Retooling for 802.11n - Wider Range of Rates (
Page 3 of 3 )
Wider Range of Rates
The available data
rates offered by 802.11n networks,
which now extend all the way up to 300M bps (or up to 600M bps in future 4-by-4
antenna implementations), are determined by several factors: the number of
simultaneous streams, the width of the WLAN channel and the length of the guard
interval (a waiting period built into the standard to avoid echo effects, which
has been shortened for 802.11n).
802.11n analysis tools will
need to identify not only the data rates advertised for an 802.11n WLAN but
also the reasons why those particular rates are available, in order to help
administrators identify possible mis-configurations in the new standard’s wide
variety of tunable knobs.
Beamforming
Beamforming—an
optional feature of 802.11n under
which APs dynamically switch antenna combinations to focus wireless
transmissions on a particular wireless client to maximize reception for that
station—will also cause headaches for wireless administrators.
Current models for overlay
networks fail to provide accurate coverage information in a network using
beamforming.
A stationary WLAN sensor will
struggle to accurately predict client coverage in this sort of network because
the AP’s antennae are not pointing at the sensor. Combating this problem will
be tough, as vendors will need to include either client-side analysis
(requiring some kind of agent on all WLAN
clients) or denser wireless sensor deployments. The latter scenario seems
untenable because 802.11n is often billed as a way to lessen the number of
sensors deployed in an enterprise due to the technology’s improved performance
at longer distances.
Frame Aggregation
Normal WLAN activity includes a lot
of management frames, and this is why throughput totals reported in tests are
significantly less than the advertised data rate. 802.11n introduces two
flavors of frame aggregation—8KB MAC Service
Data Unit Aggregation and the 65KB MAC
Protocol Data Unit Aggregation—both of which allow for more data to be sent
over the air with less management overhead.
802.11n-enabled analysis tools should be able to clearly
identify when frame aggregation is in use and which type is being employed—as
each type has different rules surrounding re-transmits if the packets are
damaged. ´
eWEEK
Labs Senior Analyst Andrew Garcia can be reached at agarcia@eweek.com.
Mobile & Wireless 
/ 8 
