PLC Access Networks

2.3.1 Structure of PLC Access Networks
The low-voltage supply networks consist of a transformer unit and a number of power
supply cables linking the end users, which are connected to the network over meter units.
A powerline transmission system applied to a low-voltage network uses it as a medium
for the realization of PLC access networks. In this way, the low-voltage networks can be
used for the realization of the so-called “last mile” communications networks.
The low-voltage supply networks are connected to medium- and high-voltage networks
via a transformer unit (Fig. 2.10). The PLC access networks are connected to the backbone
communications networks (WAN) via a base/master station (BS) usually placed within
the transformer unit. Many utilities supplying electrical power have their own telecommunications
networks linking their transformer units and they can be used as a backbone
network. If this is not the case, the transformer units can be connected to a conventional
telecommunications network.
The connection to the backbone network can also be realized via a subscriber or a
power street cabinet, especially if there is a convenient possibility for its installation (e.g.
there is a suitable cable existing that can be used for this purpose at low cost). In any case,
the communications signal from the backbone has to be converted into a form that makes
possible its transmission over a low-voltage power supply network. The conversion takes
place in a main/base station of the PLC system.
The PLC subscribers are connected to the network via a PLC modem placed in the
electrical power meter unit (M, Fig. 2.10) or connected to any socket in the internal electrical
network. In the first case, the subscribers within a house or a building are connected
to the PLC modem using another communications technology (e.g. DSL, WLAN). In the
second case, the internal electrical installation is used as a transmission medium that leads
to the so-called in-home PLC solution (Sec. 2.3.2).
The modem converts the signal received from the PLC network into a standard form
that can be processed by conventional communications systems. On the user side, standard
communications interfaces (such as Ethernet and ISDN S0) are usually offered. Within
a house, the transmission can be realized via a separated communications network or
via an internal electric installation (in-home PLC solution). In this way, a number of
communications devices within a house can also be connected to a PLC access network.
2.3.2 In-home PLC Networks
In-home PLC (indoor) systems use internal electrical infrastructure as transmission medium.
It makes possible the realization of PLC local networks within houses, which connect some
typical devices existing in private homes; telephones, computers, printers, video devices,
and so on. In the same way, small offices can be provided with PLC LAN systems. In both
cases, the laying of new communications cables at high cost is avoided.
Nowadays, automation services are becoming more and more popular not only for
their application in the industrial and business sectors and within large buildings, but also
for their application in private households. Systems providing automation services like
security observation, heating control, automatic light control have to connect a big number
of end devices such as sensors, cameras, electromotors, lights, and so on. Therefore,
in-home PLC technology seems to be a reasonable solution for the realization of such
networks with a large number of end devices, especially within older houses and buildings
that do not have an appropriate internal communication infrastructure (Sec. 2.2.4).
Basically, the structure of an in-home PLC network is not much different from the
PLC access systems using low-voltage supply networks. There can also a base station
that controls an in-home PLC network, and probably connects it to the outdoor area
(Fig. 2.11). The base station can be placed with the meter unit, or in any other suitable
place in the in-home PLC network. All devices of an in-home PLC network are connected
via PLC modems, such as the subscribers of a PLC access network. The modems are
connected directly to the wall power supply sockets (outlets), which are available in the
whole house/flat. Thus, different communications devices can be connected to the in-home
PLC network wherever wall sockets are available.
An in-home PLC network can exist as an independent network covering only a house
or a building. However, it excludes usage and control of in-home PLC services from a
distance. On the other hand, a remote controlled in-home PLC system is very comfortable
for the realization of various automation functions (e.g. security, energy management, see
Sec. 2.2.4). Also, connection of an in-home PLC network to a WAN communication
system allows the usage of numerous telecommunications services from each electrical
socket within a house.
In-home PLC networks can be connected not only to a PLC access system but also
to an access network realized by any other communications technology. In the first case,
if the access network is operated by a power utility, additional metering services can be
realized; for example, remote reading of electrical meter instruments saves the cost of
manual reading, or energy management, which can be combined with an attractive tariff
structure. On the other hand, an in-home PLC network can be connected to the access
networks provided by different network operators as well. Thus, the users of the in-home
network can also profit from the liberalized telecommunications market.
