Linux and WiFi

In general, the newer a technology in computing the lower the cost and quality of the experience, as the main driving factor in recent years (writing at the end of 2004) has been cost reduction, at any cost :-).

Also, the base of Linux developers expands with people who are enthusiastic but not correspondingly skilled or wise, and therefore prone to creating awkward solutions. This has already been apparent as to X11, fonts, printing, ALSA, the USB subsystem, and many distributions, and Linux WiFi is particularly messy because the WiFi side of things is also particularly messy.

Therefore these notes will contain two types of information, which summarize a lot of the (extremely messy as a rule) documents on WiFi that I have read: brief information on what's what in the hardware side of WiFi, and what's what on the Linux side of WiFi.

Some similar and very good notes, and much more detailed than these (for now) as to what is and is not easily supported, are available in this page by Nicolai Langfeldt.

WiFi in general

WiFi is subset of the general area of wireless networking, which includes for example Bluetooth, iRDA, and carrier pigeons. Currently it is by far the largest subset, because to a large extent WiFi is just Ethernet, actually on the ether, that is by radio, and the combination of a fairly familiar mode of operation with the removal of wires is fairly compelling.

Note: the original research that gave rise to Ethernet was actually the Aloha radio network at the University of Hawaii, so the wired version was called Ethernet even if it was actually wired.

WiFi is based on low power transmitters in unlicensed regions of the radio spectrum, usually in the 2-3GHz range. Low power is a necessary consequence of the spectrum being unlicensed (and it is unlicensed because it is reserved for equipment that might emit radio noise, like microwave ovens). This is similar to other local area radio technologies, like DACT for wireless phones, and walkie-talkies.

Given that even at low power the range (dozens to hundreds of meters) is such that multiple networks may be in range of each other, that region of the radio spectrum is subdivided into channels, and each network is supposed to use just one channels, but there is a twist, where some turbo modes use two channels.

Jean Tourrilhes has written some more detailed overview notes.

Types of WiFi networks

WiFi networks are of several types, depending on:

They are managed or unmanaged

A network in managed (also called infrastructure) mode has an access point that all other nodes communicate with, a bit like a hub or switch for Ethernet. In unmanaged (also called ad-hoc) mode WiFi nodes communicate directly with each other.

They have one cell or multiple cells

WiFi defines a cells as being the set of nodes that communicate over a single channel directly via the same access point. A logical network (also called a ESSID or SSID by the attribute that names it) may contain a single cell or multiple cells.

Which standard they use

In practice there are only three relevant standards:

802.11b has a raw top bandwidth of 11 megabits/s, runs on a carrier around 2.6GHz, it is also called just 11b, and is almost always supported. There is also a double speed variant called turbo mode or 802.11+.
802.11g has a raw bandwidth of 54 megabits/s, also runs on a carrier around 2.6GHz, it is also called 54g (it is unfortunate that the name of the standard contains the number 11, which is also the speed of 802.11b), and is very commonly supported. Almost all equipment that supports 54g also supports 11b. There exists at least one double speed turbo or nitro variant, usually supported by TI chipsets.
802.11a has a raw bandwidth of 54 megabits/s, runs on a carrier around 5GHz, and therefore has a rather shorter range than 11b or 54g, but there is much lower likelyhood of noise around that frequency.

There have been earlier standard, but in practice only the 802.11 family of standards matters.

Which encryption they use

Encryption can be:

disabled, in which case all nodes communicate in the clear over the radio channels. This at the very least allows anybody within reception range to analyze the IP traffic, even if the payload is encrypted. Using IPsec is the only way to avoid it, but IPsec requires a dedicated gateway and is another huge hassle.
open, in which case nodes may communicate with or without encryption.
WEP shared static key, with various lengths of keys (usually 64, 128, 256 bits minus 14) in which traffic is encrypted with a key that must be known to all nodes. The encryption is not that strong, the 64 bit key length is very very weak (it is actually 40 bits too), and encryption often slows down traffic considerably.
WPA is stronger than WEP, as it changes the transmission key periodically, but it is a more recent standard which is less often supported. It has two modes of operation, with and without Radius.

Types of WiFi client devices

WiFi devices can be either usually access points or network interface cards for desktops or laptops, and their main aspects are:

The chipset they use

This is the single most important attribute of a WiFi card, because Linux drivers are per-chipset, not per-card. Also, the quality of various chipsets is widely different.

The Linux driver that supports them

Usually some chipset has a single driver, but some chipsets are supported by multiple drivers, and most chipsets have no native Linux driver. Even where a driver exists it may be only half-finished.

Whether firmware has to be downloaded to the card

Most chipsets are in effect radio softmodems, and like most softmodems their firmware is often not stored in a flash/ROM chip on the card, but to save hardware cost it has to be downloaded on activation.
Cards that don't need firmware to be downloaded are vastly preferable, for the same reasons as dialup modems, but they are rather rare nowadays.

Transmission and encryption standards supported

Which of the 802.11 standards they implement, and which of the encryption standard they support.

