A SCADA Comunications

8/3/2019 A SCADA Comunications

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COMPONENTS OF A SCADA SYSTEM

SCADA communication system :

encompasses the transfer of data between a SCADA central hoscomputer and a number of remote sites (Remote Terminal Units or RTUsand the central host and the operator terminals. Figure 1 shows a generSCADA system that employs some form of data multiplexing (process osending more than one message simultaneously through a multiplesystem )

multiplexers (MUXs) : between the central host and the RTUs. Thesmultiplexers serve to route data to and from a number of RTUs on a locanetwork, while using one or very few physical links on a Wide AreNetwork (WAN) backbone to pass data back to the central host computer

SCADA systems consist of

One or more field data interface devices usually called Remote StationRemote Terminal Units (RTUs), or Programmable Logic Controllers (PLCswhich interface to field sensing devices and local control switchboxes anvalve actuators.


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A communications system used to transfer data between field datinterface devices and control units and the computers in the SCAD

central host. The system can be radio, telephone, cable, satellite, and son, or any combination of these.

A central host computer : (computer that provides services to othecomputers that are connected to it )server or servers (sometimes called SCADA Center, master station, master terminal unit, or MTU).

A communications system to support the use of operator workstationsthat may be geographically remote from the central host computer.

A collection of standard and/or custom software [sometimes calledHuman Machine Interface (HMI) software or Man Machine Interface (MMI)software] systems used to provide the SCADA central host and operator

terminal application, support the communications system, and monitorand control remotely located field data interface devices.

Field Data Communications System

The field data communications system is intended to provide the meansby which data

can be transferred between the central host computer servers and thefield-based RTUs.

Bandwidth. An important property of a communications channel is itscapacity to


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carry data. The term bandwidth is used to describe this capacity.Originally, the term

bandwidth applied to the width, in Hertz, of an analog channel. Forexample, a telephony

voice channel that occupies the nominal band 0.3 to 3.4 kHz has abandwidth of 3.1 kHz and a radio channel that occupies the spectrumfrom 929.88875 to 929.8875 MHz has a channel bandwidth of 12.5 kHz.With digital transmission, the term bandwidth has been extended toinclude the data transmission rate in bits per second (bps).

SCADA Communications Availability and Protocols. The availability

imposed by the communications infrastructure is an important aspect ofthe SCADA

system. Because SCADA systems are typically deployed over largegeographical

areas, links to remote SCADA outstations from the central host computer

are often

multilayered, meaning that there may be several physical and logicalpaths through

which data must be routed before it reaches the intended destination.Such longhaul

links may impose a heavy financial consideration on the type ofcommunication

systems used and the bandwidth used on those links.

What the differences between SCADA communication system and backbone

communication system?

As a result of cost constraints, SCADA communications links generallyoffer less


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bandwidth and lower reliability than that offered by communicationsbackbones commonly used on a process plant, where there are fewgeographical constraints and highspeed fiber optic LAN infrastructuresmay be employed. Process plant communication backbones may exhibit

99.9 percent availability (less than 9 hours outage per year) and bit errorates of better than 10-9 (1 error in every 109 bits). Comparatively onSCADA links, where a combination of data radio, telephone line, andsatellite link

technology may be employed, the availability may be as low as 99.0percent (an average

of approximately 90 hours outage per year) and bit error rates of 10-6 (1error in every 106 bits) or greater.

The availability discrepancy can be attributed to the fact that themultilayer SCADA

links traverse a greater number of media conversion and data routingports as compared

to high-speed optic fiber LAN backbones. Therefore, there are manysingle points of failure in a diverse SCADA communications network.Communications outages typically result from equipment and powersupply failures and human interference.

Better availability is possible through the use of redundantcommunications paths to outstations; however, such designs cancontribute significantly to the cost of a

communications system and therefore may not be financially viable if th

communications link is not crucial to operational security.

