Wireless access point
A wireless access point [ ˈwaɪəlɛs ˈæksɛs pɔɪnt ] (English for wireless access point ), also known as an access point (AP) or base station , is an electronic device that acts as an interface for wireless communication devices. Terminal devices use a wireless adapter to establish a wireless connection to the wireless access point, which can be connected to a permanently installed communication network via a cable. Usually, wireless access points connect notebooks and other mobile devices with built-in wireless adapters via a wireless local area network (WLAN, radio network) to a local area network (LAN) or another wired data network ( telephone network , cable television network ).
A wireless access point can also be used in the so-called ad-hoc mode (not to be confused with ad-hoc networks that work without a wireless access point) as a central interface between several end devices. In this way, devices such as computers and printers can be connected wirelessly.
Wireless access points mainly perform the same tasks as bridges and switches : They connect different devices with each other on a hardware level. On the one hand, they avoid data collisions using techniques such as CSMA / CA and, on the other hand, they bridge differences between different transmission media .
In the OSI model , wireless access points such as switches and bridges are located in the security layer (layer 2, English data link layer ). For comparison: the well-known protocol Ethernet also covers the security layer , but also the underlying bit transmission layer (layer 1, English physical layer ).
WLAN routers are devices made up of a router with an integrated wireless access point. By providing functions such as routing , packet filters and DHCP servers , these devices also affect higher layers of the model.
802.11 access points
A radio network name ( ESSID ) is assigned to a radio network based on the IEEE standard 802.11 . Clients can use the ESSID to distinguish between different networks. An access point can now regularly send data packets with the ESSID (and other wireless LAN data ) via broadcast telegram so that clients can identify an existing network.
A key distinguishing feature of 802.11 base stations is which operating modes they support: Some access points can establish connections to other access points and clients at the same time.
The possible operating modes of an 802.11 access point are the Basic Service Set (BSS), the Extended Service Set (ESS), the Ethernet Bridge, the Wireless Bridge, the Wireless Repeater and the Wireless Distribution System . These topologies are also referred to as infrastructure mode ( infrastructure networks ).
- In the Basic Service Set , a single access point is operated at which any number of end devices can log in and exchange data. The clients of such an access point form an independent intranet.
- If an access point supports the Ethernet Bridge mode , in addition to the radio interface, it also offers a network interface with an RJ-45 socket as an interface to the wired Ethernet and transmits the data between Ethernet and wireless LAN. This mode (bridging between wireless LAN and Ethernet on OSI layer 2) corresponds to the capabilities of a normal, currently commercially available access point. Can mediate on OSI layer 3 of the Access Point, however, particularly TCP / IP packets route , it is called a wireless router.
- For the Extended Service Set , two or more access points are wired via an Ethernet and the same wireless network name (ESSID) is set on all access points. This increases the range of the wireless network, as the clients are automatically transferred between the access points ( roaming ) as soon as the client's location has changed accordingly.
- The wireless distribution system makes it possible to wirelessly connect several access points to one another. A distinction is made between point-to-point mode ( wireless bridge ) and point-to-multipoint mode ( wireless repeater ). Access points from the same manufacturer should be used.
Some access points can be operated in client mode - in the sense of this article, however, they are no longer access points, but functionally wireless adapters : Such reconfigured access points are useful when a client (PC, printer, etc.) only has an Ethernet connection and is to be integrated wirelessly into the LAN via an access point.
Not every product supports all operating modes. Not every access point has a socket for the simple connection of an external antenna.
In Extended Service Set mode: There is often a problem with the compatibility of base stations from different manufacturers. If these are to be connected to a common network and roaming between the individual APs is to be enabled, the devices must exchange information via the logged-in clients. A network protocol is required for this, but the IEEE 802.11f standard was only adopted with a time lag . In the meantime, many manufacturers had already created their own incompatible solutions.
Point-to-Point or Multipoint: Since the WDS used is not a recognized standard, devices from the same manufacturer should be used, at least devices with the same chipset.
Problems are reported with individual WLAN cards or drivers that cannot communicate using repeaters. The buyer should therefore research in advance in the usual internet forums and on the manufacturer's website whether there have already been successes or failures for the combination of the respective products.
802.11 access points as a software solution
There is software that enables a WLAN-enabled computer (PC, laptop, smartphone, etc.) to work as an access point. With Mac OS X on Apple Macintosh computers, this capability (see also tethering ) is already implemented in the operating system, but without compatibility with WPA or WPA2-protected networks (status: Lion).
The software solution also has disadvantages: the WLAN 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.
- Hot Spot (WLAN) , a public Internet access point