Analog Cell Phones
Back in 1983, an analog cell phone standard called AMPS [Advanced Mobile Phone System] received approval by the FCC. It was first used in Chicago and used frequencies ranging between 824 megahertz [MHz] and 894 MHz. To create healthy competition, and keep prices low, the government required that two carriers be present in every market. One of these carriers was typically a local phone company, and the other a national carrier.
Each carrier was assigned 832 frequencies. The standard issue was 790 for voice use and 42 for data use [telephony housekeeping like registration and paging]. A pair of frequencies were used to create one channel [one to transmit, and one to receive]. Analog voice channel frequencies are normally 30 kHz wide. So, 30 kHz was selected as standard size because it gives the voice quality of a landline wired phone.
The two frequencies that make up the single channel are separated by 45 MHz. This keeps the frequencies from interfering with each other. So, there were 395 voice and 21 data channels.
After AMPS, another technology was introduced. It is known as Narrowband Advanced Mobile Phone Service [NAMPS]. This incorporates digital technology and allowed the system to carry around three times as many calls as AMPS. However, this is still considered analog because it can only operate on the 800 MHz band and doesn’t offer common digital cellular services like web browsing or email.
Enter Digital
The second generation of cell phones is 2G digital technology. These phones use the same radio technology as analog devices, but use it in a different way. The analog systems do not completely use the signal between the cellular network and the phone. Analog signal is not compressed and manipulated as easy as a digital signal. Companies can fit more channels on a given bandwidth. This makes digital systems more efficient.
A digital phone converts your voice into binary. This binary information [0s and 1s] gets compressed. This compressed information allows anywhere from three to ten digital cell phone calls to occupy the space of a single analog cell phone call.
Most of the digital cellular systems rely on frequency-shift keying [FSK]. FSK sends data back and forth over AMPS. FSK uses two frequencies. One is for 0s and the other is for 1s. The frequencies alternate rapidly between the two, and send their digital information between the phone and the cell tower. Modulation and encoding is required. This converts analog information to digital, compresses it, and converts it back again. After all of this it must maintain an acceptable level of voice quality! So, believe it or not, that little phone in your hand is quite a powerful device when coupled with cellular technology.
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