SCADA communication protocols :are designed specifically for thereduced reliability


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communications links typically employed with SCADA systems and toprovide

secure transmission of data, guaranteeing reliable delivery of data to theintended destination in most circumstances.

The protocols employ error detection and message retry techniquesusually by receive/transmit handshaking established through the use ofheaders and footers attached to the raw data under transmission. Suchextra information introduces an overhead to the transmission of data,resulting in a trade off between speed of data transmission and thereliability of the communications link. As a result, the speed of datacommunications associated with SCADA is regularly slower than thattypical of a communications backbone commonly used on a process

plant, office, or factory floor application.

Not only do the latter boast links using media such as hard wired opticfiber, which usually allows higher speeds of data transmission thanavailable over radio links, but the communications protocols associatedwith SCADA systems introduce data transmission overheads which furtheslow the rate of data transfer. Where media such as low bit error rateoptic fiber cabling is available, simple communications protocols may beused which do not require substantial transmission overheads.

Users of SCADA systems and the resulting data do not need to be awareof the communications

protocols used. In fact, the protocols should be transparent to the user.

However, it is important to understand that with the use ofcommunications links such

as radio, there is a possibility, albeit small, that communication errors wioccur.

For example, a control command could be sent to the wrong destination.SCADA systems


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often request confirmation from an operator to confirm that a controlaction is

required. This approach provides some level of protection against acontrol message

being sent to the wrong destination. It is much more likely, however, thathe operator

has made the error and the control action has been requested of theincorrect outstation.

The control confirmation check gives the operator another chance toselect the

correct outstation for control.

An example of a SCADA communications protocol includes DNP 3.0(Distributed

Network Protocol), a vendor independent protocol that incorporatesmultiple layers of

error detection and correction and allows a select/confirm regime forcontrol actions.

Modbus is another widely used protocol for SCADA, but it does not offerthe same

level of data transmission security as DNP 3.0. There are also a largevariety of protocols

that are proprietary to individual SCADA vendors and offer capabilitiessimilar

to those described in this section.


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Common Communications Media. The following communicationsmedia

are common:

Licensed radio links (UHF and VHF)

Unlicensed spread spectrum radio links

Public switched telephone networks

Mobile telephony

Microwave

Cable TV networks

Dedicated satellite links

Dedicated cable, including fiber optics (for very short distancecommunication)

Corporate WAN computer communications systems

For highly critical sites, it is not uncommon for combinations of thesedifferent media

to be used to ensure high reliability of communications to the site.

Selection of the preferred communications media depends onseveral important factors:

The remoteness of the field equipment site


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The required reliability of the communications media (primarilydetermined by the perceived operational importance of the remote site)

Availability of communications options

Cost of each option for the particular application

Availability of power (power company, battery, solar, or other)

The communications systems used for SCADA are often split into twodistinct parts:

a Wide Area Network backbone (WAN) and numerous Local Area Network(LANs).

The interface between the two parts is commonly achieved through somform of

multiplexing.

Wide Area Network Backbone.The WAN connects the central hostcomputer

to the multiplexers. It may comprise cable, radio, or satellite datacommunications

links depending on the geographic distribution of the SCADA system.

The WAN links are generally full duplex (they provide simultaneous datatransmission in

both directions) and may be configured in a staror loop topology.

The star and loop topologies employ dedicated point-to-pointcommunications links


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between multiplexers. The star configuration (as shown in Figure 1) doesnot provide

WAN redundancy. The loop configuration (see Figure 4) links adjacentmultiplexers

and provides alternative communications paths for redundancy, thereforproviding

higher reliability. Looped WANs require data traffic routers, and links musbe dimensioned

to carry all WAN traffic

In some cases, a WAN is not needed. An example is a simple SCADAsystem where

all RTUs are connected directly to the central host computer via a singlemulti-drop

communications link. These systems therefore effectively only contain anRTU local

area network.

