Table Of Contents

Site Survey RF Recommendations

AP and Antenna Placement

Improper AP and Antenna Placement

Proper AP and Antenna Placement

Interference and Multipath Distortion

Signal Attenuation

Antenna Types Recommended for Indoor Applications

Surveying Multi-Floor Buildings, Hospitals, and Warehouses

Testing Active Cisco 7920 Phones without Cisco CallManager

Site Survey RF Recommendations

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This section provides additional details about voice site surveys that are crucial for a successful deployment, and it includes additional information on RF and antenna patterns and behavior.

AP and Antenna Placement

This section gives examples of both proper and improper placement of access points (APs) and antennas.

Improper AP and Antenna Placement

Figure B-1 shows improper placement of an AP and antennas close to an I-beam, which creates distorted signal patterns. An RF null point is created by the crossing of signal waves, and multipath distortion is created when signal waves are reflected. This placement results in very little coverage behind the AP and reduced signal quality in front of the AP.

Figure B-1 Improper Placement of Antennas Near an I-Beam

Figure B-2 shows the signal propagation changes or distortions caused by an I-beam. The I-beam creates many reflections from both received packets and transmitted packets. The reflected signals result in very poor signal quality because of null points and multipath interference. However, the signal strength is high because the AP antennas are so close to the I-beam.

Figure B-2 Signal Distortions Caused by Placing the Antennas Too Close to an I-Beam

The AP and antenna placement in Figure B-3 is better because it is away from the I-beams and there are fewer reflected signals, fewer null points, and less multipath interference. This placement is still not perfect because the Ethernet cable should not be coiled up so close to the antenna.

Figure B-3 AP and Antennas Mounted on a Wall, Away from I-Beams

Figure B-4 shows the signal propagation caused by the wall on which the AP is mounted.

Figure B-4 Signal Reflection Caused by a Wall

The preceding examples also apply when placing APs and antennas in or near the ceiling in a standard enterprise environment. If there are metal air ducts, elevator shafts, or other physical barriers that can cause signal reflection or multipath interference, Cisco highly recommends that you move the antennas away from those barriers. In the case of the elevator, moving the antenna a few feet away will help eliminate the signal reflection and distortion. The same is true with air ducts in the ceiling.

Conclusion

A survey conducted without sending and receiving packets is not sufficient. The I-beam example shows the creation of null points that can result from packets that have CRC errors. Voice packets with CRC errors will be missed packets that adversely affect voice quality. In this example, those packets could be above the noise floor measured by a survey tool. Therefore, it is very important that the site survey not only measures signal levels but also generates packets and then reports packet errors.

Proper AP and Antenna Placement

Figure B-5 shows a Cisco AP1200 properly mounted to a ceiling T-bar, with the antennas in an omni-directional position.

Figure B-5 Cisco AP1200 Mounted to a Ceiling

Figure B-6 shows a Cisco Aironet 5959 omni-directional diversity antenna properly mounted to a ceiling T-bar. In this case, the Cisco AP1200 would be mounted above the ceiling tile.

Figure B-6 Cisco Aironet 5959 Antenna Mounted to a Ceiling

Figure B-7 shows a Cisco AP1200 properly mounted to a wall.

Figure B-7 Cisco AP1200 Mounted to a Wall

Figure B-8 shows the Cisco Aironet 2012 diversity patch antenna mounted to a wall. In this case, the Cisco AP1200 would be mounted above the ceiling tile.

Figure B-8 Cisco Aironet 2012 Antenna Mounted to a Wall

For areas where user traffic is high (such as office spaces, schools, retail stores, and hospitals), Cisco recommends placing the AP out of sight and placing unobtrusive antennas below the ceiling.

Interference and Multipath Distortion

The throughput performance of the WLAN network is affected by unusable signals.

WLAN interference can be generated by microwave ovens, 2.4 GHz cordless phones, Bluetooth devices, or other electronic equipment operating in the 2.4 GHz band. Interference also typically comes from other APs and client devices that belong in the WLAN but that are far enough away so that their signal is weakened or has become corrupted. APs that are not part of the network infrastructure can also cause WLAN interference and are identified as rogue APs.

Interference and multipath distortion cause the transmitted signal to fluctuate. Interference decreases the signal-to-noise ratio (SNR) for a particular data rate. Packet retry counts go up in an area where interference and/or multipath distortion are high. Interference is also referred to as noise level or noise floor. The strength of the received signal from its associated AP must be high enough above the receiver's noise level to be decoded correctly. This level of strength is referred to as the signal-to-noise ratio, or SNR. The ideal SNR for the Cisco 7920 Wireless IP Phone is 25 dB. For example, if the noise floor is 98 decibels per milliwatt (dBm) and the received signal at the Cisco 7920 phone is 73 dBm, then the signal-to-noise ratio is 25 dB. (See Figure B-9.)