On the other hand, there are also other cost-effective communications systems for
the realization of the broadband in-home networks. Wireless LAN (WLAN) systems
are already available on the market, providing transmission data rates beyond 20 Mbps
(Sec. 2.1.3). So, in contrast to the in-home PLC, WLAN allows the mobile usage of
telecommunications services, such as cordless telephony, and more convenient handles
with various portable communication devices. Nowadays, WLAN components with significantly
improved performance become cheaper making the penetration of the in-home
PLC technology more difficult.
2.3.3 PLC Network Elements
As mentioned above, PLC networks use the electrical supply grids as a medium for
the transmission of different kinds of information and the realization of various communications
and automation services. However, the communications signal has to be
converted into a form that allows the transmission via electrical networks. For this purpose,
PLC networks include some specific network elements ensuring signal conversion
and its transmission along the power grids.
2.3.3.1 Basic Network Elements
Basic PLC network elements are necessary for the realization of communication over
electrical grids. The main task of the basic elements is signal preparation and conversion
for its transmission over powerlines as well as signal reception. The following two devices
exist in every PLC access network:
• PLC modem
• PLC base/master station.
A PLC modem connects standard communications equipment, used by the subscribers,
to a powerline transmission medium. The user-side interface can provide various standard
interfaces for different communications devices (e.g. Ethernet and Universal Serial Bus
(USB) interfaces for realization of data transmission and S0 and a/b interfaces for telephony).
On the other side, the PLC modem is connected to the power grid using a specific coupling
method that allows the feeding of communications signals to the powerline medium and its
reception (Fig. 2.12).
The coupling has to ensure a safe galvanic separation and act as a high pass filter
dividing the communications signal (above 9 kHz) from the electrical power (50 or 60 Hz).
To reduce electromagnetic emissions from the powerline, the coupling is realized between
two phases in the access area and between a phase and the neutral conductor in the indoor
area [Dost01]. The PLC modem implements all the functions of the physical layer including
modulation and coding. The second communications layer (data link layer) is also
implemented within the modem including its MAC (Medium Access Control) and LLC
(Logical Link Control) sublayers (according to the OSI (Open Systems Interconnection)
reference model, see for example [Walke99]).
A PLC base station (master station) connects a PLC access system to its backbone
network (Fig. 2.10). It realizes the connection between the backbone communications
network and the powerline transmission medium. However, the base station does not
connect individual subscriber devices, but it may provide multiple network communications
interfaces, such as xDSL, Synchronous Digital Mierarch (SDH) for connection with
a high-speed network, WLL for wireless interconnection, and so on. (Fig. 2.13). In this
way, a PLC base station can be used to realize connection with backbone networks using
various communication technologies.
Usually, the base station controls the operation of a PLC access network. However, the
realization of network control or its particular functions can be realized in a distributed
manner. In a special case, each PLC modem can take over the control of the network
operation and the realization of the connection with the backbone network.
2.3.3.2 Repeater
In some cases, distances between PLC subscribers placed in a low-voltage supply network
and between individual subscribers and the base station are too long to be bridged by
a PLC access system. To make it possible to realize the longer network distances, it
is necessary to apply a repeater technique. The repeaters divide a PLC access network
into several network segments, the lengths of which can be overcome by the applied
PLC system. Network segments are separated by using different frequency bands or by
different time slots (Fig. 2.14). In the second case, a time slot is used for the transmission
within the first network segment and another slot for the second segment.
In the case of frequency-based network segmentation, the repeater receives the transmission
signal on the frequency f1, amplifies and injects it into the network, but on the
frequency f2. In the opposite transmission direction, the conversion is carried out for frequency
f2 to f1. Depending on applied transmission and modulation methods, the repeater
function can include demodulation and modulation of the transmitted signal as well as
its processing on a higher network layer. However, a repeater does not modify the contents
of the transmitted information, which is always transparently transmitted between
the network segments of an entire PLC access system (Fig. 2.15).
