Draytek Vigor 2950 User Manual page 138

Hide SSID
Long Preamble
132
selected channel is under serious interference.
Check it to prevent from wireless sniffing and make it
harder for unauthorized clients or STAs to join your
wireless LAN. Depending on the wireless utility, the user
may only see the information except SSID or just cannot
see any thing about Vigor wireless router while site
surveying.
This option is to define the length of the sync field in an
802.11 packet. Most modern wireless network uses short
preamble with 56 bit sync filed instead of long preamble
with 128 bit sync field. However, some original 11b
wireless network devices only support long preamble.
Check it to use Long Preamble if needed to
communicate with this kind of devices.
Vigor2950 Series User's Guide