I am hesitant to put pen to paper to write about
Broadband over Power Lines or BPL and Power Line Communications or PLC (maybe this should be Broadband over mains in the UK!) as I have no doubt that I am biased in my
views and have been for a long time. This does not derive from in-depth experience of the technology but because I have been a radio amateur or 'ham' since my teenage years.
In the amateur radio world BPL is seen as a ogre that could have a major impact on their ability to continue their hobby due to interference from
BPL trials or deployments. More on this later.
Today, the principle technology used to deliver broadband Internet access into homes is Asynchronous Digital Subscriber Line (ADSL)
technology delivered by local telephone companies or ISPs collocating equipment in their switching centres. As ADSL is delivered over the ubiquitous copper cables previously used to deliver only
traditional telephony services, it's rollout has experienced tremendous growth over the last decade throughout the world.
However, ADSL does have some inherent commercial and technical limitations. For example, The further away you are from your local telephone
exchange or central office the lower the bandwidth that can be delivered. This means that ADSL works best in high population areas such as towns and their suburbs. Even in the UK, there are still
country areas where ADSL is not available because BT believes it is uneconomic or technically challenging to provide the service. For many years BT ran trials using wireless (that we would
probably call WIMAX these days) to test the economics of providing Internet service to remote locations or caravan parks.
As ADSL can only be offered by telecommunications companies, whether they be old telephony providers or newer ISPs, this led to other utility
providers wanting to get into the act. Water companies installed fibre optic cables when they dug trenches and canal and railway operating companies allowed telecommunications companies to run
cables along their facilities.
We should also not forget our very own Energis (now Cable and Wireless) who started by providing wholesale backbone services by running cables
along pylons. At one time nearly every electricity company had a telecommunications division.
This neatly brings back to Broadband over Power Line technology. The logic that drove the development of BPL is quite straightforward to
understand. Every home is connected to an electricity distribution network so why should that not be used to deliver a broadband Internet service? This would mean that electricity companies could
participate in the Internet revolution and create additional revenues to fill their coffers! Moreover, maybe BPL could be used to deliver broadband access to remote locations where ADSL cannot
There is one thing about BPL that is clearly
different from all the other technologies I have written about and this may seem a little strange. There are no IETF or IEEE technical standard for BPL although there are standards
activities afoot. This makes deploying a BPL service a rather hit or miss affair.
Deployment is also challenging due to the fact there is tremendous variation in the electricity distribution networks throughout the world making
standardisation a tad difficult. For example, in the UK hundreds if not thousands of homes are connected to a local substation where the high transmission voltages are converted to the normal 240
volt house supply. Hence it should be possible to 'inject' the broadband service in front of the transformer and deliver service to many houses at the same time which helps improve service
In the USA the situation is quite different because of the distances involved. It is always more efficient to carry electricity at the highest
voltage possible over long distances to reduce losses, so in the USA it is common practice to have the transformation to 110 volts done at the last possibly opportunity by placing an individual
transformer on a pole outside of each home. This can wreck BPL service economics. However, this has not stopped many services trials taking place.
A BPL service can offer similar bandwidth capabilities to ADSL in that it supports an 256kbit/s up stream and up to 2.7M/bit/s down stream,. It
achieves this by encoding data utilising the medium and shortwave spectrum of 1.6 to 30MHz or higher. In-house modems connect back to the head-end located at the substation where fibre or radio
can be used to connect back to a central office as used in wide-area Wi-Fi services ( see
The Cloud hotspotting the planet).
The modulated radio frequency carrier is injected into the local electricity distribution network using an isolation capacitor and transmitter can have a power of 100s of watts.
BPL modems use several methods of modulation depending on the service bandwidth required:
GMSK (Gaussian minimum-shift keying) for
bandwidths less than 1Mbit/s
CDMA (Code division multiple access) as used
in mobile 3G services for greater than 1Mbit/s, and
OFDM (Orthogonal frequency-division multiplexing)
for bandwidths up to 45Mbit/s
Most modern BPL deployments use ODFM as higher bandwidths are required if the service operators are to compete with their local telephone companies
There are several organisations involved in standardisation efforts:
Consumer Electronics Powerline Communication Alliance (CEPCA):
A PowerPoint introduction to the activities of the CEPCA can be found here.
Their mission and purpose is the:
Development of specifications enabling the coexistence
Between in-home PLC Systems
Between Access PLC Systems and in-home PLC Systems
Promotion of high speed PLC technologies in order to achieve world-wide adoption thereof.
Power Line Communications Forum (plcforum):
A similar body to CEPCA with many equipment suppliers as members.
HomePlug Powerline Alliance (HPPA):
This group focuses on home networking using home electricity wiring as the distribution network - as they say, "power outlets are almost everywhere someone might want to use a networked device
According to their scope description the P1901 project will "develop a standard for high speed (>100 Mbps at the physical layer) communication devices via alternating current electric power
lines, so called Broadband over Power Line (BPL) devices. The standard will use transmission frequencies below 100 MHz."
Powernet: The main
project objective of Powernet is to develop and validate a ‘plug and play’ Cognitive Broadband over Power Lines (CBPL) communications equipment. Power net is a European Commission project.
