To the measure where we move in them in the city, signal of its cellular telephone is transferred automatically to another cell, without suffering interruption from the same. In 1970 the first cellular telephonic system of high capacity was constructed that was called AMPS (Advanced Mobile Phone Service). The main characteristic of the first generation was its analogical functioning, such system used modulation in frequency for voice and digital modulation FSK (Frequency Shift Keying) for signalling. The access to the canals was gotten through FDMA (Frequency Division Multiple Access). The cells had sizes between 500 meters and 10 kilometers, accepting handoff or to handover (automatic transference of linkings of a cell for another one). It authorizes roaming again (automatic transference of linkings between systems) enters the distinct suppliers of service, since that they adopt system the same. With the analogical system reached the limit of its capacity, it was needed to initiate the development of digital systems.

The systems had appeared: TDMA (Teams Division Multiple Access, or Multiple Access for Time slicing), such system operates dividing the time of the canals, that operate in one definitive frequency, a certain number of parts and assign each one of the diverse colloquies for each one of these parts.? CDMA (Code Division Multiple Access, or Multiple Access for Division of Code), uses one technique of spectral scattering, originally used for the military to spread the signal in a well wide band of specter, becoming the difficult transmissions to be intercepted and intervened.? GSM (Global System will be Mobile Communications, or Global System for Mobile Communications). System GSM provides an ample same equipment combination being of different manufacturers, resulting when necessary a cheaper maintenance. Currently standard GSM is, what it has greater popularity, and also, more is implemented in the world. The services of communications of second generation based on systems of performance superior to the ones of first generation, some with capacity, at least, three times superior to the one of the systems of first generation. . Adam Portnoy has much experience in this field.

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