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Wireless Information and Troubleshooting

post #1 of 8
Thread Starter 
I have been around for a little while, and have gotten a lot of good information from OCN. While I have been here, I have noticed a ton of wireless questions in this forum. I figured I would take some time out to write this, and try to give back some to the community. If you have anything you would like to add or have added to the document, please let me know.

Thank you to Biscuits_N_Gravy for his contribution to this document too.

Contents:
1.Wi-Fi
2.RF fundamentals
a.Frequencies
b.MIMO
c.RF Power measurement
i.Watt
ii.Milliwatt
iii.dB
iv.dBi
v.dBm
d.RF Math
3.Hardware
a.Antennas
i.Omni-Directional
ii.Semi-Directional
iii.Directional
b.Wireless Routers
4.Security
a.Security
b.Encryption
5.Trouble Shooting
a.Coverage
b.Interference
c.Performance
6.Extending coverage
a.Bridges
b.Repeater
c. BSSID to ESSID


1.Wi-Fi
Before we get too far along, I guess we need to know what Wi-Fi stands for, and where it came from. In the United States, the Federal Communications Commission (FCC) regulates all outgoing and incoming communications. The FCC also regulates what wireless spectrums can be used for and what power levels can be used. Next on the list is the Electrical and Electronics Engineers (IEEE). The IEEE group creates standards on how equipment communicates with each other. The IEEE group created the 802.11 standard which we use today. Last but not least is the Wi-Fi Alliance. The Wi-Fi Alliance is a nonprofit group that will certify products, and allow their logo to be displayed on them. This testing tries to ensure compatibility between vendor products.

2.RF fundamentals

A.Frequencies:
802.11a - (5.725 - 5.875 GHz). 19 non overlapping channels.
The 802.11a band has a maximum connection rate of 54 Mbits.

802.11b / 802.11g - (2.4000–2.4835 GHz). 3 non overlapping channels (1, 6, and 11)
The B and G spectrums are bundled together because they use the same frequency. 802.11b has a maximum connection rate of 11 Mbits. 802.11g has a maximum connection rate of 54 Mbits. 802.11g is also backwards compatible with 802.11b.

802.11n -
2.4000–2.4835 GHz – (22 Mhz wide and 40Mhz wide channels.)
5.15–5.25, 5.25–5.35, and 5.75–5.85 GHz
802.11n is also backwards compatible with 802.11a/b/g depending on if you are using 2.4 GHz or 5 GHz.

20-MHz wide channels:
802.11n has 52 subcarriers per channel where 802.11g has 48. This increases the total bandwidth from 54 Mbps to 65 Mbps per channel. When using 20-MHz wide channels you can have the following speeds: 130 Mbps, 195 Mbps, and 260 Mbps. To get the higher speeds, you have to have more transmitters on the access point and receiver.

The current standard only allows up to 4 transmitting streams.
(Base bandwidth with no Guard Interval (SGI). Following is based on 400 nano seconds between transmissions.)

1 Transmitting Stream = 65 Mbps
2 Transmitting Stream = 130 Mbps
3 Transmitting Stream = 195 Mbps
4 Transmitting Stream = 260 Mbps

40-MHz wide channels:
When using 40-Mhz wide channels, the subcarrier count goes up to 108 per channel. The 40-Mhz wide channel is actually two channels that have been bonded together to make one large channel.

(Base bandwidth with no Guard Interval (SGI)).
1 Transmitting Stream = 135 Mbps
2 Transmitting Stream = 270 Mbps
3 Transmitting Stream = 405 Mbps
4 Transmitting Stream = 540 Mbps

600 Mbps is possible, but it would require the use of Short Guard Interval (SGI) which reduces the time between transmissions.

