Wireless LAN 802.11 Wi-Fi
Background[edit | edit source]
The general IEEE designation for network standards in “802”, eg IEEE 802.3 for Ethernet. The "11" family of standards governs wireless local area networking. The IEEE 802.11 wireless LAN standards or Wi-Fi denotes a set of standards developed by working group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802).
The 802.11 family currently includes six over-the-air modulation techniques that all use the same Layer 2 protocols, the most popular (and prolific) techniques are those defined by the a, b, and g amendments to the original standard; security was originally included, and was later enhanced via the 802.11i amendment. Other standards in the family (c–f, h–j, n) are service enhancement and extensions, or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed by 802.11a and 802.11g.
IEEE 802.11b and 802.11g standards use the unprotected 2.4 GHz frequency band. The 802.11a standard uses the 5 GHz frequency band. Operating in an unregulated frequency band, 802.11b and 802.11g equipment suffer interference from microwave ovens, cordless phones, and other appliances using the same 2.4 GHz ISM band.
Protocols[edit | edit source]
IEEE 802.11 legacy Implementation[edit | edit source]
The original version of the standard IEEE 802.11 released in 1997 specifies two raw data rates of 1 and 2 Megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or in the Industrial Scientific Medical (ISM) frequency band at 2.4 GHz. IR channels remains a part of the standard but has no actual implementations.
The original standard also defines Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the media access method, identical to the Ethernet protocol. Maximum capacity of the channel is limited to around 65% after error correction and error handling are applied
A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realize. It is really more of a "meta-specification" than a rigid specification, allowing individual product vendors the flexibility to differentiate their products. Legacy 802.11 was rapidly supplemented (and popularized) by IEEE 802.11b.
802.11b[edit | edit source]
The IEEE 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same Ethernet based signalling protocol. Due to the CSMA/CA protocol overheads, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s over TCP and 7.1 Mb/s over UDP.
IEEE 802.11b also operates in the unprotected 2.4 GHz frequency band with an 83.5Mhz wide channel.
802.11b products appeared on the market very quickly, since 802.11b is a direct extension of the DSSS modulation technique defined in the original standard. Hence, chipsets and products were easily upgraded to support the 802.11b enhancements. The dramatic increase in throughput of 802.11b (compared to the legacy standard) along with substantial price reductions lead to the rapid acceptance of 802.11b as the definitive wireless LAN technology.
802.11b network cards can operate at 11 Mb/s, but will scale back to 5.5 Mb/s, 2 Mb/s, then 1 Mbps depending on signal quality. Since the lower data rates use less complex and more redundant methods of encoding the data, they are less susceptible to corruption due to interference and signal attenuation. Extensions have been made to the 802.11b protocol (e.g., channel bonding and burst transmission techniques) in order to increase speeds to 22, 33, and 44 Mb/s, but these extensions are proprietary and have not been endorsed by the IEEE. Many companies call enhanced versions "802.11b+". These extensions have been largely obviated by the development of 802.11g.
The first widespread commercial use of the 802.11b standard for networking was made by Apple Computer under the trademark AirPort.
Channels and international compatibility[edit | edit source]
The IEEE 802.11b and 802.11g standard divides the frequency spectrum into 14 overlapping, staggered channels whose center frequencies are 22 MHz apart. It is common to hear that channels 1, 6 and 11 (and where available by the regulator, channel 14) do not overlap and those channels (or other sets with similar gaps) can be used such that multiple networks can generally operate in close proximity without interfering with each other. The 802.11b and 802.11g standards do not specify the width of a channel. Rather, they specify the center frequency of the channel and a spectral mask for that channel. The spectral mask for 802.11b requires that the signal be at least 30 dB down from its peak energy at ±11 MHz from the center frequency and at least 50 dB down from its peak energy at ±22 MHz from the center frequency.
Since the spectral mask only defines power output restrictions up to ±22 MHz from the center frequency, some people assume that the channel's energy doesn't extend any further than that, but in reality, it does. In fact, if the transmitter is sufficiently powerful, the signal can be quite strong even beyond the ±22 MHz point. Therefore, it is incorrect to say that channels 1, 6, and 11 do not overlap. It is more correct to say that, given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel. But this is not universally true. For example, a powerful transmitter on channel 1 can easily overwhelm a weaker transmitter on e.g. channel 6. In one lab test, throughput on a file transfer on channel 11 decreased slightly when a similar transfer began on channel 1, indicating that even channels 1 and 11 can interfere with each other a little bit.
