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PAC.MARCH.2010
Technology & Applicatio
GPRS
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GPRSA
pplications
Communic
ations
by Aitor Arzuaga Munsuri, Jose Miguel Arzuaga Canals and Mikel Zamalloa Aiartzaguena, uSysCom, Spain
PAC.MARCH.2010
do so. A clear alternative is the use ofpublic communications networks,using a service provided by anoperator. One option which hasbeen used for some time is data calls,either over a fixed or mobile (GSM)network.
In recent years, with thedevelopment and progressiveimplementation of packet switchingtechnologies over mobile networks(GPRS/UMTS/EDGE), a newrange of possibilities has openedup which may make them viablefor telecontrol applications. On theone hand, a TCP/IP-based serviceis offered, which guarantees thereception of traffic and the alwaysonline nature of this type of service.Further, charges based on exchangedtraffic volume, as opposed to circuitswitching connections which arecharged by connection time, may be
a very attractive feature for utilities.Only the traffic actually exchangedis paid for.
Nevertheless, each of thetelecontrol protocols which wereonce designed for implementationusing serial ports have theirpeculiarities which make direct useover a TCP/IP network, with the
inherent latency of a GPRS network,problematic and even unviable.There are a number of determiningfactors which must be consideredfor telecontrol communicationsto be rolled out appropriately andreliably. On the other hand, the mainsource of reluctance to using GPRSnetworks on the part of electriccompanies lies in the difficulty inmonitoring and verifying the levelof service provision promised by themobile services operator, given thatthis comprises an aspect which theelectric company does not possessa sufficient level of control over ormeans to verify.
This article summarises allof the aspects which must beconsidered in order to successfullyand reliably establish a telecontrolapplication over GPRS, and detailsthe parameters and methods which
must be taken into account by theelectric company to control thequality of service provided by theoperator.
Serial telecontrol protocols
and their peculiarities
Telecontrol protocols usedover a serial port communicationinterface were initially designed for
Implementation of Telecontrol
Applications over GPRS Networks
Given that GPRS operator
n e t w o rks are public, theimplications of this fact areexamined from the utilitys pointof view, and finally we list the mostimportant aspects to be consideredin order to perform a successfulimplementation of a telecontrolsystem over GPRS.
Companies in the electricsector have a multitude of SCADATelecontrol systems whichoperate with protocols aimedat serial ports, for example IEC60870-5-101, DNP3... Historically,communication with remote siteshas been performed using the electriccompanys private communicationschannels, such as SDH networks,PDH, VHF and microwave radioor others. This option was the onlyone available capable of covering thecommunication requirements for
the provision of service.In the present day, companies are
under increasing pressure to reduceinfrastructure operating costs, but atthe same time maintain (or increase)the availability and reliability oftheir communications. As such,new methods of communicationare being explored to allow them to
Aitor Arzuaga Munsuri grad
ated at the ETSIT Bilbao
1999. He began his career a
grantee at Hewlett Packard,
then moved to Ericsson.
2002 he joined uSysCom
branch of ZIV, and design
hardware for utility commu
cations equipment. He is an a
tive member of Cigr groups
representative of the Span
committee in IEC TC57WG
and a member of IEEE. He h
many articles about comm
nications for industrial applic
tions. Since mid-2009, Aito
head of R&D activities at ZIV
This article introduces GPRS (General Packet Radio Service)technology and examines its applicability to the sphere of telecontrolcommunications in an electric utility. In order to do so we will performan analysis of the technology and its possibilities, telecontrol protocols, andgive specific examples of why traditional telecontrol applications whichfunction correctly over point-to-point serial links require a reformulation
which optimises their function over a communications technology such asGPRS. It will also be analyzed whether this technology fulfils the reliabilityrequirements expected by utility communications.
Jose Miguel Arzuaga Canals
or Txetxu for friends obtain
in 1998 his Telecommunicati
Eng. degree. He started his c
rier in Vodafone, Spain, with p
sitions in the IT area. In 2000
started a new job in an Ericss
unit, whose goal was to ma
machines talk. At present, T
etxu is the general manager
uSysCom, a spin-off compa
in ZIV, uSysCom designs a
manufactures telecom. equ
ment for the Electrical Utiliti
38
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GPRS CarrierUtility IP corporatenetwork
Control Center(60870-5- 101 ...)
