Power Line Communications: An Overview - Part I

Muhammad Salman Yousuf*, Mustafa El-Shafei*** Research Assistant, Department ofElectrical Engineering

** Professor, Department ofSystems EngineeringKing Fahd University ofPetroleum and Minerals, Dhahran, KSA.

syousuf@kfupm. edu. sa, elshafei@ c cse.kfupm. edu. sa

Abstract

We give an overview of the Power LineCommunications (PLC) technology, its importance, itsstandards and an overview of the HomePlug standardsassociated with it. This is done is two parts due topublication constraints. In this part, we willconcentrate on the PLC applications and the technicalissues regarding it. We will also see the layers andmethods that are needed to make it work.

Keywords: Latest technology, Power LineCommunications, PLC, Standards, HomePlug 1.0,HomePlug AV.

1. Introduction

Power lines were originally devised to transmitelectric power from a small number of sources (thegenerators) to a large number of sinks (the consumers)in the frequency range of 50-60 Hz. It is a fact thatpower transmission towers and lines are some of themost robust structures ever built. Historically, the PLCtechnology has very limited applications but now weare witnessing the possibility of it being acclaimeduniversally as a prime mode of long-haul datacommunication.

With the inevitable arrival of broadband access,the demand for sending digital voice, video andInternet data within the home increases continuously.While retrofitting the houses and neighborhoods withspecial wires is one option, it is expensive and timeconsuming. PLC Technology allows the use of theexisting and widespread power distributioninfrastructure to provide high speed networkingcapabilities along with many other benefits.

Section 2 gives a brief application history and theshining prospects of PLC technology. Section 3discusses the technical issues in PLC, its layers andprotocols. The second part of this overview will focuson the standards and the HomePlug standards.

2. A Treatise on PLC Applications

2.1. A Bit of History

Initially, the first application involving datatransmissions over power lines were primarily doneonly to protect sections of the power distributionsystem in case of faults. (In fact, power line protectionremains one of the primary functions of power linecommunications.) In such an event, the fast exchangeof information is necessary between power plants,substations and distribution centers so as to minimizetheir detrimental effects. The robustness of the power

lines and their ready connectivity and availability makethis technique an optimal solution.

Narrowband power line communications startedsoon after the beginning of wide-spread electricalpower supply. Around the year 1922 the first carrierfrequency systems began to operate over high-tensionlines in the frequency range 15 to 500 kHz fortelemetry purposes, and this continues to the presenttime [1]. Consumer products such as baby alarms havebeen available at least since 1940. [2]

Historically, also, a primary motivation for power

line communications has been to do load managementin future. The currently employed ripple controlsystems have the disadvantage of requiring severalmegawatts for information transmission. A secondimportant motivation has been to facilitate meterreading from a distance. An English study has shownthat a meter reader achieves an average informationrate of only about 1 bit/s [3]. The Tokyo ElectricPower Co was running experiments in the 1970'swhich reported successful bi-directional operation withseveral hundred units [4].

Considering that data transmission over power

lines has been around for quite some time, one mightwonder why it is receiving such renewed attentionrecently especially considering the data rate forprotection and telemetering purposes is at most a fewkb/sec and is not comparable to the Mb/sec data thatneeds to be supported for multimedia applications? Theanswer is a combination of effects that took placeduring the mid thru late 1990s, namely, the explosive

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growth of the Internet and the gigantic leaps in VLSI(Very Large Integrated Circuits) and DSP (DigitalSignal Processing) technology. Then was thetelecommunications market deregulation, first in theUS and then in Europe and Asia. All these events havemade power line communications a viable technologyfor numerous other applications.

2.1.1. Home Automation. Power linecommunications technology can use the householdelectrical power wiring as a transmission medium. Thisis a technique used in home automation for remotecontrol of lighting and appliances and sensors foralarm systems etc without installation of additionalcontrol wiring. This is primarily based on the X1OIndustrial Standard and has been in development since1975. A detailed article on this standard and itsapplication can be found in [5].

2.1.2. Home Networking and Internet Access(Broadband over Power Lines, BPL). It follows thatwe can use the low voltage power network as a LocalArea Network (LAN) for conveniently connectingmany different computers in the same building. Withmultiple outlets in every room, residential power linesare already the most pervasive network in the home.Using this existing infrastructure to provide high speednetworking capabilities provides several benefits. Firstof all, there is no need for expensive rewiring of thehouse. Secondly, almost all devices that need to benetworked are already connected to the AC wiring.Thus, home networking becomes as simple as pluggingthe device in the AC outlet.

