Wi-Fi technical information

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Evolution of Wi-Fi standards

The IEEE standard that governs Wi-Fi technology is IEEE 802.11; that standard has gone through several generations since its inception in 1997.

How it works

Wi-Fi networks use radio technologies called IEEE 802.11 to provide secure, reliable, fast wireless connectivity. A typical Wi-Fi set-up includes one or more access points (APs) and one or more clients. An AP broadcasts its SSID (service set identifier, or "network name") via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client that receives the beacon can communicate at at least 1 Mbit/s. Based on the settings, the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide with which of the two APs to make a connection.

The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than another. Since Wi-Fi transmits in the air, it has the same properties as a non-switched wired Ethernet network, and therefore collisions can occur. Unlike a wired Ethernet, and like most packet radios, Wi-Fi cannot detect collisions, and instead uses an acknowledgment packet for every data packet sent. If a sender receives no acknowledgement within a certain time, the sender retransmits the corresponding packet. Also, a medium reservation protocol can be used when excessive collisions are experienced or expected (request-to-send and clear-to-send used for collision avoidance or CA) in an attempt to avoid collisions.

A Wi-Fi network can be used to connect computers to each other, to the Internet, and to wired networks (which use IEEE 802.3 or Ethernet). Wi-Fi networks operate in the unlicensed 2.4 GHz (802.11b/g/n) and 5 GHz (802.11a/n) radio bands, with an 11 Mbit/s (802.11b) or 54 Mbit/s (802.11a or g) data rate. They can provide real-world performance similar to that of the basic 10BASE-T wired Ethernet networks.

Channels

See also: List of WLAN channels

Except for 802.11a, which only operates at 5 GHz, and 802.11n, which optionally operates at 5 GHz, Wi-Fi devices historically have primarily used the spectrum in 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations and maximum permitted power vary slightly in different parts of the world. Channel numbers, however, are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers. The 2.4 GHz band is also used by microwave ovens, Bluetooth devices, amateur radio, legal domestic video senders, security systems (including cordless CCTV), also (in North America) cordless phones and baby monitors.

The maximum number of available channels for Wi-Fi enabled devices are:

• 11 for North America. Only channels 1, 6, and 11 are recommended for 802.11b/g to minimize interference from adjacent channels.^[3] (Note: In Mexico channels 1 through 8 are reserved "for indoor use only")
• 13 for China and most countries in Europe. A typical channel layout for 802.11b would be 1/7/13. For traffic that is predominantly 802.11g, 1/5/9/13 provides a fourth frequency enabling a much better frequency plan. (Note: In France only channels 10 through 13 are available. A change for EU harmonization is pending.)
• 14 for Japan.^[4]

• 7 for Israel (only channels 3 through 9 are permitted)^{[citation needed]}

There are also restrictions on power levels and permitted antenna types which vary by country.^[4]

In some countries, such as the United States, licensed Amateur Radio operators may use some of the channels at much higher power for very long distance wireless access. Such extended range operation is referred to as High Speed Multimedia Radio.

Wi-Fi on various operating systems

There are two sides to Wi-Fi support under an operating system: driver level support, and configuration and management support.

Driver support is usually provided by multiple manufacturers of the chip set hardware or end manufacturers. Also available are Unix clones such as Linux and FreeBSD, sometimes through open source projects.

Configuration and management support consists of software to enumerate, join, and check the status of available Wi-Fi networks. This also includes support for various encryption methods. These systems are often provided by the operating system backed by a standard driver model. In most cases, drivers emulate an ethernet device and use the configuration and management utilities built into the operating system. In cases where built in configuration and management support is non-existent or inadequate, hardware manufacturers may include their own software to handle the respective tasks.

Microsoft Windows

Microsoft Windows has comprehensive driver-level support for Wi-Fi, the quality of which depends on the hardware manufacturer. Hardware manufactures almost always ship Windows drivers with their products. Windows ships with very few Wi-Fi drivers and depends on the original equipment manufacturers (OEMs) and device manufacturers to make sure users get drivers. Configuration and management depend on the version of Windows.

• Earlier versions of Windows, such as 98, ME and 2000 do not have built-in configuration and management support and must depend on software provided by the manufacturer
• Microsoft Windows XP has built-in configuration and management support. The original shipping version of Windows XP included rudimentary support which was dramatically improved in Service Pack 2. Support for WPA2 and some other security protocols require updates from Microsoft. There are still problems with XP support of Wi-Fi. (One simple interface problem is that if the user makes a mistake in the (case sensitive) passphrase, XP keeps trying to connect but never tells the user that the passphrase is wrong. A second problem is not allowing the user to see different BSSID's for the same ESSID; that is, it provides no way for the user to differentiate access points with the same name.) To make up for Windows’ inconsistent and sometimes inadequate configuration and management support, many hardware manufacturers include their own software and require the user to disable Windows’ built-in Wi-Fi support.
• Microsoft Windows Vista has improved Wi-Fi support over Windows XP. The original betas automatically connected to unsecured networks without the user’s approval. The release candidate (RC1 or RC2) does not continue to display this behavior, requiring user permissions to connect to an unsecured network, as long as the user account is in the default configuration with regards to User Account Control.

