SKYPHONE

Never wrote about this project in which I was involved very early on in my career when I worked for British Telecom for a bit. I thought about posting it as it was a very interesting experimental project.

So the idea was — what if you are on a plane and still receive mobile phone calls on your seat for any calls made to your mobile. Bear in mind this was in 2000 and we were still using the Nokia handsets.

Background — GSM Technology

So assuming you are not familiar with the inner workings of GSM technology I will just explain a few concepts at a high level.

There is something called a HLR (Home Location Register) and VLR (Visitor Location Register). The HLR is where your mobileID is registered. As a mobile phone moves from one cell to another, it registers on the VLR which in turn updates the HLR of where the mobile is located. So as the mobile moves from one VLR to another, the HLR is kept informed so it can route incoming calls to the user.

Note: What is interesting is how the GSM handles a mobile as it moves from one cell to another and how it cleverly chooses the best path based on the signal strength. Also, the mobile phone itself is clever in adapting to the distance from the tower by boosting it’s power.

Background — Cell Diverts

In the old days there was something called Cell Divert. One of them was when user is not present. I forget the actual name of the divert but the idea is if the mobile is not registered in any VLR, it would route the call to the given divert number.

One important point here is that the mobile keeps sending a keep alive signal every 15 minutes to the network. If a signal is not received, the network deregisters the mobile and subsequent calls cannot be routed to the mobile.

System

Process— when a user gets on a plane, they scan a card which is associated with their phone number. The system registers a generated phone number (explained below) as a “user not present” divert for the User’s mobile. The number is basically something that maps to that seat. So all incoming calls would be routed to that number. A comms system is put in place to route the call to the seat. So any incoming call would make it’s way through the network to the diverted number which in turn is routed to your seat through the comms setup. Pretty cool!

The dynamic number is generated using the following.

• Zone number — earth was divided into 4 zones so depending on which zone the aircraft was in. This is for routing via the correct satellite.
• Plane ID
• Seat number

As explained above, the dynamic phone number is generated based on the zone, plane id and seat.

A background comms is put in place to route the call to the seat on the aircraft flying in a specific zone (one of four where the aircraft may be present). The only issue now is how do you ensure that the temporary divert number is registered to the special cell so the routing can be made to the aircraft.

So for this, a software was written ( by me :) ) which would register this dynamic number into the pseudo VLR. This would ensure that the number was kept alive and prevents these numbers from getting deregistered from the network.

When a phone call would come in it would find that the user’s mobile is not registered to any VLR. It will look for any set diverts for when “user not available”. It would try and then forward to the set number (dynamic number) which will then make it’s way to the seat on the aircraft with some clever routing through the correct satellite to the correct plane and eventually to the correct seat.

There was also a switch installed on the plane tyre mechanism. This is so as the plane lands, a message is sent to the server and all dynamic numbers associated with this plane id will deregister and the diverts would be reset. The user’s phone will register onto a network VLR when the phone switches ON and that’s it.

So while this was an interesting concept to test, shortly after this Virgin Airlines had put a cell on board and created a link back to the ground GSM network. Since you are in the plane and the mobile can register to a cell in very close proximity, the power levels on the mobile would be minimal and not cause any interference with plane comms to the ground. So, there was no need for all the complicated setup anymore.

I may have made mistakes in some of the detail as it was a very long time ago but conceptually still it was an interesting one to document.