Multiplexers. Generally, some form of multiplexing is required to conneca


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WAN backbone to a local network of RTUs. Multiplexers allow differentdata streams

to share a single data link, as shown in Figure 5. Multiplexers combinecommunications

paths to and from many RTUs into a single bit stream, usually using timedivision

multiplexing (TDM) or other such bit stream manipulation techniques.

The multiplexers must be able to combine the traffic to and from tens orsometimes hundreds of RTUs for transmission over the SCADA WAN.

A simple form of multiplexer is to use a data traffic router together with a

point-tomultipoint radio, as shown in Figure 6.

In this diagram, LR refers to Local Radio, PMR to Point to Multipoint Radioand ROUT to data router.


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The multiplexer may itself be a SCADA processing device that managesthe local network

and not only combines the data, but also reduces the amount of data thamust be

interchanged with the central host.

The SCADA system may employ a tree network with multiple hierarchica

levels of multiplexer processors, as shown in Figure 7.

Local Networks. Local networks connect the RTUs to the multiplexers,or directly to the SCADA central host computer if there is no need for aWAN connection.

Like the WAN, the local network may comprise cable, radio, or satellitedata communications links depending on the geographic distribution ofthe SCADA system.

The links may be private or rented from a telephone company.


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The Central Host Computer

The central host computer or master station is most often a single computer or anetwork of computer servers that provide a man-machine operator interface to theSCADA system. The computers process the information received from and sent to thRTU sites and present it to human operators in a form that the operators can workwith. Operator terminals are connected to the central host computer by a computer

network so that the viewing screens and associated data can be displayed for theoperators. Recent SCADA systems are able to offer high resolution computer graphicto display a graphical user interface or mimic screen of the site or water supplynetwork in question. Figure 8 shows the types of display screens offered by mostsystems.

Some examples include

System overview pages displaying the entire water distribution system and oftensummarizing SCADA sites that might be operating abnormally


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Site mimic screens for each individual RTU location showing up to the minute siteinformation and offering an interface to control items of equipment at that site

Alarm summary pages displaying current alarms, alarms that have beenacknowledged by an operator, and alarms that have returned to normal but remain

unacknowledged by an operator

Trend screens enabling an operator to display the behavior of a particular variableover time Historically, SCADA vendors offered proprietary hardware, operatingsystems, and software that was largely incompatible with other vendors SCADAsystems. Expanding the system required a further contract with the original SCADAvendor. Host computer platforms characteristically employed UNIX-based architecturand the host computer network was physically removed from any office computingdomain.

However, with the increased use of the personal computer, computer networking has

become commonplace in the office and as a result, SCADA systems are now availablthat can network with office-based personal computers. Indeed, many of todays

SCADA systems can reside on computer servers that are identical to those serversand

computers used for traditional office applications. This has opened a range ofpossibilities

for the linking of SCADA systems to office-based applications such as GIS

systems, hydraulic modeling software, drawing management systems, workscheduling

systems, and information databases.


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Operator Workstation Communications System

SCADA systems, several operators may require simultaneous access

to the SCADA central host computer to view the performance of thesystem. SCADA

systems are often designed to accommodate this requirement byincluding communications

channels between the central host and the remote workstations accesseby the operators.

Operator workstations are most often computer terminals that arenetworkedwith the

SCADA central host computer. The central host computer acts as a servefor the


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SCADA application and the operator terminals are clients that request ansend information

to the central host computer based on the request and action of theoperators.

The communications system in place between the central host computerand the operator

terminals is a Local Area Network. SCADA LANs enable multiple users in relatively

small geographical area to exchange files and messages, as well asaccess shared resources, such as the central host computer.

Historically, SCADA LANs have been dedicated networks; however, withthe increased deployment of office LANs and Wide Area Networks (WANsas a solution for interoffice computer networking, there exists thepossibility to integrate SCADA LANs into everyday office computernetworks.

