Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
9-3
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 9 VoWLAN Design Recommendations
Channel Utilization
Antenna Positioning
Ceiling-mounted antennas typically have better signal paths to handheld phones. The recommended
coverage cell size takes into consideration the signal loss because of the attenuation of the head and other
obstacles. It is important to understand that the gain of antennas is reciprocal; gain applies equally to
reception and transmission. Antenna gain is not an increase in transmitted power; the radio produces the
transmitted power. The antenna is only a passive device. Gain is derived by focusing the signal of the
radio into a direction, plane, and beam width, much in the same way a flashlight reflector focuses the
light emanating from its bulb.
coverage cell size takes into consideration the signal loss because of the attenuation of the head and other
obstacles. It is important to understand that the gain of antennas is reciprocal; gain applies equally to
reception and transmission. Antenna gain is not an increase in transmitted power; the radio produces the
transmitted power. The antenna is only a passive device. Gain is derived by focusing the signal of the
radio into a direction, plane, and beam width, much in the same way a flashlight reflector focuses the
light emanating from its bulb.
Handset Antennas
The Cisco Unified Wireless IP Phone 7920 and 7921G have antennas that extend from the main body of
the phone. The way they are held in the hand does not significantly influence signal attenuation resulting
from the hand.
the phone. The way they are held in the hand does not significantly influence signal attenuation resulting
from the hand.
For phones that integrate the antenna inside the body of the phone, the way the user holds the phone in
the hand can influence signal attenuation by 4 dB. In some cases, a phone held against the head with the
hand covering the antenna can result in a signal drop of 9 dB. The general rule for indoor deployments
is that every 9 dB of signal loss reduces the coverage area in half.
the hand can influence signal attenuation by 4 dB. In some cases, a phone held against the head with the
hand covering the antenna can result in a signal drop of 9 dB. The general rule for indoor deployments
is that every 9 dB of signal loss reduces the coverage area in half.
shows an example of the
difference in radiating power from a handset when held to the head.
Handsets using the 2.4 GHz spectrum generally do not use diversity antennas because the 2.4 GHz
wavelength is nearly five inches, so there is no practical antenna diversity option that can be
implemented to improve signal reception. Therefore, the only improvement in link quality that can be
achieved is at the AP. To provide optimum link quality between the phone and the AP, the AP needs to
operate in its default configuration, which is with diversity enabled along with a diversity antenna.
wavelength is nearly five inches, so there is no practical antenna diversity option that can be
implemented to improve signal reception. Therefore, the only improvement in link quality that can be
achieved is at the AP. To provide optimum link quality between the phone and the AP, the AP needs to
operate in its default configuration, which is with diversity enabled along with a diversity antenna.
Note that 802.11a handsets such as the Cisco 7921G do have a diversity antenna solution for the 11a
radio.
radio.
Channel Utilization
The 802.11, 802.11b, and 802.11g standards use the same 2.4 GHz band. All must interoperate with each
other, which introduces additional overhead reducing channel throughput. Many sites already have
products using the Wi-Fi 2.4 GHz band. Additionally, there are many other products that use the same
2.4 GHz frequencies as used by Wi-Fi. Other products include Bluetooth, cordless phones, video game
controllers, surveillance cameras, and microwave ovens. Because the 2.4 Ghz band is so “crowded”,
coupled with its channel allocation constraints, you should consider using the 5 GHz Wi-Fi band for new
VoWLAN deployments. The channels available in 5 GHz are generally free of use at most sites (see
other, which introduces additional overhead reducing channel throughput. Many sites already have
products using the Wi-Fi 2.4 GHz band. Additionally, there are many other products that use the same
2.4 GHz frequencies as used by Wi-Fi. Other products include Bluetooth, cordless phones, video game
controllers, surveillance cameras, and microwave ovens. Because the 2.4 Ghz band is so “crowded”,
coupled with its channel allocation constraints, you should consider using the 5 GHz Wi-Fi band for new
VoWLAN deployments. The channels available in 5 GHz are generally free of use at most sites (see
. Use of the UNII-2 channels for VoWLAN traffic requires the absence of radar. Cisco
therefore recommends that there should be extra testing at any new site to see whether a channel in
UNII-2 should be blocked out by configuration. The reason for this is that if an AP detects radar during
normal use, it must leave the channel within ten seconds.
UNII-2 should be blocked out by configuration. The reason for this is that if an AP detects radar during
normal use, it must leave the channel within ten seconds.