Wireless LAN Implementation Challenges

Near/Far

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.

Troubleshooting Hidden Node

The primary symptom of a hidden node is degraded throughput over the wireless LAN. Many times you will discover that you have a hidden node by hearing the complaints of users connected to the wireless LAN detecting an unusual sluggishness of the network. Throughput may be decreased by up to 40% because of a hidden node problem. Since wireless LANs use the CSMA/CA protocol, they already have an approximate overhead of 50%, but, during a hidden node problem, it is possible to lose almost half of the remaining throughput on the system.

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.