Analysis of Quality of Service in Wimax networks

Figure 4 NS-2 architecture and set up 
Results 
Simulation 
Results 
Tracefile 
TCL Script 
Network 
Topology 
Traffic Scenarios 
ns-2 
C++
library 
oTCL Interpreter 
WiMAX 
module 
PERL 
Script for 
analysis 

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Figure 4 shows basic structure of ns-2. The network topology and traffic 
agents etc are specified in the TCL file. It is parsed by the oTCL interpreter. The 
C++ library has all the implementation details. When ns-2 is run, the resulting data 
could be obtained in a trace file format. The trace file contains time stamp and 
information about each packet that is sent, received or dropped. It also has 
information about the packet size, type of packet etc. A base station and a subscriber 
station can be set up as a node in ns-2. As the number of nodes in the simulation 
increase, the packets that are sent and received increases. This makes the trace file 
very large. To aide in extracting the right data out from the file, a PERL script was 
written, details of which are provided later. 
In case of the Chang Gung University simulation, there was no concept of a 
base station for a particular node. However the WiMAX forum simulator had the 
distinction between nodes. While setting up the node, it could be classified as a base 
station as opposed to a subscriber node. Thus it was decided to use the simulator 
from WiMAX forum. 
Version 2.31 of ns-2 and Version 2.1 of the WiMAX module from NIST was 
used. The WiMAX module simulates both physical (PHY) and MAC layers based on 
IEEE 802.16e standard.
After setting up ns-2 and compiling the WiMAX module it was discovered 
that there were several memory leaks in the simulation. For a single node the 
simulation would run properly but as the nodes increased, so would the packet traffic.
This would eventually cause the simulation to stop to run as it would run out of 

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dynamic memory necessary to allocate new packets. Several memory leak issues 
were fixed during the course of experimentation, to make the simulation useful.
These issues and fixes were reported back to the WiMAX forum. 
As mentioned earlier, simulation scenario for ns-2 is defined by a Tcl script.
This file contains all the information needed to run the simulation. In a typical 
model, the nodes in the network are setup, starting with the base station. The mobile 
nodes are created and associated with the base station they belong to at the start. A 
traffic agent is created and is attached to the source node. An example of a traffic 
agent could be a CBR traffic agent. On the top of the traffic agent, an application 
which generating required traffic created. There is a sink node created and attached 
to the base station using a wired connection. This sink node is just to accept the 
incoming packet. The focus of the study will be the path between the mobile node 
and the base station.