Horst F. Wedde und Wolfgang Freund
Harmonious Internal Clock Synchronization
EUROMICRO Workshop on Real-Time-Systems 2000 (ERTS’00), S. 175-182, IEEE Press, Stockholm, Sweden, 2000-06
Internal clock synchronization has been investigated, or employed, for quite a number of years, under the requirement of good upper bounds for the deviation, or accuracy, between a predefined master node M and slaves S1, S2, etc. It has been tacitly assumed that the accuracy range between slaves S1 and S2 was just the same as for the master/ slave. In extensive experimental studies of unicast-based protocols, however, we found a significantly worse accuracy between the slave nodes. Except for the asymmetry as such – which results in different communication conditions in a logically peer communication structure (like in our MELODY system) – this is particularly undesirable, if not harmful, if one wants to dynamically shift the master role between nodes, for the purpose of achieving fault tolerance. Nearly all previous internal clock synchronization algorithms with high accuracy are based on unicast messages (including our own efforts). In this paper we present two new broadcast protocols, the Real-Time Duplex Protocol (RTDP) and the Real-Time Burst Protocol (RTBP). With these we achieve a homogeneous clock deviation structure in that the accuracy between master and slave is the same as between two slaves. Also, despite the fact that the broadcast delay is slightly higher than the unicast transmission time, the accuracy is even better than for a master/ slave synchronization under unicast methods. We explain how both algorithms serve both real-time and fault tolerance needs. We report on our experimental findings.