A.2. The Dawn of Second Generation (2G) Digital Cellular

Roaming in Europe was obviously impossible. Partly to alleviate this problem, the European operators decided to standardize the next generation of mobile phones by forming the European Telecommunications Standards Institute (ETSI). In the early 1980s, ETSI developed a digital mobile phone standard known as GSM (originally Groupe Special Mobile, later Global System for Mobile Communications). The GSM standard included something termed Short Message Service (SMS), which used spare bandwidth on the control channel to send and receive short 160-byte messages.

The GSM system and some other digital cellular standards (such as the digital successor to AMPS in North America, D-AMPS, or IS-54) multiplex different voice callers on a common radio frequency by using time division multiplexing (Time Division Multiple Access, or TDMA). Essentially, the signal from each user is rapidly sampled, and samples from different users are interleaved and broadcast in an assigned time slot. The sampled speech is reassembled at the receiving end of the signal, and in this way multiple users can share a single radio channel.

The cellular protocols are actually quite a bit more complex than this simple explanation would imply. At the same time the radio signal is being sampled and desampled, it is also hopping around to a preset sequence of frequencies, and samples are being reordered in time, all in order to reduce mobile effects such as interference, jitter, dropouts, and multipath distortion.

In the very late 1980s, Qualcomm introduced a new digital system in the U.S. termed CDMA, for Code Division Multiple Access (later also called IS-95 and still later cdmaOne). Instead of dividing each voice signal into time-based divisions, CDMA transmitted all of the signals on multiple radio frequencies at the same time.

But how to keep the signals from interfering with each other? In CDMA, the signals make use of orthogonal "codes" that define which of the frequencies are used for which signal. The signal is transmitted on a number of frequencies defined by the code, and can be extracted on the receiving end by sampling only those frequencies assigned to this particular code. The other signals on those same frequencies are averaged out as noise because they don't appear consistently in most of the frequencies. CDMA proved to be much more efficient at spectrum use than TDMA, but GSM had already taken hold, and was the more popular standard worldwide.

The 2G mobile protocols were mainly designed for voice, but also provided the first real channels for data. At first the data rates were slow, the coverage spotty, and the technology inefficient in its use of the available bandwidth because it was based on circuit switching. The optimistically named High Speed Circuit Switched Data (HSCSD) system used multiple GSM channels and was rated at 28.8 to 64 kilobits per second, though it rarely achieved even a fraction of that speed. In the 1990s, HSCSD was replaced with the General Packet Radio System (GPRS) standard, the first packet-switched technology for GSM.