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Power Line Transmission
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Electric power supply lines offer a ubiquitous infrastructure for
the transmission of broadband within home or as a last mile
telecommunication link for bringing broadband to home or
office premises. Previously, powerline communications
systems were limited to relatively low data rates, typically less
than 500 kbit/s. However, the new broadband systems provide
significantly higher data rates of 4 - 20 Mbit/s, about ten to forty
times faster than earlier systems
Today’s homes are packed with communication devices. Various
transmission technologies are offered to provide content to home
terminals. Consumers can choose from various possibilities to get
access to the Internet and/or to enjoy TV and radio in steadily
improving digital quality. Free-to-air digital broadcasting is and will
continue to be a future-oriented approach on how to satisfy the
needs of consumers. However, Power Line Telecommunications
(PLT) is another future-oriented technology providing broadband
access via the main networks to consumers' homes or to distribute
and collect huge amounts of data inside the homes to and from
various terminals or radios and TV sets.
Power Line Telecommunications (PLT), Power Line Communications
(PLC) and Broadband over Power Line (BPL) are all terms used to
refer to the process of delivering high frequency broadband data
over existing electricity supply cables, on a secondary use basis.
PLT products are designed to provide broadband Internet access
using the electricity distribution network as a transmission medium. In
concept, PLT has some similarities with digital subscriber line (DSL)
in that it delivers high frequency broadband data using existing
infrastructure cables on a secondary use basis. They have the
potential to provide simplified in-house interconnection of computers
and peripherals, and cost effective last-mile delivery of broadband
data services.
PLT systems consist of terminal devices that are plugged into or
attached to the electrical power supply network and allow data to be
transmitted via the network to other terminal devices plugged into or
attached to the network. The use of the existing electrical power
supply network wiring reduces costs and provides convenient access
to broadband interconnection between devices. It typically uses a
number of carrier frequencies in parallel to spread the data over a
wider range of frequencies. This allows individual carriers to be
turning on or off with less impact on the overall data rate of the
system. This makes the communications less vulnerable to nulls, or
interference from noise or other devices connected to the network.
Historically, powerline communications systems had been limited to
relatively low data rates - typically less than 500 kbit/s. These low
data rate systems are still in use for such applications as the remote
control of switches in domestic installations (domestic appliance
control systems, offpeak hot water supply control systems) and
power supply authority switch yard control and monitoring systems.
However, the new broadband systems provide significantly higher
data rates of 4 to 20 Mbit/s, some ten to forty times faster than
earlier PLC systems. These data rates are comparable with early
Ethernet (a common wired local area network) systems used to
interconnect computers.
The applications for PLT systems fall into two broad categories:
• In-house applications; and
• Last-mile applications.
In-House Applications occur within a single building with both ends of
the communications link within the same building. The building might
be a house, an apartment block or an office building. The path over
which the transfer of data occurs within these buildings is relatively
short - typically it is less than 100m between devices. Broadband
powerline communications systems are seen as an attractive means
of retrofitting data services to existing buildings because no
additional wiring is required. The networking of computers, printers
and other telecommunications services by simply plugging into the
existing AC supply wiring is attractive for both, home use or small
office. Systems have been successfully developed for these short
distance in-house applications under existing Electromagnetic
Compatibility (EMC) requirements for digital information technology
(computer) equipment. This has been possible due to the relatively
low power levels necessary to communicate over the comparatively
short (cabling) distances within a building. The development of
technology for computer networking in-house applications is fairly
advanced, with industry standards for terminal devices already in
place allowing interoperability between devices from different
manufacturers (eg, HomePlug 1.0). Other applications include the
use of these systems for sound and video distribution, and appliance
switching and control, within the home.
Last-Mile Applications typically include the distribution to the home or
office of Internet and other services by broadband service suppliers.
In addition to broadband Internet connections, these broadband
distribution systems also provide voice (IP telephony), video (VHS
video quality), surveillance systems, entertainment (gaming) and
utilities metering (electricity/water/gas) services. In principle, the
terminal devices used in these systems could potentially be used to
link adjacent buildings as well. This market entry price is similar to
the market entry price of 802.11b RLANs a few years ago and
suggests in-house broadband powerline communications solutions
are likely to become a highly competitive alternative to wireless
SOHO networking. A significant proportion of the cost of providing
broadband services to the public is associated with the so called last-
mile connection between the network and the individual user.
The provision of cabling to and into each building represents a
significant cost and inconvenience due to the differences between
buildings and the lack of existing infrastructure within the building.
The technology for this category of application is less well
developed, mainly due to the need for higher signal levels for the
longer last-mile paths. Higher signal levels mean higher levels of
emissions which could not be contained within existing EMC
requirements in most cases. There are, however, some cities in
developed countries where the building density, the configuration of
the AC power line network (eg, underground) and the existing
broadband data network infrastructure have been such that it has
only been necessary to use outdoor powerline communications
systems over distances of a few hundred meters. This has allowed
the use of devices with signal levels similar to in-house systems.
