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54 Mbps based on OFDM in the 5 GHz band, failed to garner significant support because
two radios were required to maintain backward compatibility with 2.4 GHz 802.11b
devices; the cost of two radios was often too high. The real success story was 802.11g,
which provided 54 Mbps based on OFDM in the 2.4 GHz band in a way that was
backward-compatible with 802.11b.
The success of 802.11g drove the use of Wi-Fi to new heights and expanded the
demands on the technology yet again; everyon e wanted more. Fortunately, the technol-
ogy continued to develop and in 2002 the IEEE 802.11 Working Group started defining
the next generation of PHY and MAC features as part of 802.11n. 802.11n will define
mechanisms to provide users some combination of greater throughput, longer range and
increased reliability, using mandatory and optional features in the PHY (including
MIMO technology and 40 MHz channels) and the MAC (including more efficient data
aggregation and acknowledgements).
Interestingly, 802.11n operates in both the 2.4 GHz and 5 GHz bands. It is expected
that 5 GHz operation will be more popula r than when 802.11a was introduced, because
2.4 GHz is now more congested, the number of available channels in the 5 GHz band has
been expanded with the introduction of DFS and TPC technology, there is more need for
high throughput 40 MHz channels, and the cost of dual-band radios has decreas ed.
The 802.11n standard is not yet complete, and is unlikely to be ratified by the IEEE
until at least mid 2009. Until August 2006, the Wi-Fi Allia nce had a policy to not certify
802.11n products until the standard was ratified. However, some vendors decided the
market could not wait for ratification of the 802.11n standard and started releasing pre-
standard products. These products were often not interoperable at the expected perform-
ance levels because they were not based on a common interpretation of the draft 802.11n
specification. The problem for the Wi-Fi Alliance was that these products were adversely
affecting the reputation of Wi-Fi. The Wi-Fi Alliance decided the only way forward was
to certify the basic features of 802.11n from a pre-standard draft. Such a decis ion is not
without precedent. In 2003, certification of WPA started before the 802.11i standard was
ratified and in 2004 certification of WMM started before 802.11e was ratified. The Wi-Fi
Alliance commenced certification of 802.11n draft 2.0 on 26 June 2007.
The decision has turned out to be the right one for the industry and for users. The Wi-Fi
CERTIFIED 802.11n draft 2.0 programme has been remarkably successful, with more
than 150 products certified in less than five months. This represents a significantly higher
number of certified products than for the 802.11g progra mme during a similar period
after launch. The Wi-Fi Alliance’s certification progra m has helped ensure interoper-
ability for the many products that will be released before the ratification of the 802.11n
standard. This is particularly important given that the likely ratification date of the
802.11n standard has been extended by more than a year since the decision to start a
certification program was announced by the Wi-Fi Alliance. The next challenge for the
Wi-Fi Alliance is to ensure a backw ard-compatible transition path from the 802.11n draft
2.0 as certified by the Wi-Fi Alliance to the final rati fied standard.
Standards are never the most accessible of documents. The 802.11 standard is partic-
ularly difficult to understand because it has been amended so many times by different
groups and editors over a long period. A draft amendment to the standard, such as
xvi Foreword