The channels that are available for use in a particular country will differ according Regulator in that country. In the United States, FCC regulations permit channels 1 to 11 to be used. Channels 10 and 11 are the only channels which are common throughout the world.
Channel 14, where available, is restricted to 802.11b operation only.
802.11a[edit | edit source]
The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard, operates in 5 GHz frequency band, and uses a 52-subcarrier OFDM (Orthogonal Frequency Division Multiplexing) with a maximum raw data rate of 54 Mb/s. This yields a realistic throughput of approx 24 Mb/s. The data rate is reduced to 48, 36, 34, 18, 12, 9 then 6 Mb/s under difficult signal paths. 802.11ais not interoperable with 802.11b, except if using equipment that independently implements both standards.
Since the 2.4 GHz frequency band is heavily used by many users and appliances, moving to the 5 GHz band gives 802.11a the advantage of less interference. The 5Ghz carrier frequency restricts the use of 802.11a to almost line of sight, necessitating the use of more access points. It also means signal penetration through walls and foliage is much reduced compared to IEEE 802.11b.
The outcome from the 2003 World Radiotelecommunications Conference made it easier to use this standard worldwide with a channel bandwidth of 255Mhz. IEEE 802.11a is approved in the United States and Japan, but in other areas, such as the European Union, there were delays in approvals. European regulators were considering the use of the European HIPERLAN standard, but in mid-2002 they cleared 802.11a for use in Europe.
802.11a products started shipping in 2001, lagging 802.11b products due to the slow availability of the 5 GHz components needed to implement products. 802.11a was not widely adopted overall because 802.11b was already widely adopted. IEEE 802.11a's suffered disadvantages with poor initial product implementations with shorter range and regulatory restrictions. Manufacturers of 802.11a equipment responded to the lack of market success by improving the implementations (current-generation 802.11a technology has range characteristics much closer to those of 802.11b), and by making technology that can use more than one 802.11 standard. There are dual-band, or dual-mode or tri-mode cards that can automatically handle 802.11a and b, or a, b and g, as available.
802.11g[edit | edit source]
On 12 June 2003, a third modulation standard was ratified: 802.11g. Also using the 2.4 GHz band (like 802.11b) with an 83.5Mhz wide channel, it operates at a maximum raw data rate of 54 Mb/s, or about 24.7 Mb/s throughput like 802.11a. It is fully backwards compatible with “b” and uses the same frequencies. Details of the interoperability between “b” and “g” occupied much of the technical process. In older networks the presence of an 802.11b node significantly reduces the speed of an 802.11g network.
The 802.11g standard swept the consumer world of early adopters starting in January 2003, well before ratification. The corporate users held back and Cisco and other big equipment makers waited until ratification. By summer 2003, announcements were flourishing. Most of the dual-band 802.11a/b products became dual-band/tri-mode, supporting a, b, and g in a single mobile adaptor card or access point.
While 802.11g held the promise of higher throughput, actual results were mitigated by a number of factors: conflict with 802.11b only devices (see above), exposure to the same interference sources as 802.11b, limited channelization (only 3 fully non-overlapping channels like 802.11b) and the fact that the higher data rates of 802.11g are often more susceptible to interference than 802.11b, causing the 802.11g device to reduce the data rate to effectively the same rates used by 802.11b. The move to dual-mode/tri-mode products also carries with it economies of scale (e.g. single chip manufacturing). The use of dual-band/tri-mode products ensures the best possible throughput in any given environment.
A new proprietary feature called “Super G” is now integrated in certain access points. These can boost network speeds up to 108 Mb/s by using channel bonding. This feature may interfere with other networks and may not support all b and g client cards. In addition, packet bursting techniques are also available in some chipsets and products which will also considerably increase speeds.
The first major manufacturer to use 802.11g was Apple, under the trademark AirPort Extreme. Cisco joined by buying up Linksys, an early adopter, and also offers its own wireless mobile adaptors under the name Aironet.
802.11n[edit | edit source]
In January 2004 IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for local-area wireless networks. The real data throughput will be at least 100 Mb/s (which may require an even higher raw data rate at the physical layer), and so up to 4–5 times faster than 802.11a or 802.11g, and perhaps 20 times faster than 802.11b. It is projected that 802.11n will also offer a better operating distance than current networks. There are two competing variants of the 802.11n standard; WWiSE (backed by companies including Broadcom) and TGn Sync (backed by Intel and Philips). The standardization process is expected to be completed by the end of 2006.
