On the left above: Front of a Pacific Bell GSM phone. In the middle above: Same phone, showing the back. The SIM card is the white plastic square. It fits into the grey colored holder next to it. On the right above. A new and different idea, a holder for two SIM cards, allowing one phone to access either of two wireless carriers. Provided you have an account with both. :-) The Sim card is to the left of the body.
"Our call starts by turning on our GSM phone . . . "
GSM phones use SIM cards, or Subscriber information or identity modules. Memory modules. They're the biggest difference a user sees between a GSM phone or handset and a conventional cellular telephone. With the SIM card and its memory the GSM handset is a smart phone, able to do many things a conventional cellular telephone cannot. Like keeping a built in phone book or allowing different ringtones to be downloaded and then stored. Conventional cellular telephones either lack the features GSM phones have built in, or they must rely on resources from the cellular system itself to provide them. Let me make another, important point.
With a SIM card your account can be shared from mobile to mobile, at least in theory. Want to try out your neighbor's brand new mobile? You should be able to put your SIM card into that GSM handset and have it work. The GSM network cares only that a valid account exists, not that you are using a different device. In other words, GSM checks your account status first, not your equipment status. The network then bills you for charges on whatever equipment you've ued. You get billed, not the neighbor who loaned you the phone.
This flexibility is completely different than AMPS technology, which enables one device per account. No swtiching around. Conventional cellular telephones have their electronic serial number burned into a chipset which is permanently attached to the phone. No way to change out that chipset or trade with another phone. SIM card technology, by comparison, is meant to make sharing phones and other GSM devices quick and easy.
2. The Cell Site Antenna
"Depending on the best signal strength received, usually, the nearest cell site antenna makes a radio connection with our phone . . ."
Down the antenna our signal goes! Cell site towers and antennas come in all shapes and sizes. Conventional cellular and GSM use the same kind of antennas if their service is at the same frequency. Antennas are tuned or matched to the physical size of the radio wavelength they transmit. Many American carriers now operate at 800 Mhz (the conventional cellular band) and 1900 Mhz (the PCS band) if they have the operating licenses to do so.
So, you may see different antennas on the same mast for the two different radio bands. Depending on call traffic, your GSM conversation may be put on the low band at one moment and the high band another. Again, antennas are frequency dependent, not service dependent. The antennas pictured here are two thirds the way up the mast.
In the United States GSM needs more base stations and antennas because it uses lower power and has been at higher frequencies than in Europe. This means more antennas; on buildings, towers, and billboards.
