Archive for the ‘802.11n’ Tag

How Stuff Works – 802.11n and Short Guard Interval

This is a post that I wrote the other day as a “guest post” for a co-worker’s blog.   It is a Xirrus sponsored blog, titled “Geekster”.  The URL for the blog is The guest post was part of a series, “How Stuff Works”, which has been one of the most successful portions of my own blog, so I am going to re-post for my faithful readers.  🙂


In the first several “How Stuff Works” posts, I have been talking about technical improvements to 802.11n such as MIMO antennas, Spatial Multiplexing, and Channel Bonding.  In this post, I want to talk about another such technical improvement, Short Guard Interval.

The guard interval is the space between symbols (characters) being transmitted.  This is often confused with the space between packets, which is the interframe space (IFS).  The guard interval is there to eliminate intersymbol interference, which is referred to as ISI.  ISI happens when echos or reflections from one symbol interfere with another.  Adding time between symbol transmission allows these echos and reflections to settle in before the next symbol is transmitted.  In normal 802.11 operation, the guard interval is 800 ns.

With 802.11n, short guard intervals are possible.   The short guard interval time is 400ns, or half of what it used to be.  Shorter wait time (guard interval) between symbols increases throughput.  However, if it’s too short, the amount of ISI will increase, and throughput will decrease.  On the other hand, if the guard interval is too long, there is increased overhead due to the additional idle time.    If you look at an 802.11 Modulation and Coding Scheme (MCS) chart, you will see that Short Guard Interval increases the data rate by roughly 10-11%.

Check out my blog at for other “How Stuff Works” postings as well as other information and opinions on wireless networking and security!

How Stuff Works – 802.11n and Spatial Multiplexing

This is the third post in my “How Stuff Works” series.  The first two posting discussed MIMO and channel bonding.  This post looks at another technical improvement that leads to greater speed in 802.11n networks – spatial multiplexing.

It is helpful to take a quick look at a classic 802.11 transmitter.

802.11 Classic Transmitter

802.11 Classic Transmitter

In this scenario, only one data stream is sent from the transmitter to the receiver (represented by the orange line).

With spatial multiplexing, multiple data streams are transmitted at the same time.  They are transmitted on the same channel, but by different antenna.  They are recombined at the receiver using MIMO signal processing.  This is represented in the diagram above with two spatial streams – an orange colored one and a navy blue colored one.

Spatial Multiplexing - Two Streams

Spatial Multiplexing - Two Streams

Spatial multiplexing doubles, triples, or quadruples the data rate depending on the number of transmit antennas.   Remember, you may hear three numbers when referring to 802.11n or MIMO networks – the first is the number of transmit antenna, the second is the number of receive antenna, and the third is the number of spatial streams.   For example, a 3×3x2 system has two spatial streams.

How Stuff Works – 802.11n and Channel Bonding

We already discussed how MIMO works.  Let’s look at another technical improvement currently utilized in 802.11n – channel bonding:


The graphic is fairly self explanatory – traditional 802.11 channels are either 20 MHz wide (OFDM) or 22 MHz wide (DSSS).  Channel bonding combines two adjacent channels, which effectively doubles the amount of available bandwidth.

One footnote to channel bonding is that it works best in the 5GHz frequency band, as there is only space for three traditional, non-overlapping channels in the 2.4GHz frequency band.   Therefore, there is only enough space for one bonded channel in that portion of the RF spectrum.

Gigabit Wireless Expected in 2009?!?

While we are on the topic of 802.11n and speed, I thought this was an applicable story that appeared on Network World’s website earlier in the week.

It talks about Quantenna Communications plans to release a 4×4 MIMO chip set in Q3 of 2009 that is capable of 1 Gbps wireless bandwidth (600 Mbps of throughput).

With 100+ Mbps throughput currently provided by 802.11n, most desktop applications have more than enough bandwidth.   Gigabit Wireless would certainly eliminate speed as one of the obstacles to replacing Ethernet with wireless.

How Stuff Works – 802.11n MIMO

If you have been following my blog, you know that I have a poll regarding the greatest challenges to 802.11n deployment in the enterprise.  (Here’s your chance to rock the vote!)

One challenge for replacing desktop Ethernet with wireless is speed.  Perhaps the widest publicized enhancement to 802.11n is that of MIMO (“my-moe”) antennas, which stands for “Multiple Input, Multiple Output”.  How does MIMO work?

To answer that question, let’s look at how a classic 802.11 wireless transmitter operates:

802.11 Classic Transmitter

802.11 Classic Transmitter

In this case, the signal is sent out of one antenna.  The signal is received by both antennas at the other end, but only one signal is processed and sent up to the MAC layer.   Antenna diversity helps in the fact that the best signal is the one that is processed, but remember that it is still a single antenna that processes the receives and processes the RF energy.

Let’s compare that to a MIMO antenna structure:

MIMO Signal Processing

MIMO Signal Processing

In this case, we have three transmit antennas and three receive antennas (often referred to as 3×3 MIMO).  The black, green, and red lines above each represent their own signal.   With MIMO all three signals are received and processed up the stack.   This significantly improves the receiver’s “ability to hear” and it represented in the graph above by the orange line.

You may hear different implementations of MIMO such as 2×3 and 3×3.  The first number is the number of transmit antennas and the second number is the number of receive antennas.   If you hear 3x3x2, the last number refers to the number of spatial streams, which I will discuss in another post.

802.11n Challenge – Speed

First, vote on the greatest deployment challenge for 802.11n in the Enterprise (if you haven’t done so already).

Let’s take a closer look at one of these challenges – speed.

There are many technical improvements in 802.11n that make it significantly faster than 802.11 a/g.   In no particular order, here are some of the improvements:

  • MIMO (Multiple In, Multiple Out) Antennas
  • Spatial Multiplexing
  • Channel Bonding
  • Short Guard Interval
  • Frame Aggregation
  • Block Acknowledgments

It is my intent to have a blog post “tutorial” on each of these improvements, so check back shortly!