European Transactions on Telecommunications Focus on Synchronization of Digital Networks

Introduction to the subject

Network synchronization plays a central role in digital telecommunications, having a determining influence on the quality of most services offered by the network operator to his customers.

Indeed, while transmission equipment based on the Plesiochronous Digital Hierarchy (PDH) does not need to be synchronized, since a bit justification technique (pulse stuffing) allows multiplexing of asynchronous tributaries with substantial frequency offsets, digital switching equipment requires to be synchronized in order to avoid slips in the input elastic stores. And while slips do not significantly affect normal phone conversations, they can even be catastrophic on some data services! The introduction of the circuit-switched data networks, and of new advanced services such as for example those provided by the emerging ISDN, yielded first the need of more stringent synchronization requirements.

As a matter of fact, however, the ongoing spreading of Synchronous Digital Hierarchy (SDH) technology in trasmission networks has really made synchronization a hot topic in standard bodies in the last few years, as the need for adequate network synchronization facilities has become more and more stringent in order to fully exploit SDH capabilities: it is widely recognized that SDH transmission relies on a suitable and dependable timing distribution to fully meet all its benefits.

Beyond SDH needs, anyway, nowadays network synchronization facilities are indeed unanimously considered as a profitable network resource, allowing slip-free digital switching, enhancing the performance of ATM-based transport services and serviceable for improving the quality of a variety of services (e.g. ISDN, GSM,...).

For this reason, all the major network operators have set up, or are now planning, national synchronization networks, in order to distribute a common timing reference (traceable to a national Primary Reference Clock or PRC) to every equipment of the telecommunications network. On the standardization side, ITU-T and ETSI bodies are currently developing completely new synchronization standards, suitable for the operation of modern (including SDH-based) digital telecommunications networks, specifying more stringent - and complex - requirements for jitter and wander at synchronization interfaces, for clocks accuracy and stability and for the synchronization network architecture.

Hot topics of debate, both in academia and in standard bodies, have been - or still are indeed - mainly, among the others, the definition of the most suitable timing signal stability quantities to digital telecommunications purposes, the specification of synchronization equipment performance with proper criteria based on such quantities, how timing impairments impact on SDH equipment operation (i.e. on pointer actions) as well on the various services from the user's point of view, and the design and implementation of time and frequency measurement techniques which are suitable to telecommunications field applications.

In the Call for Papers the Guest Editors sought for a wide coverage of such synchronization issues and much more, in order to provide the scientific and industrial community with up-to-date contributions in this field and to achieve a wider understanding of these important issues. Most of the topics mentioned above are now well addressed by the papers featured by this Focus.