CELLULAR PHONES Each day something like 30,000 people in the United States sign up for and start using a cellular phone. With a cell phone you can talk to anyone on the planet from just about anywhere (80% of the U.S. has coverage). A cell phone is really an extremely sophisticated radio. A cell phone is a duplex device which uses one frequency for talking and a second, separate frequency, for listening. A cell phone can communicate on 1,664 channels and operate within cells. They can switch cells as they move around. Cells give cell phones incredible range. Someone using a cell phone can drive clear across a city and maintain a conversation the entire time. The way a cellular phone does this is the carrier chops up an area (such as a city) into cells. Each cell is typically sized at about 10 square miles (perhaps 3 miles by 3 miles). Cells are normally thought of as hexagons on a big hexagonal grid. As you move toward the edge of your cell, your cell’s base station will note that your signal strength is diminishing. Meantime, the base station in the cell you are moving toward, which is listening and measuring signal strength on all frequencies, will be able to see your phone’s signal strength increasing. The two base stations coordinate themselves through the MTSO, and at some point your phone gets a signal on a control channel telling it to change frequencies. This handoff switches your phone to the new cell. Roaming makes things a bit more interesting. In modern systems, the phones listen for a System ID (SID) on the control channel at power-up. If the SID on the control channel does not match the SID programmed into the phone, then the phone knows it is “roaming”. The phone also transmits a registration request and the network keeps track of your phone’s location in a database. This way the MTSO knows which cell you are in when it wants to ring your phone. As you move between cells, the phone detects cha…
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