Global Time

Overview

This chapter starts with a general discussion on time and order. The notions of causal order, temporal order, and delivery order and their interrelationships are elaborated. The parameters that characterize the behavior and the quality of a digital clock are investigated. Section 3.2 proceeds along the positivist tradition by introducing an omniscient external observer with an absolute reference clock that can generate precise timestamps for all relevant events. These absolute timestamps are used to reason about the precision and accuracy of a global time base, and to expose the fundamental limits of time measurement in a distributed real-time system.

In Section 3.3 the idea of a sparse time base is introduced to establish a consistent view of the order of computer-generated events in a distributed real-time system without having to execute an agreement protocol.

The topic of internal clock synchronization is covered in Section 3.4. First, the notions of convergence function and drift offset are introduced to express the synchronization condition that must be satisfied by any synchronization algorithm. Then, the simple central-master algorithm for clock synchronization is presented, and the precision of this algorithm is analyzed. Section 3.4.3 deals with the more complex issue of fault-tolerant distributed clock synchronization. The jitter of the communication system is a major limiting factor that determines the precision of the global time base.

The topic of external synchronization is studied in Section 3.5. The role of a time gateway and the problem of faults in external synchronization are discussed. Finally, the network time protocol (NTP) time format of the Internet is presented.