ty was increased over the preceding analogue design by dividing each 30 kHz channel pair into three time slots and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analogue scanners could not access digital signals. IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD) and an improved compression protocol. The short message service (SMS) and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion. D-AMPS used the 800 and 900 MHz frequency bands - as does the AMPS - but each 30 kHz channel (created by FDMA) is further subdivided into three TDMA, which triples the channels available and the number of calls. 1.3.2 CDMA (Code Division Multiple Access) CDMA has many variants in the cellular market. N-CDMA, i.e. Narrowband CDMA (or just CDMA), was developed by Qualcomm, known in the US as IS-95, and was a first generation technology. Its typical characteristic was high capacity and small cell radius. CDMAone (IS-95) is a second generation system, offering advantages such as an increase in capacity (almost 10 times that of the AMPS), improved quality and coverage, improved security system, etc. Enhancement of the CDMAone was IS-95B, also called 2.5G of CDMA technology, which combined the standards IS-95A, ANSI-J-STD-008 and TSB- 74. Major advantages of this system include frequency diversity (i.e. frequency dependent transmission impairments have less effect on the signal), increased privacy as the spread spectrum is obtained by noise like signals, an interference limited system, etc., while some disadvantages of this system include the air interface, which is the most complicated, soft hand-off, which is more complicated than the ones used in the TDMA/FDMA system, signals near to the receiver, which are received with less attenuation than the ones further from it, etc. 1.3.3 GSM (Global System for 