Figure B-9 Signal-to-Noise Ratio (SNR)

Changing the type and location of the antenna can reduce multipath distortion and interference. Antenna gain adds to the system gain and can reduce interference if the interfering transmitter is not directly in the boresight of the directional antenna.

Conclusion

While directional antennas can be of great value for certain indoor applications, the vast majority of indoor installations use omni-directional antennas. Directionality should be strictly determined by a correct and proper site survey. Whether you use an omni-directional or patch antenna, indoor environments require diversity antennas to mitigate multipath distortion. The Cisco Aironet 350, 1100, and 1200 Series Access Point radios include diversity support; however, the AP radio cannot provide diversity support with a single non-diversity antenna.

Signal Attenuation

Signal attenuation or signal loss occurs even as the signal passes through air. The loss of signal strength is more pronounced as the signal passes through different objects. A transmit power of 20 mW is equivalent to 13 dBm. Therefore, if the transmitted power at the entry point of a plasterboard wall is at 13 dBm, the signal strength will be reduced to 10 dBm when exiting that wall. Table B-1 shows the likely loss in signal strength caused by various types of objects.

Table B-1 Signal Attenuation Caused By Various Types of Objects

Object in Signal Path	Signal Attenuation through Object
Plasterboard wall	3 dB
Glass wall with metal frame	6 dB
Cinder block wall	4 dB
Office window	3 dB
Metal door	6 dB
Metal door in brick wall	12 dB
Human body	3 dB

Each site surveyed will have different levels of multipath distortion, signal loses, and signal noise. Hospitals are typically the most challenging environment to survey due to high multipath distortion, signal losses and signal noise. Hospitals take longer to survey, require a denser population of APs, and require higher performance standards. Manufacturing and shop floors are the next hardest to survey. These sites generally have metal siding and many metal objects on the floor, resulting in reflected signals that recreate multipath distortion. Office buildings and hospitality sites generally have high signal attenuation but a lesser degree of multipath distortion.

Antenna Types Recommended for Indoor Applications

Cisco highly recommends the use of diversity antennas for optimal throughput and performance in indoor deployments.

Figure B-10 and Figure B-12 show two types of antennas recommend for indoor applications, and Figure B-11 and Figure B-13 show the respective radiation patterns for those antennas.

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Note The antenna power level ratings are listed as decibels isotropic (dBi).

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Figure B-10 Cisco Aironet 2 dBi Diversity Omni-Directional Ceiling Mount Antenna (AIR-ANT5959)

Figure B-11 Radiation Pattern for AIR-ANT5959 Antenna

Figure B-12 Cisco Aironet 6.5 dBi Diversity Patch Wall Mount Antenna (AIR-ANT2012)

Figure B-13 Radiation Pattern for AIR-ANT2012 Antenna

Surveying Multi-Floor Buildings, Hospitals, and Warehouses

Consider the factors listed in this section when surveying multi-floor buildings, hospitals, and warehouses.

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Note There is no way of determining the distance an RF signal will travel without conducting a survey.

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Construction Methods and Materials

Many aspects of the building construction are unknown or hidden from the site survey, so you might have to acquire that information from other sources (such as architectural drawings). Some examples of typical construction methods and materials that affect the range and coverage area of APs include metallic film on window glass, leaded glass, steel-studded walls, cement floors and walls with steel reinforcement, foil-backed insulation, stairwells and elevator shafts, plumbing pipes and fixtures, and many others.

Inventory

Various types of inventory can affect RF range, particularly those with high steel or water content. Some items to watch for include printer paper, cardboard boxes, pet food, paint, petroleum products, engine parts, and so forth.

Levels of Inventory

Make sure you are performing a site survey at peak inventory levels or at times of highest activity. A warehouse at a 50% stocking level has a very different RF footprint than the same warehouse at an inventory level of 100%.

Activity Levels

Similarly, an office area after hours (without people) will have a different RF footprint than the same area full of people during the day. Although many parts of the site survey can be conducted without full occupation, it is essential to conduct the site survey verification and tweak key values during a time when the location is occupied.

The higher the utilization requirements and the higher the density of users, the more important it is to have a well designed diversity solution. When more users are present, more signals are received on each user's device. Additional signals cause more contention, more null points, and more multipath distortion. Diversity on the AP helps to minimize these conditions.

