This isn't to say maximum performance won't increase as subsequent versions of 802.11n are endorsed by the Wi-Fi Alliance. But it does mitigate the risk that current devices will be incompatible with future versions. Companies don't buy WLAN devices based on an IEEE standard but on the Wi-Fi Alliance's endorsement of interoperability and the independent certification that 802.11n Draft 2.0 has received. Also, Cisco is part of Intel's "Connect With Centrino" interoperability testing program, which gives an added comfort level.
SETTING A 5-GHZ STRATEGY
The 5-GHz frequency has been underused compared with the 2.4-GHz band, but with 802.11n that's poised to change. In the early days of WLANs, light user loads and a focus on maximizing coverage made the superior propagation characteristics of 2.4 GHz a clear choice over 5 GHz's more limited range. Now those WLANs have grown from scattered hotspots to pervasive coverage blankets with many microcells supporting a multitude of users and high-bandwidth applications, and the focus has shifted from coverage to capacity. And when you're talking capacity, nothing beats the massive amount of spectrum available in the 5-GHz band, which encompasses 21 non-overlapping channels when an AP implements full Dynamic Frequency Selection 2 (DFS2) support, compared with 2.4 GHz's modest three nonoverlapping channels.
Interference is another differentiator. WLANs in the 2.4-GHz band must contend with cordless phones, microwaves, and other 802.11b/g devices, but 5 GHz is relatively vacant except for cordless phones and a few 802.11a networks. Although the 802.11n standard supports either frequency, 5 GHz is superior when channel bonding comes into play because it supports many nonoverlapping 40-MHz channels, while 2.4 GHz supports only one.
In addition to increasing speeds with wider 40-MHz channels, 802.11n boasts more OFDM subcarriers, MAC Layer packet aggregation, and MIMO. Core to 11n's increased performance, MIMO allows 802.11n to use environmental multipath reflections. Multipath has caused dead spots for 802.11a/b/g devices because they had a limited number of antennas to sort out reflections. But in 11n, multiple antennas and multiple radios help boost reliability and allow multiple data streams, or "spatial streams," to be sent concurrently. As for nomenclature, a MIMO device with two transmit and three receive antennas supporting two spatial streams would be referred to as "2x3:2" MIMO, following a "TxR:S" convention. At its projected maximum, 802.11n using 4x4:4 MIMO will support 600-Mbps raw data rates. Current Draft 2.0 products top out at 300 Mbps using 2x3:2 or 3x3:2 MIMO.
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