Powerline
Communication, as is apparent from the name, is the exchange of meaningful
multimedia information (voice, data & video) between any two intelligent
entities via the existing network infrastructure of electrical wiring systems.
The interesting part is that these intelligent entities employ existing AC power
lines to transfer multimedia information, which would otherwise require
additional hard wire installation.
How Did it Start?
The idea of transmission of information over electrical power supply networks
is certainly not new and dates back to the beginning of this century. As far
back as the 1920s, some energy companies used the high voltage (greater than 11
kV) grids, spanning enormous distances of several hundred kilometers to transmit
signals of tens of watts of power. This technology was called "carrier
frequency technology on high-voltage lines" and its use was limited to the
realization of remote measurement and control tasks for these energy systems.
Information transmission was also done on medium voltage lines and this was
called the "audio-frequency ripple control technology".
In
the late seventies and early eighties, people started thinking about information
transfer over low voltage power lines. One of the first usages of this
technology in residential households was in baby phones. These were phones for
kids that provided a set of transmitter and receiver to talk from one room in
the house to another over power cables. All of these and subsequent systems were
based on analog technology, and due to the bad voice quality, consistent
disconnection and inadequate protection against wire tapping, the transmitted
data was only suitable for non-time critical applications.
With the advancements in digital communications and advent of new algorithms
for signal processing, some of the above mentioned problems have been overcome.
In principle, it is possible to transmit binary data at frequencies much above
the 50-60 Hz used for electricity transport. With frequencies of the magnitude
of tens of KHz, it is possible to achieve data rates of several kilobits per
second. Such rates are of no practical use for modern data processing and
communication systems. To achieve higher transmission rates (of the order of 1
Mbps or more), it is necessary to use frequencies of the order of 10-20 MHz or
even more and operating at this frequency is not simple. All this happens
because power cables were never meant to transmit information, so the physical
characteristics of this media, by themselves, are somewhat hostile to the idea
of data transmission.
Powerline as a Transmission Medium
An
obvious question is that if data rates of the order of several Mbps are
achievable with this technology, why have the telecom behemoths left this media
unutilized?
There are several reasons for this:
In the first place, power cables were made to transmit electrical power, so
it is natural that they do not transmit data as easily. The transmission
characteristic of a powerline channel depends on the time of the day and area
where the network is installed. Some of these characteristics are:
-
Dominant and widely varying noise
sources: These noise sources are electronic, electro-mechanical and even
induced by the powerlines themselves. Some noise is harmonically related to
the 50 to 60 Hz power, others are induced from electro-mechanical sources,
while still others may result from sources as strange as the corroded
junctions in building wiring! -
Multipath effect:
This is the result of the fact that the transmitted signal may reach the
destination through different paths and thus at different times, thereby
causing an echo effect. Some signals may reach as late as the subsequently
transmitted signals, thereby causing interference. -
Impedance
changes: The impedance at a point of the powerline network varies with time,
as applications on the network are alternately drawing and then not drawing
power from the network at twice the AC line frequency. Such changes in
impedance, in turn affect the transmitted information signals.
A powerline system working on high frequencies starts
exhibiting antenna behavior. This radiation causes disturbances in the existing
communication systems. A solution to this problem is to reduce the radiation,
i.e. reduce the signal power. Reducing the signal power makes the transmission
more sensitive to disturbances, thereby increasing the complexity of design of
transmitters and receivers of such a system.
A large number of wireless communication systems (commercial
as well as military) make use of frequencies in this range. If the powerline
systems also start using the same frequencies, the result would be pure
unadulterated noise for all! As a result, a very small frequency spectrum is
available for the powerline systems to operate in.
How Does it Work?
Powerline Physics say that with some "conditioning"
of the existing electricity infrastructure, it is possible to transmit regular
low frequency power signals on 50 to 60 Hz and much higher frequency data
signals on 1 MHz, without affecting either signal.
In the receiving unit, the low frequency power signals can be
extracted out of the common signal with the help of a low-pass filter.
Standardization of PLC |
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Some of the early powerline protocol standards are |
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Standard |
Operating Frequency |
Data speed |
Data-link Protocol |
X-10 |
120 KHz |
50-60 bytes/sec |
Simple cmd-response |
CEBus |
100-400 KHz |
 |
CSMA/CR-CD |
European Home |
132 KHz |
2.4 Kbps |
CSMA/ack |
Systems (EHS) |
|||
As already discussed, these older systems used low frequencies for transmission and thus data speed achieved was of no use except in some primitive applications.
