Responsible IT decision makers are wary of pre-standard technologies, having discovered the hard way that "pre-standard" is often code for "proprietary, premature, and/or prone to vendor lock-in." Still, sometimes you have to walk on the wild side, so when Cisco Systems offered to let us test an early version of its 1250 802.11n Draft 2.0 access point in our Syracuse University Real-World Labs, we were excited to get a glimpse of the future of enterprise WLANs.
From a performance standpoint, our testing revealed speed increases of four to six times what an 802.11a/g infrastructure can provide. And using an 802.11n access point for even legacy a/b/g clients delivered a measurable performance advantage thanks to the ability of a multiple input, multiple output (MIMO) approach to maintain high-bandwidth data rates for a larger portion of an AP's coverage area. For a/b/g voice-over-WLAN phones, this reliability translates into higher-quality calls and fewer dead spots.
We believe 802.11n's bug-shakedown period will be a less rocky version of what occurred when 802.11g was an IEEE draft: 802.11g's Wi-Fi Alliance interoperability certification occurred after the standard was finalized; in contrast, products based on 802.11n Draft 2.0 are being certified today, before the standard's ratification by the IEEE. As of this writing, 225 products, albeit predominantly oriented toward small and home offices, have received the seal of interoperability from the Wi-Fi Alliance. Notable names on that list include Apple, Aruba Networks, Atheros, Broadcom, Cisco, Intel, and Meru Networks--all big players with an economic interest in ensuring that the current crop of 802.11n Draft 2.0 products are backward and forward compatible.
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.