A Wireless Brick In The Wall


I had a very interesting conversation today with some friends about predictive wireless surveys. The question was really more of a confirmation: Do you need to draw your walls in the survey plan when deciding where to put your access points? Now, before you all run screaming to the comments to remind me that “YES YOU DO!!!”, there were some other interesting things that were offered that I wanted to expound upon here.

Don’t Trust, Verify

One of the most important parts of the wall question is material. Rather than just assuming that every wall in the building is made from gypsum or from wood, you need to actually go to the site or have someone go and tell you what the building material is made from. Don’t guess about the construction material.

Why? Because not everyone uses the same framing for buildings. Wood beams may be popular in one type of building, but steel reinforcement is used in other kinds. And you don’t want to base your predictive survey on one only to find out it’s the other.

Likewise, you need to make sure that the wall itself is actually made of what you think it is. Find out what kind of sheetrock they used. Make sure it’s not actually something like stucco plastered over chicken wire. Chicken wire as the structure of a plaster wall is a guaranteed Faraday Cage.

Another fun thing to run across is old buildings. One site survey I did for a wireless bid involved making sure that a couple of buildings on the outer campus were covered as well. When I asked about the buildings and when they were made, I found out they had been built in the 1950s and were constructed like bomb shelters. Thick concrete walls everywhere. Reinforcement all throughout. Once I learned this, the number of APs went up and the client had to get an explanation of why all the previous efforts to cover the buildings with antennas hadn’t worked out so well.

X-Ray Vision

Speaking of which, you also need to make sure to verify the structures underneath the walls. Not just the reinforcement. But the services behind the walls. For example, water pipes go everywhere in a building. They tend to be concentrated in certain areas but they can run the entire length of a floor or across many floors in a high rise.

Why are water pipes bad for wireless? Well, it turns out that the resonant frequency of water is the same as 802.11b/g/n – 2.4GHz. It’s how microwaves operate. And water loves to absorb radiation in that spectral range. Which means water pipes love to absorb wireless signals. So you need to know where they are in the building.

Architectural diagrams are a great way to find out these little details. Don’t just assume that walking through a building and staring at a wall is going to give you every bit of info you need. You need to research plans, blueprints, and diagrams about things. You need to understand how these things are laid out in order to know where to locate access points and how to correct predictive surveys when they do something unexpected.

Lastly, don’t forget to take into account the movement and placement of things. We often wish we could get involved in a predictive survey at the beginning of the project. A greenfield building is a great time to figure out the best place to put APs so we don’t have to go crawling over bookcases. However, you shouldn’t discount the chaos that can occur when an office is furnished or when people start moving things around. Things like plants don’t matter as much as when someone moves the kitchen microwave across the room or decides to install a new microphone system in the conference room without telling anyone.


Tom’s Take

Wireless engineers usually find out when the take the job that it involves being part radio engineer, part networking engineer, part artist, and part construction general contractor. You need to know a little bit about how buildings are made in order to make the invisible network operate optimally. Sure, traditional networking guys have it easy. They can just avoid running cables by florescent lights or interference sources and be good. But wireless engineers need to know if the very material of the wall is going to cause problems for them.

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