More reading on frequencies: http://en.wikipedia.org/wiki/List_of_WLAN_channels

More reading on 802.11n:
http://en.wikipedia.org/wiki/IEEE_802.11n-2009

B.MIMO (Multiple-input multiple-output)
MIMO is one of the key features that make 802.11n work. The transmitter actually gets receive signal information from the client. The transmitter then takes that data, so it can tweak the 802.11n signal for best performance. This transmitter can then use beamforming to improve SNR (Signal to noise ratio) which will allow for a faster data rate. MIMO can also use multipath. Multipath in 802.11a/b/g can cause the signal to reach the client 180 degrees out of phase which will make the signal unusable. MIMO actually takes advantage of multipath by using multiple streams called spatial stream. This allows a stronger signal to reach the client, and also improves SNR at the client.

C.RF Power Measurements
(Relative unites of power. Deal with power change.)
dB - (Decibel) Decibel is the difference between two signal levels. This can be seen in dB loss in cables, amplifier gain, and antenna gain.
dBi - (dB isotropic) dBi is the gain an antenna has compared to an isotropic antenna. An isotropic antenna is just theoretical since one cannot be made. It gives a starting point for measuring how much gain an antenna will give you. You will see most antenna gain measured in dBi.

(Actual measurements of power.)


mW - milliwatt
dBm - decibel

mW and dBm are both used to set power for radiators (radios) in access points. Depending on the manufacturer, you could see power measurements in either of these formats. There are also calculators on the internet that make converting back and forth between these very easy.

D.RF Math
Ok so how can I use all the units of power and change? Let’s start out with an example. Let’s say you have purchased and access point and you what to know how much total power the access point is putting out (EIRP – Equivalent Isotropically Radiated Power.). If you are lucky, you can log into the AP, and see the power measurement in mW or dBm. Let’s just say the AP is set for 50mW, but this is not your total output power. You then need to add the gain or the dB of the antenna to the power rating of the AP. (That number in mW we just found in the AP.) Most of the antennas that come with home routers run around 2.1 dBi. So first we have to convert mW to dBm so we can add the gain to it.

50mW = 16.98 dBm now we can just add the gain of the antenna. 16.98 + 2.1 dB = 19.08 dBm. We can now convert back to mW. 19.08 dBm = 80.90 mW

The rule of 3s. For every 3 dBm you raise the strength of your signal, you double your signal strength. Hypothetical example: You have a access point set for -70 mW with a 3 dBm antenna. If you replace that antenna with a 6 dBm one, you have now doubled your signal strength. This can also be done by changing the mW setting in the access point, but don’t forget to do your conversion to dBm and add the gain of the antenna.

The rule of 10 works the same way, but it increases your signal strength 10x. +10 dBm = 10x the signal strength.

Here is an online calculator I use for the conversions:
http://www.aubraux.com/design/dbm-to-milli-watts-calculator.php

So why is this important? The signal from the AP must reach the PC, and the signal from the PC much reach back to the AP. If you have an AP that is pushing 100 mW that sends a usable signal 600 feet but the PC is only running 50 mW going out to 300 feet, you will never get connected to the AP or have a weak signal. If you put a high gain antenna on the AP, you need to make sure your PC can get back to it. Laptops max out at 100 mW since it is max power allowed by the FCC for omni directional antennas, but usually run around 32 mW.


3.Hardware
A. Antennas
Omni-Directional: The Omni antenna is the most common antenna used in one’s home, and is also known as a rubber ducky antenna. It provides 360 degree radiation of signal around the antenna. If you took a really big donut and put it over the antenna, this is what the signal pattern would look like.

Semi-Directional: The Semi-Directional antenna is also known as a Patch antenna. The Patch antenna sends signal from 0 to 180 degrees. Useful for home installations.

Directional: The Directional antenna is also known as a Yagi antenna. This antenna shoots a fairly narrow beam which is good for setting up bridges.

