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802.11n Evolution or Revolution?

Evolution and revolution closely resemble each other, although their true definitions could not be more antagonistic of one another. Where evolution is a gradual process, revolution implies changes more or less sudden in their action.

What does this have to do with 11n? 11n was by far the biggest change within the IEEE 802.11 wireless networking standards. However, was this a revolution in wireless or just another incremental, evolutionary step for 802.11?

Before you decide how you want to define 802.11n, let’s take a step-by-step analysis of the five major capabilities attached to 11n: MIMO, Channel Bonding, Aggregation, Short Guard Interval, and Beam Forming.

MIMO: Multiple Input – Multiple Output

With 802.11a/g/b, most access points have two antennas and one data stream or bitstream. This bitstream is sent through both transmit antennas and received by both receiving antennas. Having this single bitstream traverse through two antennas enables diversity. Diversity helps to eliminate the effects of multipath, which can reduce the signal strength of the transmitted data.

With the introduction of 11n, multiple input, multiple output (MIMO) came into play. Now each transmit and receiving antenna (one set) have their own unique data stream, and up to four data streams can be sent simultaneously, increasing the throughput of your data fourfold. Multipath still exists, but instead of attempting to reduce it with two antennas, the time and phase delay which multipath introduces are used as an advantage with multiple streaming.

With 11n, four unique data streams are the max. However, there are very few commercially available devices that provide this level of performance. Most devices on the market today employ either two or three data streams.

Channel Bonding:

Channel bonding combines existing channels in either the 2.4 or 5GHz band to double the RF bandwidth available to send signals, from 20MHz to 40MHz. Results? This capability doubles the data rate for wireless users. This capability has been available in most 11n equipment shipped since the draft 11n standard, and it only requires a simple configuration change to enable the feature.

While you do have a higher data rate from channel bonding, it can also be obtrusive in nature. First, channel bonding reduces the channels available for legacy 802.11a/b/g equipment. So, if you are working in a mixed environment that is not exclusively 802.11n, channel bonding is not the best route to take. Secondly, channel bonding can interrupt and cause interference with your neighbor’s wireless transmissions. For example, if the company next door has tuned their closest AP to you to channel 11, and you decide to use channel bonding with channel 6 and a higher channel, you will most certainty cause interference for your neighbor on channel 11.

Aggregation and Block Acknowledgements:

Aggregation groups data together for more efficient and timely transport of packets. It is only effective, however, when both the AP and the client are compatible.

There are two different types of aggregation A-MSDU and A-MPDU. They both accomplish a similar goal, but they do it from a different perspective depending on the size of the data packets sent over your network. Understanding the difference between these two aggregation types typically only comes into play when performing detailed network analysis, but briefly, A-MSDU uses a single packet to aggregate multiple data units inside itself and A-MPDU aggregates multiple IP packets in a row without requiring acknowledgments or typical packet spacing between the packets, but requiring an overall block acknowledgement at the end of the aggregated packet stream. While some APs may give you the ability to enable or disable these capabilities, most decide for themselves when it’s appropriate to use aggregation and which type of aggregation (A-MPDU or A-MSDU) to use.

Short Guard Interval (SGI):

Short Guard Interval is a reduction in the amount of “dead” time between RF communications. With 802.11a/b/g the time is 800 nanoseconds, but with 802.11n it is reduced to 400 nanoseconds. This may seem like a relatively small change, but there are a lot of short guard intervals to account for, so this simple reduction creates an 11% performance improvement.

Like aggregation, this is something you may or may not have the ability to enable, and it’s the access point’s decision to use or not use a short guard interval.

Beam Forming:

Beam forming allows APs to determine the approximate location of a wireless client, and then actively tune the antennas to increase signal strength in the direction of the client. This is highly complex technology, and is optional for any 11n equipment. Even if your access points can handle this, we suggest you stay away, as it is not a feature that is ready for primetime.

Evolution or revolution, however you want to define 802.11n, there is no doubt that these standards have brought a slew of new capabilities and possibilities to wireless. Check out the case studies of Purdue University and Liberty University in the video below to see the full potential of 802.11n in real world deployments.

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