31 MAR, 2004 - 1705 CST
I am an infrequent visitor to the DSL Reports website, so I found
this
time yesterday. It's a pretty good description of the technical and
economic issues with BPL.
73 - Dick, W9GIG
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Posted on DSL Reports at:
http://www.dslreports.com/shownews/41546
Broadband
Fool's Gold
Editorial:
'Great broadband hope' it's not...
Written by Anthony Good, K3NG
Broadband over power-lines (BPL) has been hailed as the
"great broadband
hope" by FCC commissioner Powell, who believes it will usher in a
new age
of competition. Others lean on world-wide trial failures as evidence the
technology is doomed-for-obsolescence; during its run bringing plenty of
trouble (and interference) to areas contemplating the option.
BPL is a system that is being tested to provide broadband Internet
service
via power lines to power outlets in homes and is a last mile technology.
The system uses radio frequencies that will radiate into the air and
cause interference to several licensed communications services including
Amateur Radio. The frequencies BPL uses in general is 1 to 80 megahertz
(MHz). Of particular interest is the band of frequencies known as HF,
which is 1 to 30 MHz . This part of the radio spectrum has very special
properties not found elsewhere. With this band, one can communicate
around the world with very minute power levels. This is due to the fact
that radio waves in this band
can bounce off the ionosphere multiple times to get to a faraway
destination. Other portions of the radio spectrum, like that used by
802.11 wireless LANs, are essentially line-of-sight. This means that the
signals cannot bend or bounce off the ionosphere, but they can only
propagate like light in a straight line and shorter distances.
The medium of BPL, the power line cable, unlike other broadband mediums
such
as copper twisted pair, fiber, and coaxial cable, is inherently unsuited
for carrying the frequencies BPL uses. Power lines, twisted pair, and
coaxial cable all act like natural low pass filters, meaning higher
frequencies are attenuated more than lower frequencies when attempting to
transmit them through the medium. The exact slope of the graph of
attenuation depends on
the specific construction of the material, but in general, twisted pair
is suitable up to 100 MHz and coaxial cable can go up to about 3 GHz.
Power
lines are suitable for up to perhaps 350 kHz or so. The exact figure will
vary and is unimportant for this discussion, but note that this is
kilohertz, not megahertz or gigahertz. The medium of BPL is simply not
suited for broadband data.
The other property of the medium chosen for BPL is its radiating
capability. Unlike all other broadband mediums, power lines are excellent
radiators of
the frequencies BPL uses. Copper twisted pair, coaxial cable, and fiber
are all inherently non-radiating self-shielded mediums. Powerlines act
like a natural antenna and lose the BPL signal out into the air. The
resulting interference can vary from a noticeable noise to a deafening
roar on radio equipment which drowns out all communications.
BPL has been tested and deployed on a limited basis in other countries
and
was rejected in some places due to interference issues. BPL vendors may
claim new technology and advances have now made it possible, but the fact
is they cant change the laws of physics. High speed data must occupy a
certain amount of bandwidth and power lines which were designed to
operate at 60 hertz will radiate radio frequency energy that is applied
to them. Only changing power line construction (i.e. coaxial cable) would
eliminate this radiation. BPL proponents reject this as being too costly,
but that would be the cost to
make this a real viable technology.
Users of the affected radio spectrum cannot be relocated, or at least not
economically or in a timely manner. All of the services that use HF bands
require the characteristics that only HF spectrum exhibits. There would
also be huge international treaty implications with any relocation.
Relocating government and military services alone would take years as the
FCC would
have to structure a migration plan. Chances are it would be ten years
before this could be completed and its likely that power companies will
have run fiber to the home or DSL and cable will finally be ubiquitous.
Perhaps the largest issue to tackle though is where to move these
services in what is
an already overcrowded spectrum.
If it was determined that relocation was the way to go, this would be
very irresponsible as HF radio bands are a unique natural resource. No
other
radio spectrum can provide worldwide communications without any
supporting infrastructure. The military (and Amateurs for that matter)
have had satellites at their disposal for years, but HF is still in use
as it
provides unique capabilities that satellites just can't. Internet
technologies such as email, streaming media, and instant messaging, or
cell phones simply cannot take the place of the wireless
infrastructure-
free communications capabilities that HF provides.