802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output). The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, perhaps through coding schemes like Alamouti coding.
Hardware Certification[edit | edit source]
Because the IEEE only sets specifications but does not test equipment for compliance with them, a trade group called the Wi-Fi Alliance runs a certification program that members pay to participate in. Virtually all companies selling 802.11 equipment are members. The Wi-Fi trademark, owned by the group and usable only on compliant equipment, is intended to guarantee interoperability. Currently, "Wi-Fi" can mean any of 802.11a, b, or g. As of fall 2003, Wi-Fi also includes the security standard Wi-Fi Protected Access or WPA. Eventually "Wi-Fi" will also mean equipment which implements the 802.11i security standard (aka WPA2). Products that say they are Wi-Fi are supposed to also indicate the frequency band in which they operate, 2.4 or 5 GHz.
Standards Summary[edit | edit source]
IEEE 802.11 - The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard
IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
IEEE 802.11d - international (country-to-country) roaming extensionsNew countries
IEEE 802.11e - Enhancements: QoS, including packet bursting
IEEE 802.11F - Inter-Access Point Protocol (IAPP)
IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
IEEE 802.11h - 5 GHz spectrum, Dynamic Channel /Frequency Selection (DCS/DFS) and Transmit Power Control (TPC) for European compatibility
IEEE 802.11i (ratified 24 June 2004) - Enhanced security
IEEE 802.11j - Extensions for Japan
IEEE 802.11k - Radio resource measurements
IEEE 802.11n - Higher throughput improvements
IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars)
IEEE 802.11r - Fast roaming
IEEE 802.11s - Wireless mesh networking
IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics
IEEE 802.11u - Interworking with non-802 networks (e.g., cellular)
IEEE 802.11v - Wireless network management
Wireless Security[edit | edit source]
In 2001, a group from the University of California at Berkeley presented a paper describing weaknesses in the 802.11 WEP (wired equivalent privacy) security mechanism defined in the original standard; they were followed by Fluhrer, Mantin, and Shamir's paper entitled "Weaknesses in the Key Scheduling Algorithm of RC4". Not long after, Adam Stubblefield and AT&T publicly announcing the first verification of the attack. In the attack they were able to intercept transmissions and gain unauthorized access to wireless networks.
The IEEE set up a dedicated task group to create a replacement security solution titled, 802.11i, previously handled as part of a broader 802.11e effort to enhance the MAC layer. The Wi-Fi Alliance announced an interim specification called Wi-Fi Protected Access (WPA) based on a subset of the then current IEEE 802.11i draft. These started to appear in products in mid-2003. 802.11i (aka WPA2) itself was ratified in June 2004, and uses the Advanced Encryption Standard, instead of RC4, which was used in WEP and WPA.
The Wi Fi Alliance definition of interoperability goes well beyond the ability to work in a Wi Fi network. To gain certification under a specific program, products have to show satisfactory performance levels in typical network configurations and have to support both established and emerging applications. A user that purchases a Wi Fi enabled laptop, for instance, would not be satisfied if the laptop established a connection with the home network, only to get the throughput of a dial-up connection. Similarly, subscribers using a Wi Fi enabled mobile phone would be disappointed, if a voice call could not go thru or was dropped.
The Wi Fi Alliance certification process includes three types of tests to ensure interoperability. Wi Fi CERTIFIED products are tested for:
- Compatibility: certified equipment has been tested for connectivity with other certified equipment . Compatibility testing has always been, and still is, the predominant component of interoperability testing, and it is the element that most people associate with “interoperability”. It involves tests with multiple devices from different equipment vendors. Compatibility testing is the program component that helps to ensure devices purchased today will work with Wi Fi CERTIFIED devices already owned or purchased in the future.
- Conformance: the equipment conforms to specific critical elements of the IEEE 802.11 standard. Conformance testing usually involves standalone analysis of individual products and establishes whether the equipment responds to inputs as expected and specified. For example, conformance testing is used to ensure that Wi Fi equipment protects itself and the network when the equipment detects evidence of network attacks.
- Performance: the equipment meets the performance levels required to meet end-user expectations in support of key applications. Performance tests are not designed to measure and compare performance among products, but simply to verify that the product meets the minimum performance requirements for a good user experience as established by the Wi Fi Alliance. Specific performance tests results are not released by the Wi Fi Alliance.