The foremost advantage of this arrangement is that there is no need toinvest in a separate

computer network for SCADA operator terminals. In addition, there is aneasy path to integrating SCADA data with existing office applications,such as spreadsheets, work management systems, data historydatabases, GIS systems, and water distribution modeling systems.However, there are several disadvantages that should be considered

before integrating SCADA operator terminal LANs with office LANs:

Corporate networks are often only supported during office hours whileSCADA LANs are most often required 24 hours per day, 7 days per week


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Communications links associated with SCADA may present a networkinsecurity breach into the corporate computer network because some linksmay bypass the office networks usual security precautions

During office hours, data traffic on the network associated with thecorporate network may seriously slow the networking of the SCADAoperators

SCADA network traffic generated during emergency operationprocedures may seriously slow the corporate computer network

Linking the SCADA system with the office LAN provides ways forhackers or terrorists to interfere with operation of the system

Common Communications Media.

There are many communication methods available. Evaluation ofdifferent communication systems for data communication among thesystem elements is required at the planning stage. The fundamental

requirements for communication infrastructure are:

(i)

If it can handle the requisite amount of data and multitasking;(ii) Determination of system average message rate;(iii) Data through put and system response times should meet variousapplication requirements;

(iv) It should allow for network growth and added applications.The communication methods may be used individually or combined.

Public Telephone Communication (PTC)

Dial-up and dedicated leased telephone lines are often used for

Distribution Automation. Telecommunication services through Packet

Switching Network (PSN) ),(wide area network in which the transferred information is dividedinto segments which are sent separately with the attachment of the destination computer's address )

cellular radio or rural radio are viable, and may have the advantages ofproviding services in otherwise inaccessible places.


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Power-Line Carrier (PLC)

It has an advantage of using the power network owned by the utility. Itcan reach any point on the network and extend automatically to newly

added network elements. It has very limited bandwidth, in most casesless than 100 bits/second. This technology is useful for consumer loadmanagement and automatic meter reading of the consumer. PLCtechnologies are:

Ripple control very low-frequency signals injected at high-voltage levels

Signaling by changing the power waveform on distribution feeders, so

that the time of zero crossing is shifted.

Distribution line-carrier-signals in the range of 5-12 kHz injected intodistribution feeders.

Ripple control is slow and suitable for applications such as one way loadcontrol and capacitor control The other two technologies can provide twoway communications at up to a few dozen bits per second (bps) betweensubstations and devices on the distribution network.

Radio Communication

Radio communication systems, like PLC systems, are usually owned bythe utility, but they have the important advantage of their operationbeing independent of the condition of the power system. Furthermore,many data channels, including high-speed channels, as well as voicecommunication can be provided. Radio technologies require licensing-

generally UHF, and VHF radio is the most widely used medium. Thedisadvantages of radio are the limited channels available from licensingauthorities, the relatively low data rate possible (typically 1200 bits/sec.)

The radio technologies generally used are:

(a) UHF Point-to-point Radio


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UHF radio in the 800-900 MHz range is widely used by utilities for twoway point-to-point communications. Systems in the 400 MHz range arealso available. These systems usually support up to 15 voice and datachannels operating at 4800 or 9600 bps. They require line of sight

operation, in particular for the 900 MHz frequency bands. With flat terraiand antennas mounted at a reasonable height, reliable operation over arange of 30 to 40 km is practical. Dish antennas are used for point-to-point radios.

(b) UHF Multi Address System Radio

Multi address systems were developed as a substitute for dedicated

telephone lines, and are widely used for SCADA/EMS/distributionautomation application. They consist of master radio station whichcommunicates with several remote radios with IRTU (RTU interfaced withdistribution equipment). Each system uses a pair of frequencies (400MHz-l000 MHz)one for master-to-remote communications and the otherfor remote to master, so that duplex communication is possible. Coveragcan be increased by providing remote more sub-master stations. Anomnidirectional rod antenna is used at the master radio. At the remoteradios, directional antennas provide better performance, but less

expensive, less conspicuous and vulnerable "stick" antennas are alsoused. Typical MAS system has been shown in Fig. 5. This technology isuseful for load management.

