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I have read that dual antennas on access points are there to assist in reducing multipath interference. I have just had a look at an installation where Cisco 1200 series access points have had one of the 2.4 GHz antennas placed in another room connected via a length of cable, so 1 access point covers 2 rooms.

Is this ok or shoult I point this out as an error ?

-Thanks for any comments.

This is a poor design idea. Most likely this was done to avoid buying anotherr AP. The money saved by not buying another AP will not match the money spent in the future on troubleshooting and loss of productivity. Is this an array or just diversity misused?

Thanks, Bryan.

I can't give too much away as I'm under confidentiality, however I am taking over responsibility for tech support of the network once the installation teams leave. I have two identical geographically diverse sites. (actually, they are boats). The first has the access points fitted. The second has HP access points specified, I have requested a change to Cisco. Both sites have this odd antenna plan for three or four access points.

I know no predictions or modelling have been done, and the install team, when asked on how they perform the site survey, told me that they just "walk round with a laptop and see where they get a signal"

I have asked for a controller to be added per site, plus I have been given permission by the final customers to use my Ekahau software to see if the planning is right. I am sure that the 30 HP access points can be replaced with 15 Ciscos correctly placed and with directional antennas.

As a company, we are trying to acquire these skills and promote best practices. However, I am a small fish, even though the customers trust me, it's hard to diplomatically state that a multi million dollar turnover installation company can be making mistakes. But wifi isn't their primary business, so, like many others, they get by with minimal installation skills.

I am careflly submitting written reports, based on my recommendations, one of the things to point out is this antenna arrangement.

BTW me and my colleague are both studying for CWNA, hoping to become a primary company in our field for getting these installs professional and correct. (Lot to learn, though..)

I have also wondered about this issue. I have heard of access points fitted with one omnidirectional antenna and one directional antenna: the omnidirectiona to connect surrounding wireless clients, the directional antenna to connect wirelessly to another AP using WDS. Does this work? Are there problems with this setup?
Thanks.

This is most likely due to misunderstanding of how the diversity is used.

You'd have to understand how diversity works here (which is different to how diversity works in MIMO system)

With the two antennas, only one antenna transmit at a particular time. The AP decides which antenna to transmit based on the last received signal.

Ie if antenna 1 was determined to have the best signal for the last packet, the next packet will be sent from antenna 1.

Hope this helps

I think what Sacim is referring to here is what vendors like proxim and others that are in the metro deployments do to backhaul the signal for mesh type nodes. These units typically have 2 or more radios, at least one with an omni antenna used for client access and a second radio (within the same AP) with a separate directional antenna to backhaul (usually on 5.8Ghz) to the core.

The implementation Neilmac spoke of is just ugly. He is correct in pointing this poor design out to the customer. As Bryan mentioned, it was probably done to cut corners on hardware cost. (Under bidding a proper install comes to mind)

Actually I was referring to a basic 802.11g access point with only one radio...
Can antenna diversity handle traffic correctly in this case, or will the entire system suffer?

In effect it is the same thing, only in your case, with one radio. System performance would surely suffer. Remember everything is half-duplex, when you start cutting halves in half and adding other overhead to the equation, things could get pretty slow.

I wouldn't do this unless for basic internet web browsing for a low cost solution. (Free wi-fi networks some are installing for areas/people who cannot otherwise afford internet access for example)

From what I remember, you'll get Near-Far problems.
If the AP is talking to one station, and another station can't hear it, you'll get collisions or reseant packets.
It might be OK if one antenna is just extending the coverage a bit, but having 2 directional antennas is a bad idea.

Any vendor that can claim their radios will perform just as well with WDS enabled and using a single radio to handle both client connections and bridging to another access point would be lying! There is no good way to implement WDS for bridging and access, and WDS should only be used as a last resort (not only my advice, but Byron Putman - author of "802.11 Network Monitor Unleashed") also warns of WDS use, and says they should be used as a means of LAST RESORT ONLY.

See his warnings here and excerpt of book here:
http://tinyurl.com/2szpv3

Anyway, WDS - when oversubscribed, promotes "Hidden Node Syndrome" - which is also called Near-Far Problems are the biggest concern. Your furthest clients will have more contention for the wireless link and will produce more collisions and Page Can Not Be Displayed errors to end users than going with solution that uses two radios, one for backhaul and another for servicing clients.

I just came back from an install in a nice neighborhood in Sunny Florida, and they were using a bunch (qty 12) low end, single radio access points equivelant to the Linksys and D-Link SOHO products you would find in a residential setting. I can not articulate to you how poor of an overall design this was...the installation was relatively clean, but the antenna cables were cheap RG-58 cables and to make up for the tremendous loss, the installer used 250mW amplifiers...but installed them in a wiring closet where high voltage lines came in and all the EMI interference that was produced in the room bled through the wires and was amplified out into the network, causing the relatively good signal to become very unusable due to noise and interference due to the WDS.

Long story short, never use Diversity antenna ports to try and go different directions, unless you want to visit the site more than once to get a good install. Manufacturers everywhere tell you, and not to sell more radios, that if you use the antenna ports for external antennas, use one connector for single element antennas and use dipole antennas (two element antennas with two antenna connectors) if you plan on using both antenna connectors.

You are doubling your chances for problems when combining the drawbacks of WDS with the interference created from splitting a diversity antenna to go two seperate directions...and won't find anyone to back up the installation when something goes wrong. The vendor will say, we don't support diversity antenna links being split to form two seperate coverage areas...it just doesn't make good design sense.

Last, WDS drawback definition from Wikipedia cautions:
"It should be noted, however, that throughput (using WDS) is inversely proportional to two raised to the power of the number of "hops",[2] as all traffic uses the same channel. For example, client traffic going through one relay station before it reaches the main access point will see at most half the maximum throughput that a directly connected AP would experience and a client two hops from the directly connected AP will see at most one quarter of the maximum throughput seen at the directly connected AP.

In laymans terms, for every link you cut the speed of transmission and distance in half. So if you were getting 6Mbps off the AP, off the WDS repeater you would get 3Mbps off the first hop and 1Mbps off the second hop...and wireless doesn't go below this, so we are already talking about throughput of kilobytes instead of megabytes, and you don't want to make your users this angry! Rule of thumb is don't go beyond 3-4 WDS hops, and make sure the expectation with the end users are understood before specing out such a design.

For more info, look up Wireless Distribution System (WDS) Performance issues and Splitting out Antenna Diversity...there are a lot of great articles out there that warn against this configuration, and tell of nightmare experiences from the field.