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Frequency Hopping


Direct Sequence

First lets start out by saying that each technology has its strengths and weaknesses. It depends on your needs.

A quick overview of each technology.

Frequency Hopping (FHSS): derived from military radio technology where it was designed to be inherently secure and reliable under adverse battle conditions. Divides the available 83.5 MHz spectrum (in most countries) into 79 1 MHz channels, the Radio then hops around these 79 channels in a 'pseudo-random' sequence and using all 79 frequencies every 30 seconds.

Direct Sequence (DSSS): designed by two vendors to increase the available speed on the wireless network. Divides the available 83.5 MHz spectrum (in most countries) into 3 wide-band 20 MHz channels. Uses an 11 bit spreading code to reduce the possible interference on signals in each wide-band channel.

We wanted the most cost-effective, reliable wireless communication we could build or find.

We'll accept - reluctantly - a customer can suffer performance problems from time to time to due adjacent wireless and other radiating installations BUT NEVER a shut-out.

Reliable wireless performance is a combination of:

Transmission Power
Receiver sensitivity
Antenna design and diversity
Alternate routing
Noise (competing installations)

We control the amount of radio power we can cost-effective supply (in our case a very high 200mW). Our radios are very sensitive (-89 dB). We can and do supply very cost-effective antenna's. We can advise customers on the range and antennas to be used. We provide alternate radio access (mesh network technology).

We cannot control the 'noise' from competing installations and other 'noise' sources. For outdoor operations we believe 'noise' is the key element in the equation.

As wireless systems becomes more popular (and that is inevitable) we must start from the following worst case assumptions:

It's very noisy out there
It's going to get worse

Note: Pretending the problem disappears at 5.8 GHz is lunacy. The relative lack of 'noise' in this spectrum is at best a transient and temporary phenomena.

So let's play out the nightmare scenario with the two technologies.

Let's suppose our neighbour turns up a powerful (full 1 watt EIRP) DSSS radio within 100 meters. If you had been using a DS radio quite happily on the same channel you would now be shut out - 100%. Zero throughput. The FH radio would lose about 28% throughput (not good but not the end-of-the-world). So you reconfigure your DS radio to use another channel and you walk into another DS radio blasting away. Dead again. Zero throughput. Our FH radio is still operating (yes, we've lost 55% throughput) and we haven't done anything yet. So our DS radio is now moved to the 3rd channel and ... you've guessed we get hit again. Zero throughput.

The only solution for a DS system - Move location!

Our FH radio is now down to 25% throughput but still running. And we still haven't done anything to improve the situation e.g. directional antennas, alternate access (mesh networking) etc..

So our choice of radio technology boils down to this. If you can control absolutely all the interference and noise between your stations - use Direct Sequence technology (in-building, airports etc). If you can't - use Frequency Hopping.

But, what about the loss of performance you will say. DS is typically quoted as 11 Mbps (actually its not) whereas FH is only around 1 - 2Mbps. Guess what, 11 Mbps x 0 = zero Mbps.

In the real world of outdoor ISP access and network solutions Frequency Hopping is practical.

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Last modified: July 11 2011.

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