With the introduction of PCM technology in the 1960s, communications networks were gradually converted to digital technology during the years that followed. To cope with the demand for ever-higher bit rates, a multiplex hierarchy or plesiochronous digital hierarchy (DSn) evolved. The bit rates start with the basic multiplex rate of 1.5 Mb/s with a further stage of 45 Mb/s. In many parts of the world however the primary rate is 2 Mb/s with additional stages of 8, 34 and 140 Mb/s. This fundamental diﬀerence in developments made the set up of gateways between the networks both diﬃcult and expensive.
In response to the demand for increased bandwidth, reliability, and high quality service, SONET developed steadily during the 1980s eliminating many of the disadvantages inherent in DSn. In turn, network providers began to beneﬁt from the many technological and economic advantages this new technology introduced including:
High transmission rates
Transmission rates of up to 40 Gb/s can be achieved in modern SONET systems making it the most suitable technology for backbones – the super high-ways in today’s telecommunications networks.
Simpliﬁed add and drop function
Compared to the older DSn system, low bit rate channels can be easily extracted from and inserted into the high-speed bit streams in SONET. It is now no longer necessary to apply the complex and costly procedure of demultiplexing then remultiplexing the plesiochronous structure.
High availability and capacity matching
With SONET, network providers can react quickly and easily to the requirements of their customers. For example, leased lines can be switched in a matter of minutes. The network provider can use standardized network elements (NE) that can be controlled and monitored from a central location via a telecommunications management network (TMN) system.
Modern SONET networks include various automatic back-up circuit and repair mechanisms which are designed to cope with system faults and are monitored by management. As a result, failure of a link or an NE does not lead to failure of the entire network.
Future-proof platform for new services
SONET is the ideal platform for a wide range of services including POTS, ISDN, mobile radio, and data communications (LAN, WAN, etc.). It is also able to handle more recent services such as video on demand and digital video broadcasting via ATM.
SONET makes it much easier to set up gateways between diﬀerent network providers and to SDH systems. The SONET interfaces are globally standardized, making it possible to combine NEs from diﬀerent manufacturers into a single network thus reducing equipment costs.
The trend in transport networks is toward ever-higher bit rates, such as OC-768 (time division multiplex, TDM). The current high costs of such NEs however are a restricting factor. The alternative lies in dense wavelength division multiplexing (DWDM), a technology enabling the multiple use of single-mode optical ﬁbers. As a result, a number of wavelengths can be used as carriers for the digital signals and transmitted simultaneously through the ﬁbers (See DWDM Pocket Guide for more information).