Can you hear me now?
Last week we were asked to help troubleshoot technical issues that a company (who shall remain nameless) was having with a mobile marketing campaign.
Without giving too much away, the plan is to use a location-based system to call people on their cell phones and direct them to a special marketing event in their area. I’m not doing it justice (because we didn’t do it), but it is actually pretty cool. Anyway, the problem was that it wasn’t working. According to the system identifying the cell phones, people should be at Point A, but were really at Point B, and everything just sort of fell off from there.
Fortunately, it only took a few quick questions to diagnose the problem – no, they aren’t using GPS; yes; they had good signal strength; no, it didn’t matter which carrier; no, it worked just fine on their campus – just not downtown; and no, it wasn’t everyone – just some people.
Mystery solved! A few adjustments to the algorithm that was being used to identify the target audience and everything was back up and running. I’m told that the marketing test was fairly successful and the program will be rolled out to more locations in the near future. So don’t be too alarmed if you get a call in the near future from someone that seems to know exactly where you are invites you to come to a cool location quite near where you are.
The problem wasn’t in the campaign, it was in the technology being used to support the campaign. Cellphones use radio frequencies with short wavelengths. When radio waves reflect off of a solid object, it creates peaks and valleys in signal strength. Throw a rock into a still pool and you see the ripples that emanate from the point of entry, but (if you threw a big enough rock) the waves that hit the side of the pool reflect back into the incoming ripples and create a criss-cross pattern. In the case of a cellphone, the distance between peaks and valleys can be as little as 75 centimeters. As cellphones transmit and receive on different frequencies, the valleys from speaking may not be in the same places as those for receiving, which is why we all have those “I can hear you . . . can you hear me?” conversations.
In an urban environment, your phone can talk to any one of several base stations (towers) each transmitting and receiving on multiple frequencies or channels. Each cell site has only a limited range – partly due to buildings blocking the signals, and partly to maximize the number of channels available by allowing nearby cell sites to sue the same frequencies. When it has channels available, a tower will transmit an “available” signal. When all its channels are busy, the signal disappears and that tower becomes effectively invisible to your phone, which then has to connect to the next tower. If that tower is busy too, then you bounce to one even further away that may have a weak, but usable, signal. This can even happen during a call. The network knows if your phone has al alternative tower within range. If your tower has become saturated with calls, it will free up some channels to handle those calls that only it can see, and the network will then hand you off to another site.
So, in this particular case, the system wasn’t accounting for people that had been automatically switched by the network. It spied them on a specific tower, which would indicate their location was “X”, but those people might actually be two towers away. All it took was an adjustment to note if the cell had been switched by the network and a quick analysis of relative signal strength from the cellphone itself, and it was back to business. Stuff like this seems cool and scary all at the same time.