Draytek VigorPro 5510 Series User Manual page 208

Hide SSID
Long Preamble 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 field 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.
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.
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VigorPro5510 Series User's Guide