Multi-Floor Buildings

Keep in mind the following guidelines when conducting a site survey for a typical office building:

•Elevator shafts block and reflect RF signals.

•Supply rooms with inventory absorb signals.

•Interior offices with hard walls absorb RF signals.

•Break rooms (kitchens) can produce 2.4GHz interference through the use of microwave ovens.

•Test labs can produce 2.4 GHz or 5 GHz interference, creating multipath distortion and RF shadows.

•Cubicles tend to absorb and block signals.

•Conference rooms require high AP coverage because they are areas of high utilization.

Take extra care when surveying multi-floor facilities. APs on different floors can interfere with each other as easily as APs located on the same floor. It is possible to use this behavior to your advantage during a survey. Using higher-gain antennas, it might be possible to penetrate floors and ceilings and provide coverage to floors above as well as below the floor where the AP is mounted. Be careful not to overlap channels between APs on different floors or APs on the same floor.

In multi-tenant buildings, there might be security concerns that require the use of lower transmission powers and lower gain antennas to keep signals out of neighboring rooms or offices.

Hospitals

The survey process for a hospital is much the same as that for an enterprise, but the layout of a hospital facility tends to differ in the following ways:

•Hospital buildings tend to go through many reconstruction projects and additions. Each additional construction is likely to have different construction materials with different levels of attenuation.

•Signal penetration through walls and floors in the patient areas is typically minimal, which helps create micro-cells.

•The need for bandwidth increases with the increasing use of WLAN ultrasound equipment and other portable imaging applications. Of course, the need for bandwidth increases with the addition of wireless voice as well.

•Healthcare cells are small, and seamless roaming is essential, especially with voice applications.

•Cell overlap can be high, and so can channel reuse.

•Hospitals may have several types of wireless networks installed, including 2.4 GHz non-802.11 equipment. This equipment could cause contention with other 2.4 GHz or 5 GHz networks.

•Wall-mounted diversity patch antennas and ceiling-mounted diversity omni-directional antennas are popular, but keep in mind that diversity is required.

Warehouses

Warehouses have large open areas, often containing high storage racks. Many times these racks reach almost to the ceiling, where APs are typically placed. Such storage racks can limit the area that the AP can cover. In these cases, consider placing APs on other locations besides the ceiling, such as side walls and cement pillars. Also consider the following factors when surveying a warehouse:

•Inventory levels affect the number of APs needed. Test coverage with two or three APs in estimated placement locations.

•Unexpected cell overlaps are likely because of multipath variations. The quality of the signal will vary more than the strength of that signal. Clients might associate and operate better with APs farther away than with nearby APs.

•During a survey, APs and antennas usually do not have an antenna cable connecting them. But in a production environment, the AP and antenna might require antenna cables. All antenna cables have signal loss. The most accurate survey will include the type of antenna to be installed and the length of cable to be installed. A good tool to use to simulate the cable and its loss is an attenuator in a survey kit.

Surveying a manufacturing facility is similar to surveying a warehousing, except that there might be many more sources of RF interference in a manufacturing facility. In addition, the applications in a manufacturing facility usually require more bandwidth than those of a warehouse. These applications can include video imaging and wireless voice. Multipath distortion is likely to be the greatest performance problem in a manufacturing facility.

Testing Active Cisco 7920 Phones without Cisco CallManager

Perform the following steps to test a Cisco 7920 Wireless IP Phone without using Cisco CallManager:

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Step 1 Associate the Cisco 7920 phone with the test AP.

Step 2 Use ping commands from the AP to a statically assigned IP address on the phone. Telnet to the test AP and start a continuous ping with the following commands:

a. At the prompt (>), enter the command enable and the password Cisco.

b. Enter the command ping nnn.nnn.nnn.nnn size 256 repeat 1000 validate, where nnn.nnn.nnn.nnn is the IP address of the Cisco 7920 phone.

c. Review the reported success rate.

d. Move to the next coverage test location.

e. Repeat steps b and c.

f. After determining the performance for the new location, walk back to the previous location while pinging the phone.

g. Verify the ping success rate while walking.

h. Repeat the above steps throughout the survey of the site.

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Note When the phone is in standby mode, it will not answer every ping. To prevent this condition, place the phone in a call or temporarily disable power-save mode as described in the section on Advanced Cisco 7920 Commands, page A-1.

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For voice, the success rate for pings should be 99%. The size value of 256 in Step 2b is used because it is slightly larger than the size of a voice packet. The repeat value of 1000 is just a suggestion for a value when you are stationary. The value can be up to 2147483647. When walking, use a larger value, but the success rate should remain at 99%.