Everyone has a cell phone these days. From the multifunction, multiple application iPhone to simple one-touch emergency cell phones, the variety and selections can be overwhelming. You can synchronize your contacts with your computer, take video clips or still images and even use the bar code on products and check pricing all from your neat little cell phone. Still, with all these amazing features and selections, have you ever wondered how a cell phone works?
Cell Phone Technology
Decades ago, in the days of "Smokey and the Bandit", CB radios were the rage. Not only were truckers using them extensively, many of us had them in their everyday vehicles. They became a quick way to get in contact with friends and the perfect device for emergency situations. Cell phones work in a very similar way. The CB radio is a "half duplex" device. A person on one end can speak, but then must way for the person on the other end to reply. Both people are using the same radio frequency. Conversely, the radio used within cell phones are "full duplex". One frequency is used to talk, while another frequency is used to listen. In this way, there is no delay and no waiting.
Additionally, most CB radios come from the manufacturer with about 40 radio channels, as opposed to walkie-talkies that operate on a single channel. Cell phones, on the other hand, can handle over 1,000 channels! Walkie talkies usually can hold their signal fairly well up to one mile, depending on terrain and location. CB radios carry a larger transmitter and usually work well up to about 5 miles. With the cell-to-cell technology behind cellular networks, cell phones range is nearly limitless.
Each cell phone company or "carrier" is provided with about 800 frequencies to use across most cities. These frequencies can be used and reused within each cell. You can think of the cells in each city as a hexagonal grid. As you travel from one cell to another, the frequencies connect and reconnect to give you continual and usually uninterrupted use.
Mobile Telephone Switching Office (MTSO)
Also, each cell within the cities requires a cell tower and base station to operation. The larger cities can have hundreds of towers and base stations, all connected and reporting back to a main switching office. In addition to communicating with all the base stations in the direct area, the main switching office also handles the mobile-to-landline connections.
So, how does a cell phone know where or who you are? Each cell phone has a unique ESN or Electronic Serial Number. This number is, of course, different for every phone out there and created during the manufacturing process. It can not be changed. Working in conjunction with the ESN is the SID or System Identification Code. The SID is a five digit code assigned to each cell phone carrier by the FCC and programmed into your phone prior to purchase. When you power up your cell phone, the SID is immediately checked against the listing at the nearest base station.
When you being your phone call, two frequencies are then assigned to your phone for talking and listening. As you begin to reach the perimeter of that cell, the base station contacts the next base station in the adjoining cell and hands off the call. If you happen to reach an outlying area not covered by your cell carrier, it can be transparently handed off to another service provider.