Mac OS X and classic Mac OS

Apple was an early adopter of Wi-Fi, introducing its AirPort product line, based on the 802.11b standard, in July 1999. Apple later introduced AirPort Extreme, an implementation of 802.11g. All Apple computers, starting with the original iBook in 1999, either included AirPort 802.11 networking or were designed specifically to provide 802.11 networking with only the addition of the internal AirPort Card (or, later, an AirPort Extreme Card), connecting to the computer's built-in antennae. All Intel-based Macs either come with built-in AirPort Extreme or a slot for an AirPort card, and all portable Macs (all MacBooks and the earlier iBooks and PowerBooks) have included Wi-Fi for several years. In late 2006, Apple began shipping Macs with Broadcom Wi-Fi chips that also supported the Draft 802.11n standard, but this capability was disabled and Apple did not claim or advertise the hardware's capability until some time later when the draft had progressed further. At the January 2007 Macworld Expo, Apple announced that their computers would begin shipping with Draft 802.11n support. Systems shipped with this hidden capability can easily be unlocked through software, but due to the accounting requirements of Sarbanes-Oxley, Apple cannot freely add features to already-sold hardware and so must nominally sell an upgrade. This "upgrade" is included in the price of an AirPort Extreme Base Station for all computers owned by the purchaser, and Apple sells the "upgrade" separately (as the "AirPort Extreme 802.11n Enabler for Mac") for about US$2 in the United States and at similar prices elsewhere.

Apple produces the operating system, the computer hardware, the accompanying drivers, AirPort Wi-Fi base stations, and configuration and management software, simplifying Wi-Fi integration, set-up, and maintenance (including security updates). The built-in configuration and management is integrated throughout many of the operating system's applications and utilities. Mac OS X has Wi-Fi support, including WPA2, and ships with drivers for all of Apple’s current and past AirPort Extreme and AirPort cards. Many third-party manufacturers make compatible hardware along with the appropriate drivers which work with Mac OS X’s built-in configuration and management software. Other manufacturers distribute their own software.

Apple's older Mac OS 9 supported AirPort and AirPort Extreme as well, and drivers exist for other equipment from other manufacturers, providing Wi-Fi options for earlier systems not designed for AirPort cards. Versions of Mac OS before Mac OS 9 predate Wi-Fi and do not have any Wi-Fi support, although some third-party hardware manufacturers have made drivers and connection software that allows earlier OSes to use Wi-Fi.^[5]

Open source Unix-like systems

Linux, FreeBSD and similar Unix-like clones have much coarser support for Wi-Fi. Due to the open source nature of these operating systems, many different standards have been developed for configuring and managing Wi-Fi devices. The open source nature also fosters open source drivers which have enabled many third party and proprietary devices to work under these operating systems. See Comparison of Open Source Wireless Drivers for more information on those drivers.

• Linux has patchy Wi-Fi support.^[6] Native drivers for many Wi-Fi chipsets are available either commercially or at no cost,^[7] although some manufacturers don't produce a Linux driver, only a Windows one. Consequently, many popular chipsets either don't have a native Linux driver at all, or only have a half-finished one. For these, the freely available NdisWrapper and its commercial competitor DriverLoader allow Windows x86 and 64 bit variants NDIS drivers to be used on x86-based Linux systems but not on other architectures.^[8] As well as the lack of native drivers, some Linux distributions do not offer a convenient user interface and configuring Wi-Fi on them can be a clumsy and complicated operation compared to configuring wired Ethernet drivers.^[9] This is changing with NetworkManager, a utility that allows users to automatically switch between networks without using the command line.

• FreeBSD has Wi-Fi support similar to Linux. Support under FreeBSD is best in the 6.x 7.x versions, which introduced full support for WPA and WPA2, although in some cases this is driver dependent. FreeBSD comes with drivers for many wireless cards and chipsets, including those made by Atheros, Ralink, Cisco, D-link, Netgear, and many Centrino chipsets, and provides support for others through the ports collection. FreeBSD also has "Project Evil", which provides the ability to use Windows x86 NDIS drivers on x86-based FreeBSD systems as NdisWrapper does on Linux, and Windows amd64 NDIS drivers on amd64-based systems.

• NetBSD, OpenBSD, and DragonFly BSD have Wi-Fi support similar to FreeBSD. Code for some of the drivers, as well as the kernel framework to support them, is mostly shared among the 4 BSDs.

• Haiku has no Wi-Fi support at all as of April 2007.

• Solaris and OpenSolaris have the Wireless Networking Project to provide Wi-Fi drivers and support. ^[10]

See also

Electronics portal

Wikibooks has a book on the topic of

Nets, Webs and the Information Infrastructure

• Access Point
• DTIM (Delivery Traffic Indication Message)
• Hotspot
• Long Range Wi-Fi
• Switched mesh
• Public Safety Network
• Wireless security

References

External links

• Wi-Fi Alliance
• Windows XP Bedevils Wi-Fi Users
• Technical Notes on Wi-Fi