ZyXEL Communications ZYWALL USG 20 Manual page 805

An ESSID (ESS IDentification) uniquely identifies each ESS. All access points and
their associated wireless clients within the same ESS must have the same ESSID
in order to communicate.
Figure 451 Infrastructure WLAN
Channel
A channel is the radio frequency(ies) used by wireless devices to transmit and
receive data. Channels available depend on your geographical area. You may have
a choice of channels (for your region) so you should use a channel different from
an adjacent AP (access point) to reduce interference. Interference occurs when
radio signals from different access points overlap causing interference and
degrading performance.
Adjacent channels partially overlap however. To avoid interference due to overlap,
your AP should be on a channel at least five channels away from a channel that an
adjacent AP is using. For example, if your region has 11 channels and an adjacent
AP is using channel 1, then you need to select a channel between 6 or 11.
RTS/CTS
A hidden node occurs when two stations are within range of the same access
point, but are not within range of each other. The following figure illustrates a
hidden node. Both stations (STA) are within range of the access point (AP) or
ZyWALL USG 20/20W User's Guide
Appendix C Wireless LANs
805