How to reduce the impact of performance anomaly problem in an IEEE 802.11 WLAN?
1. Refer to http://www.cs.utexas.edu/~lili/classes/F11/reading2/5.pdf to know what the performance anomaly problem is.
2. One of Legacy methods that can reduce the impact of performance anomaly problems include is to increase the CWmin value of lower bit rate stations to lower the contention ability.
1. In order to set the CWmin value in the tcl file, we have to modify the mac-802_11-dcf.h/.cc files under NCTUns-6.0/src/nctuns/module/mac
(mac-802_11-dcf.h: change u_int16_t aCWmin; à u_int32_t aCWmin;)
(mac-802_11-dcf.cc: create a connection between tcl and cc files.)
1. First, I will show what the performance anomaly problem is.Then, I will set the CWmin value of lower bit rate station to higher value in order to reduce its contention ability.
2. Create a topology. (Node 3 which is a lower bit rate station sends packets to Node 5 while Node 2 which is a higher bit rate station sends packets to Node 6.)
(remember to click Node 1, Node 2, and Node 3 to form a subnet)
3. Click “E” to edit property.
(change the Link Bandwidth of Node 3 to 1Mbps.)
(start stg program at Node 3 from 10 ~20 seconds)
(start stg program at Node 2 from 1 ~30 seconds)
(turn on the incoming throughput log function in the mac layer of Node 5 and Node 6.)
4. Click “R” and Simulation/Run to start simulation.
5. Assume that this name of this project is test2. After simulation, you can find test2.802_N5_P1_InThrput.log and test2.802_N6_P1_InThrput.log under test2.results.
6. Use the gnuplot to draw the figure.
From the above figure, we can find that the throughput of Node 2 – Node 6 is down to a lower value when the lower bit rate node 3 starts to transmit at the 10th second. After the 20th seconds, the lower bit rate node stops transmission and the throughput of node 2 – 6 is back to a higher value.
7. Increase the CWmin value of lower bit rate Node
7.1 Click “R” again but don’t click Simulation/Run. We have to modify the tcl file to set the CWmin value.
7.2 Click Simulation/Run to start simulation.
From the above figure, we can see that the throughput is higher during 10-20 second. The impact of the performance anomaly problem is reduced.
Department of Computer Science and Information Engineering, National Quemoy University, Taiwan