An Introduction to IEC 60870-5-101--GBC Report 011-2

7/30/2019 An Introduction to IEC 60870-5-101--GBC Report 011-2

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A

GENERALINTRODUCTION TO THE IEC

60870-5-101 STANDARD

FOR A COMMUNICATION PROTOCOL THAT SUPPORTS BASICTELECONTROL TASKS

1. FOREWORD

The IEC 60870-5 series of standards documents consists of a number of basestandard sections and an open number of companion standards. Eachcompanion standard contains a selection of provisions taken from the basestandards sections. These are tailored to suit a particular application by addingfurther provisions that are defined within the companion standard itself. Theapplications covered are all related to performing tasks needed by electricpower systems, although companion standards for other types of system couldalso be produced.

The subject of this report is an international standard that provides rules forcommunicating between telecontrol stations. Such rules are known as atelecontrol communication protocol. Several stations, which use this protocol,may be assembled into an interconnected installation for controlling andmonitoring the operational equipment of a widely distributed electric powersystem, from a central point.

The protocol is defined with reference to a simplified version of the BasicReference Model (ISO 7498) for system interconnection.

The present report starts by giving an outline of the reference model used,then continues by describing how the particular protocol works and givingsome of the reasoning behind the standard. Finally some questions, which havebeen asked about the standard, are answered.

NOTE: The views expressed in this report are those of the author. They areintended to help the reader understand the standard described. The report

does not give or imply any guarantee by the author concerning the use of thestandard or its suitability for any particular installation.

2. REFERENCE MODEL

The Basic Reference Model divides a protocol into seven layers. The top threelayers are directly concerned with the actual Application messages being sentbetween stations. The bottom four layers are concerned with the method usedto Transport these messages between stations.

The simplified reference model used in the IEC 60870-5-101 standard (andseveral other protocol standards) has fewer layers, because some of thefacilities supported by the full seven layer model are not required and

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enhanced working of the remaining facilities is desired. Hence the model isoften called the Enhanced Performance Architecture (EPA) Model.

Each station in an installation performs its own local application tasks, calledApplication Processes. For example the central (Controlling) station would drivethe keyboard/display or other operator interface equipment and manage the

database containing all the information about the installation, such as currentvalues of measured variables obtained from remote outstations etc.

Each outstation would have Application Processes for scanning, reading andstoring its local measurements and performing local control actions etc.Communication between Application Processes in the central station and thosein remote outstations is performed according to the communication protocol.

The following figure shows two stations communicating using the EPA model.Each station has a stack of protocol layers providing communication servicesto the station Application Processes at the top and accessing the

communications medium at the bottom.

STATION A STATION BAPPLICATION

LAYER(7)

APPLICATIONLAYER

(7)

LINK INTERFACELINK INTERFACE

LINKLAYER

(2)LINK

LAYER(2)

PHYSICAL INTERFACEPHYSICAL INTERFACE

PHYSICALLAYER

(1)PHYSICAL

LAYER(1)



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COMMUNICATIONS MEDIUM

In general Application data is accepted at the top of the protocol stack in one

station (say station A) and passes down through the stack, acquiring in eachlayer any necessary extra data needed to control the working of the protocol,until it emerges in serial form at the bottom. It is then transmitted to the otherstation (say station B) where it enters at the bottom of the protocol stack. Thedata passes up this stack having the control data stripped off layer by layeruntil the original Application data emerges at the top and is passed to theApplication Processes in station B.

This is called peer to peer communication because all data originating in aparticular layer is transported to the same layer in the remote station.

The model is used to define a protocol in an abstract way. A real protocol maybe implemented in any way at all provided that, when viewed from outside thestations, it works exactly as specified by the model. Thus the protocol standardimposes minimum constraints on the hardware and software used inside thestations. This allows stations to be chosen according to economic or otherpractical considerations.

The layer interfaces shown in the protocol stack do not have to be present in areal station. However the Physical Interface is usually present because thePhysical layer is often implemented using a separate MODEM. The LinkInterface needs to be present if it is desired to implement the Application layersoftware and the Link layer software separately.

3. MESSAGE STRUCTURE

Serial messages, as viewed outside of the stations, have a nested structurewhich derives from the layered structure of the protocol, see the followingfigure.

Start End

S L L S C A ASDU CS E

APDULPCI LPCI

LPDU

All data fields shown in the figure consist of octet strings of one or more octets.

The ASDU (Application Service Data Unit) is a block of data being sent from theApplication Processes in one station to the Application Processes in another

station.



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According to the EPA model some APCI (Application Protocol ControlInformation) is in general added to the ASDU to form the APDU (ApplicationProtocol Data Unit). However the APCI is not needed in the IEC 60870-5-101protocol, so the APDU is equal to the ASDU.

The Link layer adds its own LPCI (Link Protocol Control Information) to the APDUto form the LPDU (Link Protocol Data Unit). In addition it prepares each dataoctet in the LPDU to be transmitted as an Asynchronous start/stop serialcharacter having one start bit (value=0), eight data bits (the data octet), oneeven parity bit and one stop bit (value=1).

