Hidden Nodes Could Be Degrading Your Wi-Fi Network Performance
So many issues can wreak havoc on your connectivity - and it's not necessariliy the Wi-Fi at fault. In this final installment of our "Don't Blame the Wi-Fi" series, Eastman discusses hidden nodes and the impact of AP transmit power on Wi-Fi usability.
Hidden-node issue happens when two stations that are connected to the same AP cannot hear each other. As learnt previously, a node will need to listen to the medium before it starts transmitting. Since both stations cannot hear each other, both nodes may transmit packets at the same time, as a result, the packets may collide and nodes need to do retransmission.
High collision and retransmission may lead to a performance issue. This issue usually happens when there are obstacles between the clients or the AP’s coverage is much higher than clients’. In addition, a poor wireless design is often found to be the cause of this issue.
A common mistake made by less experienced wireless engineers is that the APs are placed based on the RSSI measurement alone. As long as the complete area that needs to be covered is covered, the WLAN will work perfectly.
Well, that is not always correct. Figure 1 shows an example of a hidden-node issue due to poor AP placements. In this environment, the signals of the APs and clients can only penetrate one wall. In order to be efficient, one AP was placed in Room-2 and another AP in Room-5 so that these APs can provide coverage to their adjacent rooms.
Using this approach ensures all the rooms are perfectly covered with the minimum number of APs. However, the problem here is that since the station can only penetrate one wall, Station-1 and Station-2 that are connected to AP1 cannot hear each other and therefore, you will have a hidden-node issue.
Another example is shown in Figure 2 where the AP’s coverage is much higher than the clients’ coverage. This issue normally occurs in an outdoor area where a high-gain antenna is used in order to provide a larger coverage or low data rate is used as mandatory data rate.
In this scenario, the Station-1 and Station-2 that are connected to the same AP cannot hear each other and therefore they may perform transmission at the same time that may cause collisions, which may then degrade the performance.
Figure 2 - hidden node - high coverage
How do we resolve the issue? On top of the carrier-sense method discussed above, wireless devices can also utilize RTS/CTS method in order to avoid collision. Although deploying RTS/CTS may prevent packet collision, the additional overheads of RTS/CTS may also lead to throughput and performance issues. Therefore, deploying RTS/CTS may not always be the effective way to address this issue.
The best option is to get the design right, such as, reposition or deploy additional APs, adjust the transmit power and data rate. You may also need to deploy external antennas on the client when high gain antenna is used on the AP. Furthermore, to be efficient is important, however, deploying a stable and functional wireless network is far more important.
I hope these five "Don't Blame the Wi-Fi" posts have been helpful. If you still have unanswered Wi-Fi questions, feel free to tweet me.