In a first network segment, between a base station placed in the transformer unit and
the first repeater, the signal is transmitted within the frequency spectrum f1. Another
frequency range (f2) has to be applied in the second network segment. Independent of
the physical network topology, the signal is transmitted along both network branches.
Theoretically, frequency range f1 could be used again within the third network segment.
However, if there is an interference between signals from the first segment, a third frequency
range f3 has to be applied to the third network segment and frequency f4 to the
fourth segment.
However, there is a limited frequency spectrum that can be used by the PLC technology
(approximately up to 30 MHz), which is (or will be) specified by the regulatory bodies.
So, with the increasing number of different frequency ranges, the common bandwidth is
divided into smaller portions, which significantly reduces the network capacity. Therefore,
a frequency plan for a PLC access network has to provide usage of as low a number of
frequencies as possible. Application of the repeaters can extend network distances that
are realized by the PLC technology. However, the application of repeaters also increases
the network costs because of the increasing equipment and installation costs. Therefore,
the number of repeaters within a PLC access network has to be kept as small as possible.
2.3.3.3 PLC Gateway
There are two approaches for the connection of the PLC subscribers via wall sockets to
a PLC access network:
• Direct connection
• Indirect connection over a gateway.
In the first case, PLC modems are directly connected to the entire low-voltage network
and with it to the PLC base station as well (Fig. 2.16). There is no division between
the outdoor and indoor (in-home) areas, and the communications signal is transmitted
through the power meter unit. However, the features of indoor and outdoor power supply
networks are different, which causes additional problems regarding characteristics of PLC
transmission channel and electromagnetic compatibility problems (as is explained later in
the book). Therefore, the indirect connection using a gateway is a frequently used solution
for the direct connection of the wall sockets to entire PLC access networks.
A gateway is used to divide a PLC access network and an in-home PLC network.
It also converts the transmitted signal between the frequencies that are specified for
use in the access and in-home areas. Such a gateway is usually placed near the house
meter unit (Fig. 2.17). However, a PLC gateway can provide additional functions that
ensure a division of the access and in-home areas on the logical network level too. Thus,
PLC modems connected within an in-home network can communicate internally without
information flow into the access area. In this case, a PLC gateway serves as a local base
station that controls an in-home PLC network coordinating the communication between
internal PLC modems and also between internal devices and a PLC access network (see
Sec. 2.3.2).
Generally, a gateway can also be placed anywhere in a PLC access network to provide
both signal regeneration (repeater function) and network division on the logical level.
In this way, a PLC can be divided into several subnetworks that use the same physical
transmission medium (the same low-voltage network), but exist separately as a kind of
virtual network (Fig. 2.18). Both gateways (G) operate as PLC repeaters converting the
transmission signal between frequencies f1 and f2 (or time slots t1 and t2), as well as
between f2 and f3 (or t2 and t3). Additionally, the gateways control the subnetworks II
and III, which means that internal communication within a subnetwork is taken over by a
responsible gateway and does not affect the rest of a PLC access network, similar to that
within in-home networks using a gateway. The communication between a member of a
subnetwork and the base station is possible only over a responsible gateway. However, the
network can be organized so that the base station directly controls a number of subscribers
(subnetwork I).
The gateways are connected to the network in the same way as the repeaters (Fig. 2.14).
Also, an increasing number of gateways within a PLC access network reduces its network
capacity and causes higher costs. However, where the repeaters provide only a simple signal
forwarding between the network segments, the gateways can provide more intelligent
division of the available network resources, ensuring better network efficiency as well.
2.3.4 Connection to the Core Network
A PLC access network covers the so-called “last mile” of the telecommunications access
area. This means that the last few hundred meters of the access networks can be realized
by PLC technology applied to the low-voltage supply networks. On the other hand, PLC
access networks are connected to the backbone network through communications distribution
networks, as is shown in Fig. 2.19. In general, a distribution network connects a
PLC base station with a local exchange office operated by a network provider.