Type of connection to the PC

Wireless cards can usually be attached to a PC in one of these ways:

PCMCIA is a 16 bit bus that vaguely resembles PCI, and physically PCMCIA cards resemble thick credit cards or small pocket calculators. It is very common on laptops, and there are a few and rare PCMCIA adapters for desktop PCs; since wireless is popular with laptop owners, most WiFi devices come in PCMCIA format. PCMCIA devices are hot pluggable, unlike PCI ones.
Cardbus is the 32 bit version of PCMCIA, and is otherewise identical. Most current laptops have slots that support both Cardbus and PCMCIA. The 32 bits means it is faster, but this is largely irrelevant for WiFi cards. However some WiFi devices are Cardbus only, and some (old) laptops don't have Cardbus slots, or not all PCMCIA slots also support Cardbus, so beware.
mini-PCI is like PCI (and it is not hot pluggable either), but in a smaller physical form factor, for cards to be put inside laptops.
PCI cards are usually mini-PCI cards riding in an adapter for the full PCI form factor.
USB or USB2. WiFi devices have limited bandwidth in practice, so usually USB2 is not necessary, but sometimes useful, in ideal conditions. They are hot pluggable. Their huge advantage is that they are convenient to attach, and the same card can be used both with laptops and desktops. However sometimes one needs a powered portable hub to use a USB device with a laptop, because some laptops supply too little power to their USB ports.

Some practical hints

As a matter of practical advice, here are some hints:

As a rule, 802.11a does not matter, and the only standards that matter are 11b and 54g, which is a pity as these are subject to microwave oven interference.
Unless in a medium-to-large corporate environment, a WiFi network will consist of a single cell.
The access point will usually be a gateway to a wired network, and/or to a long distance broadband link, usually ADSL/cable. Devices with Ethernet, WiFi, and broadband usually don't cost much more than more specialized ones and are fairly convenient.
It is rather involved to achieve security on a WiFi network (or in any other context, security is a much more difficult and harder subject than most people realize), and it is usually pointless. Modest but sufficient degrees of safety are usually obtained by:
- Not publishing the ESSID of your local network, but this is almost pointless.
- Using MAC based access control on the access point. This prevents leeching but listening is only made a little harder.
- Using 128 bit (actually 114 bit) or higher WEP static shared encryption to prevent casual listening and some traffic analysis.
- Lowering the power of your transmitters to reduce the range in which your radio traffic can be listened to; this also usually lengthens the battery life of laptops, and the useful life of cards.
- Using SSH/SSL/IPsec/... for sessions that need some degree of security (either end-to-end or using the advanced modes of WPA).

WiFi Under Linux

WiFi devices under Linux need a driver and some configuration, like all network devices.

The main issues are:

While the chipsets for wired network cards are usually fairly well documented and there are Linux drivers for practically all, this is not the case for wireless network devices. As a rule, many 11b-only cards are supported, but very few 54g are.
The wireless cards need link level configuration before being configured for IP. To some extent the wired cards can also be configured for the specific type of link level parameters, via the MII configuration tools, but usually this is not necessary. Also, link level WiFi configuration can be fairly complicated because there are several variants of WiFi network setups.
There are at least two different types of WiFi drivers, the wireless-tools type and the wlan-ng type. They differ in the API they offer for link level configuration (and internal organization). The first type is mostly popular for 11b cards, the second for (some) 54g cards.
As a result of there being at least two types of drivers, there are at least two widely different sets of WiFi link level configuration utilities, the iw tools and the wlan tools. They are mostly equivalent, but each works only with a driver of its type.
Once configured, a WiFi network card will behave from the Linux point of view largely like a wired network card, with two important differences:
- All the traffic will be radio broadcast over the neighbourhood.
- Performance may strongly depend on distance from the AP, thickness and type of the walls within, and other impediments to radio transmission.
Therefore usually traffic will be encrypted, at one level or another, and some moving around of the AP and the devices may help to obtain best signal quality, a bit like with mobile phones.
Another less important difference is that the name of the network interface will usually not be like ethN but the slightly different wlanN or similar variants.

Various features of a WiFi Linux driver and chipset

First of all some important details on how WiFi devices work and their interactions with the Linux drivers:

A typical WiFi device has three parts: a host bus (USB, PCI, Cardbus) interface, a signal/protocol processor and a radio.
Usually the host bus interfaces and the radio are hardware based, while usually the signal/protocol processor is software based (running usually on a MIPS or ARM based embedded microprocessor, like a mobile phone), and this software is called the device's firmware. In other words, almost all WiFi cards are software modems.
Most cards don't have a ROM or a EEPROM or a Flash ROM to store the firmware; it has to be downloaded every time the card is powered up. The Linux kernel contains a firmware downloader that tries to get the firmware file loaded from one of a set of hardcoded directories, usually /lib/firmware, /usr/lib/hotplug/firmware and /usr/local/lib/firmware.
Usually it is the Linux hotplug scripts that trigger the loading of the driver and this requests the loading of the firmware.
Different products based on the same chipset can use different radio chips. As a rule there is a different firmware version for each chipset and radio chip combination.

