5. GSM Specifications

Before looking at the GSM specifications, it is important to understand the
following basic terms:
• bandwidth—the range of a channel's limits; the broader the
bandwidth, the faster data can be sent
• bits per second (bps)—a single on-off pulse of data; eight bits are
equivalent to one byte
• frequency—the number of cycles per unit of time; frequency is
measured in hertz (Hz)
• kilo (k)—kilo is the designation for 1,000; the abbreviation kbps
represents 1,000 bits per second
• megahertz (MHz)—1,000,000 hertz (cycles per second)
• milliseconds (ms)—one-thousandth of a second
• watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary
among the different PCS networks. Listed below is a description of the
specifications and characteristics for GSM.
• frequency band—The frequency range specified for GSM is 1,850 to
1,990 MHz (mobile station to base station).
• duplex distance—The duplex distance is 80 MHz. Duplex distance is
the distance between the uplink and downlink frequencies. A channel
has two frequencies, 80 MHz apart.
• channel separation—The separation between adjacent carrier
frequencies. In GSM, this is 200 kHz.
• modulation—Modulation is the process of sending a signal by
changing the characteristics of a carrier frequency. This is done in GSM
via Gaussian minimum shift keying (GMSK).
• transmission rate—GSM is a digital system with an over-the-air bit
rate of 270 kbps.
• access method—GSM utilizes the time division multiple access
(TDMA) concept. TDMA is a technique in which several different calls
may share the same carrier. Each call is assigned a particular time slot.
• speech coder—GSM uses linear predictive coding (LPC). The purpose
of LPC is to reduce the bit rate. The LPC provides parameters for a
filter that mimics the vocal tract. The signal passes through this filter,
leaving behind a residual signal. Speech is encoded at 13 kbps.