By Phil Belanger

August 09, 2007

802.11a, the first 5 GHz Wi-Fi spec, lacked effective coverage. N changes that equation.

The IEEE 802.11 wireless LAN standards have long included service on multiple frequency bands—2.4 GHz and 5 GHz.

However, largely due to the disappointing coverage of existing 5 GHz products (802.11a)—the use of 5 GHz Wireless LANs has been limited to a few high-capacity enterprise networks, consumer networks, and wireless backhaul for metropolitan area networks. 5 GHz radio signals just do not propagate as well—particularly indoors, through walls—as 2.4 GHz radio signals. It’s basic physics.

For most enterprises, a 5 GHz 802.11a infrastructure required too many access points. When 802.11g came along and delivered the 54 Mbps data rate of 802.11a in the 2.4 GHz band, most networks stayed at 2.4 GHz due to the better coverage of 802.11g access points. One consequence of this is the overloading of the 2.4 GHz band. With only three non-overlapping 802.11 channels in this band; Wi-Fi networks are increasingly contending with their neighbors as well as microwave ovens, cordless phones, and other devices that share this spectrum. Over time, the congestion in the 2.4 GHz band will only get worse. Where do we go from here?

802.11n to the rescue

802.11n supports both 2.4 GHz and 5 GHz bands. It has a single MAC protocol that operates with a multiple frequency physical layers. And it dramatically improves the range, coverage and throughput in both frequency bands. Frequency Band(GHz) Independent 20 MHz Channels Possible 40 MHz Channels 2.40-2.485 3 1 Indoor/outdoor 5.15-5.25 4 2 Indoor only 5.25-5.35 4 2 Indoor/outdoor 5.47-5.75 10 5 Indoor/outdoor, dynamic frequency selection and power control 5.75-5.85 4 2 Outdoor Total 25 12

Frequency Band(GHz)	Independent 20 MHz Channels	Possible 40 MHz Channels
2.40–2.485	3	1	Indoor/outdoor
5.15–5.25	4	2	Indoor only
5.25–5.35	4	2	Indoor/outdoor
5.47–5.75	10	5	Indoor/outdoor, dynamic frequency selection and power control
5.75–5.85	4	2	Outdoor
Total	25	12

802.11n makes use of the legacy 2.4 GHz band and constructs three largely non-interfering 20 MHz channels or one 20 MHz channel and one 40 MHz channel. It is backward-compatible with 802.11b/g stations and channelization. 802.11n makes use of the existing 802.11a channel set in the 5 GHz band at (5.15–5.25, 5.25–5.35, and 5.75–5.85 GHz) to construct twelve non-overlapping 20 MHz channels or as many as six non-overlapping 40 MHz channels. 802.11n will also take advantage of new worldwide regulatory changes making the 5.47–5.75 GHz band available for unlicensed WLAN use.

If 802.11n users could only tap the potential of the 5 GHz band they would have access to 25 channels in the combined bands—potentially delivering over seven gigabits per second of raw wireless capacity in an enterprise network.

The realization of that potential requires 802.11n to do what seems impossible—substantially increase the range of 5 GHz operation to match the range of 802.11g, while delivering the performance advantages of 802.11n. Novarum decided to test Draft 802.11n products to see if the performance improvements of multiple antennas, smart radios, and multiple spatial streams offered by 802.11n would be enough to overcome coverage limitations of 802.11a.

The test setup

To make the comparison, we selected a few standard 802.11g clients (The same clients we use for the Novarum Wireless Broadband Review of metropolitan wireless networks), several after-market 2.4 GHz 802.11n clients, a classic 802.11g access point, the new Apple dual-band draft 802.11n clients (embedded in Intel based MacBooks) and the new dual-band Airport Extreme N access point.

Our testing location is a classic San Francisco Victorian house with four floors and many small rooms. To illustrate the effects of wall and floor penetration, we picked seven locations of gradually increasing distance and numbers of walls and floors between the access point and the client test location. This residence has always needed several 802.11g 2.4 GHz access points to provide adequate Wi-Fi coverage. We used our standard Chariot delay, upstream throughput, downstream throughput test scripts from the Novarum Wireless Broadband Review to capture the data in a consistent fashion.

The results

The pure 802.11n 5 GHz connections (between Apple’s MacBook and Airport Extreme access point) had at least three times the throughput of the legacy 802.11g system in all but one location (where all systems performed equally). 802.11n in the 5 GHz band also delivered twice the throughput of 802.11n in the 2.4 GHz band in these tests.

These tests, while not exhaustive, illustrate the potential of 802.11n in the 5 GHz band. The extended range provided by 802.11n overcomes many of the real world deployment challenges of 5 GHz 802.11a networks. Novarum testing of Draft 802.11n products shows that 802.11n operating at 5 GHz will have similar range to legacy 802.11g networks in the 2.4 GHz band—at the maximum data rates.

The combined performance benefits of 802.11n enable more practical enterprise deployments at 5 GHz. The result is that 802.11n will be able to operate effectively across many more channels and therefore deliver much higher capacity in a given area.

Deployment strategies

While some have proposed that 802.11n will allow enterprise networks to operate with fewer APs, we think a better deployment strategy is to use the same AP density as current 802.11g networks but operate the entire 802.11n network in the 5 GHz band. Legacy 802.11 b/g clients and guest network access should stay in the 2.4 GHz band served by legacy APs or new 802.11n APs operating in legacy mode. With legacy 802.11 a/b/g clients and unknown guest clients isolated in the 2.4 GHz band, the 802.11n network at 5 GHz can operate at the highest possible performance. Our testing shows that the range and coverage of 802.11n will be sufficient to deliver maximum data rates across an enterprise with the access point density that we expect from 802.11g networks.

802.11n will accelerate the transition of wireless LANs from the 2.4 GHz band to the 5 GHz band, and users will benefit from the additional capacity that is available at 5 GHz.

Look for future Novarum reports on enterprise 802.11n testing and outdoor 802.11n testing where increased range and capacity could have a positive impact on the metro Wi-Fi business opportunity.