What is an Access Point?
An AccessPoint is a wireless Internet access service that serves a contiguous physical area from a single 802.11b wireless transceiver. Common hardware that provides this service includes any number of dedicated or multifunctional machines.
A dedicated AccessPoint is traditionally just an Ethernet bridge-- that's actually in the IEEE spec. So the Apple Airport base station falls into this.
Linksys makes a product that is essentially their normal broadband router (not wireless) but with wireless ap functionality too-- this could be accomplished by bridging the wireless segment to the NAT'd wired segment, or having both interfaces be NAT'd. (Please edit if you know how it works.)
Can I build one?
Roll-your-own wireless access points are typically mulitfunctional and can take on many forms. See the LinuxAccessPoint page. Having cacheing proxies, enterprise authentication (e.g. NoCatAuth), etc. etc. is possible.
Traditionally there has been operation only in ad hoc mode but there are ways to get true Master/Infrastructure functionality out of a plain old card, depending on what hardware it is.
One involves semi-contraband firmware for the Prism II chipset. The other involves an expanded (though not yet complete) driver that uses a Host AP mode where software on the PC takes over some of the functionalities. This may actually be preferrable as more control may potentially be had at the MAC layer... this could be a more efficient way to manage QOS. (Please edit if anybody knows more.)
What is a Public Access Point?
A Public Access Point is a wireless internet service area that is available for public use, whether through membership in a co-op or through a standard public access login that is publicly available. Or, of course, if there is no login or control whatsoever.
What Hardware is used for constructing and operating an Access Point? What Software is used for an Access Point?
What are the minimums for calling your system a PublicAccessPoint?
How do you keep your own network secure?
A feature with rolling-our-own e.g. LinuxAccessPoints is that the wireless network can be configured to be directly NAT'd out into the cloud, not just bridged to the wired network. Indeed if one is bridging, subnetting is probably a good idea. At any rate, run appropriate firewalling code on the gateway and/or anything you can see with the wireless clients. Maybe have MAC-addy-specific rules in your firewall so, say *I* can hit my Samba share for my MP3 stash but joe off the street can't (until he jacks my MAC addy).
How do you keep your publicly accessible network connection from becoming a liability because of people doing bad things through your network?
This is an issue of reverse IDS or ExtrusionDetection.
How do you present a 'Welcome' message?
This is an issue of the ActivePortal.