Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Guia Do Desenho
9-14
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 9 VoWLAN Design Recommendations
Dual Band Coverage Cells
Figure 9-13
Channel Power Assignment
The maximum permissible transmit power across the 5 GHz band varies by as much as 6 dB. This means
that when using the maximum allowed transmit power throughout a site that allows all channels, there
will not be equal cell coverage on all channels. It also means that if dynamic channel selection is used,
the cell coverage edge may change based on the channel number. However, dynamic channel selection
can be tuned (see
that when using the maximum allowed transmit power throughout a site that allows all channels, there
will not be equal cell coverage on all channels. It also means that if dynamic channel selection is used,
the cell coverage edge may change based on the channel number. However, dynamic channel selection
can be tuned (see
) The default mode of
dynamic channel selection accounts for the difference of maximum transmit power level by channel.
Cell transmit power on all APs should not exceed the maximum or desired transmit power of the phone.
If the phones maximum or set transmit power is 13 dBm, Cisco recommends that all APs have a
maximum transmit power of 13 dBm. Therefore, the maximum transmit power on the AP should be set
to an equal level or, if not possible, the next higher transmit power level. Equal transmit power is
recommended to avoid one-way audio. The AP generally has better receiver sensitivity and diversity
support than the phone, so it should be able to receive the slightly lower strength phone signal. See
If the phones maximum or set transmit power is 13 dBm, Cisco recommends that all APs have a
maximum transmit power of 13 dBm. Therefore, the maximum transmit power on the AP should be set
to an equal level or, if not possible, the next higher transmit power level. Equal transmit power is
recommended to avoid one-way audio. The AP generally has better receiver sensitivity and diversity
support than the phone, so it should be able to receive the slightly lower strength phone signal. See
for more information on equal transmit powers.
Dual Band Coverage Cells
illustrates 2.4 GHz and 5 GHz band
channel coverage design. For a dual mode AP to provide equal cell coverage on both the 2.4 GHz
channel and the 5 GHz channel, the 2.4 GHz channel must have an equal (or more likely lower) transmit
power than the 5 GHz channel. At most sites, the noise level in the SNR formula will be lower by perhaps
10 dB. The receiver sensitivity of 802.11g radios is generally 2 dBm better than the same data rate on
the 11a radio. As an example, the data sheet for the 7921G has the receive sensitivity of -78 dBm at the
data rate of 36 Mbps for 802.11g, and -76 dBm for 802.11a. Therefore, given the anticipated better noise
floor of 10 dB, the 802.11a cell can do better by 8 dBm. Other details such as the difference in path loss
between 802.11g and 802.11a keep this from being a direct ratio. However, if the same coverage cells
are desired, reducing the 802.11g network by one or two power levels from the 11a network should
accomplish this goal.
channel and the 5 GHz channel, the 2.4 GHz channel must have an equal (or more likely lower) transmit
power than the 5 GHz channel. At most sites, the noise level in the SNR formula will be lower by perhaps
10 dB. The receiver sensitivity of 802.11g radios is generally 2 dBm better than the same data rate on
the 11a radio. As an example, the data sheet for the 7921G has the receive sensitivity of -78 dBm at the
data rate of 36 Mbps for 802.11g, and -76 dBm for 802.11a. Therefore, given the anticipated better noise
floor of 10 dB, the 802.11a cell can do better by 8 dBm. Other details such as the difference in path loss
between 802.11g and 802.11a keep this from being a direct ratio. However, if the same coverage cells
are desired, reducing the 802.11g network by one or two power levels from the 11a network should
accomplish this goal.
Dynamic Transmit Power Control
Cisco Aironet APs by default have DTPC enabled. DTPC is automatic with Wireless LAN Controllers
and is configurable on the standalone APs. Clients need to support a minimum of Cisco Compatible
Extension v2 capabilities to use DTPC.
and is configurable on the standalone APs. Clients need to support a minimum of Cisco Compatible
Extension v2 capabilities to use DTPC.
DTPC accomplishes the following:
•
Sets the phones transmit power to match the transmit power of the AP
•
The AP advertises its transmit power for the clients to learn
•
Prevents one-way audio; that is, RF traffic is only being heard in one direction
220353
5GHz
Channel
Max Tx Power
Channel
Max Tx Power
UNII-1
UNII-2
UNII-3
New UNII-II
36
40
44
48
52
56
60
64
149
153
157
161
11
11
11
11
11
17
17
17
17
100
11
104
17
108
17
112
17
116
17
132
17
136
17
140
17
17
14
11