alpha s_n + (1 - \alpha) \bar{d} + \sqrt{(1-\alpha^2)} d_{x_n}\]

where \(\bar{s}\) and \(\bar{d}\) are constants representing the mean value of speed and direction as \(n \to \infty\); and \(s_{x_n}\) and \(d_{x_n}\) are random variables with Gaussian distribution.

Totally random walk (Brownian motion) is obtained by setting \(\alpha=0\), while \(\alpha=1\) results a linear motion.

To ensure that the node does not remain at the boundary of the constraint area for a long time, the mean value of the direction (\(\bar{d}\)) modified as the node enters the margin area. For example at the right edge of the area it is set to 180 degrees, so the new direction is away from the edge.

MassMobility

This is a random mobility model for a mobile host with a mass. It is the one used in .

“An MH moves within the room according to the following pattern. It moves along a straight line for a certain period of time before it makes a turn. This moving period is a random number, normally distributed with average of 5 seconds and standard deviation of 0.1 second. When it makes a turn, the new direction (angle) in which it will move is a normally distributed random number with average equal to the previous direction and standard deviation of 30 degrees. Its speed is also a normally distributed random number, with a controlled average, ranging from 0.1 to 0.45 (unit/sec), and standard deviation of 0.01 (unit/sec). A new such random number is picked as its speed when it makes a turn. This pattern of mobility is intended to model node movement during which the nodes have momentum, and thus do not start, stop, or turn abruptly. When it hits a wall, it reflects off the wall at the same angle; in our simulated world, there is little other choice.”

This implementation can be parameterized a bit more, via the changeInterval, changeAngleBy and changeSpeedBy parameters. The parameters described above correspond to the following settings:

  • changeInterval = normal(5, 0.1)
  • changeAngleBy = normal(0, 30)
  • speed = normal(avgSpeed, 0.01)

ChiangMobility

Implements Chiang’s random walk movement model (). In this model, the state of the mobile node in each direction (x and y) can be:

  • 0: the node stays in its current position
  • 1: the node moves forward
  • 2: the node moves backward

The \((i,j)\) element of the state transition matrix determines the probability that the state changes from \(i\) to \(j\):

\[\begin{split}\left( \begin{array}{ccc} 0 & 0.5 & 0.5 \\ 0.3 & 0.7 & 0 \\ 0.3 & 0 & 0.7 \end{array} \right)\end{split}\]

Replaying trace files

BonnMotionMobility

Uses the native file format of BonnMotion.

The file is a plain text file, where every line describes the motion of one host. A line consists of one or more (t, x, y) triplets of real numbers, like:

t1 x1 y1 t2 x2 y2 t3 x3 y3 t4 x4 y4 ...

The meaning is that the given node gets to \((xk,yk)\) at \(tk\). There’s no separate notation for wait, so x and y coordinates will be repeated there.

Ns2MotionMobility

Nodes are moving according to the trace files used in NS2. The trace file has this format:

# '#' starts a comment, ends at the end of line
$node_(<id>) set X_ <x> # sets x coordinate of the node identified by <id>
$node_(<id>) set Y_ <y> # sets y coordinate of the node identified by <id>
$node_(<id>) set Z_ <z> # sets z coordinate (ignored)
$ns at $time "$node_(<id>) setdest <x> <y> <speed>" # at $time start moving
towards <x>,<y> with <speed>

The Ns2MotionMobility module has the following parameters:

  • traceFile the Ns2 trace file
  • nodeId node identifier in the trace file; -1 gets substituted by parent module’s index
  • scrollX, scrollY user specified translation of the coordinates

ANSimMobility

It reads trace files of the ANSim Tool. The nodes are moving along linear segments described by an XML trace file conforming to this DTD:

<!ELEMENT mobility (position_change*)>
<!ELEMENT position_change (node_id, start_time, end_time, destination)>
<!ELEMENT node_id (#PCDATA)>
<!ELEMENT start_time (#PCDATA)>
<!ELEMENT end_time (#PCDATA)>
<!ELEMENT destination (xpos, ypos)>
<!ELEMENT xpos (#PCDATA)>
<!ELEMENT ypos (#PCDATA)>

Parameters of the module:

  • ansimTrace the trace file
  • nodeId the node_id of this node, -1 gets substituted to parent module’s index

Note

The AnsimMobility module processes only the position_change elements and it ignores the start_time attribute. It starts the move on the next segment immediately.

TurtleMobility

The TurtleMobility module can be parametrized by a script file containing LOGO-style movement commands in XML format. The content of the XML file should conform to the DTD in the TurtleMobility.dtd file in the source tree.

The file contains movement elements, each describing a trajectory. The id attribute of the movement element can be used to refer the movement from the ini file using the syntax:

**.mobility.turtleScript = xmldoc("turtle.xml", "movements//movement[@id='1']")

The motion of the node is composed of uniform linear segments. The movement elements may contain the the following commands as elements (names in parens are recognized attribute names):

  • repeat(n) repeats its content n times, or indefinitely if the n attribute is omitted.
  • set(x,y,speed,angle,borderPolicy) modifies the state of the node. borderPolicy can be reflect, wrap, placerandomly or error.
  • forward(d,t) moves the node for t time or to the d distance with the current speed. If both d and t is given, then the current speed is ignored.
  • turn(angle) increase the angle of the node by angle degrees.
  • moveto(x,y,t) moves to point (x,y) in the given time. If \(t\) is not specified, it is computed from the current speed.
  • moveby(x,y,t) moves by offset (x,y) in the given time. If \(t\) is not specified, it is computed from the current speed.
  • wait(t) waits for the specified amount of time.

Attribute values must be given without physical units, distances are assumed to be given as meters, time intervals in seconds and speeds in meter per seconds. Attibutes can contain expressions that are evaluated each time the command is executed. The limits of the constraint area can be referenced as $MINX, $MAXX, $MINY, and $MAXY. Random number distibutions generate a new random number when evaluated, so the script can describe random as well as deterministic scenarios.

To illustrate the usage of the module, we show how some mobility models can be implemented as scripts.

RectangleMobility:

<movement>
    <set x="$MINX" y="$MINY" angle="0" speed="10"/>
    <repeat>
        <repeat n="2">
            <forward d="$MAXX-$MINX"/>
            <turn angle="90"/>
            <forward d="$MAXY-$MINY"/>
            <turn angle="90"/>
        </repeat>
    </repeat>
</movement>

Random Waypoint:

<movement>
    <repeat>
        <set speed="uniform(20,60)"/>
        <moveto x="uniform($MINX,$MAXX)" y="uniform($MINY,$MAXY)"/>
        <wait t="uniform(5,10)">
    </repeat>
</movement>

MassMobility:

<movement>
    <repeat>
        <set speed="uniform(10,20)"/>
        <turn angle="uniform(-30,30)"/>
        <forward t="uniform(0.1,1)"/>
    </repeat>
</movement>