How 802.11n backward compatibility works

It has been widely reported that 802.11n, the wireless LAN IEEE draft standard that uses multiple input/multiple output technology to boost Wi-Fi speeds to over 100Mbps, is “backward compatible” with today’s 802.11a, 11b, and 11g networks. This can seem confusing, given that 802.11a, which runs in the 5-GHz frequency band, is not compatible with 802.11g and 11b, which operate in the 2.4-GHz band.

So how can 11n be backward compatible with all these networks, when they are not compatible with each other?

5-GHz networking support is currently an optional component to the 802.11n draft standard. As such, the MIMO scheme of 802.11n does not require simultaneous use of both the 5-GHz and the 2.4-GHz bands, according to Mark Hung, director of strategic marketing at Wi-Fi chipmaker Atheros. That’s how a standard that potentially makes use of both bands can be backward-compatible with networks that use a single band only: Only one band is in use at a time.

For interoperability, 802.11n supports three modes, explains Hung:

* Legacy mode.

Addresses 11n APs and an 11a/g/b client environment. In this case, the client decodes the first three fields of a six-field header, which is specific to 11a and 11g. If the 5-GHz option isn’t supported in the 11n AP, there will be no backward compatibility with 802.11a.

* Mixed mode.

Addresses 11n APs and a mixed 11n/a/g/b environment. As in legacy mode, 11a and 11g/b clients will read the first three fields of the AP’s 11n header, while 802.11n will also parse the additional three “high throughput” fields of the full 11n header. Again, if the 5-GHz option isn’t supported in the 11n AP, there will be no backward compatibility with 802.11a.

* Greenfield mode.

Addresses a pure network of 802.11n APs and clients, taking full advantage of the high-throughput capabilities of the 11n MIMO architecture.

What if you have some 802.11a/g APs, and some 802.11n clients creep into the picture?

Because MIMO technology leverages multiple pairs of antennas to boost speed, 11n clients mixed with 11a/g networks should experience a 30% to 50% improvement in either range or throughput, Hung says because of the client’s “extra pair of ears.”

In the wake of 802.11n being approved as an IEEE draft specification last month, both Atheros and competing chipmaker Broadcom have announced the availability of 802.11n chips that will be software-upgradeable to comply with the final specification. Both support the 5-GHz option, which much of the industry agrees is needed to deliver the 40-MHz of spectrum in adjacent spectrum required for 11n’s promised speeds.