Configuration of a Point to MultiPoint link with Cisco Aironet 1310 bridges

[igor.sotelo]

Hi All,

The previous problem of which I started another conversation here:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=General&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddba023

somehow dissapeared. It could have been a problem of interferences.

I have another issue with other (multipoint) wireless WAN link, which I hope has a solution.

On the central node, we have an Cisco Aironet 1310 bridge configured as root-bridge. It has a panel of four vertical polarity 17 dBi panel 90? antennas, with more than enough gain (there is a 250 mWatts 802.11 b/g amplifier, before the 4-way splitter) and excellent line of sight to three remote bridges.

The three (03) remote bridges are also Aironet 1310 models, confidured as non-root-bridges.

The problem we have is that it seems that when the three remote links operate concurrently the amount of lost packets is huge. When I shutdown the radio interfaces of two bridges, the remaining bridge makes an excellent link with the central node.

It seems that some hours are more critical than others, also the links operate much worse when there is some (small) network traffic in them.

I have read the 1310 manuals, and I can't find a sample configuration for point to multi-point links.

Does someone knows what radio interface configuration should I need to use to establish better quality communication?

I mean, perhaps the 1 x root - 3 x no root configuration is not recommendable for the multipoint link configuration.

Any hints will be welcome.

Best Regards,

Igor Sotelo.

[oj88]

How far are the remote stations from the central node? Have you tried lowering the connect speed?

Based on the information you gave and the fact that you used an amplifier leads me to think that the remote sites might be too far for the ethernet spec to cope. You may need to tweak the RF timing parameters or lower the connect speed.

You may also try to enable "optimize for range" or something to that effect on all radios.

[igor.sotelo]

Hi,

Thank you for the information. The distance from the central node is less than 0.8 milles.

I have disabled the OFDM speeds at the central node and it's working at 11 Mbps, so far very stable, with almost no lost packets.

It seems that the problem was due to the fact that two remote nodes ware using OFDM speeds (18 and 12 Mbps, respectively) and the third node was using a non-OFDM speed (5.5 Mbps).

The constant switching from one modulation to another generated the loses. According to Hyperlink it would be an issue of the Cisco equipment. But it could be the amplifier too.

I will try to tweak the largest link, doing a better allignement, to be able to use OFDM speeds at all sites, and will try it again at better than 11 Mbps speeds.

With which parametars should I tweak the RF timings?

Best Regards,

Igor Sotelo.

[oj88]

You can use the distance command on the root briodge. More info here: http://www.cisco.com/en/US/partner/products/ps5861/products_configuration_guide_chapter09186a00804ed70e.html#wp1053605

"The distance setting adjusts the access point/bridge's time out values to account for the time required for radio signals to travel from access point/bridge to access point/bridge."

[scottmac]

Personally, first, I'd drop the amp. For the distance you're running, it shouldn't be necessary.

It may also be causing the system to interfere with itself (signal transmitted on one antenna is being picked up on the other(s)).

If possible, you may want to separate the sector antennas from each other. Even the most directional antennas have a little back-end lobe ... the extra power you are using can cause the back-end lobe(s) to distort the primary signal on the other two antennas.

Next, if you *absolutely* decide you have to use the amp, and it's a "transmit only" amp (versus a receive amp, or one that amplifies in both directions), you want to put the amp as close to the transmitter (the AP) as possible, so that the additional signal keeps the signal level above the noise floor.

Putting it at the antenna end (for transmit) means that the signal amplitude / level above the noise floor has diminished, and when you amplify, you are amplifying the noise floor right along with it.

For a "receive only" amp, you want it closest to the antenna, to boost the signal above the noise before you add the losses of the cabling, couplers, connectors etc.(maintain the signal-to-noise ratio).

So, IMHO, 1) lose the amp 2) separate the antennas, if possible 3) tweak, tweak, tweak.

Good Luck

Scott

[igor.sotelo]

Hi All,

Thank you for the information. I configured the distance on the root bridges, but the links showed instability.

I'm using a bi-directional amplifier. It has two pieces. According to the manuals, one is installed indoors, the other outdoors. I'm not sure if the indoors piece has the transmition module or it's only the injector.

We could establish connection at 7 km (around 4 milles) distance from the central point, using 24 dBi antennas on the other side.

However, we have issues with a near located point that is only 1.2 Km (around 0.8 milles) away and has a 13 dBi integrated patch antenna. The signal strenght value we get there is in the -62 to -68 dBm range, and is noticiably (5-10 dBm) lower than the strenght we get at other points of the link. And I have trouble establishing a high quality link with that point, using OFDM modulation. I tend to think that if I remove the amplifier I'm not going to reach that point at all. The EIRP on the central iste is 34 dBm / 2.5 watts, without amplifier it would be 26 dBm / 0.4 watts.

On the opposite sites the EIRP is 33 dBm / 2 watts using CCK or 28 dBm / 0.63 watts using OFDM.

When one looks at the central site from that point, an Motorola Canopy with passive reflector (EIRP 48 dBm or around 64 watts) can be seen. It doesn't have the same direction, but the opposite site must be large distance and could interfere with my wireless network. Attached is an amplified photo of the view. It's safe to assume that the Canopy operates in the 2.4 GHz frequency range.

Once I connect the point at 1.2 Km, the multipoint link loses its quality, and soon the lost packets get too frequent.

The CCK seems to be much more interfered than OFDM, I guess because of that canopies.

Another thing I'm wondering about is if the Aironet 1310 can continuosly switch CCK-OFDM over the same point - multipoint link, without losing packets.

What other parameters should I tweak? Is there a way to avoid interferences fromt the canopy?

I would like to apply 100 mWatts local power using the radio with OFDM, but it seems that's not possible.

Best Regards,

Igor Sotelo.

[scottmac]

I would check the cabling and connectors on the remote end (with the 24dbi antenna). Try another section of cabling ... if only for diagnostic / rule out troubleshooting. Cables / connectors are extremely critical at these freqs.

Also, with a beamwidth that narrow, aim will be critical (azimuth and elevation ... depending on the altitudes involved).

It might also be a reflection problem (multipath).

Check / recheck your calculations for the "freznel zone" as well; even though the beamwidth is likely to be tighter, the freznel zone doesn't shrink / doesn't shrink as fast ... you still need some clearance, espeically at distance.

If you're shooting over a large body of water, that can also cause some problems.

The two parts of your amp are likely to be a power injector and a mast-mounted amp. If it's truely bi-directional, check the spec sheet closely. It is possible / likely that the amp is meant to be used with a set of cans (very tight notch filters to prevent the transmit signal from blanketing the receive side .... i.e, desensitize the the receiver).

Again, it's much better to tweak your antennas than to just dump a bunch of power into air. With standard APs and good dishes, you can go ~26km (something like that). Using power, especially at these freqs, frequently create more problems (for you and other wireless users in your neighborhood) than it solves.

(people do moonbouce and satellite coms with less power than you're using)

Good Luck

Scott