Summary:

Internet architects are realizing that timing is becoming more and more important on packet-based networks. The question is how they can implement precise timing on a distributed architecture.

dalitime

The past few years have seen low-latency networking get a lot of attention, driven primarily by high-frequency traders looking for an edge for their algorithms. However, the importance of communication latency and timing accuracy in general isn’t new. From the dawn of homo sapiens, when cave people first scratched lunar cycles on their cave walls, to the birth of telecommunications, accurately knowing what time it is has been important — for people and for networks.

Yet, in the move to packetized information, and the internet as we know it, timing got left behind. In a fatal mix of both enthusiasm and arrogance, synchronous timing was seen as irrelevant. After all, the world was moving to asynchronous packetized information switched by routers. Why would anyone still need old-fashioned synchronous information? Ma Bell was dead. And what did she know anyway? Fast forward to today and the current standard Network Time Protocol offers timing only to within tens of milliseconds and only within 2 whole seconds in the Windows implementation!

One only need look at the OPERA physics experiment in Gran Sasso to see the critical importance of timing. A single loose optical connector in their timing network produced a 75 nanosecond error, which led to global press coverage of their announcement that neutrinos travel faster than the speed of light. Timing will always be important, as all information is time-variant. There is no way to accurately know the what without knowing the when.
clocks

The evolution of timing standards.

With synchronous networking, you got the timing for free, as both the frequency and phase of the clock was buried in the carrier signal. Maintaining accurate timing and synchronization over a network that communicates with variable lengths packets spaced randomly apart is much more challenging.

So challenging, in fact, that network architects are taking a new look at the old approach: timing distribution networks. A throwback to analog phone calls and T1 internet service, the basic premise is that timing is once again embedded in the data being transported, with a clear protocol on how it may and may not be used. What might have been blasphemous to evangelical packet proponents at the start of the asynchronous packet age — making asynchronous networks more synchronous — is now seen as an urgent necessity.

clocksIn a modern timing distribution network, there is still an atomic “Master” master clock that serves as the single reference point for the entire network. The challenge is in maintaining that accuracy as the clock is distributed across a transport network. There is nothing that can be done about time of flight of the clock signal, as the speed of light is the speed of light (except in Gran Sasso).

However, if that transport time is accurately known, an offset may be applied, and relative clock accuracy is maintained. GPS and other local clock references do not go away. Rather, they all interconnect and are carefully synchronized, with all available information used to statistically narrow the uncertainty of the exact time at each node in the network. A loss of any source is easily compensated by group knowledge. If a more severe timing outage occurs, all that happens is the standard deviation of existing clock sources may spread a little. Math to the rescue.

Meet the standards.

There is a complex interplay of industry standards making all this happen. IEEE 1588v2 Precision Time Protocol (PTP) defines both how the timing is embedded in packets, as well as how each node should pass or modify the information. ITU-T G.8261/2/4 Synchronous Ethernet (SyncE) locks an output packet signal to the incoming signal in both frequency and phase. For SyncE to work, all links in the chain must support it and be in a locked state; PTP is much more forgiving, as it only requires all nodes to be transparent to the packets, a much lower bar. When both PTP and SyncE are combined, the ultimate in accuracy can be achieved.

While the packet timing standards and technology may be complex, implementation is surprisingly simple. Network devices that support timing are merely added at each node. Where existing customer premises equipment does not support timing, SyncProbes can be added either in series or in parallel to existing links. The timing protocols start working instantly in the background to improve all aspects of packet transport.

Why it matters

mastwheat
These recent advances in network timing have come none too soon. As mobile network operators make the transition to LTE Advanced, the required frequency and phase accuracy can only be achieved with timing distribution networks. LTE Advanced needs not only microsecond timing accuracy, but tight phase alignment as well. An often overlooked fact of LTE Advanced is the sheer number of antenna sites. Even though GPS clock sources continue to drop dramatically in price, it is simply not practical to place a GPS clock source at all sites, nor would they be accurate enough without the additional timing information provided by the timing distribution network .

However, the most important need for accurate timing is the one that goes unnoticed by even the most prognostic of soothsayers: Data centers. In the second of these articles on Sunday, we will look at the growing importance of timing in data centers.

Jim Theodoras is director of technical marketing at ADVA Optical Networking, working on Optical+Ethernet transport products.

Comments have been disabled for this post