Cellular telephone system, though with limited functionalities, features, and scale of deployment, existed as early as the 1920s. The commercial cellular networks, as we know them today, started in the late 1970s. The growth in mobile communication from then onward is amazing. Within 30 years of introduction, cellular telephony is now so popular that many will find it difficult to imagine life without it. The initial deployments of cellular networks were based on different variations of analog technologies and standards. These included:
■ Advanced Mobile Phone Service (AMPS) was initially a United States standard but was later adopted by many other countries such as Australia, South Korea, Singapore, and Brazil. It operates in the 800-MHz band. Later Narrowband AMPS (NAMPS) was introduced by Motorola and adopted by operators in the United States, Russia, and other countries. NAMPS was developed as an interim technology between first and second generations of mobile networks. NAMPS, like AMPS, is based on analog technology. The significant difference lies in its voice channel bandwidth, i.e., 10 kHz instead of 30 kHz in AMPS.
■ Total Access Communication System (TACS), a variation of AMPS, was initially introduced in the United Kingdom and later was deployed in many other countries such as Italy, Spain, and the United Arab Emirates. It operates in the 900-MHz band. A variation of TACS, known as JTACS, was later deployed in Japan.
■ Nordic Mobile Telephone System (NMT) was initially introduced in the Scandinavian countries. NMT operates in the 450- and 900-MHz bands.
The NMT-450 and NMT-900 systems were deployed later in many countries in Asia and Europe and in Australia.
In addition, there were a few other limited implementations of cellular networks such as NETZ-C, which was deployed in Germany, Portugal, and South Africa.
Limitations of early cellular technologies
Early deployed cellular technologies caught the imagination of the users; they were a great success and registered phenomenal growth. However, they were limited by many factors. A few significant limitations of early cellular technologies are as follows:
■ Restricted spectrum, limited capacity
■ Cost of ownership
■ Limited roaming
■ Low mobility and cost of mobile phones
■ Inherent speech quality issues
■ Lack of internetwork standardization
■ Compatibility issues with ISDN
Roaming and early cellular networks
The early standards for cellular networks were focused on standardizing the air interface, i.e., the Common Air Interface (CAI). There was not much work done in standardizing internetwork communication. The result was a variety of vendor-dependent proprietary protocols. This means the roaming was possible only between two networks supplied by the same vendor. This was surely a serious limitation when it came to roaming. As the demand for roaming increased, the need for a standard for communication between the home and visited networks was felt. The IS-41 standard was introduced as a standard protocol for internetwork communication to enable roaming in AMPS-based networks. Later, as part of GSM standardization, Mobile Application Part (MAP) was developed. Both IS-41 and GSM MAP were enhanced several times to ensure seamless roaming for the next-generation networks.