zBoost Cell Extender: Refinement Post

This is a refinement, or follow-up, to my first post about cell signal boosting with zBoost:

While the base station allows decent communication over a specific area once a data or voice call is established, it seems to have little or no ability to propagate the signaling part of the GSM protocols when there is no connection established.

This symptom means that the network is unaware of a handset, and a handset thinks there is no service. As a result, it often will not even try to make a call.

Once the handset decides there's a network -- usually by picking up a very faint signal from a real tower -- it will attempt the call and then discovers a near-perfect connection.

zBoost may not be responsible for this behavior -- the product docs specifically say that you have to have some real signal level in order to use the repeater. I had imagined this restriction was solely due to the zBoost device needing a tower to talk to (it cannot bridge to another backhaul) -- but in fact it may also be due to zBoost's inability to simulate the idle signaling between the handset and the tower. That is, you need a tower to make or get a call, but the zBoost can boost the data in the call stream.

So what does this all mean?

Well basically that if your RF visibility to a tower is marginal, as mine is, zBoost may still be able to help. But you don't want to rely on this to reach 911 or make any other kind of need it right now phone call, as it might take you a few minutes of moving around or monkeying with the phone to get it to realize there's any service.

Likewise, if you absolutely positively cannot miss an incoming call, this may not be the solution for you, although, interestingly, incoming signaling (calls and SMS) seems to be stronger/prioritized traffic from the tower than idle updates, and so gets to the handset more frequently, without any kind of booster present.

Extending a Cell Signal With zBoost

Although I'm curious about the AIRAVE, I'm not on Sprint and not convinced it's worth switching just for the AIRAVE capability.

But I do have a signal problem at my house, and I'm currently testing out the zBoost Wi-Ex signal booster to see how much of a dent it can make.

Unlike AIRAVE, which creates a local CDMA network and bridges it to another backhaul, the Wi-Ex is an RF repeater/amplifier. So you need to feed it a signal -- from somewhere nearby that has a signal -- and it extends the signal via a new base station and antenna.

So far I'm inconclusive on the quantitative results. Certainly, in my house, the coverage area it creates is closer to 600 square feet than the 2500 square feet advertised. But as with any RF setup, there are so many variables that this doesn't prove a heck of a lot.

I can report positive results on a qualitative measure.

Normally, the signal is so weak inside my house that, without this unit, I can receive a call signal (e.g., ring or text message) but as soon as I press "talk" it drops. 100% of the time. Also, my Blackjack had never successfully retrieved a web page from inside the house. With the zBoost unit, there is  qualitative difference: The data connection can retrieve web pages -- sometimes slower, sometimes faster, but pretty much successfully. And if I receive a call, I can answer without the call dropping instantly. Again, depending on various things the call quality may be better or worse -- but there is always a call there, struggling to get through. Which despite being reminiscent of 1995 is a major improvement.

What I'm wondering is ... if, as zBoost claims, the device is FCC legit and is designed not to create trouble for the existing cell network, why don't the cell carriers themselves sell this device? It seems like it could help them extend their reach geographically and satisfy a few more customers.

And before you suggest that they don't offer it because doing so would be tantamount to admitting their existing network footprint isn't solid, note that both T-Mobile and Sprint are already selling personal femtocells and rumors are that AT&T isn't far behind. And anyway, spotty coverage inside buildings has never been any kind of secret.

Airave and Network Sniffing for Fun and Profit

Sprint's AIRAVE BYOB (bring-your-own-backhaul) femtocell went on sale nationwide this week. Reviews seem mildly positive; it kind of costs a bunch considering you're subsidizing Sprint's infrastructure, but it's marketed toward people who have little other choice.

I don't have one (I'm not on Sprint at the moment). But if I did, the second thing I'd do with it is to start sniffing the traffic it sends back up the wire.

But all that data would be encrypted, right? Well, maybe ... and maybe not. Since Excel files seem to be an enterprise data store of choice these days for companies handling sensitive personal information, and since telcos are not the science-fair winners of the class but more like that kid in the back that just giggles all day keeps getting his lollipop stuck in his own hair, it wouldn't surprise me if they think ADPCM is encryption.

Even if some or all of the call content is encrypted, though, one could probably learn a lot about the signaling layer of the system -- i.e., how the network identifies and talks to the phones to indicate presence or session initiation.

Why would this be useful?

Well, for one thing, it would be interesting to use the AIRAVE as a presence detection mechanism. Once I can determine that the base station has recognized a known cell phone ID, I can automate all sorts of things that are supposed to happen when I'm nearby.

Applications include home automation (open locks, turn off the alarm); media (access and/or license to all of the media I own "appears" on any connected device when I'm nearby with my cell phone, and goes away when I leave); or for commerce: my hotel reservation, preferences at a restaurant, or online search history for a retailer can cue up automagically as I enter range.