From a distribution network company's perspective, the possibility of
utilizing its existing network asset to generate more revenue by
providing value-added services such as Internet access is not such a
bad idea especially when there is an increasing pressure from the
regulatory regime to decrease the network tariff. However, there are
a number of issues associated with the broadband PLT systems.
These issues fall into three categories.
• Compatibility between private and public networks;
The two application types, that is, in-house and last-mile, are not
necessarily compatible. The non-isolated electrical interconnection
of private in-house networks and broadband data distribution
supplier networks, both attempting communications over the same
physical wiring, can lead to interference between systems. Also,
separate in-house systems in high density areas such as apartment
buildings are likely to suffer mutual interference leading to degraded
performance even when using interoperable equipment. Since the
last-mile powerline infrastructure is owned by a Utility, the Utility might
be particularly concerned at the potential pollution of its
infrastructure, or the diminished utility of that infrastructure, from
signals introduced by private households. There are other
compatibility issues associated with existing supply authority systems
and the electrical power supply network. For example, electrical
safety requirements, protection of signaling needs of powerline
authorities, and potential effects on other electrical devices.
Arguably, resolution of these aspects will need detailed
consideration by all stakeholders, including manufacturers, Utilities
and relevant regulatory bodies.
• Telecommunications policy issues
Last-mile broadband PLC systems have the potential to provide an
alternate means of delivering broadband telecommunications
services to the public. The electrical power distribution network is,
however, unlike other guided public telecommunications networks.
The communications characteristics of the network are less
controlled with large numbers of non-telecommunications devices
being constantly added and removed from the network. The lack of
isolation between private networks and public networks raises such
questions as interference to public networks, compatibility of systems
and carrier licensing requirements. The full extent of public
telecommunications issues arising from widespread use of
broadband powerline communications systems will need further
investigation.
• Radio communications interference issues
The main radio communications issue raised by the widespread use
of broadband powerline communications systems is the risk of
interference to radio communications services caused by their
generation of electromagnetic emissions from the powerlines over
which they operate. In particular, open wire aerial powerlines freely
radiate high frequency signals across their entire length. Radio
communications services using frequencies above 30 MHz may also
be at risk from some broadband powerline communications systems,
though this does not seem to be as significant an issue, judging by
current overseas concerns and developments. Broadband PLC
systems use much higher data rates which cannot be accommodated
in the bandwidth available below 525 kHz. Instead, these systems
typically use carrier frequencies in the range 2 to 30 MHz. The use of
higher frequencies leads to greater attenuation of the signals along
the cable. The shorter wavelengths associated with these
frequencies are now more typical of cable lengths found in the
network and this increases the likelihood of nulls from open circuits
occurring. These effects rapidly increase with frequency, effectively
limiting the use of frequencies higher than the typical values
indicated above. As well, the cables are becoming more effective
radiators of electromagnetic waves. The level of general electrical
noise on the electrical power network requires PLC systems to utilize
high levels of error protection coding and robust modulation
techniques.
Since 2002, Internet trials using the electricity power cabling system
have been conducted in the USA and Europe (Austria, Belgium,
France, Germany, Luxembourg, Switzerland, etc) by private and
national electricity companies. Several critical success factors for the
next generation of PLT have emerged. Assuming that the uncertainty
on EMC regulation could be resolved, future generation of PLT must
be able to deliver integrated services transparently to the customers.
For instance, integrating Internet access with home automation.
Furthermore, PLT vendors must learn from their counterparts in the
telecommunication industry to quickly co-operate with their
competitors and reach a consensus so that some kind of "standards"
could be established to promote and speed up product development
and availability. This must happen before the market is captured by
a particular technology. Finally, the market conditions, for example,
the costs for accessing the backhaul network and PLT hardware and
software must decrease to make the business opportunity related to
PLT more attractive.
References:
1) Powerline Telecommunication – Practical Results From the
Field Trials by Dr. Kam-Hoog Chong & Olle Hansson, Anders
Perterson
2) Why broadband PLT is bad for EMC by Tim Williams, Elmac
Services
3) PLT, DSL, CABLE Communications (Including Cable TV),
Lans and their effect on Radio Services by Caytat
4) Broadband Powerline Communications Systems – A
Background Bried by Spectrum Planning Team, Radiofrequency
Planning Group, Australian Communications Authority
5) BR Circular CA/195 by International Telecommunication
Union (ITU)
6) www.wikipedia.org
7) A brief guide to new technologies and new networks in
telecommunications by Rogers Darlington (source: internet)
8) Broadband Power Line telecommunications & the amateur
service (source: internet)
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