[afnog] Losing wireless link between two APs

Tue Jan 31 14:16:53 EAT 2006

Mangaliso,

<LONG WIRELESS LECTURE>

         <Disclaimer>    This is a simplistic approach to wireless 
Networking :-) </Disclaimer>

 From your mail, it should work, but there are other (unknown) factors also 
in your mail, that would ensure it does not work. Sustaining a good link, 
is a function of the following amongst many other parameters:

1. Power of Transmit Card
2. Loss of cable and connectors at the transmit end
3. Gain of Transmit Antenna
4. Free Space Loss due to distance between the two points
5. Gain or Receive Antenna
6. Loss of cable and connectors at the Receive end
7. Receive Sensitivity of Receiver

All expressed in Decibels (db). When you add all of the above, it 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). so 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 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:

1. Maximum power of 100mw, so that A1=20db
2. Very short cable of about 10Metres and 2 connectors, so that A2 is (10 
*.22 + 2* 0.2) A2=-2.6db
3. +12db
4. -112db
5. as in 4: +12db
6. as in 3: -2.6db
7. -85db

20-2.6+12-112+12-2.6-(-85)
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.

1. Use an amplifier, within the safe legal limits in your country
2. 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
3. Use shorter length cable and fewer connectors. Meaning you will mount 
your radio on your tower.
4. Use a radio with better receive sensitivity.
5. 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.

</LONG WIRELESS LECTURE>

Cheers.

Sunday.

At 08:40 AM 1/31/2006, Mangaliso Jere wrote:
>Guys
>
>I have a problem on my wireless network. Its giving me an unreliable
>to link to my customers.
>A brief background is that we have connected two Full cisco 350 series bridges
>
>1. Acts as root bridge
>
>2. the other is a non-root w/clients
>
>The distance between the is less than 4km and the light of sight is
>very clear and the antennas used are 12dbi which from its specs it
>should work.
>
>Rebooting solves the problem for a small period.
>
>The problem
>
>1. I lose the link within a short period.
>2. The log error i get is as 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 more info required to solve this problem will be readily be
>provided. Any assitance will be greatly appreciated.
>--
>Mangaliso Jere
>P/O Box 1120
>Lilongwe
>Malawi
>(265)- 9 - 953079
>
>Failure is an option -- it comes bundled with Windows.
>
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>afnog mailing list
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