Wednesday, December 8, 2010

How Mobile Channels Work

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A single Mobile in an analog Mobile-phone system uses one-seventh of the available duplex voice channels. That is, each Mobile (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:
  • A Mobile-phone carrier typically gets 832 radio frequencies to use in a city.
  • Each Mobile phone uses two frequencies per call--a duplex channel-- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)
Therefore, each Mobile has about 56 voice channels available. In other words, in any Mobile, 56 people can be talking on their Mobile phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many Mobiles as an analog system, so each Mobile has about 168 channels available.
Mobile phones have low-power transmitters in them. Many Mobile phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:
  • The transmissions of a base station and the phones within its Mobile do not make it very far outside that Mobile. Therefore, in the figure above, both of the purple Mobiles can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.
  • The power consumption of the Mobile phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld Mobile phones possible.
The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using Mobile phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.

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