ABSTRACT
When the PDH hierarchy was developed, it was considered to have enough capacity, which was then good enough for the requirement of voice and data traffic. However, as time passed and the demand for the services grew, the need for higher rates of transmission emerged very fast, largely because of the boom in data traffic and a steep growth in the consumer base of mobile telephones. Designing still higher rates in PDH was highly complex and uneconomical-uneconomical because more stages of multiplexing would have to be added to achieve a higher data rate, and consequently, a similar number of stages of demultiplexing were to be deployed wherever even one tributary was required to be dropped or inserted (see Chapter 8 for more details on PDH). In fact, the International Telecommunication Union (ITU) did not work out the standards for PDH systems beyond 140 Mbps, and whatever systems existed for higher data rates were proprietary of the vendors. After the emergence of optical fiber technology in the 1980s, which had enormous channel capacity, the limited capacity systems of PDH standards became a real bottleneck in transport of larger number of channels on the optical media.
