There are several kinds of Wi-Fi and 802.11 family equipment. Generally, a computer, printer, handheld, or similar independent device is equipped with a low-power radio which connects to a central access point.
Computer-to-computer connections are also possible, and generally supported by manufacturer's software. A standard revised in 2001 from The Wi-Fi Alliance formalized "ad hoc" computer-to-computer networking standards, which were previously supported only across specific firmware sets, not all Wi-Fi devices.
Some access points can connect to each other, or connect via specialized wireless bridges, to create daisy-chained networks to hop to a wired or wireless high-speed connection to the Internet or a corporate network. Others combine bridging and client access in a single device.
Access point (AP). These devices are typically standalone units, some of which can even be supplied with power by Ethernet (PoE or Power over Ethernet). Many access points feature routing support to handle subnet network assignment. Some access points are configured via the Web; others require proprietary software or a USB connection. For larger networks, several software and hardware packages allow configuring many units at once and monitoring them centrally. See, for instance, Proxim's Harmony AP Controller system and Sputnik's Enterprise Gateway. Many access points also allow external antennas to improve directionality or signal clarity. The 802.1x standard for user authentication -- incorporated in the WPA (Wi-Fi Protected Access) and 802.11i security revisions -- allows access points to pass login information to a RADIUS, AAA, or similar authentication server rather than handling it directly or through a local system connected to the AP.
Home gateways. This class of access points is tailored for the home market, and may omit management features, routing, or a robust architecture. In most respects, however, they work identically with access points. Many gateways offer two or more Ethernet ports to fully isolate both a wired and wireless network from a broadband connection to filter traffic via a built-in firewall. See Cheap Residential Gateways.
Wireless bridges. Wireless bridges can span wireless networks by connecting the traffic from one or more AP to another, or by connecting multiple wired networks. In a hub-and-spoke model, a central access point could serve many wireless bridges as if they were normal clients (with equipment from Alvarion, for instance), allowing longer outdoor distances or indoor deployments. In the case of wired networks, a set of bridges can connect islands of wired devices or even entire networks in separate buildings or parts of a city. (See also Ethernet adapters and bridges, below.)
PC Cards. Laptops and certain desktop units can take PC Cards, a standard form factor that has an external projecting antenna in a small case, typically less than an inch long by two inches by a few eighths.
PCI cards. For ease of regulatory approval of emissions, older PCI cards, which slip into slots inside a tower or desktop computer, were essentially a simple holder for a PC Card. This unfortunately limited the kind of external antenna that can be used with most units. Newer PCI cards, however, are designed from the ground up to work as a PCI device, improving integration and functionality. (Most of these PCI cards use the same components found in mini-PCI cards.)
Mini-PCI cards. Many laptop manufacturers have adopted the mini-PCI form factor to add support for wireless networking and modems methods while leaving their PC Card slots free.
USB adapters. Legacy equipment, printers, and other devices which lack any slots into which an enabling radio could be added may use a USB-based radio. These devices are currently limited to USB speeds of 12 Mbps. They also require special drivers which won?t work on every platform or system release.
PDA modules. The PocketPC-based Toshiba e740 comes with integral Wi-Fi support (plus slots for Compact Flash and Secure Digital cards). Other PocketPC systems also handle Wi-Fi Compact Flash cards. On the Palm side, support lags.
Ethernet adapters and bridges. Several companies provide hardware that can bridge an Ethernet network, even if that "network" is just a single machine -- or up to 30 or 50. These devices masquerade as a single radio adapter, and translate or map the Ethernet behind it.
Apple AirPort and AirPort Extreme Card. Apple builds a card slot and an antenna into all of its computers since 1999, as they phased in wireless support. Computers introduced before 2003 have an PC Card-like slot for an AirPort Card, about $80; machines starting in 2003 have a mini-PCI-like slot which handles an AirPort Extreme Card, about $100. The AirPort standard is 802.11b; Extreme 802.11g.