How to Calculate Network Buffer Length in NS2

To calculate the Network Buffer length in NS2 represents the amount of packets that can be saved in a network node’s queue (usually at routers or switches) as they wait to be transmitted. It is an important metric that impacts network performance when it can impact packet loss, delay and entire throughput. If the buffer or queue turns out be full, incoming packets will be dropped, leading to packet loss and decreased performance.

The buffer length (queue size) in NS2 can be set up and observed. Use the given guide to calculate the buffer length of the network in ns2:

Steps to Set and Calculate Network Buffer Length in NS2

  1. Set the Buffer Length (Queue Size)

In NS2, buffer length is built at the link level, using the queue-limit command to state the extreme amount of packets that can be stored in the queue.

When designing a link amongst nodes, you define the bandwidth, delay, and queue type (such as DropTail, RED, and so on). You can also set the queue limit (buffer length) based on the amount of packets.

Example to Set Buffer Length:

# Create a duplex link between node(0) and node(1) with 10Mbps bandwidth, 10ms delay, and DropTail queue

$ns duplex-link $node(0) $node(1) 10Mb 10ms DropTail

# Set the queue limit (buffer length) to 50 packets

$ns queue-limit $node(0) $node(1) 50

In this example:

  • A link is generated amongst node(0) and node(1) with a 10 Mbps bandwidth, 10 ms delay, and a DropTail queue.
  • The queue limit (buffer length) is set to 50 packets.
  1. Monitor Buffer Usage During the Simulation

You can observe the buffer usage (queue length) during the simulation by logging the count of packets in the queue at regular breaks or after each event (such as packet arrival or departure).

You can write a TCL procedure to print the current queue length during the simulation:

TCL Script to Monitor Buffer Length:

# Function to monitor and print the current buffer length

proc monitor_queue_length {node1 node2} {

set qlen [$ns monitor-queue $node1 $node2]

puts “Time: [$ns now], Queue Length: $qlen packets”

 

# Re-schedule to monitor the queue length every 1 second

$ns at [expr [$ns now] + 1.0] “monitor_queue_length $node1 $node2”

}

# Schedule the first call to monitor the queue length

$ns at 0.0 “monitor_queue_length $node(0) $node(1)”

This script:

  • Sees the queue length amongst node(0) and node(1) and prints it at 1-second intervals.
  • Re-schedules the function call every 1 second to incessantly track the buffer length.
  1. Analyze Queue Length from the Trace File

You can also assess the queue behavior from the NS2 trace file. The trace file logs packet events, and you can monitor when packets are enqueued and dequeued from the buffer.

  • +: Packet enqueued (added to the buffer).
  • -: Packet dequeued (removed from the buffer and transmitted).

Here’s an example of trace file entries:

+ 0.12345 0 1 tcp 1040 ——- [0 0 1 0] ——- [1:0 0:0 32 0] [0] 0 0

– 0.23456 1 0 tcp 1040 ——- [0 1 0 0] ——- [0:0 1:0 32 0] [0] 0 0

In this trace:

  • + represents that a packet is enqueued (included to the queue).
  • – signifies that a packet is dequeued (removed from the queue and transmitted).
  1. Calculate Buffer Length Using an AWK Script

AWK script is used to evaluate the trace file and estimate the buffer length at any given time by summing up how many packets are currently enqueued versus dequeued.

AWK Script to Calculate Queue Length:

awk ‘{

if ($1 == “+”) {  # Packet enqueued

qlen++;

}

if ($1 == “-“) {  # Packet dequeued

qlen–;

}

print “Time: ” $2 “, Queue Length: ” qlen ” packets”;

}’ out.tr

This script:

  • Rise the queue length (qlen) for every + (packet enqueued) event.
  • Minimizes the queue length for every – (packet dequeued) event.
  • Prints the queue length at each packet event.

You can use this script to track how the buffer (queue) is being used over time.

  1. Handle Packet Drops Due to Full Buffer

If the buffer (queue) converts full, NS2 will drop packets when they arrive. You can see this in the trace file with d (packet dropped) events.

Example of Packet Drop in the Trace File:

d 0.45678 0 1 tcp 1040 ——- [0 0 1 0] ——- [1:0 0:0 32 0] [0] 0 0

You can observe the number of packet drops due to full buffers using an AWK script.

AWK Script to Count Packet Drops:

awk ‘{

if ($1 == “d”) {  # Packet dropped

dropped_packets++;

}

} END {

print “Total Dropped Packets: ” dropped_packets;

}’ out.tr

This script counts the total amount of dropped packets because of full queues or other issues. If your buffer size is too small, you may see a higher count of dropped packets.

  1. Analyze the Impact of Buffer Length on Network Performance

You can experiment with various buffer sizes by modifying the queue limit and observing its effect on:

  • Throughput: A larger buffer can help prevent packet drops and increase throughput though may launch more delay.
  • Delay: A larger buffer can maximize the time packets spend waiting in the queue, leading to higher delays.
  • Packet Loss: A smaller buffer might cause packets to be dropped if the queue becomes full during periods of blockage.

Example Workflow

Here’s a workflow to compute and observe the network buffer length in NS2:

  1. Set the buffer length using the queue-limit command in your NS2 script.
  2. Monitor the buffer length during the simulation using a TCL process that prints the current queue size at regular intervals.
  3. Analyze the buffer behavior using an AWK script to estimate the queue length over time from the trace file.
  4. Count packet drops because of buffer overflow using the trace file and assess how frequently packets are being dropped.
  5. Experiment with different buffer lengths and evaluate their influence on network performance metrics like throughput, delay, and packet loss.

In this process, we have expoundedly delivered the valuable instructions and examples with formulas to calculate the network buffer length using ns2 simulator by setting the buffer length. To know more about this calculation, reach out to us so that we can help you.

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