A wireless phone that receives signals from cell phone towers. A cell is usually an area (several miles) around the base station from which it can receive a signal.
Cell phones have many amazing functions. Depending on your phone model, you can:
• Store contact information
• Create a task or to-do list
• Track appointments and set reminders
• Use the integrated calculator for simple calculations
• Send and receive e-mail
• Get information from the Internet (news, entertainment, stock prices)
• Play game
• Watch tv
• Send text message
• Integrate with other devices like pdas, MP3 players, and GPS receivers
Cell phones are full-duplex devices. This means they use one frequency to talk and a different frequency to listen. Both participants in the call can talk at the same time. Dividing a city into smaller cells allows spectrum to be reused widely throughout the city, allowing millions of people to use their cell phones at the same time. Cell phones work inside cells, and cells can change as you move along. The cells give cell phones incredible range. Thanks to the cellular approach, cell phone users can drive hundreds of miles and talk the entire time. Each cell consists of a base station, which consists of a tower and a small building containing radio equipment.
In analog cellular systems a single cell uses 1/7 of the available duplex voice channels. This means that each cell uses 1/7 of the available channels, so it has a unique set of frequencies and no collisions.
A mobile phone provider is typically given 832 radio frequencies to use in a city.
Since each mobile phone uses two frequencies (duplex channels) per call, there are typically 395 voice channels per provider. (The remaining 42 frequencies are used for control channels.) Therefore, approximately 56 voice channels are available in each cell. In other words, each cell can have 56 people talking on their cell phones at the same time. Analog cell phone systems are considered the first generation of cell phone technology, 1G. With digital transmission methods (2G), the number of channels available increases. For example, TDMA-based digital systems (more on TDMA later) can handle three times as many calls as analog systems, giving each cell about 168 channels. Cell phones have low-power transmitters. Most cell phones come in two signal strengths: 0.6 watts and 3 watts. Base stations also communicate at low power. There are two advantages to a low power transmitter:
Transmissions from the base station and the phones in its cell do not reach very far outside that cell. Therefore two different cells can reuse the same 56 frequencies. Therefore the same frequencies can be reused throughout the city. Cellular phones consume relatively little power because they are usually battery operated. Lower power consumption means smaller batteries, which is exactly what makes cell phones possible. Cellular systems require a large number of base stations to cover a city of any size. A typical large city may have hundreds of cell towers. But because so many people use cell phones, the price per user remains low. Each network operator in each city also operates a central office, the Mobile Telephone Switching Office (MTSO). This office manages all telephone connections to the regular landline network and controls all base stations within the area.
Every mobile phone is given a unique code. These codes are used to identify the phone, the phone owner, and the service provider. The mobile phone codes used are:
1. Electronic Serial Number (ESN): This is a unique 32-digit number that is programmed into your phone when it was manufactured.
2. Mobile Identification Number (MIN): A 10-digit number derived from your phone number.
3. System Identification Code (SID): A unique 5 digit number assigned to each carrier by the Federal Communications Commission (FCC).
The ESN is an integral part of your phone, but when you purchase a service plan and activate your phone, both the MIN and SID codes are programmed into your phone. Some of the terminology required for cellular connectivity is explained below:
1. Mobile Telephone Switching Office (MTSO): The switch to which all base stations are connected. It is a sophisticated computer that has the ability to monitor all cellular calls, track the location of all cellular-equipped vehicles in the system, arrange handoffs, track billing information, etc. The MTSO is in turn connected to the PSTN through a link to the Network Control Center.
2. Public Switched Telephone Network (PSTN): A worldwide network of public circuit-switched telephone networks, just as the Internet is a worldwide network of public IP-based packet-switched networks. The PSTN was originally a network of analog landline telephone systems, but is now almost entirely digital and includes both mobile and landline telephones.
3. If you have a cell phone, turn it on. Someone calls you. What happens with the call:
• When you turn on your phone for the first time, it listens for the SID on the control channel. The control channel is a special frequency that your phone and base station use to communicate with each other about call setup, channel changes, etc. If your phone can’t find a control channel to listen to, it thinks it’s out of range and displays a “No Service” message.
• When the phone receives the SID, it compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of the home system.
• The phone also sends a registration request with the SID, and MTSO keeps track of the phone’s location in its database. This way, when they want to call your phone, the MTSO knows which cell you’re in.
• MTSO receives the call and tries to determine your location. It looks in its database to see which cell you are in. The MTSO selects the frequency pair that phones located in its cell will use to answer calls.
• MTSO communicates with your phone over a control channel and tells it which frequencies to use. When your phones and the tower turn on those frequencies, your call connects. Now you are talking to your friend on the radio.
• As you approach the edge of the cell, your cell’s base station notices that your signal strength is decreasing. At the same time, the base station of the cell you are visiting (which listens and measures signal strength on all frequencies, not just its 7th frequency) notices that your phone’s signal strength is increasing. The two base stations coordinate via MTSO, and at some point the phone receives a signal on the control channel telling it to change frequency. This handover switches the phone to the new mobile phone.
If you move from one mobile phone to another while on a call, and the new mobile phone is serviced by a service provider different from your phone, the call is not dropped but is forwarded to the other service provider .
If the SID on the control channel does not match the SID programmed into the phone, the phone knows it is roaming. The MTSO of the cell you are roaming on contacts the MTSO of your home system and checks its database to make sure that the SID of the phone you are using is valid. The home system authenticates your phone with the local MTSO and tracks your phone as you move around the cell. All this happens within a few seconds.
On most mobile phones, when you leave your provider’s service area and enter another provider’s service area, the word “roaming” appears on your phone’s display. If you want to roam internationally, you’ll need a phone that works both domestically and abroad. Different countries use different technologies for mobile access.
Read Also:
- Radio Frequency Radiation and Cell Phones
- Rapid Growth of Smartphones and Gaming Review
- History and Uses of Mobile Phones
- 6G and 7G Cellular Network Technology
- Introduction To Mobile Phones
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