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802.11n: n = not yet

04 декабря 2006
IEEE 802.11n - promising more than double the speed and up to 12 times the throughput of 802.11g systems - is on the horizon. While a lot of the attention 802.11n has been receiving is for increased voice over IP (VoIP) usage and multimedia applications, speed isn't everything. It's also important to be able to "go the distance." And that's where 802.11n may really pay off - potentially offering users up to four times the effective range of current 802.11b/g systems.

Whether it's outside in a container yard or inside where reflected signals can create interference and limit range, 802.11n is designed to offer improved performance with the same number of access points (APs) or equal performance with far fewer APs.

With existing 802.11 equipment, the data transmission signal can be reflected off various surfaces resulting in multiple versions of the signal being received with slight time differences (multipathing). This has the potential to cause data reception problems and has, in many applications, limited the effective range of WLANs.

The secret behind 802.11n's extended range and throughput capabilities is something that may look like a bad hair style: multiple antennas. Called MIMO (multiple input/multiple output), the use of multiple antennas to receive data streams not only solves the problem of multipathing, it can actually benefit from it.

MIMO equipment sorts out the various signals and compares them to each other. If one received data transmission isn't complete, the system looks for that data in a different (multipathed) signal. There is a high probability that the missing data can be found in one of the other signals, enabling the system to effectively "splice" together the complete data stream. This increases overall transmission robustness and reduces the need to retransmit data in "noisy" environments.

Base-line 802.11n equipment will use one radio with a 20 MHz bandwidth. More complex equipment may use two or more radios - and as many sets of antennas - to expand the bandwidth (by 20 MHz increments) to more readily meet the minimum 108 Mbps requirement of the draft 802.11n standard.

That's right. The standard is still in draft form and is not expected to be completed in the very near future. In fact, it has been delayed a year beyond its originally projected completion date. Because the standard has been delayed, the WiFi Alliance has announced it will begin "certifying" pre-standard equipment beginning early in 2007.

A number of vendors are already offering pre-802.11n devices that are supposed to be backwardly compatible with 802.11b/g and offer significantly improved speed, throughput and range but that also employ different MIMO implementations. That means there's no guarantee of interoperability (and a high probability that they won't be interoperable). In fact, according to one source, some don't interoperate on existing WiFi networks. And not all pre-standard 802.11n equipment supports all the currently proposed provisions in the standard. There's some concern that many of these devices won't be able to fully comply with 802.11n when it is published, even with firmware upgrades.

Even after the technical issues have been settled, the increased power demands of 802.11n equipment is also something that still needs to be addressed before 802.11n can be effectively deployed in mobile computing devices. Intel® has committed to supporting mobile 802.11n next year, incorporating additional power conservation capabilities with its Core 2 Duo processor but that too is a bit over the horizon.

LXE is committed to providing robust, high performance, versatile mobile computing devices and WLAN systems based on the best international standards available. So, even though an increasing number of pre-standard 802.11n devices is becoming available, 802.11n is not yet a standard and not yet stable.

Once the 802.11n standard has been published, power issues have been solved, and radios have been certified for full 802.11n compliance and backward compatibility with existing 802.11b/g devices, you can be sure that LXE will offer solid, reliable, and usable mobile computing devices that offer the benefits of 802.11n.

Jim Childress, President & GM, LXE Inc.

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