There are about a dozen channels defined for 802.11b, which also apply, in principle, to 802.11g. However, the sad truth is that the dozen channels only apply at very poor protection ratios. No more than half-a-dozen access networks can, realistically, simultaneously be active in the same geographic area at once.
The channelisation does, however, improve co-existence between adjacent networks.
When a mobile terminal wishes to receive Wireless LAN service at a hotspot, it must first register with the service. The terminal communicates with the access point, which in turn communicates with the AAA functionality. Traditionally, this happens through a web page with encrypted access, although other methods could, in principle, be used.
After registration a terminal remains registered until a timeout expires.