(c) VHF Radio

One-way VHF radio (typically, the 154 MHz band widely used in the USA)is generally used to send load control commands. Inexpensive radio

receivers / switches are available for giving load control switchingcommands.

(d) Packet Switching Network (PSN)


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A packet switching network uses low power UHF spread spectrumtransmission (typical 900 MHz band) and does not require licensing offrequencies. It is a unique solution to frequency management usingspread spectrum technology. Due to its very low power density spectrum

it minimises interference to existing services. The packet switching sendshort burst of digital information over low power radios. These are datatransmission networks with computerized nodes that perform specificcommunication tasks such as routing. Many power utilities in the worldare using packet switching networks for many purposes, ranging fromreal-time SCADA, EMS to line and capacitor switching, administrativedata; and from inter-utility to intra utility communications. The typicalPSN system is shown in Fig. 6. for WAN and LAN through PLC. PSNtechnology is convenient for use in inter-utility communication, lineswitching in energy management systems and in data transmission

between control centers. A number of utilities are using CCTIT's X.25protocol in these applications. X.25 is the ideal method to harmonize thedifferent hosts.

(e) Cellular Radio

This system is widely used for AMR, DSM and DA. Its benefits includewide-area broadcast and monitoring capability, and independence fromcommunication utilities. This technology offers very high-speed (9600

baud) real-time, bidirectional data gathering systems control, and shouldbe ideal for field operations. The typical cellular radio architecture isshown, in Fig. 7. The system has two levels of communication. A digitalcellular for wide area network (WAN) at 900 MHz range and a low powerspread spectrum RF as well as PLC channel for local area network (LAN).

Fiber Optics

Fiber optics, with its explicit downward cost trend in terns of product as we

as installation and maintenance costs, has become a widely accepted

choice, as it offers both technical and commercial advantages over

conventional systems that use metallic cables and radio links.


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The communication basically consists of a transmitter, and a receiver forinformation signals coming from the user's device, which is connectedthrough copper wires to the switching center or exchange, where it ischanged into a digital signal like 1s or 0s for easy handling. The signal isthen transferred to the transmitter. In the transmitter, the information


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signal which is electrical, drives an optical source: Laser or a lightemitting diode (LED), which in turn, optically modulates the informationsignal, which get coupled into the optical fiber. The receiver located atthe other side of the link detects the original signal and demodulates or

converts back the optical signal to the original information signal(electrical). The signal is then connected through copper wires to theswitching device or exchange for selection and connection to the properuser or user device. This type of communication is important for trunkroutes power transmission lines. In such a situation, the problem ofelectrical interference is not encountered. This has the advantage of highdata rate (9600 bits and much more) and f immunity from noise.

Optical fibers are available in cable form. Such cables can be laid in cabltrenches. There is a trend to associate optical fibers with power cables o

conductors themselves. Optical fibers are embedded within thecables/conductors, either earth or phase conductor. Optical fibercommunication is expected to play an important role in future protectionsignaling, either " as a dedicated protective relaying link, or as channelswithin a multiplexed system which carry all communication traffic such avoice, telecontrol, telemetering and protection signaling.

SatelliteCommunication


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This technology is useful for load management controls. Satellite-basedVSAT (Very Small Aperture Terminal) network communication is used forpower systems. It has a 99.5% reliability factor. A communication signal sent from a VSAT to the satellite at a particular frequency (see Fig.9). Thsignal is amplified, shifted to a different frequency and sent back to theVSA T at the destination. A VSAT network is distance insensitive. One hasto pay only for the cost of VSATs and service charges which are veryreasonable. The whole network can be monitored and controlled centrallyIf there is any problem in the network, it can be diagnosed from the hub.Primarily, there are three types of technologies for VSAT networks. The

'Star' technology is approved by the Government of India. In thisconfiguration, at the center of the network is a big earth station known athe hub with a dish diameter of 6-11 metres. The control of the network at the hub and it maintains a communication link between VSATsscattered all over the country or region. The configuration is thereforecalled the Star.


