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Most onboard embedded systems have real-time requirements. Real-time mechanisms can be used, for example, to synchronize the execution of applications, to ensure guaranteed data delivery time. Time synchronization in routers and terminal nodes is required to implement real-time mechanisms.
The severity of real-time requirements may vary, depending on the purpose of the networks. For some systems, a time synchronization accuracy and a time synchronization precision of several ms is acceptable, for other systems accuracy and precision of several μs is required, in some cases a higher accuracy and precision may be required. The authors use the term “accuracy” to refer to maximum offset of a clock in any network device with respect to reference clock in the clock source, and the term “precision” to refer to maximum offset between clocks in any two network devices.
Typical clocks in network device consist of internal or external PLL and a time counter that is incremented or decremented with every rising or falling edge of clock waveform from this PLL. Synchronization of time counting in devices is necessary for several reasons. The generated by the PLL clock waveform is not ideal. This affects the accuracy of the timing.
Different PLLs may take different times after power-on to start generating the clock signal. Several devices may turn on after some time after start of system. For example, this can happen if cold redundancy is used in the network or some devices are not used during several periods of system operation. Due to these factors, time synchronization in devices is necessary throughout the entire operation of the network.
The SpaceFibre standard is developed for onboard local networks. However, the current version of the SpaceFibre standard does not specify any time synchronization mechanisms. This standard has several mechanisms that can be used, in particular, for time synchronization in network devices. In this paper, the authors review the time synchronization standards and protocols currently used for local networks with real-time requirements.
The authors consider the mechanisms of time synchronization that are used in the data transmission standards, which are currently used for onboard networks such as TTEthernet, Fiber Channel, Serial Rapid IO, SpaceWire. The authors also reviewed modern standards for time synchronization for wireless networks.
The main characteristics of time synchronization protocols are synchronization accuracy and precision, synchronization period, network loading by transmission of service information (most important for wired networks), energy consumption (most important for wireless networks), hardware implementation costs, in particular, memory costs, fault tolerance, presence/absence of restrictions on the structure of the network, scalability of characteristics with an increase in the size of the network. The last two characteristics are very relevant for the SpaceFibre standard, since large networks with arbitrary topology can be formed on the basis of this standard, in particular, containing loops at the physical layer.
Based on this consideration, the authors proposed possible time synchronization mechanisms for the SpaceFibre network.
In different networks, depending on the required time synchronization accuracy, precision, depending on the admissible overhead costs for the transmission of service information, one of these mechanisms can be selected. In the future, new mechanisms of time synchronization for the SpaceFibre network can be developed, taking into account new requirements and restrictions.
The network equipment can exploit from several years to several tens of years, therefore the ability to update the set of supported synchronization algorithms in terminal nodes and routers is necessary. Thus, the Local time controller in terminal nodes and routers must have the property of dynamic reconfiguration. Common way to provide dynamic reconfiguration is to use FPGA technology. However, chips developed using this technology have significantly higher power consumption, slower performance, and limited operating conditions compared to chips developed using ASIC technology.
Therefore, in this paper, the authors propose several approaches to the design of a dynamically reconfigurable Local time controller based on ASIC technology.
The main goals of this paper are development time synchronization mechanisms for the SpaceFibre network, evaluation their achievable synchronization accuracy and precision, evaluation network loading by transmission of service information; development of reconfigurable Local time controller for implementation of these and future mechanisms with ASIC technology, evaluation of it’s hardware cost.