SCADA Communication & Protocols

1.      Introduction

Supervisory control and data acquisition system or SCADA refers to the combination of telemetry and data acquisition. SCADA includes the collecting of the information via a RTU (remote terminal unit), PLC's (Programmable Logic Controllers) and IED's (Intelligent electronic devices), transferring it back to the central site, carrying out any necessary analysis and control and then displaying that information on a number of operator screens or displays. Three of the most important part of a SCADA system is Master Station, Remote Terminal (RTU, PLC, IED) and the communication between them. In order to have good communication between them, there must be a communication protocol. DNP3 and T101 are two of the most common protocols today. It is important to determine which protocol should be applied if you are planning a SCADA system. In the next sections the DNP3 and T101 will be discussed and compared.

2.      Protocols in SCADA Communication

In order for SCADA systems to obtain its functionality, it needs a protocol for transmitting data. Some of the SCADA protocols include Modbus RTU, RP-570, Profibus and Conitel. These communication protocols are all SCADA-vendor specific but are widely adopted and used. Standard protocols are IEC 61850 (in which T101 branched out), IEC 60870-5-101 or 104, and DNP3. These communication protocols are standardized and recognized by all major SCADA vendors. Many of these protocols is now improved and contain extensions to operate over TCP/IP. It is good security engineering practice to avoid connecting SCADA systems to the Internet so the attack surface is reduced. RTUs and other automatic controller devices were being developed before the advent of industry wide standards for interoperability. The result is that developers and their management created a multitude of control protocols. Among the larger vendors, there was also the incentive to create their own protocol to "lock in" their customer base. This paper discusses and compares T101 and DNP3. These two open communication protocols that provide for interoperability between systems for telecontrol applications. Both are now competing within the world market. DNP is widely used in North America, South America, South Africa, Asia and Australia, while IEC 60870-5-101 or T101 is strongly supported in the Europe.

3.      IEC 60870-5

IEC 60870-5 is the collection of standards produced by the IEC(International Electrotechnical Commission). It was created to provide an open standard for the transmission of SCADA telemetry control and information. It provides a detailed functional description for telecontrol equipment and systems for controlling geographically widespread processes specifically for SCADA systems. The standard is intended for application in the electrical industries, and has data objects that are specifically intended for such applications. It is also applicable to general SCADA applications in any industry. But IEC 60870-5 protocol is primarily used in the electrical industries of European countries. When the IEC 60870-5 was initially completed in 1995 with the publication of the IEC 870-5-101 profile, it covered only transmission over relatively low bandwidth bit-serial communication circuits. With the increasingly widespread use of network communications technology, IEC 60870-5 now also provides for communications over networks using the TCP/IP protocol suite. This same sequence of development occurred for DNP3.