**4.2.3 Communication system of SPMS**

The communication systems of SPMS may be similar to the SCADA communications in terms of technology, architecture and utilized media. Although these communication systems may be the same, their streamed data are different. The phasor data, which is provided by PMUs, have different nature in comparison with the data of RTUs. Phasor data is continuous and streaming in nature while RTU data is transmitted to the master station either in a specified time intervals or when master station requests it. Another difference between PMU and RTU data is their volume. In general, data of a PMU has more value than data provided by a RTU.

As a result of the above facts, two aspects are of major importance in SPMS communications: communication bandwidth and communication latency (Phadke & Thorp, 2008). High bandwidth communications guarantee that all phasor data can be transmitted to PDCs without any packet drops. On the other hand, low latency communications provide real time streaming between PMUs and PDCs.
