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New Member

wireless propagation distance

Hi,

I have heard of interference in wireless AP's and wanted to get some clear idea about this.

Assuming, if two AP's are nearby to each other, about 20 mts distance, how will the

near distance affect its capabilities?

and how can the issues resulting be avoided?

Thanks for all inputs.

7 REPLIES
Hall of Fame Super Gold

wireless propagation distance

Read this:  Cisco 1140 Series Access Point Deployment Guide

This should give you a pretty good picture how to deploy APs. 

Cisco Employee

Re: wireless propagation distance

You might be familiar with the concept  of frequency tuning in radio. When you tune your receiver to a certain  frequency, you are able to hear the programs from a particular channel.  So, when you use an analog rotary tuner to switch channels, you might  have noticed that as you rotate the tuner, first a faint sound appears,  then you get a strong signal, and then the signal weakens. So, the  signals are received (with varying range of amplitudes) over a range of  frequencies. When you consider many channels, the used range of  frequencies becomes wider.

Similarly, Wireless (Wi-Fi networks)  operate mainly in two major frequency bands (ranges) – 2.4Ghz and 5 Ghz.  Both are unlicensed ISM band frequencies (Industrial, Scientific and  Medical RF band) – Which means, any device / technology can use that  band for communications.

2.4 Ghz & 5 Ghz are frequency bands  (range of frequencies). The actual communications happen in  sub-frequencies called channels, within each spectrum (frequency band).  For example, in the 2.4 Ghz spectrum, Channel center frequencies might  be like : Channel 1 – 2.412 Ghz; Channel 2 – 2.417 Ghz…… Channel 13 –  2.472 Ghz, etc. A Wireless Radio (on wireless access point) & client  radio (wireless client on a laptop) operates in one of these channels  to transmit information between them.

Every channel (sub-frequency) overlaps  with its adjacent channels. So, Channel 6 for example, might overlap  strongly with channels 5, 4 but weakly with channels 3, 2. In the 2.4  Ghz spectrum, Channels 1,6 & 11 are non-overlapping channels. That  brings us to the next topic – Interference.

Wireless Interference:

Consider that there are three  operational access points situated at a distance of 1 meter from each  other (for example). If they operate in channels 1, 2 & 3  (respectively) or channels 1, 1 & 1 (respectively) – there would be a  lot of interference that will affect all the clients connecting to  these three access points. That’s because, generally access points and  clients receive all the communications that are transmitted and reject  those that are not in its frequency (channel) of operation. But if  different access points operate in same channels (or) adjacent channels,  they get confused if messages sent to them were meant for them or not!

But if the three access points are  operating in channels 1, 6 & 11 (respectively), even if they are  placed very close to each other, there would not be much interference  because, the sub-frequencies used by each channel are far apart. In  other words, these three channels are non-overlapping channels.

Interference might not allow you to  connect to a wireless access point/ network, disconnect you from an  existing connection (requiring you to re-connect to the network) or  might slow down/ choke the wireless connectivity. Wireless Interference  causes noticeable problems with real time applications like voice/ video  transmitted over the wireless network. Interference is both a  performance issue and a security concern (Rogue Access Points, Wireless DOS attacks, etc).

New Member

wireless propagation distance

Thanks.

But shouldn't the wireless AP/ controller adjust the power levels/channels dynamically to factor for these potential issues?

or need this to be done manually?

Hall of Fame Super Silver

Re: wireless propagation distance

It can manage that, but the question is... if you leave default settings, how will that affect your coverage. The AP's can hear each other, if within hearing distance, and does calculate. The AP doesn't know what your coverage area should be on your outer edges of your building. So it adjust power to accommodate the RF from what it hears and you might have to change default setting to provide you with coverage you want. This is why a site survey done right is important. AP's in the parameter of the building helps out too.

Sent from Cisco Technical Support iPhone App

-Scott
*** Please rate helpful posts ***
Hall of Fame Super Gold

wireless propagation distance

But shouldn't the wireless AP/ controller adjust the power levels/channels dynamically to factor for these potential issues?

Depends on your overlap.  If you have good wireless coverage overlap, the WLC will only adjust ONCE.  The next time the WLC will adjust is when there's an event.  Events like:

1.  An AP in your domain has failed;

2.  A new AP has appeared; and

3.  If your AP supports CleanAir, then another event is when the controller detects an interferrence.

New Member

wireless propagation distance

Thanks. So what if we configure the channels/power statically rather than the controller deciding it.

In this case, if there is interference or overlaps, will it be auto-adjusted by controller/ap's.

wireless propagation distance

Hello,

As per your query i can suggest you the following solution-

This device complies with Part 15 rules. Operation is subject to the following two conditions:

1. This device may not cause harmful interference, and

2. This device must accept any interference received, including interference that may cause undesired

operation.

For more information please refer to the link-

http://www.cisco.com/en/US/docs/wireless/access_point/1140/quick/guide/ap1140gsg.pdf

Hope this will help you.

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