Wireless Access Point

Called a Wireless Access Point [ waɪəlɛs æksɛs pɔɪnt ] (English " wireless access point " ), and the access point (AP ) or base station, is an electronic device that acts as an interface for wireless communication devices. End-user devices via a wireless adapter ( wireless adapter ) a wireless connection to the wireless access point forth, which can be connected via a cable to a communication network fixed again. Usually connect wireless access points notebooks and other mobile devices with built-in wireless adapter via a Wireless Local Area Network (WLAN, wireless network) to a Local Area Network ( LAN) or other wired data network ( telephone network, cable television network ).

A wireless access point can also be used in the so-called ad hoc mode as the central interface between multiple devices. In this way, can be wirelessly interconnected devices such as computers and printers.

Technical classification

Wireless access points mainly fulfill the same tasks as bridges and switches: connect different devices to hardware -level level together. Here, on the one hand to avoid data collisions. Using techniques such as CSMA / CA and bridge the other hand, differences between various transmission media

In the OSI model (data link layer layer 2, engl. ) Are wireless access points such as switches and bridges in the data link layer located.

Wireless routers are composite devices from a router with an integrated wireless access point. These devices concern by providing functions such as routing, packet filtering, and DHCP server also higher layers of the model.

802.11 Access Points

A wireless network using the IEEE 802.11 standard is assigned a radio network name (ESSID ). About the ESSID can distinguish between different client networks. An access point can now regularly send data packets with the ESSID (and other data of the wireless LAN) via broadcast -Telegram, so that clients can identify an existing network.

A key differentiator of 802.11 base stations which modes they support: Some access points can establish connections to other access points and clients simultaneously.

The possible modes of operation of an 802.11 Access Points are the Basic Service Set (BSS ), the Extended Service Set (ESS ), the Ethernet Bridge, Wireless Bridge, Wireless Repeater, and Wireless Distribution System. These topologies are also referred to as an infrastructure mode ( infrastructure networks).

• The Basic Service Set a single access point is powered on which any number of terminals Check and exchange data. The clients of such access points form a self-contained intranet.
• Supports one Access Point mode Ethernet Bridge, he offers not only the radio interface nor a network interface with RJ -45 connector as an interface to the wired Ethernet and routes data between Ethernet and wireless LAN. This mode ( bridging between wireless LAN and Ethernet on the OSI layer 2 ) match the capabilities of an ordinary, currently commercially available access points. Can the access point, however, convey on OSI layer 3 routing in particular TCP / IP packets is called a wireless router.
• For the Extended Service Set you wired two or more access points over an Ethernet and provides the same wireless network name ( ESSID) one at all access points. This increases the range of the wireless network, because the clients are automatically transferred between the access points (roaming) as soon as the client's site has changed accordingly.
• The Wireless Distribution System allows you to wirelessly connect multiple access points together. A distinction is made between the point- to-point mode (Wireless Bridge ) and the Point-to- multipoint mode ( wireless repeater ). It should in this case be used access points from the same manufacturer.

The possibilities of the above Infrastructure modes is the ad-hoc mode ( ad hoc network ) with respect to:

• The operation of a wireless LAN as an Independent Basic Service Set ( IBSS ) does not require an Access Point: In IBSS two or more terminals can build an ad -hoc network. The stations communicate directly with each other here (instead of through an access point such as in infrastructure mode ) and must be located in this mutual radio range.

• In a mesh network (ad hoc network MANET ) is ( generally ) to attendees at a network node; it is no longer a terminal. By overlapping radio cells of any node creates a spontaneous meshing: An ad - hoc network where each participant passes by a neighbor A incoming data packets to the nearest neighbors B (routing). This two communicating end nodes need no longer in mutual radio range. Network nodes can thereby spontaneously open and disappear when about a notebook is on or off. There are already IEEE standard, software and hardware for establishing mesh networks. From a societal perspective, mesh networks are on WLAN technology is currently very exciting: You have the potential to create of expensive infrastructure largely independent and therefore completely free to access citizens' networks. The software engineering challenges are just as exciting for grid- affine people. Projects that deal with these problems, are free radio in Germany or in Austria and Funkfeuer.at Openwireless.ch in Switzerland.

Outside of infrastructure and ad - hoc modes still a client mode is often supported:

• Some access points can operate in client mode: A device with Ethernet port ( PC, printer, etc.) connected by straight-through cable to the Access Point. The access point in client mode is a pure replacement for a wireless Ethernet adapter ( and of its function, thus no access point more, but only a wireless adapter).

Compatibility issues

Not every product supports all operating modes. Not every access point provides a jack for easy connection of an external antenna.

The Extended Service Set Mode: Often there is a problem with the compatibility of base stations from different manufacturers. If these are connected to a common network and roaming should be allowed between the individual APs, the devices need to exchange information about their associated clients. For this purpose, a network protocol is needed for the time delay, but only the standard IEEE 802.11f was adopted. Meanwhile own, mutually incompatible solutions had already created many manufacturers.

Point -to-Point or Multipoint: Since this is not a recognized standard in the WDS used, should be equipment from the same manufacturer used at least devices with the same chipset.

It is reported of problems with individual wireless cards or drivers that can not communicate using repeaters. The buyer should therefore investigate in advance in the popular Internet forums and on the manufacturer's website if already present successes or failures for the combination of the respective products.

802.11 access points as a software solution

There is software that allows a PC with an ordinary wireless card teaches the necessary logic to work as an access point. Only certain wireless cards are supported by the software. In Mac OS X on Apple Macintosh computers is this (see also tethering ) without additional software already possible with the system software of the operating system, but not compatible with WPA or WPA2 protected networks (as Lion ). This theoretically flexible solution also has drawbacks: the wireless cards rarely have a connection for an external antenna, and a PC has far greater power, cooling and space requirements than a hardware-based Access Point, the practical operated even in bad weather with solar panels can be and is easy to protect against environmental influences.

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