The evolution of flows of different services, the need for flexibility transmission network, the need to improve the functions of exploitationmaintenance, the continuous increase of transmission capacity on fiber optics and the need for interconnection between operators to high flows and Standardized have shown the limitations of the current hierarchy and led to standardization.
SONET or SDH? Research on SONET lead each time on another Technology: SDH (Synchronous Digital Hierarchy). It should be clearly introduce the origins of SONET and the links between SONET, PDH (Plesiochronous Digital Hierarchy) and SDH.
Introduction to SONET SONET (Synchronous Optical Networks) is an initial proposal Bellcore, which defines the transport layer of a physical architecture broadband. SDH (Synchronous Digital Hierarchy) is a vision specific SONET, requested by the Europeans and adapted to the ATM. We will focus our research rather SDH which is used in
Europe and more media on the internet. However, the differences between these
two technologies are very slim.
PDH to SDH PDH is the technique that preceded SDH. It consists of multiplexer and carry bits of the lower flow transmitting at rates higher. The lower rates are high at a value higher order of bits of justification, with an indication of their presence in the resulting frame. The flow is not exact multiple of the returns but slightly more. It is what was described as plesiochronous (in Greek, Plesio = almost plesiochronous = almost synchronous). The main shortcoming of this technique is that multiplexing does not have access to information in a way directly without demultiplex all channels.
For example, to provide a line to 2Mbit / s more multiplexing and demultiplexing must be made to extract a fast channel 140Mbit / s. It was a defect in telephony but acceptable for use on optical networks, it becomes unacceptable.