The market for PLC for consumers is thus two-fold: to the home, or "last mile" access; and in thehome, or "last inch" access. [6]

The development of the "last inch" by Home-networking companies in the form wireless networkadapters and power-line adapters is gradually leadingto widespread home networking; i.e., a wide array ofdevices connected inside the home in an intra-homenetwork. This "in-home networking" could transformall power outlets in the household into broadbandconnections for PCs, telephones and their accessories,as well as other 'enabled' electric appliances.

Figure 1 illustrates the concept of "last inch" or in-home networking, while Figure 2 illustrates the "lastmile" concept.

Broadband over power lines (BPL), also known aspower-line internet or Powerband, is the use of PLCtechnology to provide broadband Internet accessthrough ordinary power lines. A computer (or anyother device) would need only to plug a BPL "modem"into any outlet in an equipped building to have high-speed Internet access.

L

Figure 1: The "last-inch" networking through PLC.

However, due to several issues hampering theimplementation and usefulness of BPL, one possiblealternative is to use BPL as the backhaul for wirelesscommunications, by for instance hanging Wi-Fi accesspoints or cellphone base stations on utility poles, thusallowing end-users within a certain range to connectwith equipment they already have. In the near future,BPL might also be used as a backhaul for WiMAXnetworks.

Subst?tOnLocal Lbop DistRbulion Center

Links toPSTNWAN

Figure 2: The "last-mile" broadband access tohomes and offices through the local power

distribution center.

Much higher speed transmissions usingmicrowave frequencies transmitted via a newlydiscovered surface wave propagation mechanismcalled E-Line have been demonstrated using only asingle power line conductor. These systems haveshown the potential for symmetric and full duplexcommunication well in excess of 1 Gbit/s in eachdirection [7].

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Multiple WiFi channels with simultaneous analogtelevision in the 2.4 and 5.3 GHz unlicensed bandshave been demonstrated operating over a singlemedium voltage line. Furthermore, because it canoperate anywhere in the 100 MHz - 10 GHz region,this technology can completely avoid the interferenceissues associated with utilizing shared spectrum whileoffering the greater flexibility for modulation andprotocols found for any other type of microwavesystem.

At present there is no universal standard for powerline communication for this purpose. However,HomePlug Powerline Alliance has defined enduringstandards, the detailed exposition of which is inSections 5 and 6.

2.1.3. Narrowband PLC - Radio Broadcasting.PLC for radio transmission was and is widely used inGermany (Drahtfunk), Switzerland(Telefonrundspruch), Norway (Linjesender), USSRand some other countries. In all cases the radioprogram was fed by special transformers into the lines.In order to prevent uncontrolled propagation, filters forthe carrier frequencies of the PLC systems wereinstalled in substations and at line branches. Thenarrowband powerline communications channelpresents many technical challenges. A mathematicalchannel model and a survey of work can be found in[8].

2.1.4. Automotive. Power-line technology enablesin-vehicle network communication of Data, Voice,Music and Video signals by digital means over DirectCurrent (DC) battery power-line. Advanced digitalcommunication techniques tailored to overcome hostileand noisy environment are implemented in a small sizesilicon device. One power line can be used for multipleindependent networks.

Prototypes are successfully operational in vehiclesusing automotive compatible protocols such as CAN-bus, LIN-bus over power line (DC-LIN) and DC-busdeveloped by Yamar. Automotive applications includeMechatronics (e.g. Climate controls, Door modules,Immobilizers, Obstacle detectors), Telematics andMultimedia.

2.1.5. Further Reading on Specific Applications.Nowadays, we are seeing novel applications beingdeveloped using PLC and there is a wealth of literatureavailable on the topic. The demand for homeautomation systems / intelligent homes has fueledmuch research and practical intelligent homes havebeen designed, as in [9]. Cavdar has presented asolution to remote detection of illegal electricity usage

in [10]. The quality of power can also be measuredusing PLC [11].

3. Issues in PLC

3.1. The Powerline Channel as TransmissionMedium

3.1.1. Design. First off, the power line carrier wasnot specifically designed for data transmission andprovides a harsh environment for it. Varyingimpedance, considerable noise that is not white innature and high levels of frequency-dependentattenuation are the main issues.

3.1.2. Varying Channel Model. For successfulcommunication, the communication channel must befirst modeled and analyzed accordingly. The channelbetween any two outlets in a home has the transferfunction of an extremely complicated line network.Power line networks are usually made of a variety ofconductor types, joined almost at random, andterminating into loads of varying impedance. Oversuch a transmission medium, the amplitude and phaseresponse may vary widely with frequency. While thesignal may arrive at the receiver with very little lossover some frequencies, it may be completelyindistinguishable over other frequencies. Worse, thechannel transfer function itself is time varying sinceplugging in or switching off of devices connected tothe network would change the network topology.Hence, the channel may be described as random andtime varying with a frequency dependent signal tonoise ratio (SNR) over the transmission bandwidth.

