How to Implement Network AP Selection in NS2

To implement the Network Access Point (AP) selection in NS2, we have to simulate a wireless network with several Aps (Access Points) in which the mobile nodes can dynamically pick that AP to link to according to the policies include signal strength, bandwidth or load.

The following approach has the necessary details for the implementation of AP selection in ns2:

Key Concepts for AP Selection in NS2:

  1. Multiple Access Points: Arrange multiple APs (such as Wi-Fi access points) in the network.
  2. AP Selection Criteria: Apply logic for nodes to pick an AP depends on signal strength (RSSI), existed bandwidth, or load.
  3. Mobility Model: Replicate nodes forwarding inside the coverage area of various APs, triggering AP selection or handoff by using node mobility.

Steps to Implement AP Selection in NS2

  1. Install NS2

Make sure that ns2 is properly installed on your computer.

  1. Create a TCL Script for AP Selection

The following sample shows how to simulate AP selection depends on signal strength. We’ll use a simple network with mobile nodes that picks amongst two APs (AP1 and AP2).

Example: Simulating AP Selection Based on Signal Strength (RSSI)

# Define the simulator object

set ns [new Simulator]

# Define trace and nam files for output

set tracefile [open out.tr w]

set namfile [open out.nam w]

$ns trace-all $tracefile

$ns namtrace-all $namfile

# Define a ‘finish’ procedure to close trace files and visualize in NAM

proc finish {} {

global ns tracefile namfile

$ns flush-trace

close $tracefile

close $namfile

exec nam out.nam &

exit 0

}

# Set up node configurations for wireless nodes

$ns node-config -adhocRouting DSDV -llType LL -macType Mac/802_11 -ifqType Queue/DropTail/PriQueue \

-ifqLen 50 -antType Antenna/OmniAntenna -propType Propagation/TwoRayGround -phyType Phy/WirelessPhy

# Create two Access Points (APs)

set AP1 [$ns node]

set AP2 [$ns node]

# Create mobile node (that will select AP based on signal strength)

set mobile_node [$ns node]

# Set the topography object

set topo [new Topography]

$topo load_flatgrid 1000 1000

# Configure wireless channel

set chan [new Channel/WirelessChannel]

$ns node-config -channel $chan

# Create links between APs and mobile node (these are virtual, APs are wireless)

$ns duplex-link $AP1 $mobile_node 11Mb 10ms DropTail

$ns duplex-link $AP2 $mobile_node 11Mb 10ms DropTail

# Traffic configuration for mobile node (TCP to AP1 and AP2)

set tcp [new Agent/TCP]

set sink1 [new Agent/TCPSink]

set sink2 [new Agent/TCPSink]

$ns attach-agent $mobile_node $tcp

$ns attach-agent $AP1 $sink1

$ns attach-agent $AP2 $sink2

# Traffic flow starts with connection to AP1

$ns connect $tcp $sink1

# Create FTP traffic

set ftp [new Application/FTP]

$ftp attach-agent $tcp

$ftp start 1.0

# Define mobility: mobile_node moves within AP coverage areas

$mobile_node set X_ 100 ;# Start near AP1

$mobile_node set Y_ 100

$ns at 3.0 “$mobile_node set X_ 600; $mobile_node set Y_ 600”  ;# Move towards AP2

# AP Selection procedure based on signal strength (RSSI)

proc select_ap {mobile_node AP1 AP2} {

global ns

# Simulate signal strength (RSSI) for AP1 and AP2

set rssi_ap1 [expr 1000 – [$mobile_node set X_]]

set rssi_ap2 [expr [$mobile_node set X_] – 500]

# Check which AP has the strongest signal

if { $rssi_ap1 > $rssi_ap2 } {

puts “Selecting AP1”

$ns detach-agent $mobile_node $tcp

$ns connect $tcp $sink1  ;# Connect to AP1

} else {

puts “Selecting AP2”

$ns detach-agent $mobile_node $tcp

$ns connect $tcp $sink2  ;# Connect to AP2

}

# Schedule next AP selection check

$ns at [expr [$ns now] + 1.0] “select_ap $mobile_node $AP1 $AP2”

}

# Schedule AP selection checks every 1 second

$ns at 1.0 “select_ap $mobile_node $AP1 $AP2”

# Schedule simulation end

$ns at 10.0 “finish”

# Run the simulation

$ns run

  1. Explanation of the Script
  • APs and Mobile Nodes: We generate two APs (AP1 and AP2) and a mobile node. The mobile node will pick the AP based on simulated signal strength (RSSI).
  • RSSI Calculation: In the select_ap technique, the signal strength for both APs is computed depends on the distance of the mobile node from each AP. The node selects the AP with the stronger signal.
  • Handover Between APs: When the mobile node travel nearer to AP2 (at time 3.0), the select_ap procedure verify the RSSI values and switches the connection from AP1 to AP2 if AP2 has a stronger signal.
  • Mobility: The mobile node begins near AP1 and moves towards AP2, simulating mobility inside a multi-AP network.
  1. Run the Simulation

Store the script as ap_selection.tcl and execute it using:

ns ap_selection.tcl

This will produce a trace file (out.tr) and a NAM file (out.nam). Use NAM file to visualize the node’s movement and AP selection.

  1. Analyze the Results
  • Trace File (out.tr): Assess the trace file to monitor the traffic flow and handover events amongst APs.
  • NAM Visualization (out.nam): Visualize how the mobile node swaps among APs by using the nam tool.

To open the NAM file:

nam out.nam

  1. Customize AP Selection Criteria

6.1 AP Selection Based on Load

You can optimize the script by picking an AP depends on its load (count of linked clients). This involves keeping track of the amount of nodes connected to each AP and choosing the AP with the minimal load.

Example of load-based AP selection logic:

proc select_ap_based_on_load {mobile_node AP1 AP2} {

global ns load_ap1 load_ap2

# Check which AP has the least load

if { $load_ap1 <= $load_ap2 } {

puts “Selecting AP1 (least load)”

$ns detach-agent $mobile_node $tcp

$ns connect $tcp $sink1

incr load_ap1

} else {

puts “Selecting AP2 (least load)”

$ns detach-agent $mobile_node $tcp

$ns connect $tcp $sink2

incr load_ap2

}

}

6.2 AP Selection Based on Available Bandwidth

You can also replicate AP selection in terms of existed bandwidth. In this case, you would observe the current throughput of each AP and pick the one with the utmost existed bandwidth.

6.3 Handover between APs

Execute smoother handover by launching a hysteresis margin, guarding against frequent switching amongst APs when signal strengths are alike.

Example:

if { abs($rssi_ap1 – $rssi_ap2) > threshold } {

# Perform handover only if the signal difference is significant

}

  1. Conclusion

By using this approach, you can mimic multi-AP networks in which nodes dynamically select the best AP depends on signal strength, load, or existed bandwidth. The TCL script can be altered to attach more sophisticated handover mechanisms, mobility models, and selection policies to reflect real-world environment.

Follow the delivered demonstration on how you can establish the network AP selection in NS2 environment by replicating a wireless network environment and allowing mobile nodes to select the best AP. You can also customize the simulation depends on your requirements.

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