A good guide that also shows the foot print of the different types of antennas:
http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008008883b.html

B. Wireless Routers:
There has been a lot of talk about wireless routers in the forum, so I will not get into this. There are so many different types and manufacturers; this would be a very long section. I will say there are some aftermarket firmware upgrades that can add a ton of features to your routers, but they will not install on all wireless routers. You will have to do research to see if either of these will load on your router.

- DD-WRT : http://www.dd-wrt.com/site/index
- Tomato : http://www.polarcloud.com/tomato


4.Security
A. Security
WEP – (Wired Equivalent Privacy)
WEP was introduced in 1999, and uses either a 40-bit or 128-bit key. It uses open or shared authentication. Even though shared authentication may look more secure due to the challenge and response hand shake it is not.

WPA – (Wi-Fi Protected Access)
WPA was the temporary solution designed in 2003 to replace WEP and is vulnerabilities. WPA used TKIP (Temporal Key Integrity) as its encryption. TKIP uses rotating security keys to help with key exposure. TKIP security was cracked in November 2008 at a security convention in Japan.

WPA2 -
WPA2 was made mandatory by the Wi-Fi Alliance for any device to be certified by them in 2006. WPA2 also came with new encryption methods. CCMP and AES encryption took the place of TKIP. Even though WPA2 with AES encryption using PSK for authentication is the highest form of security for wireless home users. There is a new security hole out for WPA2 though.

http://www.airtightnetworks.com/WPA2-Hole196

B. Encryption
I am not going to get too detailed in the encryption. There are links below if you want to do more reading.

(WEP) - Wired Equivalent Privacy
WEP was introduced as part of the original 802.11 protocol in 1997. WEP is a very weak encryption, and can be cracked in minutes with the right tools.
More information: http://en.wikipedia.org/wiki/Wired_Equivalent_Privacy

(TKIP) - Temporal Key Integrity Protocol
TKIP was introduced in 2002 to replace WEP.
More information: http://en.wikipedia.org/wiki/Temporal_Key_Integrity_Protocol

(AES) - Advanced Encryption Standard
AES was introduced to replace TKIP.
More information: http://en.wikipedia.org/wiki/Advanced_Encryption_Standard

From strongest to least secure.
-WPA2/AES
-WPA/TKIP
-WEP
-Open – (No security)

5.Trouble Shooting
A. Coverage
One of the two main wireless issues you will run into is cell coverage. Cell coverage can be modified by type of antenna used, antenna dBi, and radio power settings. Cell coverage is measured in either dBm or RSSI. dBm is the preferred method because there is no standard for RSSI measurements. Each wireless network card manufacturer has their own RSSI scale, so there is not constant between them. RSSI can be used, but it is not the best. The problem is finding a free program that will do dBm measurements. Most enterprise grade tools use dBm as their measurement. The problem with these tools is they are expensive.

Free tools:
InSSIDer: http://www.metageek.net/products/inssider
WLANInfo: (Windows XP only.) http://www-pc.uni-egensburg.de/systemsw/tools/wlaninfo.htm
NetStumbler: http://www.netstumbler.com/

Comerical tools:
AirMagnet WiFi Analyzer: http://airmagnet.com/products/wifi_analyzer/
AirMagnet Survey: http://airmagnet.com/products/survey/
Spectrum XT: http://airmagnet.com/products/spectrum_xt/

I design for data cells to have a minimum -70 dBm in strength. This should give you a good 54 Mbps connection rate. Just for knowledge, you also need to shoot for a signal to noise (SNR) of 20 dBm. SNR is the distance your wireless signal is from the noise floor. This isn’t as important in home installations as it is in a work environment.

B. Interference
I mentioned in the previous section that coverage is one of the two main wireless issues you will run into. The second biggest problem you will run into will be interference. You will see more issues with the 2.4 GHz spectrum than you will with the 5 GHz spectrum.