The LPDU is transmitted as a contiguous frame with no idle line (gaps)between the asynchronous characters.

LPCI = S+L+L+S+C+A+CS+E

S = Start character which has a fixed defined bit pattern.L = Length character which specifies the length in octets of the ASDU+C+AC = The Link Control character.A = The Link Address field which is one or two characters chosen to suit the

installation.CS = The Check Sum character.E = End character which has a fixed defined bit pattern.

The protocol specifies that, for transmission speeds up to 1200 bits/second, thePhysical layer shall convert each transmitted bit directly into one of two

frequencies, representing the binary one state and the binary zero staterespectively. This form of modulation is called Frequency Shift Keying (FSK) andit is both symmetrical and memoryless. It is suitable for most voice frequency(v.f.) analogue channels on base band transmission line, power line carrier orradio communications media.

4. FRAME INTEGRITY

The LPDU frame, as shown in the above figure, provides a very high dataintegrity (IEC Integrity Class I2). There must be at least four bit errors in areceived frame before an undetectable frame error is possible. This

corresponds to a code hamming distance of four (hd=4).

The frame consists of two parts which may be called the header and thebody. The header contains the S+L+L+S characters and the body containsthe remaining characters. Each frame is preceded and succeeded by a periodof idle line (continuous binary one state). The header specifies the length of theASDU+C+A and hence the length of the body.

Due to the parity protection, at least two bit errors in the contents of anyasynchronous character are required to cause an undetectable character error.



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In the frame body, at least two characters with undetected errors are requiredto produce an undetectable Sum Check error. Thus a total of four data or paritybit errors are required to produce a received frame body with a possibleundetectable error.

If the start bit of any asynchronous character is erroneously received as a

binary one, then the start of that character will be delayed until the next binaryzero bit is received. This will delay (and corrupt) the received character and,because there are no gaps between characters, it will also delay (and corrupt)all subsequent characters in the frame. This is sometimes called a sync slip.

If such a sync slip occurs in the body of a received frame, the End character willbe pushed into the idle line period, which succeeds the frame. This will bedetected by the receiver, with a high degree of certainty, because the patternof bits in the End character is specially chosen to differ from idle line andshifted versions of the character itself by at least four bits.

The frame header has a fixed format and length. It is separately protected withhd=4 integrity against both sync slip and data/parity bit errors. This means thatthe receiver can rely on the length information it provides. Hence the receiverknows exactly where to look for the End character when checking the integrityof the frame body.

Because the FSK modulation in the Physical layer is symmetrical, there is anequal probability of receiving erroneous binary one bits and erroneous binaryzero bits. But more importantly the modulation is memoryless, which meansthat it does not degrade the integrity of the Link layer frame by introducing any

particular erroneous bit more than once into a received frame.

It is sometimes desired to transmit messages at speeds in excess of 1200bits/second. To do this synchronous MODEMs may be used. These have bitscramblers to ensure that the receiver clock does not lose synchronisationduring prolonged periods without change of binary state in the data being sent.The scrambler introduces memory into the modulation and potentiallydegrades the integrity provided by the Link layer. However, it is has recentlybeen proved that certain faster non-memoryless modulation methods may beused, without loss of integrity, due to particular properties of the Link frame.See

IEC 60870-1-5 technical report when it is published.

In some circumstances faster transmission is possible on directly connecteddata circuits using digital signal multiplexers. Speeds up to 19,200 bits/sec forasynchronous characters and faster for the same characters sent isochronouslyare possible.

5. THE USE OF THE LINK AND PHYSICAL LAYERS

As explained above the LPDU (Link frame) contains the ASDU (Applicationmessage) whilst it is being transmitted using FSK modulation on a v.f. analoguechannel between stations.



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The protocol is intended for use with permanent directly connected datacircuits between the central station and the outstations. These may beimplemented as follows:-

A multi-drop party line connection which is time shared between several or all

outstations, using different Link addresses to identify individual outstations.This would normally be operated in a half-duplex mode on a single channel,sending to and receiving from each outstation in turn, using unbalanced(speak when you are spoken to) media access for the Link protocol.

CENTRALSTATION

OUTSTATION

OUTSTATION

OUTSTATION

Individual full duplex connections to some or all outstations, with a single v.f.channel for each direction of communication in each connection. The individualconnections permit balanced media access for the Link protocol, enablingspontaneous sending of data in both directions.

CENTRALSTATION

OUTSTATION

OUTSTATION



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OUTSTATION

The protocol provides Link functions for supporting balanced and unbalancedmedia access. However practical considerations (including higher cost) maylimit the extent to which full duplex balanced connections are used.

6. PROVISIONS OF THE APPLICATION LAYER

The Application layer of the protocol includes all those parts of the stationApplication Processes which are concerned with communicating withApplication Processes in a remote station. These parts are referred to as the(protocol) User Process in the IEC 60870-5-101 standard.

