Wireless Networking: Unreliable Links a Case Study
This information is based on a question posed to the African Network Operators Group (AfNOG) mailing list, January 2006.
Question:[Edited for content]
I have a problem on my wireless network. Its giving me an unreliable link to my customers. A brief background: We have connected two Full cisco 350 series bridges
1. Acts as root bridge.
2. The other is a non-root with clients.
The distance between them is less than 4km and the line of sight is very clear. The antennas used are 12dbi which from their specifications means that things should work.
Rebooting solves the problem for a small period.
1. I lose the link within a short period.
2. A sample log error is below
01:19:20 (Warning): Station [Kang'ombe AP]000d2962732a Associated with Encryption, then attempted to send an Unencrypted packet to [Kamuzu Central A.P]004096538097 (length 112) 01:19:20 (Warning): Station [Kang'ombe AP]000d2962732a Associated with Encryption, then attempted to send an Unencrypted packet to [Kamuzu Central A.P]004096538097 (length 112)
3. Client gets an unreliable connection
Any assistance will be greatly appreciated.
Answer:[Provided by Sunday A Folayan to the AfNOG mailing list. Content has been edited for this page.]
Sustaining a good link, is a function of the following amongst many other parameters:
- Power of Transmit Card
- Loss of cable and connectors at the transmit end
- Gain of Transmit Antenna
- Free Space Loss due to distance between the two points
- Gain or Receive Antenna
- Loss of cable and connectors at the Receive end
- Receive Sensitivity of Receiver
All expressed in Decibels (db). When you add up all of the above, the sum must be more than 20db, which is called a safe System Operating Margin (SOM).
1. Power of Transmit card: A cisco 350 series bridge can be configured to any of 30mw, 50mw or 100mw. You need to convert the selected power in mw to db with the formula 10log(P). Thus, 30mw is 14db, 50mw is 17db while 100mw is 20db. What is this value in your case? call it +A1. Since it is a gain.
2. Loss of cable and connectors at the transmit end: The type of cable and its length matters. The most common cable would be LMR400 and it has a loss of about 0.22db per metre. I do not know how long your cable is, but whatever the length in Metres, multiply it by 0.22. Next Count the connectors at your transmitter, for each connector, another 0.2db will be lost. What is this value in your case? Call it -A2. Since it is a loss.
3. You have provided this. It is 12db. Call this +A3 since it is a gain
4. Free space Loss (FSL) in db is given by the formula 20 * LOG(F) +20 * LOG(0.625 * Distance in Km) + 36.6.
In this case, you have a distance of 4km, and are running at 2400Mhz, so your FSL is 112db. Call this -A4 since it is a gain.
I have left our Fresnel loss at this point :-)
5. You have provided this. It is 12db. Call this +A5 since it is a gain
6. Same method of calculation as in 2. Call this -A6 since it is a loss.
7. Receive Sensitivity. Cisco will do 1Mbps at -94db, 2Mbps at -91db, 5.5Mbps at -89db amd 11mbps at -85db.
Safe to work with -85db since this is the sensitivity that has the highest throughput. Call this -A7.
Add all your values together. If you do not get more than 20db, your link will be unstable.
+A1-A2+A3-A4+A5-A6-A7 must be greater than 20!!
I made the following (extreme) assumptions:
- Maximum power of 100mw, so that A1=20db
- Very short cable of about 10Metres and 2 connectors, so that A2 is (10 *.22 + 2* 0.2) A2=-2.6db
- as in 4: +12db
- as in 3: -2.6db
Adds up to 11.8db.
Since this is less than 20db, Your link will be unstable.
Things you can do, to increase your link stability.
- Use an amplifier, within the safe legal limits in your country.
- Use a more powerful antenna. You need just 8db more, which is 4db per antenna, so a 16db antenna will do just fine in my hypothetical example.
- Use shorter length cable and fewer connectors. Meaning you will mount your radio on your tower.
- Use a radio with better receive sensitivity.
- Use a radio with 200mw, if allowed in your country. 200mw will give you an additional 20db ... just like that :-)
If you have your link budget sorted out as above, and your link is not OK, check your crimps.
Response[From Jim Forster on the AfNOG mailing list]
There's another possibility. I'm not sure but will mention it so you can check it. Standard 802.11 does not work over long distances, due to certain timer settings. Many products have a configuration setting to change these timer settings. If nothing else helps, check for this.
- Wireless Networking in the Developing World, website with free, detailed book, and a wiki with useful information.
- NSRC Wireless Networking Resources.