Today's advanced computer based systems require more and more precise time synchronization. Examples of this are telecom operators that need to provide high quality transmission of voice, video, data, gaming in real time or financial institutes that need to have very accurate time stamps on transactions in international funds trade.

GPS is a technique used today not only for navigation, but also for time and frequency transfer. The latter is a wide field of application for GPS in our modern and high tech society. The GPS signals have relatively low signal strengths and are therefore sensitive for jammers and consequently the systems that use GPS for time synchronization becomes vulnerable. To make the society less dependent on GPS, alternatives becomes necessary to find.

Experimental field trials on two-way time transfer over fiber have been presented of researchers in the US and in Japan in the past, however using dedicated fibers and hence require access to the infrastructure. This makes it difficult to install these systems in non dedicated or existing networks.

The new solution can instead use the data already transmitted in the fiber optic network and recover time information from this data. This technique could be used in any network based on SDH/SONET, which are the standards used both in Europe and in North America. By using a new specifically designed high speed communication unit “Header Recognizer” a certain frame header in the SDH/SONET protocol can be detected and then be used as a reference.

The realization of this new high speed communication unit has been done by the company InformAsic facing several technological challenges. Firstly the precise time for a certain sequence needs to be detected. This has to be done by analyzing incoming data at a speed of 10 Gbit/s to find the pattern that exactly match the defined sequence. Then a signal with a similar pattern has to be generated and transmitted in the right time slot at a speed of 10 Gbit/s.

Another challenging requirement is that the researchers at SP requested that the output signal must be 500 mV p-p with a rise time of less than 25 ps and a jitter less than 10 ps. These combined and extreme requirements made it impossible to find any standard circuits/components on the market to solve the problem. To get a view of how small 10 ps really is, it takes the light 10 ps to travel 3 mm and in comparison the light travels 7,5 times around the globe in only one second.

Instead InformAsic had to design a new solution based on Demux and Mux from Vitesse and a Stratix II FPGA from Altera in between. Firstly the 10 Gbit/s communication was divided into 16 parallel signals at 622 Mbit/s. These signals were then analyzed by the FPGA to find the sequence and the FPGA then created a 20 ns long signal and sent it with extremely precise timing to the time counter.

During the development the design team discovered that one consequence of the high speed communication was that even a minimal change in temperature created delays that made the extreme requirements unreachable. This was solved by using clock signals in an innovative way and the solution includes in total seven different clocks generated by PLLs, whereof two internal PLLs in the FPGA are used to create a predefined time delay to make all signals synchronized.

All requirements have been fulfilled and confirmed by lab tests, but also by real life tests in the GigaSUNET fiber optic network, which is a fiber optic network connecting all Universities in Sweden.

The new solution can smoothly be installed in any existing network based on SDH/SONET. By integrating the new solution into fiber optic networks a new alternative to GPS time synchronization is available to make a less vulnerable environment both for the industry and the society.