 | What is a cellphone? |
Technical Answer:
At the most basic level, a cell phone is a radio. The special features that make it as useful as a landline are the full-duplex transmission-receiving method, and the cellular division of land. The first feature, full-duplex, means that you can talk and hear at the same time. One radio frequency for receiving, another for transmitting. That would mean that for the million people using a cellphone during the busiest times of day in some cities, that two million frequencies would be needed. But this is not the case. The cellular division of land solves this problem and another. As the city is divided into cells, each cell can use all the available frequencies without affecting the other cells ability to use the frequencies. That means, that someone on 8th Street can use the same frequency as someone on 101st Street. But that is not the only advantage. Because each cell is small, no more than a kilometer and a half (one mile), the transmitter in each phone can be less powerful. That saves on battery life, and makes cellphones smaller. In addition, there are those that believe that these devices create less radiation than traditional high-powered transmitters. Cell switching technology allows a user to pass from one cell to the next, and even to change frequencies, without even knowing it. You could drive from one end of the city to the other end and not know how many times that you switched cells.
Cells are typically arranged in a hexagonal pattern. Because adjacent cells cannot use the same frequency, each cell has access to one seventh of the available frequencies for a particular carrier. In the United States, most carriers are assigned 832 frequencies. divide that by the seven cells, and then again by two because of the full duplex transmission, and we come to the conclusion that 59 people can talk at the same time in any given cell. Actually, there are usually only 56 channels available for voice, as some of the channels are control channels. At least, that was the case with first generation, or analog, phones. The digital second generation phones could transmit up to three phones per channel- so we got 168 people talking in each cell at the same time.
Great, you ask, so how does the network know who's who? Remember that control channel that I just mentioned? When you power on your phone, it starts listening for a SID code (System Identification Code) on one of those control channels. If it can't find a control channel, you get the out-of-range message. If it does find a control channel, the phone compares the SID it receives to the SID that was programmed into it. If they match, your phone knows that it is on it's home network (if they don't, then you're roaming). At this point the phone sends a registration request to the base station. The MTSO (Mobile Telephone Switching Office) remembers in what cell your phone is sitting, so that you can receive calls.
But what happens when you travel form one cell to another? Each cell measures your signal strength not only on the frequencies that it serves, but on all frequencies. When you move away from a tower it sees your signal getting weaker, and another tower sees your signal getting stronger. The MTSO at some point decides that your are better served by the new tower, and on one of the control channels the instruction to use a new frequency is commanded to the telephone. This all happens without the user even noticing. You can even switch providers and go into roaming this way.
2G phones, or second generation phones, use digital technology to improve voice quality and save bandwidth. This means that more phones can use the same frequency band. 3G phones progress in the opposite direction: instead of using less bandwidth than their predecessors, they offer increased functionality. Standard in 3G phones will be a camera capable of capturing video at VGA resolution and a screen (usually color) that can display the image from the camera. The phones are capable of handling video-calls with this equipment and the underlying software and supporting networks. 3G phones are backwards-compatible with all GSM services.
Short Answer:
A cellphone is a portable telephone, not to be con...
Long Answer:
A cellphone is a portable telephone that provides voice calls and other functions. Indispensable ...