Wireless access point

Wireless access point 無線接入點 (WAP 或 AP) , 是指一類可以讓無線設備(比如筆記本電腦)通過WI-FI,藍牙等連接到無線網絡的設備, 例如: 無線路由. WAP通常連接著有線網絡, 通常在無線設備與有線網絡之間起中繼的作用. 大多數WAP都是使用IEEE 802.11標準.

In?computer networking, a?wireless access point?(WAP) is a device that allows wireless devices to connect to a wired network using?Wi-Fi,?Bluetooth?or related standards. The WAP usually connects to a?router?(via a wired network) if it's a standalone device, or is part of a router itself.

Contents ??[hide]? 1?Introduction 2?Common WAP applications 3?Wireless access point vs. ad hoc network 4?Limitations 5?Security 6?Specialized WAPs 7?See also 8?References 9?External links

[edit]Introduction

Linksys "WAP54G" 802.11g Wireless Access Point embedded?RouterBoard 112 withU.FL-RSMA?pigtail and R52?mini PCI?Wi-Ficard widely used by?wireless?Internetservice providers (WISPs) across the world

Prior to?wireless networks, setting up a computer network in a business, home or school often required running many cables through walls and ceilings in order to deliver network access to all of the network-enabled devices in the building. With the creation of the Wireless Access Point (WAP), network users are now able to add devices that access the network with few or no cables. A WAP normally connects directly to a wired Ethernet connection and the WAP then provides wireless connections using radio frequency links for other devices to utilize that wired connection. Most WAPs support the connection of multiple wireless devices to one wired connection. Modern WAPs are built to support a standard for sending and receiving data using these radio frequencies. Those standards, and the frequencies they use are defined by the?IEEE. Most WAPs use?IEEE 802.11?standards.

[edit]Common WAP applications

A typical corporate use involves attaching several WAPs to a wired network and then providing wireless access to the office?LAN. The wireless access points are managed by a?WLAN Controller?which handles automatic adjustments to RF power, channels, authentication, and security. Further, controllers can be combined to form a wireless mobility group to allow inter-controller roaming. The controllers can be part of a mobility domain to allow clients access throughout large or regional office locations. This saves the clients time and administrators overhead because it can automatically re-associate or re-authenticate.

A?hotspot?is a common public application of WAPs, where wireless clients can connect to the Internet without regard for the particular networks to which they have attached for the moment. The concept has become common in large cities, where a combination of coffeehouses, libraries, as well as privately owned open access points, allow clients to stay more or less continuously connected to the Internet, while moving around. A collection of connected hotspots can be referred to as a?lily pad network.

WAPs are commonly used in?Home wireless networks. Home networks generally have only one WAP to connect all the computers in a home. Most are?wireless routers, meaning?converged devices?that include the WAP, a?router, and, often, an?Ethernet switch. Many also include a broadband modem. In places where most homes have their own WAP within range of the neighbours' 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 although this does not negate the requirement for a wired network.

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 called?CSMA/CA?instead.

[edit]Wireless access point vs. ad hoc network

Some people confuse Wireless Access Points with?Wireless Ad Hoc networks. An Ad Hoc network uses a connection between two or more devices?without?using a wireless access point: the devices communicate directly when in range. An Ad Hoc network is used in situations such as a quick data exchange or a multiplayer LAN game because setup is easy and does not require an access point. Due to its peer-to-peer layout, Ad Hoc connections are similar to?Bluetooth?ones and are generally not recommended for a permanent installation.^{[citation needed]}

Internet access?via?Ad Hoc networks, using features like Windows'?Internet Connection Sharing, may work well with a small number of devices that are close to each other, but Ad Hoc networks don't scale well. Internet traffic will converge to the nodes with direct internet connection, potentially congesting these nodes. For internet-enabled nodes, Access Points have a clear advantage, with the possibility of having multiple access points connected by a wired?LAN.

[edit]Limitations

One?IEEE 802.11?WAP can typically communicate with 30 client systems located within a?radius?of 103?m.^{[citation needed]}?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 hundred kilometers.^[1]

Most jurisdictions have only a?limited number of frequencies?legally available for use by wireless networks. Usually, adjacent WAPs will use different frequencies (Channels) to communicate with their clients in order to avoid?interference?between the two nearby systems. Wireless devices can "listen" for data traffic on other frequencies, and can rapidly switch from one frequency to another to achieve better reception. However, the limited number of frequencies becomes problematic in crowded downtown areas with tall buildings using multiple WAPs. In such an environment, signal overlap becomes an issue causing interference, which results in signal droppage and data errors.

Wireless networking lags wired networking in terms of increasing?bandwidth?and?throughput. While (as of 2010) typical wireless devices for the consumer market can reach speeds of 300 Mbit/s (megabits?per second) (IEEE?802.11n) or 54 Mbit/s (IEEE802.11g), 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 faster speeds, and people using wireless connections keenly want to see the wireless networks catch up.

By 2012, 802.11n based access points and client devices have already taken a fair share of the marketplace and with the?finalization of the 802.11n standard in 2009?inherent problems integrating products from different vendors are less prevalent.

[edit]Security

Main article:?Wireless LAN Security

Wireless access has special?security?considerations. Many wired networks base the security on physical access control, trusting all the users on the local network, but if wireless access points are connected to the network, anyone on the street or in the neighboring office could connect.

The most common solution is wireless traffic encryption. Modern access points come with built-in encryption. The first generation encryption scheme?WEP?proved easy to crack; the second and third generation schemes,?WPA?and?WPA2, are considered secure if a strong enough?password?or?passphrase?is used.

Some WAPs support hotspot style authentication using?RADIUS?and other?authentication servers.

[edit]Specialized WAPs

Industrial Wireless Access Point

Industrial grade WAPs are rugged, with a metal cover and a?DIN rail?mount. During operations they can tolerate a wide temperature range, high humidity and exposure to water, dust, and oil. Wireless security includes:?WPA-PSK,?WPA2, IEEE 802.1x/RADIUS,?WDS,?WEP,?TKIP, and?CCMP?(AES) encryption. Unlike some home consumer models, industrial wireless access points can also act as a?bridge, router, or a client.

[edit]See also

• Hotspots?- access points or wireless networks open to the public
• Wireless LAN?- networks consisting of zero or more access points plus devices
• Wireless bridge?- between networks sharing same Service Set Identifier (SSID) and radio channel
• Wi-Fi Array?System of multiple APs
• LWAPP?- Lightweight Access Point Protocol used to manage a large set of WAPs
• WarXing?- searching for open networks
• Femtocell?- a local-area?base station?using?cellular network?standards such as?UMTS, rather than?Wi-Fi
• WiMAX?- wide-area wireless standard that has a few elements in common with Wi-Fi

[edit]References

^?Ermanno Pietrosemoli.?"Setting Long Distance WiFi Records: Proofing Solutions for Rural Connectivity".?Fundación Escuela Latinoamericana de Redes?University of the Andes (Venezuela). Retrieved March 17, 2012.

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