Firewall Serial Link
RTU (60870-5-10
Air
section
4SIP serial to IP
Serial lLink
4SIP GPSR toSerial
PAC.MARCH.2010
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Communic
ations
GPRSA
pplications
by Aitor Arzuaga Munsuri, Jose Miguel Arzuaga Canals and Mikel Zamalloa Aiartzaguena, uSysCom, Spain
Unlimited total traffic volumeor usage rate of the serial link, notrelevant
By means of a simple analysis
we can conclude that a GPRScommunications channel hascharacteristics very different to thosementioned above:
Latency can be considerable,reaching values from 200 ms perlink (typicalforUMTS), 500-700 ms(typicalforGPRS) and up to severalseconds (GPRS with low coverageand a traffic-saturated network)
Although the BER is very highin radio traffic, the modulationsemployed are robust, and coding anderror correction ensure that trafficsent to the application is error-free.However, all of these errors manifestthemselves as variance in linklatency. Sending delay depends onradio transmission conditions at anyspecific time
Lost frames are common.However, the TCP/IP protocol takescare of recovering them in theappropriate order after a few seconds.There are transport, network andlink levels below applicationcommunication
Traffic cost is calculated byvolume. The total amount of trafficis important.
Some telecontrol protocolswhich are commonly used intelecontrol applications andwhich are appropriate for use overa GPRS connection include IEC60870-5-101, DNP3, GESTEL,S A P 20, P ID 1 In g e n e r a l
any protocol which fulfils thefollowing criteria can be adjusted totransmission over GPRS:
Frame format correctly defined
by characters or binary sequences atthe beginning and end of the frame,avoiding ambiguities (predefinedescape sequences)
Predetermined maximum andminimum frame lengths, and wherepossible not too long as this impactsnegatively on latency. Satisfactoryresults have been obtained with avalue of 256 bytes
Error detection mechanism inthe protocol frame itself (CRC orsimilar) which allows for validationof the integrity of the protocol frameat both ends
Example of a telecontrol
application over GPRS
I n a t y p i c a l t e l e c o n t r o lapplication which we will take as anexample, there is a serial connectionbetween a Control Center, generallylocated in an office, and a telecontrolRTU located in a remote installation.The serial Control Center RTUconnection is realized by means ofa radio communications channel,microwave links (private channels)or by satellite and telephonenetworks (third-party channels). Inthe proposed solution over GPRSshown in Figure 1, the serial link isreplaced by a TCP connection overGPRS.
In order to implement atelecontrol solution over GPRS itis necessary to add encapsulators toperform the following functions:
transmission methods characterisedby the following properties:
Negligible latency in relation tothe bit speed used (
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Response 1 is lostin transit
Response OK
Response 1
GPRS connection withoutsequence control
Control Center
TCP recoversResponse 1
Request 2
Request 1
IncorrectResponse re-ceived, Error!
Link ResetRequest
RTU
Response 2
Response 1 is lostin transit
Response OK
R
GPRS connection withsequence control
Control Center
TCP recoversResponse 1
Request 2
Request 1
The device detects the
out-of-sequenceResponse and filters it
RTU
R
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Establish an end-to-end TCPconnection over GPRS and maintainthe same using connectionmanagement mechanisms (keep-alive or similar)
Analyze protocol framesreceived by the serial port, verifytheir integrity and IP encapsulation,broadca st over a LAN or GPR Snetwork
Manage the sending of framesover GPRS, filter unnecessaryprotocol traffic (for example,QuickCheck messages or similar), inorder to minimize the cost of trafficsent (only relevant traffic is sent overthe costly GPRS link)
Manage the sequence ofprotocol messages so that lostprotocol messages which aresubsequently retransmitted by theTCP protocol are removed, giventhat this tends to produce droppedconnections in Control Centers andRemote installations (Figure 2)
As can be observed in Figure 2,problems can arise in telecontrolapplications over GPRS whichuse protocols initially designed forapplication on serial ports if all lostmessages are allowed to be recoveredby the TCP protocol. If the recoveryoccurs after the Control Centerhas retransmitted the originalprotocol message, it will receive twoanswers, one of which will be outof sequence. As a result, dependingon the configuration of the controlcenter, this will cause the link to be
reset and recovered.In practice, this makes the GPRS
link unusable for a telecontrolapplication under low coverageconditions because the protocol linkis dropped and recovered constantly.By implementing sequence controlon the end encapsulators, theproblem is avoided and the link
is reliable, without the need toreconfigure the protocol timeparameters of the Control Center(which may not even be viable ordesirable).
