The 802.11n technology offers great potential and enables high speed applications that may transform end-user mobility experience and change the way people work and communicate. It gives a big boost to the vision of a truly wireless enterprise. But to realize the full potential from the technology, one needs to be aware of various factors that affect performance and do careful deployment and network planning.

We discuss three areas that should be considered while evaluating the performance improvements promised by 802.11n.

Improvement in physical layer that delivers improved data rates

Let’s deconstruct the data rates in 802.11n. The following PHY and MAC enhancements help 802.11n realize data rates of upto 300 Mbps:

-         Improved OFDM

-         Channel bonding ( 40MHz)

-         Short Guard Interval (SGI)

-         MIMO technology using multiple spatial streams

802.11 a/g devices

Use of HT-OFDM

Effect of channel bonding

Effect of  using SGI

MIMO with 2 spatial streams

54 Mbps

65 Mbps

135 Mbps

150 Mbps

300 Mbps

Protocol Improvements that make use of the medium efficiently:

While 802.11n delivers maximum data rate of 300 Mbps, not all frames make use of it. The management frames use basic rates, so do the multicast and broadcast frames, which brings down the overall throughput. Besides, each frame that is transmitted incurs an overhead in the form of inter frame spacing, random backoff, acknowledgement and preamble. So, to get the most out of 802.11n, there should be an attempt to maximize use of the medium and high data rates. Frame aggregation (A-MSDU, A-MPDU) help reduce the protocol overhead by sending data packets back to back at high data rates.

Backward compatibility with legacy clients and deployment considerations

Since it is not practical to replace the entire infrastructure and client devices at once, there will be a period of overlap where the network will have a mix of 802.11n and legacy devices. Since the legacy devices cannot recognize 802.11n transmissions, 802.11n devices need to use protection mechanisms that will incur some overhead. Secondly, the legacy devices will use the legacy data rates, slowing down the entire network.

So, the network should be planned such that the legacy devices are kept separate by dedicating the 2.4 GHz band and/or some channels for legacy devices. The 5GHz band is reserved for 802.11n devices since there are lesser 802.11a devices, 5GHz band offers more 40 GHz channels (11), and there is lesser interference in this band.

 
We will discuss each of these in more detail in the attached doc.regds,sukhdeep