t is covered with digital service. The roaming agreements used between cellular carriers in North America use AMPS for roaming. In many cases, the analog networks are starting to wind down in the major metropolitan areas; however, in rural communities, AMPS is still predominant. GSM system was the first digital cellular system created from the onset. Different reasons explain the transition from analog to digital technology. Cellular systems experienced phenomenal growth. Analog systems were not able to cope with this increasing demand. To overcome this problem, new frequency bands and new technologies were suggested. Many countries rejected the possibility of using new frequency bands because of the restricted spectrum (even though later on, other frequency bands were allocated for the development of mobile cellular radio). New analog technologies were able to overcome some of the problems, but were too expensive. The digital radio was the best option (but not the perfect one) to handle the capacity needs in a cost-effi- cient manner. The decision to adopt digital technology for GSM was made in the course of developing the standard. During the development of GSM, the telecom- munications industry converted to digital networking standards. ISDN is an example of this evolution. In order to make GSM compatible with the services offered by ISDN, it was decided that digital radio technology was the best option available. Quality of service can also be improved dramatically by using digital rather than analog technology. From the beginning, the planners of GSM wanted ISDN compatibility in the services offered and control signaling used. The radio link imposed some limitations because the standard ISDN bit rate of 64 Kbps could not be practically achieved. Using the International Telecommunication Union-Telecommunication Standardization (ITU-T) definitions, telecommunication services can be divided into the following categories: Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.co