It is also extremely important toadjust all of the application timeoutsin telecontrol applications to thenew transmission technology. Thelink latency increases to the order ofseconds, and it is therefore essentialto increase time values accordingly.For example, in some implementedsystems, application timeoutsof around 1 second have beenincreased to 10 seconds, obtaininggood results.
GPRS networks and their
applications
The GPRS system (GeneralPacket Radio Service) is an additionalmobile data service offered over
GSM networks. GPRS is a packetswitching service, directed at the useof TCP/IP applications. As such, thissystem is suitable for applications
such as Internet navigation or fileexchange.The GPRS service is offered on
a number of GSM frequency bands(800, 900, 1800, 1900 MHz) asa complimentary service, withpractically worldwide coverage(except for Japan and Korea).
The GPRS service allows forpacket mode data transmissionspeeds from 9 to 40kbps (uplink)and 9 to 80kbps (downlink).This transmission technology istherefore asymmetric in termsof data speed. There are evolvedversions of the technology such asEDGE (Enhanced Data rate for GSMEvolution) which allow highertransfer rates (up to 236 K bpsdownlink and 60 kbps uplink), andthese are available in many countries.GPRS technology therefore presentsmultiple applications in the world ofTelecontrol, given its reliability andhigh coverage. Nevertheless, thereare some aspects of GPRS networks
2 Sequence control of protocol messages in aGPRS encapsulator
GPRS can be applied in telecontrolapplications, either as a backup for privatecommunications channels or as a maincommunications technology.
When ap
as a bac
for priva
commun
tions cha
GPRS pr
redunda
the utilit
commun
system.
When ap
as a ma
commun
technolo
consider
best alte
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Firewall
GPRS modem
GPRS modem
GPRS router
Event recorder
Metering concentrator
TelecontrolSubnetwork
Control Center(60870-5-104...)
Control Center(60870-5-101...)
Remote metering60870-5-102
Generationnetwork
Meters (60870-5-102...)
Serial Link
Serial Link
RTU (60870-5-101...)
Meter (60870-5-102...)
Meter (DLMS)
RS-485
RTU 60870-5-104
4SIP serial to IP
4SIP serial to IPGPRS
MeteringSubnetwork
Ethernetswitch
4SIPSerial to IP
GPRS Network
PAC.MARCH.2010
Communic
ations
GPRSA
pplications
42
by Aitor Arzuaga Munsuri, Jose Miguel Arzuaga Canals and Mikel Zamalloa Aiartzaguena, uSysCom, Spain
which must be considered in order toachieve effective implementations:
Latency is extremely high inGPRS networks, in the order of 500
ms to 2s per GPRS link, is notdeterministic and also has highvariance, even for successive packets.This must be taken into account fortelecontrol applications.
The GPRS network is a radionetwork, and as a result of varyingtransmission conditions and theirvariation over time, packet lossesoccur. If the TCP/IP suite ofprotocols is used, packets aresubsequently recovered anddelivered to the application in order.This leads to retransmissions and aconsiderable increase in trafficlatency.
Traffic cost. Traffic is chargedper byte transmitted, or by large
blocks of traffic bought from theoperator. This leads to a change withrespect to traditional wired networks,where traffic is free once the
infrastructure has been rolled out.Some network operators offertheir customers the possibility ofobtaining a private IP domain (GPRSintranet), meaning they obtaina subnet of private IP addresses,isolated from the internet and otherGPRS users, and accessible onlyfrom other points in the privateIP subnet. Figure 3 shows sometelecontrol applications and othersof interest to electric companieswhich can be implemented over aGPRS network.
