|
Despite the similarities
between the 802.11a/b/g and 802.11n technologies, there are some
peculiarities in 802.11n networks that influence the way such
networks should be monitored efficiently. Without going deep into
the technical details of the standard (they are publically
available from many sources on the Internet), this chapter
overviews the best monitoring practices for 802.11n
networks.
MIMO and Transmit Beamforming
The use of the MIMO and
Transmit Beamforming technology in 802.11n networks is a serious
challenge for wireless analyzers. 802.11n networks create a very
complex, adaptive signal intensity map, with dips and bumps, some
as small as a few centimeters in volume. Because a monitoring
device is passive, the WLAN being monitored does not attempt to
adapt to it. Signals travelling at high rates (currently up to 300
Mbps) transmitted by multiple antennas are also hard to intercept
without CRC errors. All of the above means that, generally, you
should expect a considerably higher percentage of broken frames
when monitoring 802.11n networks vs. the older 802.11a/b/g ones.
While this is not a problem when you're performing a site survey or
measuring signal strength of particular devices, examining
individual TCP streams or troubleshooting problems on the
per-packet level may become problematic when too many frames are
damaged.
To mitigate these
802.11n-specific factors, consider applying the following
techniques:
·Find
the best position for the notebook running CommView for WiFi.
Rotating it or moving it just a few inches in a different direction
may dramatically increase or decrease the signal quality. In fact,
even the position of your body or a raised arm may affect the
percentage of CRC errors.
·Try to
make sure that that the WLAN devices do not operate at their
maximum rates. Successful capturing of packets with rates of 100
Mbps and below is far more likely than successful capturing of
packets with higher data rates. Although this sounds
counter-intuitive, if your monitoring notebook is located next to
the AP, moving the client devices a few meters further from the AP
will increase rather than decrease the reception quality, as a
client device located one or two meters from the AP will almost
inevitably transmit packets at the rate of 300 or 270 Mbps, whereas
the same device located five meters from the AP will drop the rate
down to 130 or 108 Mbps, which is beneficial for our
purposes.
Channel Bonding
In 802.11n WLANs, the data
rate is increased by bonding two 20 MHz channels (40 MHz
operation). The 40 MHz operation uses wider bands, compared to 20
MHz bands in 802.11a/b/g, to support higher data rates. While a
WiFi network analyzer equipped with an 802.11n card doesn't have a
problem with capturing two channels simultaneously, it's important
to pay attention to the regulatory domains of the hardware being
used. In brief, the frequency of the secondary channel in 40 MHz
mode depends on the frequency of the primary channel. For example,
selecting channel #1 in your hardware means that the primary 20 MHz
channel will operate at the frequency of channel #1, while the
secondary 20 MHz channel will operate four channels above the
primary one, i.e. at the frequency of channel #5. When operating at
higher channel numbers, e.g. 10 or 11, adding four to the channel
number would mean that the frequency of the secondary channel would
go outside of the regulatory domain constraints: in the US, the top
channel in the 2.4 GHz band is 11; in most European countries the
top channel is 13. In such cases, the secondary channel uses the
frequency that is below the frequency of the primary channel. For
example, selecting channel #10 in your hardware means that the
primary 20 MHz channel will operate at the frequency of channel
#10, while the secondary 20 MHz channel will operate four channels
below the primary one, i.e. at the frequency of channel #6.
The potential problem that
a field engineer may encounter when working internationally is that
the regulatory domain of his monitoring network adapter may be
different from the regulatory domain of the WiFi network being
monitored. For example, a Germany-based 802.11n WLAN working on
channel #9 would bond channels #9 and #13. A monitoring adapter
bought in Canada would expect the secondary channel to be #5. This
will prevent the adapter from "seeing" the 40 MHz data streams in
the wireless analyzer. To handle this situation, consider using
hardware that belongs to the same regulatory domain or use the
"Secondary channel is below the primary channel in 802.11n 40 MHz
mode" box in the CommView for WiFi options dialog. Checking this
box will force the adapter to use the secondary channel frequency
that is below the primary channel frequency even if the regulatory
domain of the network adapter does not require that.
|
