Last week I talked about the importance of doing site surveys. You may be saying that it makes sense given all the possible obstacles, but I’m installing a wireless bridge outside and I don’t need one.
Not so fast. While granted a short point to point connection between two buildings may be easy, there are still plenty of things outside to cause you to stumble.
Wireless bridging is a great alternative to wired site connectivity that saves, installation time, cost, and increased bandwidth and by knowing what to look for, can be a very reliable solution.
1. Having an UNOBSTRUCTED line of sight between the 2 locations.
This is the first thing I mention to someone who is looking at wireless bridging. Notice the capital letters above.
Many people think that if they can see part of the antenna that’s good enough. That’s not the case.
As the wireless signal travels it creates what is known as a “Fresnal zone”. This is an elliptical field that radiates out from the center on the signal.
courtesy of www.vias.org
This field must be at least 60% clear of obstructions for the link to function properly. This is also known as “radio line of site”
There are several online fresnal zone calculators online, just use your favorite search engine
2. Keep an eye on the future.
Shooting a signal across a field may seem like good idea at the time, but things tend to grow. During the early days of 2.4 Ghz bridging it wasn’t usual to get calls in the spring with the customer saying “it worked fine over the last couple of years”. The problem was the small trees they were shooting over were now grown and it full bloom. Water filled entities absorb the wireless signal taking the link down.
When shooting across a street, make sure your antennas are high enough. Having an eighteen wheeler park in the middle of your signal path isn’t a good thing.
3. Know your antennas
Antenna’s come in 2 basic types.
Directional antennas have a beam that is formed and concentrated in one direction with some energy being sent out the sides and rear. The smaller the angle the more concentrated the beam.
An omni-directional and sends a signal out in all direction. Think about a donut with a stick in the middle and you’ll get a basic idea.
Antennas are rated in “dbi”. The higher the number the more concentrated the beam. It doesn’t really amplify the power. It’s like a flashlight with an adjustable lens. The tighter the beam, the brighter the light even though is the same amount of energy coming from the light.
The dbi rating on the antenna doesn’t just rate how it transmits but also how it receives.
An antenna will not only concentrate the wanted signals it receives, but interfering signals as well.
This why you would use an omni-directional antenna as the center of a point to multipoint implementation, but directional antennas for point to point applications.
4. Understanding EIRP and the power of 3db
The power of the radio is rated in “milliwatt” or “dBm”. This is the starting point but there are a couple of things that can affect the final output. This is known as EIRP (Effective Isotropic Radiated Power) and is calculated using the formula:
EIRP = transmit strength (dBm) + antenna gain (dBi) – cable loss (dB)
So if my radio is capable of 30dBm and my antenna is rated at 6dBi my EIRP is -36dBm.
36dBm = 3981.072mw
3dB is a magic number in these calculations. Add 3db and your EIRP doubles, if 3dB of loss is added, the EIRP is halved.
If we take the example above and add 50ft on cable that adds -3dB loss, our EIRP is now 33dBm.
33dBm = 1995.262mw
So you can see that if you are close the end of your power budget (next week’s topic), adding loss into the system can stop you before you even get started.
The dB to milliwatt conversion is beyond this article, but many online wireless calculators exist and I recommend using them.
5. Can you hear me now – Receiver sensitivity
Receiver sensitivity is how well the radio can hear. Receiver sensitivity is ALWAYS expressed as a negative dBm (- dBm) and is the lowest power of signal your radio can handle
One thing to also keep in mind about signal strength. The lower the number, higher the signal, so -40 dBm is a stronger signal then -80dBm
The ultimate goal is to have the sending signal be received at a level higher that the receiver sensitivity.
Once again there are many online wireless calculators that can help you find all the variables involved and give you an idea if your design is feasible.
6. Location, Location, Location
I already touched on this a couple of times, but it’s very important to make sure the locations of the radios/antennas will work before the installation.
I was once on a site where the customer already had decided where they wanted the radios to be mounted. After digging deeper I found that neither radio was within the 328 ft (100m) distance that Ethernet is allowed. Both radios needed be relocated. Fortunately we caught it before everything was mounted to the building.
Sharing tower/building space can be another issue. Depending on the equipment involved, your signal could be interfered with or wiped out totally.
It’s also a good idea to look into what kind of signals are in the area. The 2.4ghz band is heavily utilized and if you are in a city or multi-tenant building, this could interfere with your signal. 5Ghz is good alternative.
While this just scratches the surface, hopefully it will help point out a few things to look for when installing a wireless bridge link.