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Circuit-switched networks based on time-division multiplexing require synchronization to deliver information, whereas packet-switched networks can deliver information in an asynchronous environment. However, all real-time services require that synchronization and timing information be delivered over the network. Performance of timing distribution is quantified using particular metrics and adherence to requirements determined by using masks. The traditional metrics, TDEV and MTIE, have extensions to packet-switched networks for addressing the corruption of timing information by packet delay variation. The principles of metrics and masks and these extensions are presented here.