Some devices that interfere with the 2.4 GHz spectrum:
- Bluetooth
- Microwaves
- Cordless Phones
- Other access points
- Baby Monitors
- Electric Motors
- Some Florescent Lights

Some devices that interfere with the 5 GHz spectrum:
- Other access points
- Cordless Phones
- Radar
- Perimeter Sensors
- Digital Satellite

You can find other access points using the tools listed above to see what channels are free to set your access points to, but it will take a Spectrum Analyzer to find the other devices. Unfortunately Spectrum Analyzers are expensive.

You can see more info on microwaves and phones here: http://www.overclock.net/networking-security/876953-microwave-causing-lag.html

If you want to read more on how other access points will affect your wireless network, Google “wifi co-channel interference”. Please note if you live in an apartment or the houses in your neighborhood are close together, there is a very good chance for co-channel interference. Remember 803.11n bundles channels, so there are fewer channels available for everyone to use. As you can see above, the 5 GHz band is preferred if possible due to the fewer devices that interfere with it.

C. Performance
Performance can be degraded by coverage or interference. Once you have verified your coverage with the tools above, look around to make sure you don’t have any devices list above in use that would interfere with your signal. Also note wireless signal and wireless speed go hand in hand. If you have -70 dBm you should be getting a 54 Mbps connection rate. If you have less than -70 dBm or so, you could have any of the following 802.11 G connection rates. -60 dBm is stronger than -70 dBm.

•54 Mbps
•48 Mbps
•36 Mbps
•24 Mbps
•18 Mbps
•12 Mbps
•9 Mbps
•6 Mbps

Now just because you have a 54 Mbps connection rate, that doesn’t mean you will get 54 Mbps throughput. Provided you have no interference, your actual throughput should be around half of your connection rate.

6.Extending coverage
A. Bridges
One of the ways a wireless network can be extended to another building is by using a bridge. It is best to use either a directional or semi-directional antenna on both antennas. If your router supports it, DDWRT can be used on the far end to connect to the main access point. Tomato may have this function, but I have never used it. This will allow you to use the switch ports on the wireless router that is on the far side.

B. Repeater
Extenders work similar as bridges, but every time a repeater is used you lose half of your bandwidth.

C. BSSID to ESSID
BSSID (Basic Service Set Identifier) is a wireless network made up of one wireless cell. ESSID (Extended Service Set ID) is multiple wireless cells that make up a wireless network. To convert you BSSID to an ESSID follow the following list.

1.Pug another access point into your network somewhere.
2.Set the new access point to a static ip address that is on the same subnet as your primary wireless router.
3.Disable DHCP if you are using a wireless router for your second AP.
4.Configure the second access point with the same SSID, Security, and Encryption.
5.Configure the second access point to a different channel as the primary access point.
(Use one of the tools above to figure out what channels you need to be using.)
This option will allow you to extend your wireless network along with seamless roaming between the two access points.


This document was written with knowledge we retrieved from the web, our wireless training, and on the job experiences. Hopefully this will bring a lot of the wireless information to one place so others on this board can find it easier.
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post #2 of 8
Doubt many people will read it. Since they'd rather post questions then find out information themselves. But this deserves a sticky. Some good information in there.
post #3 of 8
this is worth a bump.
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post #4 of 8
bump your link to the calculator doesn't work frown.gif
http://www.aubraux.com/design/dbm-to-milli-watts-calculator.php
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post #5 of 8
Thread Starter 
Looks like they are doing an upgrade or maintenance..



http://www.aubraux.com/
"We'll be back..."
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post #6 of 8
what you think bout this cheap extender antenna and it works as well
_42912487_cantenna203.jpg
post #7 of 8
Quote:
Originally Posted by Spooony;14242110 
what you think bout this cheap extender antenna and it works as well
_42912487_cantenna203.jpg

I've used them for fun. I can't say I'd use it for a permanent setup.
    
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post #8 of 8
Thread Starter 
I have heard of people using them, but I have never made one myself. I have been lucky enough to have access to all types of Cisco antennas to play with.
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