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 |
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.
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802.11n and g compared
(click to open in a new window)
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.
Article courtesy of Wi-Fi Planet
