The near/far problem in wireless LAN implementation results from the scenario in which there exists multiple client nodes that are (a) very near to the access point and (b) have high power settings; and then at least one client that is (a) much farther away from the access point than the aforementioned client nodes, and (b) is using much less transmitting power than the other client nodes. The result of this type of situation is that the client(s) that are farther away from the access point and using less power simply cannot be heard over the traffic from the closer, high-powered clients, as illustrated in Figure 9.8.
Near/far is similar in nature to a crowd of people all screaming at one time into a microphone, and one person whispering from fifty feet away from that same microphone. The voice of the person 50 feet away is not going to reach the microphone over the noise of the crowd shouting near the microphone. Even if the microphone is sensitive enough to pick up the whisper under silent conditions, the high-powered close-range conversations have effectively raised the noise floor to a point where low-amplitude inputs are not heard.
Getting back to wireless LANs, the node that is being drowned out is well within the normal range of the access point, but it simply cannot be heard over the signals of the other clients. What this means to you as an administrator is that you must be aware of the possibility of the near/far problem during site surveys and understand how to overcome the problem through proper wireless LAN design and troubleshooting techniques.
Troubleshooting Near/Far
Troubleshooting the near/far problem is normally as simple as taking a good look at the network design, locations of stations on the wireless network, and transmission output power of each node. These steps will give the administrator clues as to what is likely going on with the stations having connectivity problems. Since near/far prevents a node from communicating, the administrator should check to see if the station has drivers loaded properly for the wireless radio card and has associated with the access point (shown in the association table of the access point).
The next step in troubleshooting near/far is use of a wireless sniffer. A wireless sniffer will pick up transmissions from all stations it hears. One simple method of finding nodes whose signals are not being heard by the access point is to move around the network looking for stations with a faint signal in relation to the access point and nodes near the access point. Using this method, it should not be too time-consuming to locate such a node, depending on the size of the network and the complexity of the building structure. Locating this node and comparing its signal strength to that of nodes near the access point can solve the near/far problem fairly quickly.
Solutions for Near/Far
Although the near/far problem can be debilitating for those clients whose RF signals get drowned out, near/far is a relatively easy problem to overcome in most situations. It is
imperative to understand that the CSMA/CA protocol solves much of the near/far problem with no intervention of the administrator. If a node can hear another node transmitting, it will stop its own transmissions, complying with shared medium access rules of CSMA/CA. However, if for any reason the near/far problem still exists in the network, below is a list of remedies that are easily implemented and can overcome the near/far problem.
- Increase power to remote node (the one that is being drowned out)
- Decrease power of local nodes (the close, loud ones)
- Move the remote node closer to the access point
One other solution is moving the access point to which the remote node is associated. However, this solution should be viewed as a last resort, since moving an access point will likely disrupt more clients than it would help. Furthermore, the need to move an access point likely reveals a flawed site survey or network design, which is a much bigger problem.
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