IEC 60870-5-101

IEC 60870-5-101 (also known as IEC 870-5-101) is an international standard, released at the beginning of the 90ies by  the IEC (International Electrotechnical Commission). The protocol has found widespread use in the energy sector, and is still used today. It is based on the EPA architecture (Enhanced Performance Architecture) and defines only the physical link and application layers of the OSI model.

IEC 60870-5-101 is primarily used with relatively slow transmission media on the asynchronous V.24 interface. The standard promises baud rates of up to 9600 bit/s, with much higher baud rates (<115200 bit/s) actually being used. X.24/X.27 interfaces with baud rates up to 64000 bit/s, also part of the standard's description, could not establish themselves and are rarely used.

IEC 60870-5-101 is a 'Companion Standard', extended by these further standards:

• IEC 60870-5-1
  defines different frame formats, though IEC 60870-5-101 uses only the FT1.2 format
• IEC 60870-5-2
  defines the link layer transmission mode
• IEC 60870-5-3
  defines the basic application data structure
• IEC 60870-5-4
  defines how information is encoded
• IEC 60870-5-5
  defines basic application layer functions
• IEC TS 60870-5-601
  defines "Conformance Test Cases" for IEC 60870-5-101

IEC 60870-5-101 was extended and more precisely defined in 2001 by Amendment 2.

Interoperability between devices by different vendors is ensured by the interoperability list, which is defined by the standard. In the list, the function range is defined for each device by marking the applicable functions. The common denominator between different vendor lists limits the possible function range.

As the original standard left more room for interpretation, there were many different implementations on a wide range of different devices many of which are mutually incompatible. To remedy this situation, major energy utilities, such as RWE, Bayernwerke (E.ON), have developed their own IEC 60870-5-101 based standards. The "Norwegian IEC 870-5-101 User Convention", managing line redundancy in particular, has found wide acceptance in Scandinavian countries.

IEC 60870-5-101 is known for the resilience of its link layer and for its simply structured application layer. The main focus was placed on on performance definition, so that some information needed for data decoding is not transmitted. For data decoding to work, parameter settings such as information object length, ASDU address length etc. need to be correct. This is not much of a problem, as interoperability lists help to achieve this.

Fragmentary protocol definition has its disadvantages, especially with respect to line redundancy where a range of diverging implementations exist. A clear definition of project-specific requirements is recommended.

In addition to standard protocol functionalities, IEC 60870-5-101 also allows for extensions with proprietary vendor-specific functions. Though not used too often, this also carries the risk of incompatibilities.