Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Design Guide
9-13
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 9 VoWLAN Design Recommendations
Cell Edge Design
Note
The -86 dBm separations shown in
is simplified and is considered ideal. It is very unlikely
that this 19 dBm of separation can be achieved in most deployments. The most important RF design
criteria are the -67 dBm cell radius, and the 20 percent recommended overlap between cells. Designing
to these constraints optimizes channel separation.
criteria are the -67 dBm cell radius, and the 20 percent recommended overlap between cells. Designing
to these constraints optimizes channel separation.
For 5 GHz cells, there is less concern about same channel separation because of the number of available
channels. There are 20 channels in 802.11a, so a two-channel separation is almost always possible, in
contrast to the 2.4 GHz band where there are only three channels that do not overlap in frequency.
channels. There are 20 channels in 802.11a, so a two-channel separation is almost always possible, in
contrast to the 2.4 GHz band where there are only three channels that do not overlap in frequency.
For both 5 GHz and 2.4 GHz, the cell edge needs to be at the floor level where a packet error rate of
1 percent is maintained at the highest data rate desired for a given channel. In the case of 802.11b, that
data rate is 11 Mbps. Thus, from the center of the AP location to a point on the floor where the phone
signal is seen by the AP, the cell edge is -67 dBm.
1 percent is maintained at the highest data rate desired for a given channel. In the case of 802.11b, that
data rate is 11 Mbps. Thus, from the center of the AP location to a point on the floor where the phone
signal is seen by the AP, the cell edge is -67 dBm.
802.11g and 802.11a phone clients may be capable of rates up to 54 Mbps. Current chip sets support
54 Mbps, but transmit power capabilities do differ. Cisco highly recommends that all links between
phone clients and APs be established using matching transmit power levels. (see
54 Mbps, but transmit power capabilities do differ. Cisco highly recommends that all links between
phone clients and APs be established using matching transmit power levels. (see
Coverage cells can be created for specific data rates. For a high density deployment or a deployment
where a large number of calls are required within a small floor space, 802.11a is recommended because
of the number of channels and the 54 Mbps data rate. The lower data rates in 802.11a can be disabled,
the 24 Mbps data rate can be set to “required”, while the rates of 36 to 54 can be left enabled.
where a large number of calls are required within a small floor space, 802.11a is recommended because
of the number of channels and the 54 Mbps data rate. The lower data rates in 802.11a can be disabled,
the 24 Mbps data rate can be set to “required”, while the rates of 36 to 54 can be left enabled.
After setting the cell edge of -67 dBm, determine where the error rate of 1 percent occurs, and then
examine the SNR value.
examine the SNR value.
The -67 dBm cell edge may be determined as follows:
•
Set the phone to its desired transmit power.
•
Set the AP to a matching transmit power.
•
Place the AP and the desired antenna in the location where the phone will be used.
•
With an active call, or while sending and receiving packets equal in size to the G711 codec, measure
the signal level out to the –67 dBm cell edge.
the signal level out to the –67 dBm cell edge.
Carefully examine the data sheets of the particular phone device to determine the transmit power levels
and data rates supported by the phone device in a particular Wi-Fi band. The data sheets for Cisco
Unified Wireless IP Phones can be found at
and data rates supported by the phone device in a particular Wi-Fi band. The data sheets for Cisco
Unified Wireless IP Phones can be found at
. Consult the vendor website for
phones from other vendors.
The 80211a maximum transmit power levels vary on different channels and with different AP models.
The 802.11g maximum transmit power levels vary by model. Cisco Aironet AP data sheets should be
carefully examined to determine which AP model supports which data rates.
The 802.11g maximum transmit power levels vary by model. Cisco Aironet AP data sheets should be
carefully examined to determine which AP model supports which data rates.
shows an
example of the maximum 802.11a transmit power in dBm by channel.