Sunday, August 15, 2010

Wi-Fi Tip: Multipath Troubleshooting

Ever been in a problem situation where Wi-Fi clients are having all sorts of trouble staying connected reliably in a specific location. Their connection to your network is like a bad relationship: on-again, off-again. You've racked your brain, consulted your peers, verified adequate coverage, looked for interference and found none, updated client drivers, verified clients work in other locations without issue, but still can't figure out what is going on?

Perhaps you even suspected a multipath issue? But how can you verify?

Multipath is notoriously hard to identify with any degree of accuracy. The best we can do is evaluate symptoms and rely on gut instinct most of the time (unless your company has TONS of mullah and is willing to buy an oscilloscope).

So, what are the symptoms of multipath:

  • Everything listed in the first paragraph :) no really!
  • Clients have issues only in a localized area
  • Area is complex, with large amounts of metal or other reflective, refractive, or scattering material
  • Clients connect, seemingly with great signal at a high data rate
  • The data rate deteriorates quickly, with seemingly nothing changing. This is called excessive data rate shifting, and is bad, bad, bad for wireless network performance!

So you ask, how can we verify multipath is the issue? Really, without an oscilloscope to identify slight variations in RF signal propagation (called inter-symbol interference), the best we can do is test solutions and see if they resolve the issue.

Try these solutions, and see if they help client performance:

1.) Disable the higher data rates (5.5, 11, 9, 12, 24, 36, 48, 54 Mbps).

Wait, what!? Don't we want clients to send data at the highest rates for best performance? Normally, yes. But if multipath is causing transmission errors, then the higher data rates may perform worse than lower data rates because of signal encoding, even if coverage and signal strength are excellent. Remember, signals encoded with lower data rates contain more error correction bits (coding-ratio). This is counter-intuitive unless you understand the fundamentals of the physical layer encoding mechanisms.

Leaving only 1, 2 or 6 Mbps enabled helps the client and AP by maximizing error-correction capabilities and reducing the complexity of signal amplitude and phase shifting that must be recognized.

Here's an example of what I mean with 802.11a/g encoding. Focus on the modulation and bits per transition columns for encoding complexity, and on the coding ration column for error correction. The coding ratio dictates how many bits are dedicated to data versus error correction.

2.) Install 802.11n equipment

Newer 802.11n equipment handles multipath much better than legacy 11a/b/g equipment due to the benefits of MIMO and MRC (maximal ratio combining). Even if clients cannot be replaced, upgrading only the infrastructure should help communication improve.

3.) Reduce the amount of overlapping coverage

If there are too many APs in one area, and multipath is present, transmissions from every AP will create even more multipath. Ensure only 2-3 APs can be heard at a signal level of -85dBm or greater in any one location. This will help reduce the amount of multipath in the environment. This can be accomplished by reducing AP transmit power, or by completely turning off APs. If turning off APs creates coverage holes in other locations, a re-design of the network layout may be required.

4.) Install material that absorbs RF signals

If you can cover the metal or other reflective surfaces with material that will absorb the RF signals, rather than let them bounce around, then multipath should be reduced, improving network performance. This may be hard, if not impractical, to achieve since most environments that have multipath issues require those problem surfaces to operate in a specific fashion that prevents absorptive material from being installed. But it's worth a last-ditch effort!

Hope this helps some of your troubleshooting efforts!


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