With other postings about communications technologies I guess I would go on to say that although there is much work to be done, BPL is a
complimentary technology to ADSL and it has its place in the Internet marketplace. My commercial reservations are quite strong however in that it is difficult to see how BPL can effectively
compete with the now ubiquitous ADSL utilised by every local telephone company on the planet. Maybe there are niche markets where BPL could work and these would be geographical areas where ADSL
cannot reach - yet.
However, as I indicated in my opening paragraph there are other concerns about BPL that are not encountered with any of the other ways of providing
Internet service to homes whether they be delivered over wires such as ADSL or wireless such as Wi-Fi or WIMAX.
BPL has a dark side which I believe to be unacceptable and could prevent other legitimate users of the shortwave radio frequency spectrum to
pursue their interests and hobbies without interference.
Interference is the issue which can be better understood by looking at the following video of a BPL service trial currently taking place in
To quote the ARRL:"The Commission's obsessive
compulsion to avoid any bad news about BPL has clearly driven its multi-year inaction," the League continued. "Had this been any other experimental authorization dealing with any technology other
than BPL, the experimental authorization would have been terminated long ago."
Many amateurs see BPL as the biggest threat to their hobby that they have ever been seen.
So why should there be this level of interference from BPL?
It might be good to start answering this question
by looking at ADSL as this does not have any major interference issues despite its deployment in many millions of homes. ADSL is delivered into peoples homes via the copper telephone line. This
cable is not just a single copper cable as it might have been in the early 19th century but rather it is a twisted pair.
A twisted pair cable is like a rather crude coaxial cable. It is balanced in that the signal flows forward through one wire and returns
through the other. This means that the bidirectional signals cancel each other out and the cable does not radiate the signal it is carrying to the outside world. Twisted pair cable are not
as lossless as coaxial cables so there is a little loss but it is quite small for the length of cable usually used to connect a home to a telephone pole.
In general ADSL has been immune from creating interference because of the use of twisted pair cables. Imagine the consumer furore that would occur
if there was was interference from ADSL to FM or TV services it does work.
It's interesting to remember that cable companies also use broad band RF encoding but as services are delivered using high quality coaxial cables
or fibre there is generally no interference (The
tale of DOCSIS and cable operators).
On the other hand, electricity power lines that brings electrical power into houses are not shielded and are not twisted pair. They are
standard three or four core cables that we are all familiar with when we connect our kettles to plugs although they are of a heavier gauge.
BPL transmissions are spread over the shortwave spectrum with a head-end power of possibly 100s of watts and the lossy distribution cables
effectively act as an antenna or aerial so the wideband BPL signal radiates quite effectively over a wide area causing the not inconsiderable interference as seen in the video above.
Surely, the regulatory bodies such as
OFCOM or the FCC would not allow a service that significantly interfered with other spectrum users to go ahead - would they? That is not so easy to answer today as it would have been a decade ago
when anti-interference regulations were very strong. Nowadays, in this commercial world we live in, there is far more flexibility given if there is a potential commercial benefit. For example,
even in the UK the old guard band (allocated unused spectrum between services to provide isolation) have been sold off for use in picocell GSM services as discussed in GSM pico-cell’s moment of fame .
The level of interference from a service such as BPL would not - could not - have been tolerated a few years ago when everyone used the
shortwave bands for entertainment. But in this modern 'digital age' shortwave seems an anachronism and who really cares if it not usable...
At least two groups of individuals do and they are radio amateurs and short wave listeners. BPL vendors and service providers and have attempted to
suppress their criticisms of BPL by what can only be described as a sticking plaster solution. This solution is to put filters on the BPL transmitter so that notches are inserted in the broadband
spectrum to coincide with the amateur bands.
However the general consensus by amateurs who have been involved in notching trials is that they do indeed reduce interference but not by a sufficient amount
for workable co-existence.
Another concern is that BPL is not just used for the provision of Internet access services but it is also possible to buy modems to provide
in-house LAN capabilities in competition to Wi-Fi. This could be a another worrying source of interference to shortwave services. Bearing mind there is no filtering in a mains or power socket, the
use of a BPL modem in one house will radiate in all homes connected to the same substation.
I really am unable to see any real benefit in this technology when compared to cable operator DOCSYS or telephone ADSL delivered Internet services
whose access infrastructure is designed for purpose. Just slapping a broadband transmitter on a local electricity distribution network is crude and is definitely NOT fit for purpose - even if
filter notches are applied.
If the electricity industry redesigned their supply cables to be coaxial or twisted pair, which in practice is not really technically or
commercially achievable, then the concept may work.
I doubt that BPL is viable in the long term and my view is that it's use will fade with time. In the mean time if I am asked for a financial
contribution to fight BPL, I reckon I would dig deep into my pockets.
One example of one of the up and coming trials is TasTel in Hobart, Australia, a partnership between Aurora Energy and AAPT who say they have a unique service. To quoute
their web site:
"Because BPL is brought to you by TasTel and eAurora, we can give you something nobody else can offer: fast Internet
access and cheap broadband phone calls through a single service, on one bill which is sent to you electronically."
Where have I heard this before - time move away from Hobart?