As mentioned in Sec. 2.1, the application of PLC technology should save the costs on
building new telecommunications networks. However, the PLC access network has to be
connected to the WAN via backbone networks that cause additional costs as well. Therefore,
a PLC backbone network has to be realized with the lowest possible investments to
ensure the competitiveness of PLC networks with other access technologies.
2.3.4.1 Communications Technologies for PLC Distribution Networks
The cheapest solution for the realization of the connection between a PLC access and the
backbone network is usage of communications systems that are available in the application
area. Some transformer units are already connected to a maintenance network via standard
communications cables (copper lines). Originally, these connections were provided for the
realization of remote control functions and internal communications between a control
center of the supply network and the maintenance personnel and equipment. However,
they can be used for the connection of PLC networks to the backbone by applying one
of the DSL technologies (Sec. 2.1.3).
During the last decade, many electrical utilities realized optical communications networks
along their supply lines, which can be applied for connection to the backbone
as well. In this case, an access network consists of an optical and a PLC network part
(Fig. 2.19), which leads to a hybrid solution similar to HFC networks (Hybrid Fiber Coax),
in which an optical distribution network connects CATV access networks to WAN. A
further solution for the realization of the backbone connection is application of PLC technology
in medium-voltage supply networks (Sec. 2.3.5), which are, in any case, connected
to the low-voltage networks.
Application of a particular communications technology to the PLC backbone connection
depends also on technical opportunities of a network provider operating PLC access
networks. Usage of existing communication systems, of a supply utility or an independent
network provider, is always a privileged solution. Generally, there are the following
possibilities for the realization of the connection to the core network:
• Usage of the existing or new cable or optical networks
• Realization of wireless distribution networks; e.g. WLL (Sec. 2.1.2), application of
satellite technology, and so on.
• Application of PLC technology in the MV supply networks.
Communications technology applied to the PLC distribution networks has to ensure
transmission of all services that are offered in the PLC access networks. Also, PLC
backbone networks must not be a bottleneck in the common communications structure
between PLC subscribers and the backbone network. Therefore, an applied backbone
technology has to provide enough transmission capacity (data rates) and realization of
various Quality of Service (QoS) guarantees.
2.3.4.2 Topology of the Distribution Networks
A reasonable solution for the connection of multiple PLC access networks, placed within
a smaller area, is the realization of a joint distribution network connecting a number
of PLC networks, as shown in Fig. 2.20. The distribution networks can be realized in
different topologies independent of applied communications technology (bus, star, ring).
A chosen network topology has to ensure a cost-effective, but also a reliable, solution
(including a redundancy in the case of failure), and this depends primarily on the location
of PLC access networks in a considered area and on the position of the local exchange
office (Fig. 2.19).
Bus network topology is one of the possible solutions that can be realized at low costs
within adequate application areas (Fig. 2.20). However, the cost factor is not the single
criterion for the decision about the topology of the distribution network. A very important
criterion is the network reliability in the case of link failures. So, in the bus topology,
if a link between two PLC access networks breaks down, all access networks placed
behind the failed link are also disconnected from the WAN. Therefore, meshed network
topologies have to be considered for application in the PLC distribution networks. A
possible solution is a network with a star topology connecting each PLC access network
separately (Fig. 2.21).
The star network topology is adequate for application of DSL technology in PLC distribution
networks. However, failure of a single link in the star network disconnects only
one PLC access network and there is no possibility for the realization of an alternative
connection of the affected PLC access network to the backbone over a redundant transmission
link. Therefore, the application of ring network topology (Fig. 2.22) seems to
be a reasonable solution for increasing the network reliability. In the case of a failure
in a single link between the ring nodes, there is always an opportunity for realization
of the alternative transmission paths. Of course, reorganization of the transmission paths
between the PLC access networks and the backbone has to be done automatically within
a relatively short time interval (maximum several seconds). Thus, applied transmission
technology in the backbone networks has to support the implementation in a ring network
structure (e.g. Distributed Queue Dual Bus (DQDB), Fiber Distributed Data Interface
(FDDI)).
