Overview
There are a lot of reasons a wireless network does not perform well. Some of
them are obvious. Other are complex. I want to take you through some of the most common scenarios that result in poor performance in this article. Some you can fix yourself. Others,
you may have to call in some help. Wireless networks depend on radio frequency (RF) transmission - which is a science and can be monitored, modelled, etc. RF propagation also interacts with humans and their environment - an ever-changing scenario. So if there is a "black art" to tuning a wireless network, this is where it would be - dealing with the unpredictable and
ever-changing.
When Too Much is Bad For You
You can never have enough of a good thing - can you? You most certainly can with multiple access points for a wireless network. Unless you are laying out your home network with 1 access point, you will likely need to have multiple access points to guarantee coverage for your users. What is your first instinct - let's add a few more access points. Should work! Reality is that it's likely to cause you more headaches than you think. If you get too much overlap between cells, you get channel flapping. Clients will bounce back and forth between access points and you will have some unhappy users on your hands.
Unfortunately, just dropping out some access points is not always the solution. Then you get large coverage gaps where nobody can get a signal. So how do you deal with this. The key to a good network mesh is careful placement of your access points. Anytime you have more than 2 access points, especially if you are using VOIP over wireless, you should do a site survey. Examine where you are going to place the access points, look at their range and signal strength
and identify the overlaps. There are a lot of guidelines out there for how much overlap is best practice. If you listen to the folks at AirMagnet, who sell monitoring tools, 15-20% is best practice. If you read Cisco's 802.11 Wireless Network Site Survey and Installation, they recommend a typical overlap of 10-15%. Your mileage may vary depending on your local conditions.
Site surveys are a complex thing to do properly, but there are some tools to help out. Cisco has the Aironet Desktop Utility. A subset of features of the ADU come with the monitoring tools with most Cisco wireless cards. If you have an Intel Centrino system, it likely includes the Intel ProSet utility. It offers signal strength, noise-floor readings, transmitter retries, beacons missed and throughput. Orinoco and Netgear products also come with some basic tools. At the enterprise level AirMagnet offers both a sniffer tool (AirManget Laptop Analyzer) and a survey tool (AirManget Planner and Survey). In the open-source world you can give
NetStumbler a try.
What Happened Now?
Well, it worked before. Or it works only after 3PM. You had it all planned and tuned perfectly. Unfortunately, wireless networks rely on RF propagation. RF propagation is subject to both losses and gains as it leaves the transmitter and eventually finds it way to the client. Interference comes in many types from many sources. Did the environment change? Do you have some new walls or cubicles that are causing multipath or changing the propagation? Did someone stick up their own rogue access point? Did someone stack something on top of your access point? What about the antennas you are using? Did the people on the floor above you go wireless? Did the network security policy change - is there something in there that is WLAN specific?
If the environment changed there is no substitute for doing a walkabout. Even if everything is working normally and users are not complaining, you should periodically check on infrastructure. A lot of offices use cubicles, personnel change, things get moved around. Be proactive.
Friend or Foe?
Putting up an access point is almost a no brainer today. Pop on over to your favourite store and pick up a D-Link or Linksys, follow the instructions and you will be in business in 10 minutes. Unfortunately, sometimes access points pop up like mushrooms. That has an impact on your environment and performance. Did someone stick up their own rogue access point? Did
the neighbouring business suddenly go wireless. Do they know what they are doing or have they been listening to the local family geek (which sometimes is a good thing)?
Using the scanning tools mentioned above, take another walkabout and look for rouge access points. Look at the SSID's. Are they yours? Do they have security enabled? A lot of times they don't! I like to classify my SSID's' as either mine, neighbours, friends or rogues. The SSID's can often give you some excellent clues. Yours you should recognize. Seeing multiple access points with the same name, or systematically named, with security enabled may indicate a business. One that has the default name enabled, e.g. Linksys-A, has no security enabled, has a signal strength that varies, keeps coming and going, etc. is one that needs further investigation. Access points that exhibit a steady signal level but with which you cannot associate may be a neighbour. Check it out to make sure after you get done tracking down the rogues.
