Monday, October 8, 2007

Wireless access point (WAP)

In computer networking, a wireless access point (WAP or AP) is a device that connects wireless communication devices together to form a wireless network. The WAP usually connects to a wired network, and can relay data between wireless devices and wired devices. Several WAPs can link together to form a larger network that allows "roaming". (In contrast, a network where the client devices manage themselves - without the need for any access points - becomes an ad-hoc network.) WAPs have IP addresses for configuration.
Low-cost and easily-installed Wi-Fi WAPs grew rapidly in popularity in the early 2000s. These devices offered a way to avoid the tangled messes of category 5 cable associated with typical Ethernet networks of the day. Whereas wiring a business, home, or school often requires stringing many cables through walls and ceilings, wireless networking allows connecting with few or no new cables. Wireless networks also allow greater mobility, freeing users from the restrictions of using a computer cabled to the wall. In the industrial and commercial contexts, wireless networking has had a big impact on operations: employees in these areas now often carry portable data terminals integrating barcode scanners and wireless links, allowing them to update work in progress and inventory in real-time. At home with a residential gateway, any convenient chair or lawn becomes a desk for the laptop.
A typical corporate use involves attaching several WAPs to a wired network and then providing wireless access to the office LAN. Within the range of the WAPs, the wireless end user has a full network connection with the benefit of mobility. In this instance, the WAP functions as a gateway for clients to access the wired network. Another use involves bridging two wired networks in conditions inappropriate for cable: for example, a manufacturer can wirelessly connect a remote warehouse's wired network with a separate (though within line of sight) office's wired network.
Another wireless topology, a lily-pad network, consists of a series of access points spread over a large area, each connected to a different network. This provides hot spots where wireless clients can connect to the Internet without regard for the particular networks to which they have attached for the moment. The concept can become organic in large cities, where a combination of coffeehouses, libraries, other public spaces offering wireless access, as well as privately owned open access points, allow clients to roam over a large area (like hopping from lily pad to lily pad), staying more or less continuously connected.
Home wireless networks, the majority, generally have only one WAP to connect all the computers in a home. Most are wireless routers, meaning converged devices that include a WAP, Ethernet router, and often a switch in the same package. Many also converge a broadband modem. Most owners leave their encryption settings at default, hence neighbors can use them. In places where most homes have their own WAP within range of the neighbors' WAP, it's possible for technically savvy people to turn off their encryption and set up a wireless community network, creating an intra-city communication network without the need of wired networks.
A WAP may also act as the network's arbitrator, negotiating when each nearby client device can transmit. However, the vast majority of currently installed IEEE 802.11 networks do not implement this, using a distributed pseudo-random algorithm instead.
One IEEE 802.11 WAP can typically communicate with 30 client systems located within a radius of 100 m. However, the actual range of communication can vary significantly, depending on such variables as indoor or outdoor placement, height above ground, nearby obstructions, other electronic devices that might actively interfere with the signal by broadcasting on the same frequency, type of antenna, the current weather, operating radio frequency, and the power output of devices. Network designers can extend the range of WAPs through the use of repeaters and reflectors, which can bounce or amplify radio signals that ordinarily would go un-received. In experimental conditions, wireless networking has operated over distances of several kilometers.
Most jurisdictions have only a limited number of frequencies legally available for use by wireless networks. Usually, adjacent WAPs will use different frequencies to communicate with their clients in order to avoid interference between the two nearby systems. But wireless devices can "listen" for data traffic on other frequencies, and can rapidly switch from one frequency to another to achieve better reception on a different WAP. However, the limited number of frequencies becomes problematic in crowded downtown areas with tall buildings housing multiple WAPs, because enough overlap can occur between the wireless networks to cause interference.
Wireless networking lags behind wired networking in terms of increasing bandwidth and throughput. While (as of 2004) typical wireless devices for the consumer market can reach speeds of 11 Mbit/s (megabits per second) (IEEE 802.11b) or 54 Mbit/s (IEEE 802.11a, IEEE 802.11g), wired hardware of similar cost reaches 1000 Mbit/s (Gigabit Ethernet). One impediment to increasing the speed of wireless communications comes from Wi-Fi's use of a shared communications medium, so a WAP is only able to use somewhat less than half the actual over-the-air rate for data throughput. Thus a typical 54 MBit/s wireless connection actually carries TCP/IP data at 20 to 25 Mbit/s. Users of legacy wired networks expect the faster speeds, and people using wireless connections keenly want to see the wireless networks catch up.
As of 2006 a new standard for wireless, 802.11n is awaiting final certification from IEEE. This new standard operates at speeds up to 540 Mbit/s and at longer distances (~50 m) than 802.11g. Use of legacy wired networks (especially in consumer applications) is expected to decline sharply as the common 100 Mbit/s speed is surpassed and users no longer need to worry about running wires to attain high bandwidth.
Interference can commonly cause problems with wireless networking reception, as many devices operate using the 2.4 GHz frequency. A nearby wireless phone or anything with greater transmission power within close proximity can markedly reduce the perceived signal strength of a wireless access point. Microwaves are also known to interfere with wireless networks.

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