Troubleshooting Wireless Networks
by Ken Chipps
Once the network is in operation or during the site survey process problems may occur. Most of these problems will be the result of interference or limitations in the signal handling ability of the equipment.
Types of wireless network errors
Interference in all its forms
Wireless networks see the following types of interference:
- All band
- Adjacent channel
Narrowband interference is basically another signal at a single or narrow range of frequencies. As such it blocks out part of the spread spectrum signal. An advantage to spread spectrum technology is its ability to work around limited narrowband interference. To get rid of the narrowband interference shield it, turn it off, or change channels on the wireless network equipment.
All band interference is from one end of the band to the other. A microwave oven is an example of this type of interference. About the only solution to all band interference other than getting rid of the source is to change bands, such as from 802.11b to 802.11a.
Adjacent channel interference is produced by co-locating access points where the channels overlap somewhat or completely. A spectrum analyzer or a program that will identify all of the access points is required to detect this problem. To prevent this do not use channels that overlap, move the access points far enough apart so that the cells do not overlap, or turn the power down to achieve the same effect.
With co-channel interference there is a direct overlap of the channels. An example might be two different organizations using the same channels where one is on floor 1 and the other on floor 2 or in an adjacent office on the same floor. A spectrum analyzer or a program that will identify all of the access points is required to detect this problem. To prevent this do not use channels that overlap, move the access points far enough apart that the cells do not overlap, turn the power down to achieve the same effect, or change the orientation of the antennas, with one horizontal and the other vertical.
Another type of interference is multipath. When a radio frequency wave leaves an antenna it reflects off objects. This creates multiple wave fronts, one for each reflection point. Some of these waves go off in space, but others reach the receiving antenna along with the original wave front. Since the reflected waves cover the distance from the transmitter to the receiver over a different time interval than the original wave there is a delay between when the original wave front arrives and the reflected waves arrive. The time between the arrival of the original wave and the last reflected wave is the delay spread. The value for delay spreads will vary. Multipath causes several problems including decreased signal amplitude or downfade, corruption, nulling out of the signal, and increased signal amplitude or upfade. With decreased signal amplitude the reflected waves are added to the original wave. If the reflected waves are out of phase with the original wave, then a decrease in amplitude is seen. If a reflected signal is even more out of phase, then the reduction may be so great that the received signal cannot be read at all or only partially due to corruption. This is seen in a low signal to noise ratio. Nulling the phase of the reflected signal entirely cancels the original signal. When a reflected signal is in phase with the original signal then the total signal may be larger in amplitude. This causes a higher signal strength than would normally be expected at the antenna, but still lower than the transmitted signal strength.
Multipath cannot be measured directly. Only its effects can be seen and from these effects multipath is deduced. For example, if a link budget calculation is performed but the signal as measured is less, then multipath can be a reason. Holes or areas where no signal is detected when doing a site survey may be caused by multipath.
There is no direct solution for multipath problems. Moving objects that reflect the signal or moving the antennas so as to avoid the multipath path are possible solutions. Antenna diversity is another possible solution to multipath. Antenna diversity is the use of multiple antennas, inputs, and receivers. There are several types of antenna diversity that are commonly used. Non-active diversity uses multiple antennas and a single receiver input. Active diversity uses multiple antennas and multiple inputs to a single receiver. It reads the signal from one antenna at a time. Switching diversity uses multiple antennas and multiple receivers. It switches receivers based on the signal strength at each antenna. Transmission diversity transmits out the last antenna used for reception. It can alternate antennas for retransmissions.
In 802.11 based networks, interference of any type will show up as increased fragmentation, decreased transmission rates, and increased retransmissions.
In addition to interference, 802.11 type networks suffer from limitations inherent in the technology.
The hidden node problem occurs when one node cannot hear another node transmitting. This occurs when they are separated by an obstruction or when they are too far apart. Both nodes can see the access point, but not each other. This causes excessive collisions on the network, retransmissions, and therefore reduced throughput.