Public communications
channels' usage
There is a certain degree ofreluctance among companies in the
electric sector towards using publcommunications infrastructures fothe provision of telecontrol serviceThis is due to the existence of a seri
of factors which used to make thstrategy ill-advised, such as:Reduced availability an
reliability of public communicationsolutions compared to private one
Increased security of usingprivate network which is completeclosed off to the outside world
Strategic issues (autonomy iservice provision) and organizationones with respect to the electrutility
In recent years factors have alsappeared which would seem tfavor the use of operator or publcommunications networks:
Constant (growing) pressuon electric utilities to reduc
3 GPRS applications for electric utilitiesThe costof traffic
sent while
using GPRS
technology
is reducing
year by year,
and the level
of coverage
is increasing.
In fact, it has
practically
become a
worldwide
standard for IP
communication
over radio.
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CA(Certification
Entity)
InnerFirewalls
ControlCenter (104)
X.509Certificates
TLSconnection
EdgeFirewalls
Corporate IP Network
Mobile Operator A
TLSconnection
X.509Certificates
RTU (104)
Mobile Operator B
PAC.MARCH.2010
43
operational and maintenance costs.Solution scalability. The
number of connections can beincreased by contracting them from
the operator (or contracting anotheroperator)Independence from techno-
logy employed, given that theinfrastructure is owned by theoperator and only its use iscontracted
Improvement in geographiccoverage of public networks. It isincreasing in all countries and anincreasing proportion of inhabitedterritory is covered
Progressive improvement incommunications service qualityprovided
If we take all of these factors intoaccount, we can draw up a series ofgeneral recommendations whichcan make the transition to usingGPRS networks for telecontrolapplications simpler and lesstraumatic for a utility company. Welist these arguments below:
If using an external commu-nications network for telecontrolservices is considered too great a risk,GPRS networks can also be used asan alternative communicationschannel together with privatecommunications networks (forexample, private radio links and
GPRS as a backup). Using thisapproach, the utility gets improvedredundancy and enhanced reliabilitywith a limited investment and small
operation costsPerforming a preliminarycoverage study for planned sites,with each of the different GPRSoperators present in the country, inorder to avoid low signal strengthproblems or the inability to establishcommunications in the imple-mentation phase. This will provideinformation on what sites are viableout of the total number required,and as such, assess the viability ofGPRS technology for serviceprovision
Use, as far as possible, operatorredundancy policies, by usingdevices which allow commu-nications to be established throughtwo operators (double SIM card). Inthis way, it is possible to establishcommunications with a GPRSoperator, and in the event that itfails, the GPRS device switchesautomatically to the secondoperator, allowing communicationsto be re-established. Preliminaryexperience with this redundancyfunctionality in Spain demonstratesan increase in system availability toover 98%, compared to 90% with asingle operator
It is important to ensure thesecurity and privacy of telecontrolinformation. Two levels of securityshould be established:
The first consists in contra-cting the use of an Intranet or VPNwith the GPRS operators, so that therange of IP addresses assigned to thetelecontrol applications are privateand therefore inaccessible from anypoint outside of the electric companyIP network
The second level of securityentails the use of IPSec or SSL/TLStunnels with certificates betweenthe remote devices and the utilitycompanys external access firewall,as can be seen in Figure 4. Thisensures a level of total privacy, insuch a way that not even the GPRSoperator itself can access theinformation sent, at the cost ofincreased complexity in systemconfiguration, implementation andmanagement .
Requires
review &
of a num
of aspec
(protoco
configur
network
architect
terminal
element
control c
in order
ensure t
the telec
system i
capable
function
reliably,
effective
efficient
it can
complem
existing
to impro
reliabilit
The use of
traditional
telecontrol
protocols and
architectures
over GPRS:
4 Telecontrol application over GPRS with TLS security
GPRS main disadvantagelies in the fact that
coverage is reduced toinhabited areas.