21 April 2009

802.11n Performance: Radios vs. Streams

Many organizations pride themselves on being at the cutting edge of technological innovation, the first to deploy a vendors newest innovation. Indeed, >50% of organizations surveyed will evaluate other wireless vendors’ products within the next 12-18 months. Being the first to catch the hottest new innovation carries with it the risk of being burned, and a little due diligence can go a long way in making sure that a buying decision is prudent.

Take for example the matter of 802.11n performance. 802.11n performance is based in part on both the number of radio chains and the number of spatial streams. The two are often confused...at the buyers peril. The number of radio chains corresponds with the number of transmitters or receivers, and is typically denoted as “m x n” where m is the number of transmitters, and n the number of receivers. m x n need not necessarily be symmetrical, and some 802.11n access point can dynamically adjust the numbers, e.g., a 3x3 radio can operate in 3x3, 2x3 or 1x3 mode depending on configuration, mode and power profile.

While multiple transmitter and receiver chains can be used to improve the signal quality, the big increases in data rates associated with multiple input-multiple output (MIMO) access points are more dependent on the number of spatial streams. Using 1 stream, the maximum 802.11n data rate per radio, assuming 40MHz bandwidth, is 150Mbps. Using 2 streams that number doubles to 300Mbps, and so on. The number of spatial streams is typically denoted by S in “n x m : S.” There are as yet no 3 stream access points on the market, though several access points have 3 receiver and/or transmitter chains.

By way of example, Aruba's AP-124 and AP-125 Access Points are 3x3:2 devices. In contrast, Cisco's 1140 and 1250 series access points have a dual transmitter, triple receiver design and are 2x3:2 devices. If you're looking for the best performance, Aruba's 3x3:2 access points are your best bet.