IEC 60870-5-101 uses three layers: physical, link and application. The application layer exchanges units called ASDUs, which carry the data type, the cause of transmission, the common address and one or more information objects. Knowing these fields lets you read a frame, find out why a signal does not arrive and reuse the signal list when migrating to IEC 60870-5-104.
What IEC 60870-5-101 defines
IEC 60870-5-101 is the companion standard for telecontrol over serial transmission. It governs how a controlling station, such as a control center, exchanges states, measurements, counters and commands with controlled stations, such as an RTU or a substation.
The standard does not invent everything from scratch. It takes what it needs from five base documents of the IEC 60870-5 series: frame formats (IEC 60870-5-1), link transmission services (-2), general structure of application data (-3), definition and coding of information elements (-4) and basic application functions (-5). The 101 selects from each what it needs and leaves other options to the project. That is why it is called «companion»: it completes the series with the decisions for a specific case.
The three layers
The 101 follows the enhanced performance architecture (EPA) and specifies only three of the layers of the OSI model (Open Systems Interconnection, the seven-layer communications model):
| Layer | What it does | What the project selects |
|---|---|---|
| Physical | Moves the bits over the medium | RS-232, RS-485 or fiber transceiver; baud rate |
| Link | Orders the conversation and detects frame errors | Balanced or unbalanced mode; link address size |
| Application | Gives meaning to the data: what it is, where it comes from and why it is sent | Size of the address and cause fields; information types used |
Physical layer. The standard accepts any bit-serial transmission. In practice you see RS-232 over short distances, RS-485 on shared buses and fiber or modem on long links. The list of standardized rates runs from 100 bit/s to 64 000 bit/s depending on the interface type.
Link layer. It uses the FT 1.2 frame format, with a single-character variant for short acknowledgments. A link frame has a control field, a link address and, when it carries data, the ASDU. The maximum frame length is an option the project declares. The layer adds frame error checking and decides who may talk: that is balanced or unbalanced mode (a link with one master that polls each station, or with stations that initiate equally).
Application layer. This is the one the integration engineer cares about. It defines the data messages, called ASDUs, and the basic functions: general interrogation, spontaneous transmission, commands, clock synchronization, file transfer and others. Data transmission uses mode 1: the least significant octet goes first.
What an ASDU is
ASDU stands for Application Service Data Unit. Every message of the application layer is an ASDU, and every ASDU carries one or more information objects with the actual data. It has two parts: a data unit identifier and the objects.
| Field | What it is for | Size |
|---|---|---|
| Type identification (TI) | Says what kind of data follows: single-point, measured value, command, etc. | 1 octet |
| Variable structure qualifier | Tells how many objects there are and whether their addresses are consecutive | 1 octet |
| Cause of transmission (COT) | Explains why the message is sent: spontaneous, cyclic, reply to a command | 1 or 2 octets |
| Common address of ASDU | Identifies the station or sector the data belongs to | 1 or 2 octets |
| Information objects | Each with its address and its elements: value, quality, time | Address of 1, 2 or 3 octets |
The sizes of the cause of transmission, the common address and the object address are system parameters: the interoperability list of the standard forces you to choose them, and every device on the link must use the same ones. If the system uses two octets for the cause, the second is the originator address, which is zero when not used.
- Field device detects the change. A position, a measurement or a counter changes
- The ASDU is built. Data type, cause, common address and object with its address
- The link layer encapsulates it. Adds control, link address and error checking
- The physical layer transmits it. Over RS-232, RS-485 or fiber
- The control center receives it. Checks the frame, extracts the ASDU and assigns the value in its database
How the fields combine
The type determines how the object is interpreted. With the single-point type, the object carries a status bit and a quality descriptor. With the measured-value type, it carries a number. When the type ends in «TB», it adds a seven-octet time tag. The standard’s type list separates monitoring data (states, measurements, counters) from control data (commands and setpoints).
The cause of transmission changes the meaning of one and the same type. The same single point can arrive as a spontaneous event, as a reply to an interrogation or as a background scan. Two messages with the same type and the same object may not mean the same thing.
The common address plus the object address uniquely identify a signal. It is the address the control center database uses to assign the value to a variable. If either one does not match the control center configuration, the data arrives over the line but is not assigned.
Why these fields matter
- Troubleshooting. When a signal «does not arrive», the cause is usually that the type, the common address or the object address do not match between the two ends. A frame received correctly, but whose ASDU does not fit the configuration, can be ignored without the link showing any fault.
- Migration to IEC 60870-5-104. The 104 keeps the ASDU of the 101 and moves the transport to TCP/IP. The signal list is reused, but you have to review the field sizes, which are fixed in the 104. The full step is in IEC 60870-5-101: serial telecontrol protocol and move to 104.
- Testing. A protocol analyzer that decodes the ASDU lets you check point by point that the type and cause are the expected ones before connecting the control center.
What the standard does not cover
The 101 does not define the operational meaning of each signal. It does not say which object address «breaker open» must have or which type to use for a current measurement. That is decided by the project and recorded in the signal table agreed between whoever supplies the RTU and whoever operates the control center. Nor does it define communication security. For that there is the IEC 62351 series, summarized in IEC 62351: cybersecurity for IEC 61850, 104 and DNP3. How to read the parity, checksum and framing counters of a serial link is covered in IEC 101 Serial Link Errors.
We can help with your telecontrol link
At Tiza Group we do the SCADA and gateway integration of substations and plants: signal table, field sizes, tests against the control center and migration of serial links to IEC 104. We also load relay settings through remote relay configuration. Contact us for a quote.
Related guides
- IEC 60870-5-101: serial telecontrol protocol and move to 104
- IEC 60870-5-104: what it is, how it works and how it is tested
- Addresses in IEC 101 and 104: link, ASDU and object
- IEC 101 and 104 information types: the TI list
- Cause of transmission in IEC 101 and 104: what it means
- IEC 101 Serial Link Errors: Parity and Checksum
References
- IEC 60870-5-101: telecontrol equipment and systems, transmission protocols, companion standard for basic telecontrol tasks (interoperability list; physical, link and application layer clauses).
- IEC 60870-5-1 to IEC 60870-5-5: frame formats, link services, application data structure, information elements and basic functions.
Frequently asked questions
What is an ASDU in IEC 60870-5-101?
It is the data unit of the application layer. It carries a type identification, a variable structure qualifier, the cause of transmission, the common address of the station and one or more information objects, each with its address and value.
Why does the 101 use only three layers instead of the seven of the OSI model?
Because it is built on the enhanced performance architecture (EPA), designed for slow telecontrol links. It keeps only the physical, link and application layers and drops the intermediate ones to keep every message short.
Is the ASDU of the 101 the same as in the 104?
The structure is the same: types, causes and objects carry over. What changes is the transport, because the 104 carries the ASDU inside a TCP/IP frame, and the sizes of the address fields, which are fixed in the 104.