Gifts and Gadgets

Did you know that cell phones are basically radios?

The carrier divides the city into hexagonal cells, so that each cell has 6 sides. Each cell has a base station consisting of a tower and a small building containing the radio equipment. Since each cell is typically hexagonal, the carrier allows each tower to use only one/seventh of it's alloted frequencies. That way no tower will overlap another's frequencies.

Cell phone carriers typically get about 832 frequencies to use across a city. Since a cell phone is a full-duplex device (one frequency for outbound and one for inbound), there are 395 voice channels per carrier (832/2 = 416. The remaining channels are used for control purposes). So, if we split those channels into seven parts, we get 56. That means only 56 users can be on any analog cell phone at any one time!

If you have a digital service, you can get about 3X as many phones running in each cell. Which is why cities need hundreds of towers, while suburbs can get away with fewer.

Note that sometimes you'll get a busy signal on a cell phone even though the person you are trying to call is not on the phone. It's just the tower's way of saying it doesn't have any available frequencies for you to use (either for you or for the person you are trying to call).

When you move around in the cell, the tower notes that your signal strength is diminishing, while at the same time the tower in a new cell is noticing how your strength is increasing. The two towers communicate through a central switching location which makes the decision to hand you off. Your phone will get a signal to switch frequencies so that the new tower can take over responsibility.