Network synchronization plays an important role in digital telecommunications. It affects the quality of most services provided by the network operator. Presently, synchronization is primarily distributed across circuit-switched telecommunications networks, using traditional Time-Division Multiplex (TDM) links, such as E1 (2.048 Mbit/s) or DS1 (1.544 Mbit/s).

Several fixed line and mobile Operators are migrating to an IP packet-switched network infrastructure, in the framework of Next-Generation Network (NGN). In this architecture, Ethernet transport is common. This trend is driven by low operating costs, bandwidth consolidation of different services and by the convergence between fixed and mobile services. However, migrating trunk lines to IP transport poses challenges, in particular for circuit emulation and synchronization of network elements.

The evolution of communications networks towards packet-switched networks has increased interest in the distribution of synchronization using packet technologies. In particular, synchronization distribution over Ethernet has become a focus of activity in international standard bodies. The ITU-T is developing a set of four new Recommendations on these topics, namely:

• ITU-T Rec. G.8261 “Timing and synchronization aspects in packet networks”, consented in 2008, proposed update in 2010;
• ITU-T Rec. G.8262 “Timing Characteristics of Synchronous Ethernet Equipment Slave Clock (EEC)” consented 2008, proposed update 2010.
• ITU-T Rec. G.8263 “Timing characteristics of packet based equipment clocks (PEC) and packet based service clocks (PSC)”, updated in 2009;
• ITU-T Rec. G.8264 “Timing distribution through packet networks”, consented in 2008, proposed update in 2010;

Another international standard relevant to this issue is the IEEE 1588 on the Precision Time Protocol (PTP) (http://ieee1588.nist.gov/). This protocol can be used to time align hosts with sub-microsecond accuracy exchanging messages on Ethernet networks.

In this scenario, the traditional model in which synchronization distribution is engineered carefully may make people to have a greater expectation of an ad-hoc synchronization quality, i.e. to expect traditional quality of synchronization also when the underlying transport facilities are not the legacy TDM circuit-switched infrastructure. Unfortunately, this currently seems impossible to achieve and even approaching it, requires strong planning capabilities and good knowledge of all involved issues.

Such issues reach the wider audience of telecommunications engineers. An example is the distribution of synchronization to next-generation wireless base-stations, which may only have packet connectivity, but still require high accuracy of synchronization.

In this field, some key and upcoming topics of research and development in both academia and industry are:

• the key emerging standards in the area of synchronization over packet networks, including recent work at the ITU-T, IEEE and NIST;
• identifying timing scenarios in various network applications;
• implementation of devices and equipment cards achieving high-quality synchronization using new technologies for packet-based synchronization transfer;
• optimal deployment and roll-out of these solutions by network Operators;
• measurement techniques for characterizing and qualifying packet-based synchronization;
• circuit-emulation services over pseudo wire;
• delivery of synchronization to wireless base stations in 3G/4G networks using Ethernet.

It is maybe needless to say that quality of service degradations due to poor network synchronization look often sudden, unexpected and of mysterious origin for almost everybody but the (good) synchronization engineer.

The course "Synchronization of Telecommunications Networks" offered by Telefocal provides all the necessary basic knowledge that is needed by telecommunications engineers to approach these complex topics and to manage effectively all issues related to network synchronization planning and operation.