On the Necessity of Traffic Shaping for PMU Measurement Data Streams

We study the communication network of an on-campus active distribution network (ADN) that comprises phasor measurement units (PMUs) connected to medium-voltage transformers. Within stringent time delays and with minimal loss the PMUs periodically transfer fresh measurements through a phasor data concentrator (PDC) to a centralized electrical network state estimator (control point). Due to strict operational constraints, a dedicated robust communication infrastructure that withstands power shortages is needed. We use DSL technology (SHDSL) over existing telephone cables. We investigate the operating region of the system. In our experimental setup, we measure the achieved goodput (application layer throughput) for various measurement frequencies and frame sizes. We observe that goodput drops catastrophically, in some scenarios for which the PDC data transmission rate slightly exceeds the capacity of the SHDSL link for a short period of time. Specifically, when the offered traffic exceeds channel capacity by 20%, we observe up to 90% of lost packets. We explain this surprising phenomenon by the combination of IP fragmentation that splits each frame in two IP packets of significantly different sizes and the FIFO/tail-drop queuing discipline implemented within line terminal devices at the source end. We conclude that the guidelines of the C37.118.2-2011 standard are not sufficient for designing a PMU data transfer layer. Implementing traffic shaping within PMUs is essential for avoiding excessive packet losses.