Friday, December 21, 2012

Mobiles on a Plane


Why is the use of mobiles banned in planes? There are technical reasons, but what we are fed routinely -- that it might interfere with flight equipment on board -- is probably not all that important. I found this bit really interesting:

The truth is that the FCC never was concerned about the possibility of electronic interference when, in 1991, it banned the use of mobile phones on board aircraft. All it was really worried about was their impact on cellular networks on the ground. These work on the principle that, at any given moment, a mobile phone is within range of only one or two nearby masts. Each mast uses a set of channels different from those allocated to the masts closest to it, but the same as others further away. In this way, each channel can be used, and reused, to carry calls from multiple users.

Unfortunately, a mobile phone operating in an aircraft flying overhead might be within reach of any number of masts using the same channels. This could not only cause calls to be dropped, but would also confuse the network's software—reducing the mobile system’s overall capacity by blocking the reuse of channels.

There is also the added problem of an airborne phone moving too fast across the sky for the ground-based network to respond. The highest speed a mobile network is expected to cope with is that of an express train—not a passenger jet travelling at just below the speed of sound. A mobile used on an aircraft could traverse a tower too quickly to register with the network. If that happened, it would then bombard multiple towers along its route with repeated attempts to register, causing yet further network confusion.

1 Comments:

  1. inthearmchair said...

    I'm struggling to understand these two statements:

    "at any given moment, a mobile phone is within range of only one or two nearby masts...

    Unfortunately, a mobile phone operating in an aircraft flying overhead might be within reach of any number of masts using the same channels."

    With spherical ranges for the masts, I can't think of a plausible configuration of even two masts so that a cell phone anywhere on the ground is within the reach of only one mast but at higher altitudes can reach multiple masts. Perhaps the range of a mast increases with the altitude of the cell phone, for some reason.