Advanced Ethernet for Time-Aware Syntonicity

Several previous projects at ZISS (e.g. e-Wifi) made it possible to push the limits of high accurate time synchronization down to the nanosecond range. Nevertheless, for packet oriented, cable based communication infrastructures, such as 100Base-TX Ethernet, a remaining issue was the inherently present delay asymmetry of the transmission lines.

The goal of the AEtas project was to create an out-of-band communication system that allows to measure and eventually mitigate this delay asymmetry.

The main source for delay asymmetry is the different propagation delay on transmit and receive path which cannot be measured by using conventional 100Base-TX Ethernet. The reason for this is that the unidirectional communication (one wire pair for receive and another one for transmit) only allows for round-trip measurements. In contrast, AEtas uses a bidirectional approach where the point-to-point delay on each individual transmission line can be determined separately in both directions. This allows to measure static delay asymmetries induces by transceivers as well as dynamic asymmetries caused for example by changing cable lengths. As the regular Ethernet communication must not be impaired, the AEtas signal is embedded using a modulation scheme highly orthogonal to 100Base-TX, namely using a chirp spread spectrum approach. This approach creates an additional logical communication channel that is solely used by the AEtas synchronization protocol, making it independent from the network load of the regular Ethernet communication.

An important outcome of the project was an FPGA based prototype board, developed in close cooperation with the project partner Oregano Systems. The algorithms and approaches developed throughout the project were evaluated using simulation and laboratory setups. Final laboratory measurements showed that AEtas is able to achieve synchronization accuracies in the range of 100 picoseconds.

An appealing feature of AEtas is that it can provide synchronization as a service where the host requests the accurate time-of-day without knowledge of the underlying communication structure. This allows for using the system with high synchronization accuracy down to the nanosecond range in low-end performance embedded microcontroller solutions. ** This project is co-funded by FFG within the FIT-IT programme.