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Aironet Diversity

Can anybody give me a refference for the Aironet diversity algotithm ? A manual sais AP chooses the better antenna for each client to comunicate with. But Cisco recommends both antenas to the same area. It is clear it solves multipath interferrence issues. I imaginate this situation : twoo clients in a room and twoo others behind a wall. One antenna into the first room and second antenna behind the wall as well. Aironet in diversity mode will find the first two clints are better to treat by the first antenna etc. Is there some problem ?

7 REPLIES
Green

Re: Aironet Diversity

According to the docs I've read, diversity is designed to watch the same area from two viewpoints.

Since only one antenna is active at any time, separating them into two different areas, even if they're close, will significanttly impact / reduce the performance of all clients.

Good Luck

Scott

Hall of Fame Super Red

Re: Aironet Diversity

Hi Mirek,

Here is a liitle more info to add to Scott's excellent answer;

In a multipath environment, signal null points are located throughout the area. The distance an RF wave travels, how it bounces, and where the multipath null occurs are based on the wavelength of the frequency. As frequency changes, so does the length of the wave. Therefore, as frequency changes, so does the location of the multipath null. The length of the 2.4 GHz wave is approximately 4.92 inches (12.5 cm). The length of the 5 GHz wave is approximately 2.36 inches (6 cm).

In order to increase coverage, conduct a site survey to determine the RF coverage of the antennas. Place access points in the appropriate areas of the installation site. The purpose of diversity is to overcome multipath reflections. Diversity antennas that share the same physical housing are placed at an optimum distance apart. The maker of the particular antenna determines that distance based on the characteristics of the antenna. When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.Because each antenna is selected by itself, both antennas must have the same radiation characteristics and be positioned to provide similar cell coverage. Two antennas connected to the same access point must not be used to cover two different cells.

From this good doc;

Multipath and Diversity

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml

Hope this helps!

Rob

Green

Re: Aironet Diversity

Great explanation & link Rob!

Thanks!

Scott

Hall of Fame Super Red

Re: Aironet Diversity

Thanks Scott,

Coming from you, that means a Great deal to me!

Take care.

Rob

New Member

Re: Aironet Diversity

Rob,

thanks for in-depth explanation. I know the doc about the diversity and its main application. But the document doesn't describe how the AP treats frame by frame by either one atntenna or by the second one. I don't need to solve multipath issues. I know that Cisco doesn't recommend to cover two areas by this way but I don't exactly know why (except of wish to sell two radios). I am thinking about temporary usage only.

Forget the multipath nulls. Lots of radios have no divertity. Imaginate : if both antenas cover the same area AP serves each client either by one antenna or by the second one. And we will move part of the client to a different area and his better antenna as well.

Hall of Fame Super Red

Re: Aironet Diversity

Hi Mirek,

If this is a temporary setup you could give it a try, but I would probably try to do a Lab simulation first if I could. I think you will find that if the 2 Antennas are more than about 20 inches apart you will see some performance degradation.The reasoning behind Diversity Antennas is not a Cisco solution to sell more AP's, but more a fact of how radio signals work in a given environment.

Here is another good excerpt;

"You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly. By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.

A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna and use only that antenna for the remaining portion of that packet.

On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.

One caution with diversity, it is not designed for using two antennas covering two different coverage cells. The problem in using it this way is that, if antenna no. 1 is communicating to device no. 1 while device no. 2 (which is in the antenna no. 2 cell) tries to communicate, antenna no. 2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area from only a slightly different location."

From this Antenna reference guide;

http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.html

Here is the best description of why to use Diversity Antennas (in laymans terms) from Scott earlier this year. When I read his reference to being at a stadium watching a sporting event, this all finally made sense.Have a look at his great explanation;

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd7905b/6#selected_message

Maybe a good Omni (360 degree radiation pattern) would be better in the model you have described.

Let us know how this works out.

Hope this helps!

Rob

New Member

Re: Aironet Diversity

OK Rob, thanks a lot. The second link refers to a good threat. Moreover thanks for the excelent explanation with the switch. I can't get myself out of the idea. There is a collisional protocol and only just one client is served in a moment and the switch can flip according the client placement. But I am affraid that all clients need to hear each other to participate in the collisional protocol. So I will try to save a time to arrange some lab. Thanks again for patience.

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