A detailed discussion of modeling withmathematical treatment for a Power Line Channel isgiven in [12]. The signal propagation modeling in PLCnetworks is given in [13].

3.1.3. High Dependence of Transmitter andReceiver Location. The location of the transmitter orthe receiver (in this case the power outlet) could alsohave a serious effect on transmission error rates. Forexample, a receiver close to a noise source would havea poor signal to noise ratio (SNR) compared to onefurther away from the noise source. The noise sourcescould be home devices plugged into the network.

3.1.4. Reflection, Multi-path Fading andAttenuation. Just like a wireless channel, signalpropagation does not take place between thetransmitter and the receiver along a line-of-sight path.As a result, additional echoes must be considered. Thisechoing occurs because a number of propagation paths

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exist between the transmitter and the receiver.Reflection of the signal often occurs due to the variousimpedance mismatches in the electric network. Eachmulti-path would have a certain weight factorattributed to it to account for the reflection andtransmission losses. All reflection and transmissionparameters in a power line channel may be assumed tobe less than one. The number of dominant multi-pathsto be considered (N) is often not more than five or sixsince additional multi-paths are usually too weak to beof any significance. This is because the moretransitions and reflections that occur along a path, thesmaller its weighting factor would be. It has beenobserved from channel measurements that at higherfrequencies the channel attenuation increases.

3.1.5. Noise. Noise in power lines is a significantproblem for data transmission. This is because it rarelyhas properties similar to the easily analyzed whiteGaussian noise of the receiver with which we are muchfamiliar with. Typical sources of noise are brushmotors, fluorescent and halogen lamps, switchingpower supplies and dimmer switches. Apart fromthese, ingress sources such as amateur radiotransmission can render certain frequencies unfit forcommunication. The noise in power lines can beimpulsive or frequency selective in nature and,sometimes, both. Due to high attenuation over thepower line, the noise is also location dependent.

Recent studies have indicated that the noise inPLC systems can be typified into four categories.i. Colored noise: This type of noise has relatively lowpower spectral density (psd) which decreases withincreasing frequency. It is considered to be the sum ofall low power noise sources and may be time varying.ii. Narrowband background noise: This noise ismainly due to amplitude modulated sinusoidal signals.This kind of interference is from broadcast stations inthe medium and short wave bands. The interferencelevel varies during different times of the day.iii. Impulse noise that is synchronous with thegenerator's actual supply frequency: This type ofimpulse noise usually repeats at multiples of the supplyfrequency of 60/50Hz. It has a short duration of about afew microseconds and a power spectral density thatdecreases with increasing frequency. The noise iscaused from power supplies operating synchronouslyto the main's frequency.iv. Impulse noise asynchronous with the main'sfrequency: This is the most detrimental type of noisefor data transmission. Its duration varies from a fewmicroseconds to milliseconds and has a random inter-arrival time. The psd of such impulse noise may be asmuch as 50dB above the background noise spectrum.Hence, it is capable of wiping out blocks of data

symbols during high data transmission at certainfrequencies. It is caused from switching transients inthe system network.

With this discussion we are ready to generalize aPLC system into a model as shown in block diagramform in figure 3.

Transmitter Channel ReceiveFigure 3: General block diagram for PLC systems.

3.2. Layers, Access Methods and Protocols.

The communication in power lines can bedivided into two main layers: The Physical Layer andthe Medium Access Control (MAC) Layer.

The Physical Layer defines the modulationtechniques to transmit data over the power lines whilethe MAC protocol specifies as resource sharingstrategy i.e. the access of multiple users to the networktransmission capacity based on a fixed resource sharingprotocol. Communicating at the PLC Physical Layerdemands robust modulation techniques like FrequencyShift Keying (FSK), Code-Division Multiple Access(CDMA) and Orthogonal Frequency DivisionMultiplexing (OFDM). For low cost, low data rateapplications, such as power line protection andtelemetering, FSK is seen as a good solution. For datarates up to 1Mbps, the CDMA technique may providean effective solution. However, for high dataapplications beyond that, OFDM is the technology ofchoice for PLC. For MAC, there are generally twocategories of access schemes.

3.2.1. Fixed Access. It assigns each user apredetermined or fixed channel capacity irrespective ofwhether the user needs to transmit data at that time.Such schemes are not suitable for traffic in bursts, suchas data transmission that is provided by PLC.

3.2.2. Dynamic Access. These protocols may beclassified into two separate categories: Contentionbased protocols where collisions occur and Arbitrationprotocols which are collision free.

