From power grid to communications network
Highspeed Internet on the power grid
Powerline Communications (PLC)
– communications over the electricity distribution grid – has become a
hot topic in recent months. Although this technology has been in use for
special applications for several decades – street lighting is frequently
operated according to this principle, and the well-known baby intercom
sends voice signals over the internal household power network - communication
in these cases is exclusively in the narrowband range and transmission
rates are correspondingly low. The latest development was driven firstly
by the growing demand for bandwidth, and secondly by the quest for a fast,
efficient alternative to the „last mile", which in many countries is still
protected from free market forces. High bandwidths are essential for the
breakthrough to consumer-driven e-business. Only when high-speed Internet
access becomes affordable and widely available, can electronic trading
with end consumers really take off. Powerline aims to make this quantum
leap possible using the existing infrastructure.
Separation of indoor and outdoor
The idea of combining communications
signals and electricity on a single transport medium is an obvious one.
With a world-wide coverage of approximately 95 percent, the electricity
grid is the largest network bar none. Applications are divided into two
areas: procedures which are performed outside the home (outdoor), and
procedures inside the home (indoor). In the outdoor zone, the conventional
telecommunications infrastructure is used to connect the relevant local
network station with the telephone network or a specific Internet backbone.
Depending on distance and local conditions, the connection is enabled
by radio, copper lines or optical cables. The local network station combines
data and voice signals on the power grid and sends them as a data stream
to any socket in connected households i.e. to the end user via the low-voltage
network.The access point forwards incoming data streams to the indoor
network, and an indoor master in the household controls and coordinates
all (externally and internally) transmitted data signals. Intermediate
adapters separate data and power at the socket and forward the data to
individual applications. There is no need for separate telephone or data
cabling since the socket, far from being a mere electrical point, becomes
a powerful communications interface which bridges the last mile for high-speed
Internet access, thus enabling networking throughout the building or household.
Fig. 1 Overview of the PLC system
The PLC network is divided into two cascading but independent systems
for good reason. The public part of the network ranges from the network
transformer to the individual access point in the home. This section is
normally in the hands of and under the responsibility of energy supply
companies, who are ensure correct operation and high-level system quality.
The electrical system from the access point to the individual socket is
the householder’s responsibility and is not subject to consistent checks.
Another advantage of the division is that the outdoor system is used by
several buildings or households for access to the backbone network and
must therefore guarantee high-level reliability. By the same token, network
traffic inside the household does not affect the outdoor zone. There are
also solid technical reasons for the separation. Signal distribution in
the public power grid behaves differently from signalling inside a building.
Low frequencies are more suitable for the outdoor zone and higher frequencies
for the indoor zone.
Powerline technology brings data
streams and voice signals to sockets inside a building via the low-voltage
network. The concept is based on a master-slave principle and uses a small
number of standard units: The outdoor master acts as administrator for
the outdoor system and as a gateway connecting the PLC system with the
backbone network. The access point connects the outdoor and indoor systems.
Outwardly it performs the functions of an adapter (slave), while inwardly
it acts as the master and is responsible for administration of the indoor
system.The indoor adapter provides the interface between the internal
data network, PC, printers and telephones on the one hand, and the backbone
network for Internet, telephony, video conferencing and similar applications
on the other hand. Adapters which communicate on the outdoor system’s
frequency are also available for connection to the outdoor system. The
adapters are equipped with standard interfaces (Ethernet, USB, analog
A/B telephone interface). Repeaters amplify the signal over long distances.
Fig. 2 Ascom Powerline APA 45.
This adapter is switched between the socket and terminal. The outdoor
units (master, access point and repeaters) are normally connected with
all phases using fixed cables. The PLC signal is switched between two
of the three phases. As a result, signalling can be phase-optimised –
an option not offered by the adapters. These are directly connected to
the socket via a conventional electric cable, with the signal connected
between phase and neutral conductor. At first glance this disparate linking
may not appear particularly beneficial, but measurements have proved otherwise.