You Realize That You Are Not Alone
I hate to tell you this but you are not alone. If you take a gander over at Wikipedia, you can see what frequencies work at the 802.11 specifications. In summary the different standards and operating frequencies are:
- 802.11a - 5 GHZ
- 802.11b - 2.4 GHZ
- 802.11g - 2.4 GHZ
- 802.11n - 2.4 and/or 5 GHZ
short paper from the NIST about it. Microwave ovens also operate in the 2.4 GHz unlicensed band (2400 - 2483.5 MHz) for ISM (industrial, scientific, medical applications). Home ovens are less of a problem than commercial ones because they are operated infrequently and usually for very short bursts. However, in an office environment with a lot of employees using it, especially around lunch times, this can become a factor. An interesting paper on the subject was written by Kamerman and Erkoçevic of Lucent in the Nederlands. Another factor that you may not think about is 2.4GHZ cordless phones - although cordless phones do operate on other frequencies as well. To make things even more interesting, we now have things like wireless cameras. Oh joy!
Unfortunately there is no panacea to cure this issue other than constant vigilance. Th environment around you constantly changes. Once again, perform a site survey. Don't just do it once when you set things up. Make it an annual if not semi-annual event. In particularly problematic instances you may have to call in a professional with a spectrum analyzer to untangle the mess.
Did You Bother to Configure your AP?
You would be surprised how many people accept the common default values on most instruments. It works doesn't it? One of the great features of 802.11 technology has been backward compatibility. You can setup an 802.11g access point and still use your 802.11b card. However, you pay a price for this convenience. The key to tracking this down is to realize that the different standards work at different data rates:
| Standard | Operating Frequency (GHZ) | Typical Data Rate (Mbits/s) | Maximum Data Rate (Mbits/s) |
| 802.11a | 5 | 23 | 54 |
| 802.11b | 2.4 | 4.3 | 11 |
| 802.11g | 2.4 | 19 | 54 |
| 802.11n | 2.4 and/or 5 | 74 | 248 |
If you take your wireless scanner and look at the distribution of frames at each data rate, you may see that the 802.11g access point that you thought was operating at a maximum data rate of 54 Mbits/s is actually transmitting only 40% of the frames at the advertised rate. Why? Many access points, especially Cisco, accept all data rates in their default configuration. So now, instead of operating at one data rate, you are in mixed mode. Cisco has a slightly more technical treatise on what has to be added to an 802.11n transmission in order to maintain legacy compatibility. What is clear is that there is an impact while operating in a mixed mode but no easy answer on the amount of impact. In a 802.11n design guide from Cisco, they state:
- You will always realize some performance gain with 11n APs because all 11n devices will be able to transmit at 11n rates, even if legacy devices slow down aggregate performance when they operate in their legacy rates. Also, even if you have no 11n devices at all associated to your 1250 Series AP, the MIMO portion of 11n will still help the cell realize some gain.
- The higher your percentage of 5-GHz devices (even if they're legacy 802.11a
clients) compared to 2.4-GHz devices, the better your overall performance will
be. - As time goes by, we'll see more 11n devices out there. As this occurs, your
aggregate throughputs will rise accordingly. This is how the transition to 11g
happened, and all signs point to the client transition toward 11n happening a
whole lot faster. Just look at the number of laptops shipping today with
built-in 11n support; we're far past where we were at a comparable point with
11g adoption.
The solution to dealing with this issue sounds simple but may not be depending on whether or not you need to be backward compatible and what type of infrastructure you can deploy or must support.
I Can Save a Few $ - What's The Difference?
We have all heard the phrase "all men are created equal". Well, it does not apply to radios.
A client, i.e. user, with a good radio in their system can hookup with an access point much easier than one whose power level is lower. Depends on the manufacturer. Worse, there is no standard for radio power levels. While you may have your access points properly deployed, the overlap perfectly accounted for, no interference from "wireless cowboys", etc., you may still get coverage gaps because of client radios .
About the only way to deal with this is to try and keep your clients homogenous. Set some corporate governance standards and stick to them. It also goes without saying that this is another thing that needs to be continually monitored and analyzed.
Security
Everyone's favourite topic raises it ugly head again. Yes you need it. You should never be without it. And yes, sometimes having layers of security makes logging on and accessing corporate resources more challenging. And yes, there may be some security overhead if the access point has to go out to a Certificate Authority server or a RADIUS server. The best you can do is to make sure the rest of your network is optimized and functioning properly. I have seen networks where, due to misconfiguration, it took a packet 6 hops to get to a server when it should have been 3. In other words, do not more do than you have to.
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