"Best Practices" for positioning Access Points (AP's) and Antennas

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May 23, 2011 8:06 AM
May 23rd, 2011


Introduction:-

This document provides "Best Practices" or setup guidelines for positioning AP's, Antennas etc along with useful information, Tips and Tricks.

Core Issue:-

Connectivity issues either due to randomly signal drops or the connection speed.

Resolution:-

The first thing we need to look is the factors that affect the Radio Cell. We need to tweak the Radio Transmit power on AP's because the default power setting is Maximum, so we can try to turn it down to various settings to see the outcome and effect on the cell size.

Few Tips:-

You can change the radio coverage area of an AP when you modify the transmitter power level parameter.


The transmitter power (mW) setting determines the power level of the radio transmitter.** The default power setting is the highest transmit power allowed in a regulatory domain**. Government regulations define the highest power level for radio devices.


Caution: The transmitter power level setting must conform to the established standards of the country in which the setting is used.


This CLI command under the radio interface changes the transmitted power level to the maximum on an AP:


Access Point (config)# interface dot11radio 0

Access Point (config-if)# power local maximum


Use this command to set the power level to maximum. Then, check how much throughput you have, and move the power level to a lower value until you reach a high throughput rate that stays consistent. You can also start from the lowest possible power level and increase the level until you reach a consistent throughput. This is because in some cases, if you do not boost the signal to the maximum level, the throughput and signal strength can change continuously and not remain consistent

Change the Transmitter Power Level Parameter of the Existing AP to Extend the Coverage

Configuring Radio Transmit Power

Also have a look at these document:-


Use of Nonoverlapping Channels


When you have multiple APs in a wireless LAN (WLAN), ensure that the channels that the adjacent APs use are nonoverlapping. Nonoverlapping channels are frequency bands that do not have a frequency that is common to the other channels. For example, in the 2.4-GHz range, there are three channels that do not overlap (channels 1, 6, and 11). Therefore, when you deploy a secondary AP in order to extend the radio coverage, you can use:


Channel 1 for the first AP


Channel 6 for the next adjacent AP


Channel 11 for the third AP

Troubleshooting Connectivity in a Wireless LAN Network

Requirements for using two Antennas connected to the 1200 AP;


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.***

" ** Multipath ** - 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 Cisco Aironet Antennas and Accessories Reference Guide

Another good document - Multipath and Diversity

You should try to perform a site survey to help determine the AP/Antenna types and placement. Make sure the site survey is done when the facility is in full activity mode and Medical equipment is up and running. Here is some info to help get you started.

"In a Wireless network, many issues can arise which can prevent the radio frequency (RF) signal from reaching all parts of the facility. Examples of RF issues include mulitpath distortion, hidden node problems, and near/far issues. In order to address these, you need to find the regions where these issues occur. A site survey helps you to do this. A site survey helps define the contours of RF coverage in a particular facility. It helps us to discover regions where mulitpath distortion can occur, areas where RF interference is high and find solutions to eliminate such issues. A site survey that determines the RF coverage area in a facility also helps to choose the number of Wireless devices that a firm needs to meet its business requirements.

A proper site survey provides detailed information that addresses coverage, interference sources, equipment placement, power considerations and wiring requirements. The site survey documentation serves as a guide for network design and for the installation and verification of the Wireless communication infrastructure.

These are some of the steps that are performed during your site survey:-

Obtain a facility diagram in order to identify the potential radio frequency (RF) obstacles.

Visually inspect the facility to look for potential barriers or the propagation of RF signals and identify metal racks.

Identify user areas that are highly used and the ones that are not used. Determine preliminary access point (AP) locations. These locations include the power and wired network access, cell coverage and overlap, channel selection, and mounting locations and antenna.

Perform the actual surveying in order to verify the AP location. Make sure to use the same AP model for the survey that is used in production. While the survey is performed, relocate APs as needed and re-test. Document the findings. Record the locations and log of signal readings as well as data rates at outer boundaries."

Wireless Site Survey FAQ



Installation Guidelines:-

Because the antenna transmits and receives radio signals, they are susceptible to RF obstructions and common sources of interference that can reduce throughput and range of the device to which they are connected. Follow these guidelines to ensure the best possible performance:

•Mount the antenna to utilize its propagation characteristics. One way to do this is to orient the antenna vertically and mount it as high as possible.

•Keep the antenna away from metal obstructions such as heating and air-conditioning ducts, large ceiling trusses, building superstructures, and major power cabling runs. If necessary, use a rigid conduit to lower the antenna away from these obstructions.

•The density of the materials used in a building's construction determines the number of walls the signal must pass through and still maintain adequate coverage. Consider the following before choosing the location to install your antenna:

–Paper and vinyl walls have very little affect on signal penetration.
–Solid and pre-cast concrete walls limit signal penetration to one or two walls without degrading coverage.
–Concrete and wood block walls limit signal penetration to three or four walls.
–A signal can penetrate five or six walls constructed of drywall or wood.
–A thick metal wall causes signals to reflect off, causing poor penetration.
–A chain link fence or wire mesh spaced between 1 and 1 1/2 in. (2.5 and 3.8 cm) acts as a harmonic reflector that blocks a 2.4-Ghz radio signal.

•Install the antenna away from microwave ovens and 2-GHz cordless phones. These products can cause signal interference because they operate in the same frequency range as the device your antenna is connected to.

•Install the antenna in a vertical orientation to maximize signal propagation.

1.

Related Information:-

Wireless Site Survey Best Practices - White Paper


Performing a Site Survey


download the ACU (you need CCO Login)


ACU's site survey tool

Source:-

This Document is Created from following Discussion.

https://supportforums.cisco.com/thread/333422

Average Rating: 5 (1 ratings)

Comments

Rajan Parmar Thu, 06/02/2011 - 16:13

This link can be virtually sent in every wireless troubleshooting case in the signature of an email of the Engineer

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