Cabling inside a building produces a high level of crosstalk between the
individual wires. The initially selected link counteracts this after a
Fig. 3 Connection diagram for
PLC equipmentFlexible bandwidth management
Powerline is a shared medium: it
works on the point-to-multipoint principle. A local network station supplies
power to a specific number of households and also delivers data or voice
to several terminals. Users who communicate simultaneously share the available
bandwidth. Currently this is 4.5 Mbit/s but speeds of 20 Mbit/s are envisaged
in the medium term. Flexible bandwidth management and packet-switched
data transfer ensure sufficient bandwidth availability on the shared transmission
medium for voice and data services, even during peak times – so all users
can derive equal satisfaction from performance.Part of the bandwidth is
prioritised for delay-sensitive traffic such as voice and video conferencing.
Ascom PLC equipment ensures the quality of various services such as voice
and data without prioritisation, in compliance with Standard 802.p.
As with other systems, good planning
ensures efficient installation and optimal operation of the PLC system.
In the case of powerline installations, the optimum installation points
also need to be determined, deployment plans must be drawn up for the
available carrier frequencies, the IP addresses and VLAN identifications
must be assigned, and the backbone connection set up.Under normal circumstances
the installation points are already in place: the outdoor master is sited
in the transformer station, the access point beside the electricity meter,
the adapters in a socket defined by the user. Determining the optimum
distances is not quite as simple. These vary depending on power output,
loss during power distribution, and the noise level at the receiving end.
However, by applying the results of extensive measurements it is possible
to predict the average distance in a concrete situation with sufficient
accuracy. Without repeaters this can be between 200 and 300 meters, but
only for public electricity networks with aerial or underground lines.For
indoor networks with a number of different interference sources, which
in many cases are unstructured, the average values provide no useful indicators
since different types of installation affect the transmission distance
of up to 100 meters in very different ways. With increasing experience,
however, reliable estimates can also be produced for indoor systems. Thanks
to extensive field-trial experience, the Ascom installation crew is able
to make reliable predictions of distance.
Fig. 4 Signal attenuation as
a function of distance
Connecting PLC systems to the backbone
is a more complex task. This generally calls for close collaboration with
the network provider and above all concerns aspects of system security,
connection of PLC equipment to the network management system, IP addressing,
the allocation of VLAN Ids, and integration of voice traffic in the system.
Only when all this preliminary work is completed and the associated questions
are clarified can the actual installation begin. A top-down approach is
recommended: beginning with the backbone and proceeding to the outdoor
master and access point, down to the adapter inside the building and the
required settings on the user’s PC.
Practical deployment now follows
Proof that the PLC concept also
works in practice was furnished by a series of field trials in 16 European
countries from Portugal to Scandinavia, as well as in Hong Kong and Singapore.
These trials fulfilled all expectations of reliability, functionality
and the practical applications of Powerline Communications. The first
installations are now already up and running or about to go live.Users
in Germany include the electricity companies RWE Energie Essen and EnBW
Energie Baden-Württemberg, while in Spain the energy and telecoms
group Endesa uses PLC technology. Lina.Net of Iceland, a subsidiary of
Reykjavik Energy, has just begun introducing PLC technology with the declared
objective of providing private households with fast Internet access over
the power grid rather than the telephone network. In Sweden Sydkraft,
one of the leading energy providers in Scandinavia, uses PLC for bridging
the last mile as well as for networking inside buildings.
New business areas
Internet browsing, simultaneous
transmission of faxes and voice calls with first-class voice quality are
only a few of the possibilities offered by Powerline technology. All equipment
within a household – PC, printer, phone, fax – can be interconnected simply,
flexibly and without additional cabling by using existing power lines.
Electricity, gas and water meters can be read on-line, and alarm systems,
heating and household appliances can be controlled over the internal power
network and maintained via Internet. Powerline is also opening up new
possibilities in other areas – for instance care for the elderly and disabled.For
energy suppliers, the potential inherent in Powerline technology is enormous.
Gaining direct access to end customers provides them with an opportunity
to enhance their market image with individual, innovative offerings, and
penetrate new business areas. In view of the deregulation of the electricity
market and the associated intense pressure on pricing and competition,
this is a welcome development as well as a new way of using existing assets
at a relatively low investment cost. A new business area is also being
opened up for electrical installation companies: PLC offers them the opportunity
to grow in their home market by expanding into data communications, and
to play a direct role in the information economy. PLC components must
be installed professionally and correctly before they are able to bring
broadband communications into the home.Author’s
addressBeat Hübscher, Ascom Powerline Communications AG, E-mail: