GSM - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 275

4 G and the Future
4 G is currently the most advanced technology readily available, with extremely
high speeds exceeding 50 Mbit/s. The 4 G standard allows for theoretical speeds
much higher than that, but even that isn't fast enough for the future, and work
has already begun on the 5 G network. Time will tell just how far the mobile
network will progress.
Modems
A modem (short for modulator-demodulator) is a device that can send and
receive digital data through an analog carrier. Most veteran computer experts
remember modems as the trusty 56-k modem—a device that connected to a
computer through a serial port and allowed the computer to connect to the
Internet (or a company network) through a telephone line. Where does the 56 k
come from? The speed, 56 thousand baud or 56 Kbit/s data rate. If everything
went well (which it usually didn't) this meant that users could download data
at a blistering 4 to 5 kilobytes per second. Don't laugh; they were fast modems,
yet most were slower.
Although the trusty 56-k modems have been mostly replaced by broadband,
it is interesting to know how they work. Modems are serial devices, and most
were instructed to operate using the Hayes command set: simple ASCII mes-
sages instructing the modem to perform specii c actions. Most commands start
with “AT”, short for Attention. A modem is instructed to coni gure itself in a
specii c way, to call a number and to get information using simple text messages.
When the connection is made, the modem is switched from command mode to
data mode , and from there on, the modem sends each byte of data it receives. It is
also possible to change from data mode to command mode again to issue more
instructions to the modem (for example, to hang up). Again, this is performed
by sending AT commands.
The 56-k modem is indeed a dying technology, but its legacy is still with us
and will be for a long time. The AT command idea was so well implemented
that most radio peripherals still use them; Bluetooth devices, for example, are
coni gured using AT commands. Bluetooth does not connect through telephone
lines, but the modem principle is the same; a digital device transmits digital data
over an analog medium—in this case, radio waves. Even the most modern 4 G
telephone is also a modem, accepting serial data, transmitting and receiving
data over radio waves. GSM devices are exactly the same.