In our last article, we discussed the basics of cell phone technology. In this article, we’ll get into the details of cell phone frequencies, channels, and digital systems technologies.
Did you know that a single cell in an analog cell phone system uses one-seventh of its available duplex voice channels? Each cell of seven on a hexagonal grid uses one-seventh of its available channels, so it has a plenty of unique sets of frequencies to avoid collisions.
• Cell phone carriers get around 832 radio frequencies per city
• Each cell phone uses two frequencies per call. So there are usually 395 voice channels per carrier. The other 42 frequencies get used as control channels.
This means that each cell has around 56 voice channels available. In any cell, 56 people can talk on their cell phones at one time. These analog cellular systems are first generation mobile technology [1G]. Digital transmission
methods (2G) increases the available channels. For example, a TDMA-based digital system can typically carry three times as many calls as its analog sister technology. This means each cell has about 168 channels available.
Cell phones contain low power transmitters. These cell phones have two signal strengths: 3 watts and 0.6 watts. The base station transmits at low power and this creates two advantages. The base station transmissions and the phones
within its cell don’t make it very far outside the cell. Therefore, cells can reuse the same 56 frequencies all across a city. Power consumption is low. Cell phone power consumption comes from battery power. Battery power is low and batteries are small. This is what made handheld cell phones possible.
Using cellular technology requires a huge number of base stations in any size city. So, a normal city can have hundreds of base towers. However, since there are so many people using cell phones the costs are low per user. Each city runs one central office called the Mobile Telephone Switching Office [MTSO]. The MTSO handles the phone connections to the land-based phone systems, and controls the base stations in the region.
Cell phones have special codes assigned with them. The codes identify the phone, the phone’s owner, and the service provider.
Here’s what happens when you turn on your cell phone and someone attempts to call you. [This is what happens to the phone call]:
1) When the phone powers up it listens for an SID on the control channel. The control channel is a special frequency that the phone and base station use to communicate.
2) When the phone receives the SID the phone compares it to the SID programmed in the phone. If they match, the phone knows the cell it is communicating with its home system.
3) The phone transmits a registration request and MTSO keeps track of your phone’s location in a database.
4) The MTSO receives the call and attempts to find you. It looks to see what cell you are in.
5) The MTSO picks up a frequency pair for you to take the call.
6) MTSO communicates with your phone over the control channel to communicate which frequencies to use. Once the phone and tower turn on those frequencies the call connects.
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