How to balance downlink and uplink traffic in an IEEE 802.11 infrastructure WLAN?
The IEEE 802.11 Distributed Coordination Function (DCF) provides equal medium access probability to all transmitters that cause the access point (AP) to obtain less bandwidth than that of the wireless mobile stations to download traffic when the number of mobile stations is larger than one. Therefore, in this example, we will allow the AP to continuously transmit packets when the AP gets the right to access channel. The number of packets sent by AP is equal to the number of uplink flows in the system. In this way, the uplink and downlink traffic can be balanced.
Please refer to C. H. Ke, C. C. Wei, K. W. Lin*, “A Dynamic and Adaptive Transmission Scheme for Both Solving Uplink/Downlink Unfairness and Performance Anomaly Problems in a Multi-Rate WLAN”, Applied Mathematics & Information Sciences, vol. 6, no. 2S, pp. 531S-537S, Apr. 2012 (SCI) for more detail.
1. Download the modified mac-802_11-dcf.cc/h from here. This is a zip file. Decompress it and replace the mac-802_11-dcf.cc/h that are under NCTUns-6.0/src/nctuns/module/mac.
2. Recompile the nctuns.
1. Original IEEE 802.11 DCF MAC. Create a topology. (Uplink Flows: Node 2 and Node 3 are the sending packets to Node 5. Downlink: Node 6 is sending packet to Node 4).
2. Click ‘E’ to edit property.
(turn on the log function to log the throughput of incoming packets at the Node 2)
(turn on the log function to log the throughput of incoming packets at the Node 3)
(turn on the log function to log the throughput of incoming packets at the AP)
3. Click “R” and Simulation/Run to start simulation.
4. After simulation, we can plot the graph for throughput comparison.
Theoretically, when the Node 2, Node 3, and AP are all transmitting packets, they can get almost equal throughput. However, from the above figure, the AP throughput (downlink, blue line with points) is lower than that for Node 2 (uplink1, red line with points) and Node 3 (uplink 2, green line with points). This is mainly due to the beacon frames sent by the AP. Every 100 ms, the AP needs to send out a beacon. It also counts to one channel access. Therefore, the AP throughput is somewhat lower than that of Node 2 and Node 3. Needless to say, the total uplink throughput (uplink1 + uplink 2) is much higher than the downlink throughput.
5. Use the modified MAC. Please use the downloaded mac-802_11-dcf.cc. Open this file. If there are two uplink flows in the simulated scenario, the “noflows” must set to 2. It will allow the AP sends out 2 DATA packets when it gets the right to access channel. (DATA-SIFS-ACK-SIFS-DATA-SIFS-ACK).
6. Redo the simulation.
Firstly, you can see the animation that the AP continuously sends out 2 DATA packets when it gets the right to access the channel. Secondly, you can also see that the downlink traffic is equal to or better than the total uplink traffic (uplink 1 + uplink2).
Department of Computer Science and Information Engineering, National Quemoy University, Taiwan