GSM and DECT

The Global System for Mobile Communications (GSM) has established itself as the de facto global standard for digital cellular communications. It fully specifies the cellular network as well as access to it.

GSM base stations enable communications with mobile terminals. Each base station forms a cell; a network of overlapping cells constitutes a WAN. These networks are currently associated with voice communications and GSM phones, but data services are now available in many countries.

The first service started in Denmark in July 1994 and operated over a Nokia cellular data card (a PC Card) and a Nokia phone; thus, it was a proprietary solution. In a move toward standardization, Ericsson, Hewlett -Packard, and Nokia have formulated a wireless AT command set, which is likely to be incorporated in a revised GSM standard.

Digital European Cordless Telecommunications (DECT) is well placed to become the leading standard for digital cordless communications. It defines the interface between mobile cordless terminals (i.e., phones and notebook PCs) and a fixed base station. DECT is independent of the type of network being accessed.

Both GSM and DECT use Time Division Multiple Access (TDMA) technology to form a communications channel as a combination of a particular carrier frequency and a time slot. Each duplex (i.e., two-way) speech channel is compressed in time so that several channels can be multiplexed into a single frequency.

GSM operates in the frequency range of 890 to 960 MHz and has 125 duplex carriers (i.e., 250 carriers total) with eight channels per carrier; the bit rate is 270.8 Kbps. DECT operates in the 1880- to 1900-MHz range and has 10 carriers with 12 duplex channels per ca rrier; the bit rate is 1152 Kbps.

Code division multiple access (CDMA) is a competing technology that has been zealously promoted in the U.S. for use in the new personal communications services (PCS) networks. It has variable bit rates up to 1230 Kbps. Practical network experience of CDMA is still very limited, while TDMA on GSM has been in place for several years.

CDMA operates in the frequency range of 824 to 894 MHz and has 20 duplex frequencies. These are spaced six times farther apart than in GSM, and instead of being divided into a fixed number of time slots, they carry a variable number of spread-spectrum channels.

In CDMA spread-spectrum technology, the digital data stream is modulated by a pseudo-random code sequence to expand the signal over the full bandwidth. This allows multiple conversations to simultaneously share the same carrier frequency (each one using a different code) and thereby use the entire 1.25-MHz spectrum. The base station reconstructs a particular data stream fro m the code used by the terminal and sees all other channels as background noise.