Degraded throughput on the network is the common sign of hidden node. Examining the layout of the network may show hidden nodes. Moving or disconnecting possible hidden nodes and then examining the throughput may show these as well. This is a trial and error process.
The solutions for hidden node depend on the type of network. For a LAN solutions these include use of RTS/CTS, adjusting the point where the wireless packets are fragmented, increasing the power used by the far nodes and decrease the power used by the nearby nodes, removing the obstacle, moving the node closer, or using a polling mechanism to control access. RTS/CTS does not solve the hidden node problem, but it may improve the throughput if the node or obstacle cannot be moved.
To enable RTS lower the RTS threshold. Cisco recommends adjusting the RTS/CTS parameter by reducing the packet size from its default value of 2048 to a value where CRC errors become acceptable. Adjusting the fragmentation level to a value where more and more packets are fragmented may increase throughput as well. By being smaller in size the packet may make it to the access point before colliding with another packet.
The near/far problem occurs when there are nodes near the access point that have high power settings and other nodes far from the access point with low power settings. The near, high power nodes overwhelm the far, low power nodes. To detect this, check the network design. Look at the power output level of the nodes. Possible solutions to the near/far problem include reducing the power of the nearby nodes, increasing the power of the far-off nodes, moving the far-off nodes closer to the access point, and moving the access point to a more central location.
The throughput of a wireless system is dependent on:
- Amount of interference
- Type of interference
- Security solutions that add overhead
- Distance, since the data rate falls off as distance increases
- Older, slower computers
- Use of RTS/CTS
The most common solution to low throughput is the co-location of access points in a single area. For 802.11b, for example, three non-overlapping channels are possible - 1, 6, and 11. A single AP will provide from 4.5 to 5.5 Mbps in practice. In theory three APs should provide 15 Mbps or so. Of course it is possible to use fewer than three APs; two may be used on channels 1 and 11. This may make sense if three access points each producing 4 Mbps are compared to two producing 5.5 Mbps each. It may also make sense to force fragmentation so as to produce smaller frames. This means that the lost frames when retransmitted are smaller. When a packet must be fragmented this adds overhead as each fragment requires an ACK.
Fragmentation can be adjusted to improve efficiency on the network. If the network is experiencing high packet error rates, then increase the fragmentation threshold. This is done by starting with the maximum size and gradually dropping the threshold until an improvement is seen. As the frame size is increased, there is less overhead, but increased chance of collision. As the frame size decreases there is more overhead, but less chance of collision. Start with a setting of 1024 bytes. In a network where the average packet size is greater than 800 bytes, it may benefit the network to lower the fragmentation setting. Then see if performance improves. As Oppenheimer and Bardwell point out, this can be determined by transferring a large file, such as 1GB, as the test data must be larger than the fragmentation threshold, and timing how long it takes. Adjust the value in 100 byte increments above and below 1024 bytes and see when the most improvement occurs.
The main troubleshooting tool for any network problem is the network or protocol analyzer. There are currently several on the market. On the paid side Network Associates Sniffer Wireless and Wild Packets AiroPeek are very good. (http://www.networkassociates.com/us/products/sniffer/home.asp) and (www.wildpackets.com) For a free version many use Ethereal. (www.ethereal.com) Let's look at Sniffer Wireless to see what one of these programs can do. When Sniffer Wireless is started it looks like the standard program used for wired networks, except for a few additions. Starting at the top we see it has found a wireless network on the 802.11b channel 6 with a signal strength of 100%.
The other changes from a wired networks analyzer are mostly additional information. For example, on the Dashboard screen is a tab that when clicked leads to details on the wireless network.
The last major change is seen on the host table display where it can be set to show only the wireless devices.
Of course, like any protocol analyzer, Sniffer Wireless can be used to capture the packets crossing the air as well as the wires. Here is one example of such a capture. This is the beginning of a beacon frame.
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