Specifications
14 General
Due to the regulations of the DFS channels, a client that does not support radar detection is not
allowed to actively scan for APs in these channels. The client will then have to perform passive
scanning which means that it only listens for beacons. For a voice client, this will affect an ongoing
call to some degree by introducing a slight increase in jitter in the voice stream.
The handset can use the DFS channels, but the voice quality may be distorted and roaming de-
layed. The DFS channel scan algorithm is optimized and uses both passive scanning and active
scanning when it is regulatory ensured that transmitting is allowed.
NOTE: : Since the passive part of the scan phase is limited to 70 ms, a beacon interval of less
than 70 ms (e.g. 60 ms) will give the best roaming performance.
802.11 n-radio Support in the Handset
The 802.11n standard uses advanced radio technology to boost high throughput levels and more
resilient communications links. This is achieved by using multiple antennas and multiple radios in
the WLAN equipment (MIMO). The technology can be used to achieve higher speeds or extend
the coverage area, where higher speeds will be available further from the AP, and thus the trans
-
mission will take shorter time compared with a 802.11a/g transmission.
In the 802.11n specification, a tighter use of the protocols has resulted in less overhead and better
use of the channel. This will improve the max speed from 54 Mbps to 75 Mbps.
In 802.11n networks it is also possible to double the throughput by using channels twice as wide
(40MHz) than the 802.11b/g/a standards are using (20 MHz). The technique is called channel
bonding and combines two adjacent channels into a wider channel, and thus effectively reduces
the amount of channels to half.
The standard allows the use of clients that support single channel or double channel width at the
same time, but with a reduced set of channels.
The 802.11n standard also allows the use of very large frames to reduce the amount of ACKs
needed. This reduces the large overhead known in WiFi, and throughput is raised dramatically
from the traditionally 50% up to 90% of the max bandwidth.
The 802.11n builds on the same frequency bands as the 802.11b/g and 802.11a radios and is de-
signed to coexist with older clients. Legacy clients will use lower speeds than the 802.11n clients.
To really benefit from 802.11n, a WLAN that utilizes the 802.11n enhanced standards should be
configured for Greenfield mode. This means that no non-802.11n devices should be present in the
coverage area. In most cases it is impossible to create such an environment, so 802.11n will run
in what is called a mixed/protected mode which will reduce the maximum throughput.
The current 802.11n standards is really only beneficial for data clients like a laptop that are set up
for high definition video conferencing or for downloading large files from a server.
The implementation of 802.11n protocol features to be used in handsets have been carefully ex-
amined, and features which will not benefit voice have not been implemented.
* For the FCC regulatory domain US and others countries the following rules apply for the UNII-2e
band:
- Devices will not transmit on channels which overlap the 5600 - 5650 MHz band (Ch 120, 124 and
128).
- For outdoor use any installation of either a master or a client device within 35 km of a Terminal
Doppler Weather Radar (TDWR) location shall be separated by at least 30 MHz (center-to-center)
from the TDWR operating frequency. Table of current TWDR are to be found in the FCC document
“443999 D01 Approval of DFS UNII Devices v01” located at:
https://apps.fcc.gov/kdb/GetAttachment.html?id=33781