Destroying a large portion of wireless spectrum is not justifiable
because
it benefits more people. There are many examples of this in society where
reallocation of a resource would benefit more people, but it would be
detrimental long term to the people and the resource itself. Right now,
Amateur frequency allocations belong to the people internationally and
can
be enjoyed in nearly every country by simply passing a test and getting
licensed. Once they are given to a business interest, they cease to be
the public's and can only be used as a customer of that business. BPL
impacts other groups including government, military, shortwave, aviation,
maritime communications, and CBers, so this would have national security
and international implications as well. BPL has been linked in some
rhetoric
with increasing homeland security. BPL in fact takes spectrum away from
government agencies directly tasked with protecting the country.
To deploy BPL an up front investment must be made in BPL head end /
injection point equipment and repeaters -- it's not as simple as FCC
Commissioner
Powell makes it sound, as if all power lines can immediately be easily
lit
up. There's going to be significant recurring costs in backhauling the IP
traffic from the numerous BPL injection points serving an area. Neither
DSL
or Cable has this recurring cost or need for multiple network origination
points. These costs unique to BPL make it even less attractive for
deployment in rural areas that Cable or DSL as customer densities and
revenue potential is lower.
The scalability of BPL is questionable. Chunks of HF spectrum must be
reused between repeater/injection point segments. With customer bandwidth
requirements going up, over subscription ratios going down, systems will
need to be segmented in a cellular fashion. This exacerbates the
interference issue as more frequency chunks are in use in a given area.
More avoidance of frequencies will be needed, making less spectrum
available for use by BPL.
The frequency chunks in use will need to be smaller to enable tighter
frequency reuse, and the available bandwidth per injection point will get
to
a point where it won't be sufficient.
BPL is also lacking on the regulatory front. It has no protection from
interference from licensed wireless services. This means your BPL
provider
has no recourse if a licensed wireless station knocks out your BPL
service regularly. If BPL interferes with a licensed wireless station,
the BPL provider must cease to operate the system if the interference
problem cannot be solved. While on the surface one would think that this
would inherently protect wireless communications, it places a huge burden
on such services
to expend time and money identifying and seeking resolution to
interference issues. With such a weak regulatory basis and a system that
is immediately
at odds with incumbent services, why would anyone want to depend on BPL
to provide reliable broadband?
The FCC recently released Notice of Proposed Rulemaking (NPRM) 04-29
which attempts to both encourage BPL deployment and address interference
concerns. The NPRM basically proposes a national database of BPL systems
and some interference mitigation techniques. Neither solve the
interference problem. The database hopefully will aid in identifying BPL
systems operating in a given area so that interference complaints can be
filed more quickly with
BPL carriers. While in general this is a good idea, it does little for
mobile operations as it's impractical for such operators to research
ahead
of time before travelling into an area to determine if interference-free
operation is possible. The interference mitigation techniques the FCC is
proposing comes down to moving interference around in the HF spectrum
until
no one complains. The problem with this is that it's very difficult to
find open space in this spectrum that won't affect anyone. The NTIA alone
has over 18,000 frequencies in use. BPL carriers will receive an
interference complaint, reconfigure their system to use other
frequencies, only to interfere with another service. As BPL systems need
more spectrum to feed a growing number of customers, more cells, and an
increased demand for bandwidth, this interference avoidance juggling act
will become impossible.
Radio Amateurs have been the most vocal in defending the radio spectrum,
especially in Internet forums. Amateurs do not oppose broadband
deployment, and in fact welcome it as most are born techies and use the
Internet extensively. Some Amateur applications such as VHF repeater
linking systems and position reporting systems actually use the Internet
for connectivity
and messaging. It is the ill effects of BPL on wireless spectrum which
Amateurs vehemently oppose.
In summary, power companies should be building for broadband dominance in
the coming decades with viable technology such as fiber, not for the next
year or two with doomed-for-obsolescence technology. Wireless spectrum
should be used for wireless applications, not to accommodate a wired
network that pollutes the spectrum. The risk to critical licensed
communications services is too great, the technological and regulatory
foundation of BPL is too weak, and when compared head-to-head with other
technologies, BPL loses
on both the business model and technical capability sides. BPL appears
glittery, but in reality it's Broadband Fool's Gold.
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Anthony Good
(RF_Engineer) is
Director of Systems Engineering at a regional ISP and CLEC and is
experienced in RF Engineering in Wireless ISPs (ISM/UNII), Cellular,
MMDS/ITFS, licensed point-to-point microwave, and LPTV. He is an Amateur
Radio operator and holds an FCC General Radiotelephone Operator
License.