Contention protocols may not be able to guaranteea quality of service (QoS), especially for time criticalapplications, since collisions might occur and datamight have to be retransmitted. Arbitration based

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protocols are more capable of guaranteeing a certainQoS. However, contention based protocols mayactually provide higher data rates in applications whichdo not have stringent QoS requirements (e.g., Internetapplications). This is because they require much lessoverhead compared to arbitration protocols (polling,reservation, token passing).

The widely studied protocols for MAC Layer inPLC are.

3.2.3. Polling. It is a primary/secondary access methodin which the primary station asks the secondary stationif it has any data to send. Arbitration based polling canhandle heavy traffic and does provide QoS guarantees.

3.2.4. Aloha. It is a random access protocol in which auser accesses a channel as soon as it has data to send.

3.2.5. Token passing schemes. These schemes, e.g.token ring, token bus, are efficient under heavysymmetric loads. However, they can be expensive toimplement and can cause serious problems with losttokens on noisy unreliable channels such as PLs.

3.2.6. Carrier Sense Multiple Access (CSMA).CSMA with overload detection has been proposed forPLC. CSMA is a contention based access method inwhich each station listens to the line beforetransmitting data. CSMA is efficient under light tomedium traffic loads and for many low-duty-cyclebursty terminals (e.g. Internet browsing).i. Collision Detection (CSMA/CD) senses the channelfor a collision after transmitting. When it senses acollision, it waits a random amount of time beforeretransmitting again. But on power lines the widevariation of the received signal and noise levels makecollision detection difficult and unreliable.ii. Collision Avoidance (CSMA/CA). As in theCSMA/CD method, each device listens to the signallevel to determine when the channel is idle. UnlikeCSMA/CD, it then waits for a random amount of timebefore trying to send a packet. Packet size is kept smalldue to the PLC' s hostile channel characteristics.Though this means more overhead, overall data rate isimproved since it means less retransmission.

CSMA/CA is used in HomePlug Standards andwe will talk about it in more detail in part II of thispaper. For further details of all of the other protocols,see [14].

4. Conclusion

In this overview, we have seen the evolution andpotential of PLC technology and have gained some

idea of the structure of the standards involved with it.We will discuss the more important technical issuesrelated to the standards in Part II.

5. Acknowledgment

We are extremely grateful to King Fahd University ofPetroleum and Minerals, Dhahran, KSA.

6. References

[1] K Dostert, 1997, Telecommunications over the PowerDistribution Grid- Possibilities and Limitations Proc 1997Internat. Symp. on Power Line Comms and its Applicationspp1-9[2] R Broadridge 'Power line modems and networks' 4'thInternational Conference on Metering Applications andTariffs for Electricity Supply IEE conf., London.[3] B.E. Eyre, "Results of a comprehensive field trial of aUnited Kingdom customer telemetry system usingmainsbome signalling," Proceedings of the SixthInternational Conference on Metering Apparatus and Tariffsfor Electricity Supply, IEE Conference Publication.[4] M Hosono et al, Improved Automatic meter reading andload control system and its operational achievement, 4thinternat. Conf. on metering, apparatus and tariffs forelectricity supply 26-28 October 1982, IEE[5][6] Power line communications; Majumder, A.; Caffery J,Jr.; Potentials, IEEE Volume 23, Issue 4, Oct-Nov 2004[7]

(00 1 828001 htm][8] D Cooper, T Jeans, Narrowband, Low Data RateCommunications on the Low-Voltage Mains in theCENELEC Frequencies- Part I: Noise and Attenuation, IEEETrans on Power Delivery, vol 17 no 3 July 2002[9] 2006 2nd International Conference on Power ElectronicsSystems and Applications 'A Practical Intelligent HomeSystem Based on Power Line Communication' YANG Ping,YAN Heng-Ming[10] IEEE Trans. on Power Delivery, V19 #4, Oct. 2004 'ASolution to Remote Detection of Illegal Electricity Usage viaPowerline Communications', I. Hakki Cavdar.[11] 'Power Quality Monitoring System Using Power LineCommunication', Ducpyo Hong; Jinmok Lee; Jaeho Choi;Fifth Internat. Conf. Info., Comm. and Signal Process, 2005on 06-09 Dec. 2005[12] 'Modeling and Measurements for Power LineCommunication Systems', W. Q. Luo, S. Y. Tan, and BoonTiong, Tan, IEEE Conference Publishing[13] IEEE Trans. on Power Delivery, V20 #4, Oct. 2005'Signal Propagation Modeling in Power-LineCommunication Networks', Dubravko Sabolic', AlenBa zant and Roman Malaric[14] Computer Networks, 4th Edition, Tanenbaum, AndrewS., Vrije Universiteit, Amsterdam.

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