Due to the number of networked terminal devices, the data load on the internet is increasing rapidly. That is why the internet is to be gradually developed over the coming years to become a synchronous network. The challenge involved is similar to that of an orchestra: the more difficult the piece and the larger the ensemble, the greater the need for a masterful maestro. When it comes to data transmission, the maestro is what we know as the master clock. This is usually an atomic clock whose timing is transmitted to the network nodes via radio, fibre optic cable or satellite.
But just like a musician who has to refer to his sheet music every so often, an individual node cannot always see the maestro – the master clock – and must therefore have a good sense of timing of its own. In music, it is essential that every instrument starts at the right time, plays at the correct tempo and that the music score is open at the correct page. In network synchronisation, the terminology for this is synchronisation of phase, frequency and time.
Synchronisation of network nodes
Whereas previously with Ethernet it came down to frequency synchronisation (data transmission at a constant rate), synchronous data transmission also requires correct timing i.e. phase synchronicity. A common, absolute reference – synchronisation of the time or date – is often less important; it is already provided by services such as GPS, anyway. However, GPS is prone to disruption and that’s why either additional master clocks or a superb internal clock must be made available in order to bridge any interruptions. Since atomic clocks are prohibitively expensive for smaller data centres or home networks, high-precision crystal oscillators are relied on here. In smaller data nodes and mobile radio base stations, we often find what are known as oven-controlled crystal oscillators (OCXOs). With these, the temperature dependency of the quartz oscillation is compensated by warming the quartz crystal to its upper turnover point (TOP) by means of a heating element and maintaining it constantly at this temperature.