Cell phones are actually connected to multiple cell sites at the same time. A typical digital cell phone in a large city will see something like 5 cell sites; all on different channels with one being reserved solely for control of the local network. In the iDen system they use M16-QAM to actually modulate the radio signal and a bastardized version of TDDM is used to transfer the intelligence or data across the system. Your voice is actually severely compressed using a VSELP vocoder before it is modulated. A combination of cell phone logic and site logic actually determine where the phone is in a spatial environment. For instance an iDen readio is pretty damn smart. When you aren’t talking, it’s monitoring the signal quality of all it’s local cell sites. Since the system is handling literally hundred if not thousands of cell phones on a single controller base station; the phone does a major job in passing information back to the controller. For instance, when the phone notices that Cell A is better than Cell site C; it can send a request to be handed off to the Cell site with a better quality signal.

This is all monitored and adjusted by engineers who tabulate the call volumes, transient subscribers passing through the cell (cars) and number of long time subscribers (houses) as well as call drop rates (incomplete calls, busy cells etc.).

The control channel can even adjust maximum power outputs of each phone based on volume of a cell. In today’s environment cellular saturation is a problem because there are only so many frequencies. Often power output is reduced to re-use frequencies as needed on adjacent cells. For instance; on a heavily traveled piece of highway in a major city, you may pass through 15 cell sites in as little as 2.5 miles of road. They are hidden everywhere from light poles to schools to church steeples. Look for them, they could be little flat rectangles on the side of your local Wall Mart or Bank, aimed at specific pieces of land, not to mention the omni directional di-poles or Yagis hanging from Towers, but you rarely see those these days in high concentration areas. They are to broadband in their selectivity.

In fact some companies have begun to pioneer cellular switching into a 3rd dimension: altitude. Meaning not only do you switch cells in a horizontal measure, but also in a vertical measure. Meaning you can switch 2 or 3 times while riding an elevator or climbing a mountain, and be on the same controller site. The antennas are aimed at specific altitudes and designed to minimize RF reflection. Cool stuff.

It’s is true that a Sim card interprets the phone for retrieval of a call and the systems does broadcast the request over all frequencies, but once the connection is made, only an assigned channel carries the data and it’s usually assigned a slot on the T1 into a phone switch. The radio channels are actually multiplexed so a single frequency is actually carrying several phone calls. The level of multiplexing is actually determined by signal characteristics for a site. For example a site with bad problems might only carry 4 channels per frequency but a site in a good RF environment may carry 12 or 15 channels per frequency.

There is also another layer below the Sim card. The terminology varies among different types of systems but in the GSM model it is an equipment identifier. Like a MAC address it’s hard coded and that is the 1st level of authentication and a lot of communication happens at that layer. After that the Sim comes in to play, which is a storage device that contains a soft pin, a hard pin and the phone number, I guess this is sort of like a digital certificate.