The available WiFi chipset drivers differ widely in their ability to support various types of devices and features.

As to the device and its chipset:

Support for 11b only or both 11b and 54g.
Support for monitor mode.
Support for unmanaged mode (without an AP).
Support for being an access point.
Socket for plugging in an external antenna.
Ability to set different levels of power.
Firmware stored permanently, or downloaded every time.

As to the driver:

Whether the driver is included as part of the mainline (Linus, vanilla) kernel.
Whether the driver is included as part of the kernel for your distribution.
Whether the driver allows setting different levels of transmitter power.
Whether the driver, if the card supports it, also supports monitor mode.
Whether the driver, if the card supports it, also supports unmanaged (without AP) mode.
Whether the driver, if the card supports it, also supports running it as an access point.
Whether the driver supports WEP at all, and which key lengths.
Whether the driver supports WAP at all, and which type of WAP.
Whether the driver requires the wireless-tools or the wlan-ng configuration utilities.

Well known drivers and their chipsets

The drivers currently (051003) known to work well with Linux are:

The orinoco driver: This is for a very popular 11b only chipset, the PrismII or Hermes chipset; the name of the driver of the driver comes from the original line of Proxim devices with the Orinoco brand, which is now used also for devices using other chipsets. Devices with this chipset are 11b only and are becoming hard to find.
The prism54 driver: This driver supports the Prism (GT|Duette|Indigo|Frisbee) chipset for 54g/11b operation. Unfortunately supported devices are becoming difficult to find, and the driver does not support the new Prism SoftMAC or Xbow/Javelin chipsets. The project maintainers write:
If you can't test a card and want linux support, I can recommend you just not buy a prism 802.11g based chipset for now. So now what? Well we have the centrinos and ralink.
The madwifi driver: This driver supports many Atheros chipsets for 54g/11b and usually 11a operation. Since this is one of the very few chipsets that supports 11a, there are good chances that if a card supports all three standards it is based on this class of chipsets.
This driver contains some parts of the firmware, which is as usual proprietary. Thus because of this it is not fully open source. But the other drivers require closed source firmware too, which has to be obtained separately.
The atmel driver: This driver supports the Atmel AT76C5xxx chipsets for 11b USB and Cardbus devices.
The at76c503a driver: Another driver that supports the Atmel AT76C503A chipset (and its Intersil radio variant) for 11b USB and Cardbus devices.
The rt2x00 driver, for the rt2400 and rt2500: These are all related drivers, being merged in one rt2x00 project, an they support the RT2400 (11b) and RT2500 (54g) USB chipsets by Ralink Technology and they have been released thanks to Minitar whose products are therefore particularly well supported, as well as those of donated for testing.
The Intel sponsored Centrino ipw2100 driver: This driver supports the Intel Centrino 2100 chipset, which only supports 11b.
The Intel sponsored Centrino ipw2200 driver: This driver is meant to support the Intel Centrino 2200BG and 2915ABG chipsets, which as suggested by their names, support 11b and 54g, and the latter also supports 54a.
The zd1201 driver: This driver supports only the ZyDAS 1201 low cost chipset for 11b USB network interfaces. Thanks to manufacturer support versions 0.14 and later work pretty well and painlessly. This driver has been incorporated into the mainline kernel starting with kernel version 2.6.12. Devices with this chipset are currently (050315) easy to find and very cheap.
There is a new driver for the ZyDAS 1211 chipset, which does 54g, but it does not quite work yet (050822).
The Broadcom 43xx driver: Starting with Linux 2.6.17 there is a new driver that supports for the first time a recent Broadcom series of WiFi chips.

There are other Linux drivers, but they are far less reliable than those listed above.

Also note that not all the drivers above support all the possible features, for example unmanaged mode or access point mode, WAP encryption or even WEP encryption.

Virtually all however do support managed mode operation and static WEP encryption up to to 128 bits (actually 114).

So, what are my impressions? Well, for 54g the Atheros and Ralink RT2500 chipsets look like the best bet, by far, and the Ralink RT2500 seems particularly nice and rich of features.
For 11b the ZyDAS chipset is available in several really cheap USB devices, the Ralink RT2400 is also available from many sources, and the driver is particularly nice. There are still a few Hermes based cards, and Atmel does not see too bad.

The Centrinos are well supported, but I think they are only available built into laptops, so if you have a Centrino laptop there is not much choice but the situation is lucky.

Online lists of well supported device models

There are some online lists of devices known to work with the drivers easily available for Linux; it is important to double check the exact model number, as manufacturers often change the chipset while making very small changes to the model number. Sometimes the chipset gets changed but the model number does not change at all...

MADWiFi project supported devices list.
Another MADWiFi list.
List of Atheros cards from the manufacturer.
List of Ralink cards used to test the Linux drivers.
List of cards supported by the Prism54 project.
List of ZyDAS 1201 supported devices.
List of Atmel devices among other things.
Jean Tourrilhes' list of 802.11b drivers and card.
Jean Tourrilhes' list of 802.11g drivers and card.