The standard defines two sets of provisions for the Application protocol.

a) Application FunctionsStation InitialisationData acquisition by pollingCyclic data transmissionAcquisition of eventsGeneral interrogationClock synchronisationCommand transmissionTransmission of Integrated TotalsParameter loadingTest procedureFile transfer (for simple files)Acquisition of transmission time delay

b) Application Service Data Units (ASDU)The protocol offers an assortment of different types of ASDU suitable for theApplication. However they all have the same general format as shown in thefollowing figure.

T Q C CA OA IE TT ------------------------- OA IE TT

IO 1 IO n

T = Type Identification (1 data octet)



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Q = Variable Structure Qualifier (1 data octet). Indicates the number ofInformation Objects in the ASDU or the number of Information Elementsin a single Information Object.

C = Cause of Transmission (1 or 2 data octets, fixed per installation)Causes include: periodic/cyclic, spontaneous, Request/Requested,Activation (of a Control action), etc.

CA = Common Address (1 or 2 data octets, fixed per installation). Distinguishesthe Station Address/Station Sector Address housing the InformationObjects (IO1 to IOn).

OA = Information Object Address (1, 2 or 3 data octets, fixed per installation).IE = Set of Information Elements (as defined for the type of ASDU specified in

the T field).TT = Time Tag of Information Object (if specified for the type of ASDU specifiedin

the T field).

USING THE PROTOCOL

The protocol is mainly concerned with standardising provisions so that differentsuppliers of stations can agree a common set of provisions for a particulartelecontrol installation, to ensure station interoperability.

The parameters associated with the Link layer interface within stations are notdefined explicitly in the standard, because the standard is not concerned withstandardising the structure of the software used to run the communicationprotocol.

The standard permits the user of the telecontrol installation to specify/choosehis own system strategy for using the protocol provisions in ways that solve hissystem problems and adhere to his economic, practical and technicalconstraints.

For example if economic considerations dictate that some or all outstationstimeshare a multi-drop party line, it is necessary for the central station to havea suitable outstation data polling process. Such a polling process would resideat the Application level. However, no detailed polling process is defined in thestandard because it has no peer to peer communication of its own with theoutstations. It merely controls the sequence in which the standard Link

functions are used to acquire any Application data that is waiting for transfer tothe central station.

At the end of the standard there is a checklist to enable all parties concernedwith an installation to agree exactly which protocol provisions are to be used,to ensure interoperability.

QUESTIONS AND ANSWERS ABOUT THE STANDARD



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Q1. Why is the asynchronous FT1.2 frame format used rather than the moreefficient FT2 isochronous format also defined in the base standardIEC 60870-5-1 ?A1. It is generally agreed by both suppliers and users that it is worth some lossof efficiency to be able to readily obtain industry standard hardware on whichto run the Link protocol. Unfortunately the FT2 frame format requires special

hardware, which is never likely to be made in large quantities and will thereforealways be expensive and difficult to obtain.

Q2. Why does the standard not use the ISO Abstract Syntax Notation (ASN.1)for defining its ASDUs ?A2. At the time the ASDUs were being defined by the IEC, the only method ofencoding ASN.1-defined data units for transmission was by using the BasicEncoding Rule (BER). This is very general and flexible but is very inefficient. Atypical encoded ASDU would use three to four times as many bits as arecurrently used. More recently the ISO has introduced a Packed Encoding Rule(PER) which has the same order of efficiency as the encoding used in the IEC60870-5-101 standard. However the latter is now published and has alreadybeen satisfactorily implemented by several suppliers, so it is too late to changethe encoding method used.

Q3. The standard states that there shall be no gaps between characters inthe transmitted frames. Is there any tolerance allowed ?A3. Yes. It has recently been proved that a small gap, of no more than onetransmitted bit duration, is permissible without any reduction in integrity.

Q4. Is it possible for a user or a supplier to specify a special ASDU of a type

not listed in the standard ?A4. Yes. The use of privately defined ASDUs is allowed, provided they havethe same general format as the standard ASDUs. The standard providesreserve codes for compatible private use. The use of privately defined ASDUsshould be made clearly visible to all parties concerned with a particularinstallation. Because without such visibility the required degree ofinteroperability may not be obtained.

Q5. Is it possible to use the IEC 60870-5-101 standard in installations wherethe user wishes to use standard digital data networks (which do not havepermanent direct connections between stations) ?

A5. This has recently become possible using a new standard called IEC60870-5-104. This will allow the Application layer of IEC 60870-5-101 protocolto be used with such networks via an existing standard Transport interface.

FOOTNOTE

The author is prepared to attempt to answer any further questions readers mayhave about the IEC 60870-5-101 standard.

Author: G Barry ColeEmail: [email protected]



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An Introduction to IEC 60870-5-101--GBC Report 011-2

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