D-Link DWR-921 User Manual page 37

Section 4 - Configuration
Short GI:
Check this box to reduce the guard interval to 400 ns. This can
increase the throughput rate provided that the delay spread of the
connection is also low. However, it can also increase error rate in
some installations, due to increased sensitivity to radio-frequency
reflections. Select the option that works best for your installation.
HT 20/40
Enable this option to reduce interference from other wireless
Coexistence:
networks in your area. If the channel width is operating at 40 MHz
and there is another wireless network's channel over-lapping
and causing interference, the router will automatically change to
20 MHz.
Click Apply to save your settings, or Refresh to revert to your
previous settings.
D-Link DWR-921 User Manual 33