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WLAN selection and deployment considerations; Why legacy clients will exist for a long time?
802.11n standard includes several enhancements in PHY – MIMO (multiple-input, multiple-output), use of 20MHz or 40 MHz channels with multiple spatial streams per channel, more efficient OFDM and short guard interval (GI) – and MAC layers – frame aggregation and block acknowledgment. With the imminent ratification of 802.11n standard by IEEE, at its Sep 11, 2009 meeting, organizations that may be waiting for the 802.11n standard to be finalized will now look to either upgrade their existing wireless LAN networks to a 802.11n based WLAN or will consider a 802.11n based WLAN for new Wi-Fi projects.
Legacy client issue was first introduced when 802.11g was added as a faster alternative to 802.11b in 2.4 GHz and it was limited to the 2.4 GHz band. In a mixed client environment with 802.11g and 802.11b clients, faster 802.11g clients’ performance was degraded by presence of legacy 802.11b clients. When 802.11n clients are added to the mix, the presence of legacy clients and their impact on performance becomes even more important. More than 5 years after the standardization of 802.11g, we still find many PDAs, Smart Phones, and other Wi-Fi devices, including some VoWiFi handsets, which support 802.11b only in use by customers. We should expect to see mixed clients in both 2.4 GHz (802.11n, 802.11g, 802.11b) and 5 GHz (802.11n, 802.11a) bands for some time.
Some of the performance impact is attributable to the backward compatibility requirements for 802.11n, 802.11g and 802.11b clients to co-exist in the 2.4 GHz band. However, majority of the performance impact is attributed to the use of packet based fairness where the slower 802.11g/802.11b clients took much longer to send the same amount of data compared to a 802.11n/802.11g client sending the same amount of data and the slower 802.11g/802.11b clients used a disproportionately larger portion of the air time. With 802.11n, this performance degradation also occurs in 5 Ghz band with 802.11n and 802.11a clients.
An alternative approach used in a well designed WLAN solution implements time based fairness where airtime used by the various clients is taken into consideration to allocate airtime fairly among the clients contending for the use of precious RF resources. This approach eliminates the performance degradation suffered by the faster (802.11n, 802.11g) clients in a mixed client (802.11b, 802.11g, 802.11n in 2.4 GHz band and 802.11a, 802.11n in 5GHz band) environment.
Legacy clients and the impact on performance for newer devices should be a key consideration in the evaluation of 802.11n WLAN solution as enterprises move to leverage their wireless network to offer enhanced mobility while continuing to provide access to all the corporate resources.
