Introduction to the Power Line Communications (PLC) technology

The history of a system which allowed power companies to control consumption, street lighting ignition and the value of electricity rates using a low frequency signal (100 Hz) that travelled one-way through network cables is known since the 50s.

In the mid 80s, research studies were initiated as regards the use of electric cables as a way of transmitting data, and by the end of that decade information could be transmitted in both directions. At the end of the 90s, this transmission was conducted in a sufficiently high speed. This made it possible to glimpse an application in the telecommunications field: Internet access. Recently, a new communication system called Power Line Communication (PLC) has emerged in order to meet the demand of a “Real Broadband”. This technology consists of using electricity distribution lines for the transmission of information. 

PLC concept

It consists of data and voice transmission through the electrical network. Companies and engineers have been trying for many years to make the idea of taking advantage of the existing electrical network come true (half of the world’s population has electrical power). In this way, power companies would increase their cost-effectiveness when increasing the added value of their services with a minimum investment, using its infrastructure to pursue aims that initially were not conceived.

Advantages

• It uses the already existing infrastructure, that is to say the electrical wiring, so no additional work is needed. They constitute a universal wiring system.

• It lacks the drawbacks of ADSL or cable connection, which in many cases do not reach the final user. Being the electrical network already implemented, it is possible to reach any geographical location.

• Any home plug is enough to be on line.

• Possibility of creating home data networks using the already existing wiring.

• Low costs and implementation time.

• It provides a permanent connection 24 hours a day.

• Multiple services availability through the same platform.

• It is possible to combine it with other technologies.

• Microfilters are used to avoid possible interferences generated by electrical appliances.

• Its bandwidth is 45 Mbps although nowadays speeds of 135 Mbps can be reached and soon this will increase to 200 Mbps.

• A single repeater can supply 256 homes with a connection.

• In time, costs will decrease.

• Although not fixed yet, connection rates will not exceed the ADSL subscription charge.

• Speed for the “last mile” (home) users similar to the ones offers by other technologies.

In the present, companies are not totally convinced about PLC’s technology and commercial feasibility. There are technical and regulatory problems that must still be solved:

Disadvantages

• Electricity distribution circuitries have not been conceived for communication purposes. The electricity distribution network constitutes a very complex, noisy and hard to model communication channel.

• It has scarce technology competency.

• Production of necessary equipment is still scarce. PLC channel poses particular challenges to modem designers, such as the selection of appropriate modulation and coding schemes.

• Lack of technology standards for equipment interoperability.  

• Lack of security (confidentiality).

• Opposition by Telephonic companies.

• A need of a detailed control emerging from the nature of the distribution line that sends and receives electromagnetic interference.

• Interferences arising from several sources of a nature which varies through time, lead to a very poor performance of the system. As a result, transmission capability is restricted producing severe bandwidth taxes, power limits and high noise levels.

• Variability of attenuation and impedance levels originated by electric equipment switching are frequent.

In order to make a precise diagnosis of the technology and its applicability in our environment, it is necessary to include the following information while researching the above items:

• Information on the existing test facilities.

• Analysis of the real potential that the adoption of PLC technology has.

Potential of PLC Systems

Currently, the electric network covers more area than telephone and cable TV networks, for it reaches 95% of the households. In other words, through the PLC system most of the people will have the possibility to get connected to the Internet. Since the provision of this type of network is already available in offices and industries, it is estimated that the most favoured markets will be residential areas and small and middle-sized businesses. 

It must be taken into account that although the existing telephone network is smaller, its coverage comprises all the social sectors that can afford the service. Therefore, the offer of low-cost VoIP service would be competitive in low income sectors, which would cease to be customers of the local phone company. 

The expectations generated by PLC technology are high and very diverse. Some of them seek to exploit the technology from a social point of view (trying to reach equality in the access to information technologies, aimed at promoting the development of people) as well as from a commercial point of view.

The letter of introduction of this technology is the promise to cover non cost-effective geographical areas, where the users are highly scattered or where there is no easy access to the area (this concept applies to users fed by BT, since the transmission to the distribution transformer by MT is not competitive with DSL, as will explained below). This technology has several advantages, which enhances its development.

Current Major Limitations

Broadband PLC technology is still in its kicking point. Although the coding and modulation schemes that are highly used on the current PLC modems are renowned, the results obtained in other countries cannot be guaranteed for the local distribution network, for they have been tested on distribution networks with different features.

For being able to know the real capacity of the existing networks in Argentina, experimental or pilot tests must be conducted. 

Although PLC industry is in full development, it must be taken into account that the PLC channel has specific speed restrictions (less than 300 Mbps). This limitation arises mainly due to the nature of the network (i.e., line-loss, environment shared by many users, etc.) and to the need of using coding schemes to tackle the effects of noise.

This indicates that PLC is basically an access method for the last mile.

The use of PLC on MT networks to connect several transformers with the backbone has important limitations:

• Amount of additional equipment on the MT network: repeaters must be installed every certain amount of meters (500 meters) to keep the signal at an adequate level.

• Restrictions on offered speed: Although the MT network has better features for digital transmission of information than the BT network (less noise, interferences, etc.), they are not too different when it comes to their physical features. Therefore, potential speeds might be insufficient to cover the flow of information required by multiple transformer stations fed by the MT network.

• Equipment complexity: Since the couplers are powered by high voltage electricity, and in order to prevent equipment damage, special construction features and installation procedures are required.

Taking the abovementioned into consideration, PLC technology for MT networks is not expected to be a solution for the links between multiple transformer stations and backbones. It can be adopted in some areas where the configuration of the network and the capacity required allow its use (a university campus, small towns, etc).

To feed the modems located at the transformers, the most convenient solution is to use a network of optical fibre. If this particular alternative is analyzed, the energy distributor could use the resource by laying the optical fibre over its own MT station (EPEC, for example).

Due to the competitiveness of the market, PLC technology must offer a wide range of services, QoS and a reasonable price. This is why the administration of the network plays a crucial role, for it must guarantee the best use of the shared transmission medium, and it must also provide an adequate quality of service. The MAC layer is in charge of this important task, which is currently not standardized. This means that the existing protocols are custom type, which means that each manufacturer produces their own and that they are not compatible among themselves.

In conclusion, the design and the standardization of the MAC is one of the most critical steps towards the commercial and technical success of the PLC.

Currently, the application of PLC in the development of internal networks might not very attractive to small and middle-sized businesses. A PLC modem costs around U$S 130. The market price for an Ethernet 10/100 Base T network card is not higher than U$S 20, and the meter of UTP to interconnect the network is not higher than U$S2. For this reason, in some situations it is not convenient to use PLC technology to develop an internal network.

Furthermore, and perhaps more importantly, the speeds achieved through Level 5 structural networks are up to 1 Gbps, considerably higher than the maximum 14 Mbps for a PLC network functioning within a building.

Jorge Zavleon
Rapporteur
Advanced Services