To accelerate the development of powerline technology, two industry forums are developing new standards for home networking and access. These are the PLC (Powerline Communications) Forum, a combination of the PTF (Powerline Telecom Forum) and the IPCF (International Powerline Communications Forum) and the HomePlug Powerline Alliance, a group formed to develop home-networking technologies based on powerline telecom. Both of these standardization bodies enjoy the membership of big names like Cisco, Texas Instruments, Siemens, etc., to name a few. HomePlug has released its first version of specifications. These specifications are for an indoor solution. The networks in these specifications have an operating frequency of 80 MHz, capable of delivering 11 Mbps of data. The access method used is CSMA/CA. HomePlug is working on the next version of these specifications and is expected to release them by early next year. The IPCF is focusing on the last mile solution. They are in the process of finalizing the first version of their specifications, which is expected to be released soon. |
Similarly, by putting a high-pass filter, it is possible to
retrieve the transmitted information signal. This maybe achieved as shown above.
The arrangement shown in the figure is technically called ‘band-pass filter’.
Usage Scenarios
Electricity supply companies worldwide are extremely
interested in powerlines as this technology opens up enormous potential. They
can offer powerlines to their customers as an additional service and through
attractive service/price models can redefine their position in the market.
Powerline offers the end user a broad spectrum of applications and services:
high-speed Internet access (last mile), telephony, in-house LANs and special
services provided by electricity supply companies. Particularly in countries
where the conventional telephone and communications infrastructures are
insufficiently developed, powerline will permit enormous advances to be made
very quickly, in particular for telephony and access to the Internet. The
electricity supply utilities can provide special value-added services to their
customers via a global Internet portal.
The focus of all trial systems today is to provide the
facility of Internet browsing. One of the advantages of powerline based Internet
access, with respect to current dial-ups, would be that the user may stay
connected at all times and still be billed only
for the number of bytes. Also, the data rates offered would be quite lucrative.
At the data rate offered by this technology, concepts like
telephony and video-on-demand may also be easily integrated with it. A lot of
vendors are actually focusing on integration of PLCs with the existing analog
phones, while still others are working on its usage for digital telephony (VoIP
and ISDN). In fact, Ascom has been able to use analog as well as VoIP phones
over powerlines with appreciable results. One of the most effective uses of this
technology would be in home networking and home automation. Application to basic
emergency services, such as usage in gas detectors and fire alarms, is already
in field trials from some vendors.
The key is to provide a complete solution. This includes
communication within premises, communication with the external world and working
with the electric supply utilities to implement a complete end-to-end solution.
Major Players
The interest in Digital Powerline Communications (with > 1
Mbps capacity) is increasing to the extent that companies have even started work
on ASIC solutions for them. Ascom is running its field tests in different parts
of the world that include eleven European countries, Hong Kong, Singapore and
even developing nations like Chile. They have been able to achieve speeds of up
to 5 Mbps. Ascom is focusing on providing complete end-to-end solutions. Siemens
is working on the last mile access networks while Alcatel is focusing on its
"LineRunner PDSL" (Powerline Digital Subscriber Line) system, which
transmits data on medium-high voltage networks with -10-20 kV.
Promises for India
The potential markets for powerline communications are some
18,000-power utilities around the world. A majority of the world’s population
lives in low teledensity areas and the deployment of powerline communication
networks represent tremendous marketing opportunities in less industrialized
areas. The system allows existing electricity customers to enjoy fixed line
phone services without the expense or waiting period associated with a phone
line installation.
For countries like India, where electricity penetration is
high as compared to the presence of only 2.2 phone lines per 100 population
nationwide (10.6 per hundred in urban areas); this technology holds tremendous
potential. In its Asian utility valuation, ABN-AMRO estimates that the power
companies will capture twenty five to fifty percent of the fixed line
communication market by 2004.
Even though setup is easy, there may be a lot of operational
problems in India. With the incessantly existing power fluctuations,
complemented with the problems of power outages, the operation of such networks
might prove a little inconvenient for users. Moreover, with the electrical
supply infrastructure being a state owned monopoly, there might be some
reservations on part of these electrical utilities to set up extra apparatus at
distribution transformer stations.
This technology can soon be made available for the home
automation segment in areas where power supplies are regular and more reliable,
but others will have to wait a bit longer.
Manu Sharma
Ascom India Pvt Ltd