Because the nature of a wireless LAN increases mobility, you may encounter a hidden node at any time, despite a flawless design of your wireless LAN. If a user moves his computer to a conference room, another office, or into a data room, the new location of that node can potentially be hidden from the rest of the nodes connected to your wireless LAN.
Solutions for Hidden Node
Once you have done the troubleshooting and discovered that there is a hidden node problem, the problem node(s) must be located. Finding the node(s) will include a manual search for nodes that might be out of reach of the main cluster of nodes. This process is usually trial and error at best. Once these nodes are located, there are several remedies and workarounds for the problem.
- Use RTS/CTS
- Increase power to the nodes
- Remove obstacles
- Move the node
Use RTS/CTS
The RTS/CTS protocol is not necessarily a solution to the hidden node problem. Instead, it is a method of reducing the negative impact that hidden nodes have on the network. Hidden nodes cause excessive collisions, which have a severely detrimental impact on network throughput. The RTS/CTS (request-to-send/clear-to-send) protocol involves sending a small packet (RTS) to the intended recipient to prompt it to send back a packet (CTS) clearing the medium for data transmission before sending the data payload. This process informs any nearby stations that data is about to be sent, having them delay transmissions (and thereby avoiding collisions). Both the RTS and the CTS contain the length of the impending data transmission so that stations overhearing either the RTS or CTS frames know how long the transmission will take and when they can start to transmit again.
There are three settings for RTS/CTS on most access points and clients: On, Off, and On with Threshold. The network administrator must manually configure RTS/CTS settings. The Off setting is the default in order to reduce unnecessary network overhead caused by the RTS/CTS protocol. The threshold refers directly to the packet size that will trigger use of the RTS/CTS protocol. Since hidden nodes cause collisions, and collisions mainly affect larger packets, you may be able to overcome the hidden node problem by using the packet size threshold setting for RTS/CTS. What this setting essentially does is tell the access point to transmit all packets that are greater in size than “x” (your setting) using RTS/CTS and to transmit all other packets without RTS/CTS. If the hidden node is only having a minor impact on network throughput, then activating RTS/CTS might have a detrimental effect on throughput.
Try using RTS/CTS in the “On” mode as a test to see if your throughput is positively affected. If RTS/CTS increases throughput, then you have most likely confirmed the hidden node problem. You will encounter some additional overhead when using RTS/CTS, but your overall throughput should increase over what it was when the hidden node problem occurred.
Increase Power to the Nodes
Increasing the power (measured in milliwatts) of the nodes can solve the hidden node problem by allowing the cell around each node to increase in size, encompassing all of the other nodes. This configuration enables the non-hidden nodes to detect, or hear, the hidden node. If the non-hidden nodes can hear the hidden node, the hidden node is no longer hidden. Because wireless LANs use the CSMA/CA protocol, nodes will wait their turn before communicating with the access point.
Remove Obstacles
Increasing the power on your mobile nodes may not work if, for example, the reason one node is hidden is that there is a cement or steel wall preventing communication with other nodes. It is doubtful that you would be able to remove such an obstacle, but removal of the obstacle is another method of remedy for the hidden node problem. Keep these types of obstacles in mind when performing a site survey.
Move the Node
Another method of solving the hidden node problem is moving the nodes so that they can all hear each other. If you have found that the hidden node problem is the result of a user moving his computer to an area that is hidden from the other wireless nodes, you may have to force that user to move again. The alternative to forcing users to move is extending your wireless LAN to add proper coverage to the hidden area, perhaps using additional access points.
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