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The other technology is the 'Mesh' technology. The configuration permitsa group of VSATs to communicate directly with other VSATs going throughthe central hub. In this type of configuration, anyone of the VSATsfunctions as a hub and controls the whole network. This configuration

enables better voice and video communication than the Starconfiguration. This technology has also been introduced in India. In thethird type of configuration, there is point to point connection between thVSA Ts. In such a network, data, voice and video signals are carrieddirectly between the locations.

In satellite communication, an extended C-band is used forcommunicating the data. HCL COMNET, VSNL, Wipro BT, AT&T, HECL,MAX, RPG satellite and communications have emerged as the large VSATservice operators in the country. VSATs are readily available in the

market.

Transmitter

This provides output (transmittable) signals after converting andamplifying low level signals of basic sensor elements. Moderntransmitters are smart and microprocessor based, performing many othefunctions such as alarm, signal etc. .

Transducer

This is a measuring element that senses the external action. It gathersparameters and supplies through remote telecommunication capabilities


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Why TSATs PrivateSatellite Network is thebest solution for your

companyYOU GET FULL CONTROLOperating via a private HUB gives you full control overyour network. You do not have to rely on shared publicnetworks prone to malfunction and operating failure.Because the HUB is placed at your control center, the costof a leased line normally associated with a shared HUBsolution is eliminated.

YOU GET LOWER COSTTSAT features the lowest cost HUB on the market. Inaddition it requires very low bandwidth, thus making your


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space segment cost negligible.YOU GET SECURITY AND RELIABILITYOur experience shows that TSAT provides greater securityand higher reliability than radio systems, public networks andinternet solutions. It is less prone to unauthorized access(hacking), virus infections and disruption in service due to public works.

TSAT offers SatelliteBased Private Network

TSAT is a unique satellite communication solution developedfor the SCADA market. It is a tailor-made solution formanaging geographically dispersed assets through a unified,yet simple communication solution. The unique systemdesign provides low cost solutions to traditional problemareas such as frequency stability, phase noise and interferencerequirements.Our customers are electrical utilities, oil and gas companiesand water resource management companies all over theworld.

Low cost, tailor-madesolutionsEach system is tailor-made to our customers specific needs.

The unique technical solution offers flexibility and costeffectiveness due to features like:

_low cost HUB_low cost space segment_safe and reliable communications link_proprietary, dedicated HUB_flexible access schemes and protocol options_competitive with PSTN, radio links, and VSATshared HUB solutions.

Fields of applicationTSAT operates at data rates between 2400-14400 bps. Thesystem features flexible and highly reliable solutions forcollection and distribution of data in a wide array ofSCADA applications such as:ELECTRICAL POWERHydro-power generation, windfarms, load/energy management,

automated meter reading, substation automation,automatic switching operations, earth fault detection, transformermonitoring etc.OIL & GASPump and valve control, flow measurement, pressureand temperature monitoring, leak detection, corrosionmonitoring etc.WATERRiver, dam and reservoir level control, water flow,flood warning/alarm systems, dike management, coastalmonitoring etc.


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Easy, cost-effectivenetwork management

The TSAT NMS runs under Windows NT/2000/XP on anormal PC. This makes it very easy to manage your network.

Since the system is simple and uncomplicated to learn, itwill reduce the time and cost of training your staff on theoperation of the network.

Unique, optional featuresIn addition to all standard features, the following optionsare also available:_Additional inbound/outbound channels whichimproves your network capacity in case of hightraffic and high response time requirements._Redundant and load sharing HUBs with remoteterminals being able use a secondary HUB if theprimary HUB is not available.

_Protocol adapter to minimise link delays and increasenetwork capacity.

A SCADA Comunications

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