Finally, the topology of a PLC distribution network can also be a combination of any
of the three basic network structures presented above. However, the choice for a network
topology depends on several factors, among others:
• Used communications technology causing a specific network topology,
• Availability of a transmission medium within the application area,
• Possibility of the realization of reliable distribution networks
• Geographical structure and distribution of PLC access networks and a local exchange
office.
2.3.4.3 Managing PLC Access Networks
An efficient control of the PLC access networks has to be done from one or a very
small number of management centers providing an economically reasonable solution.
However, PLC access networks belonging to a network or service provider can exist in
a geographically wider area or a number of PLC networks can be distributed in several
geographically separated regions. Therefore, it is important to optimize the management
system that is used for the control of multiple PLC access networks
Management of a PLC access network includes configuration and reconfiguration of
all its elements (base station, modems, repeaters and gateways) depending on the current
network status. The management functions can be done locally by the base station or
gateways or by a management center using remote control functions. Local management
is done automatically without any action of the management personnel. On the other hand,
remote management provides both automatic and manual execution of control functions.
Transmission of management information from and to the access networks has to be
ensured over PLC distribution networks to avoid buildup of particular management communications
systems. An efficient management solution is the transfer of possibly more
maintenance functions to the base stations and gateways placed in the access networks.
However, management ability of PLC network elements increases the equipment costs.
Therefore, the division of management functions between the network elements and a
central office is an optimization task as well.
Anyway, the basic network operation has to be ensured by PLC network elements
themselves, without any action of a management center. Once the equipment is installed
in a low-voltage network, a PLC network that provides a number of self-control and selfconfiguration
procedures should operate without the aid of the maintaining personnel.
PLC access networks can be operated with economical efficiency only if the need for
manual network control is reduced, especially activities that are carried out directly on
the network locations.
2.3.5 Medium-voltage PLC
Similar to the PLC access systems using low-voltage power supply networks as a transmission
medium, the medium-voltage supply networks can also be used for the realization
of various PLC services. Generally, the organization of the so-called medium-voltage PLC
(MV PLC) is not different from the PLC in the low-voltage networks. Thus, the mediumvoltage
PLC networks include the same network elements (Sec. 2.3.3): PLC modems
connecting the end users with the medium-voltage transmission medium, base station
connecting a medium-voltage PLC network to the backbone, repeaters and gateways.
A medium-voltage electrical network usually supplies several low-voltage networks, as
is mentioned in Sec. 2.2.2 and presented in Fig. 2.7. Accordingly, an MV PLC network
can be used as a distribution network connecting a number of PLC access networks to
the backbone. In this case, several PLC access networks are connected to the MV PLC
distribution network with a network topology similar to the ring distribution network
presented in Fig. 2.22.
However, the transmission features of the medium-voltage supply networks, considered
for their application in communications, seem to be similar to the low-voltage networks.
Even the transmission conditions in the medium-voltage networks are better than in the
low-voltage networks used for the realization of PLC access networks; the data rates to be
realized over MV PLC are expected to be not significantly higher than in the PLC access
networks. Accordingly, if a MV PLC network is used to connect a higher number of PLC
access networks to the core network, the transmission part over the medium-voltage power
grids would be a bottleneck. Therefore, it is not expected that the MV PLC networks will
be used for the interconnection of multiple PLC access networks (e.g. to connect more
than two access networks). However, in the developing phase it is expected that PLC
access networks connect a fewer number of end users and in this case, the MV networks
can be used as a solution for the distribution network.
On the other hand, the MV PLC offers an opportunity for the realization of communications
networks without the need for the laying of new communications cables in a
wider covering area. So, a medium-voltage supply network can be used for the connection
of multiple LAN within a campus in a common data network, as shown in Fig. 2.24.
In the same way, the MV PLC can be applied for the realization of various pointto-
point connections, which can be used for interconnection between LAN, similar to
the campus network shown in Fig. 2.24. Nowadays, the MV PLC is mainly applied for
the realization of such point-to-point connections. An application of MV PLC is the
connection of antennas for various radio systems. In this way, an antenna used for a
wireless mobile system (see Fig. 2.2) can be connected to its base station via a mediumvoltage
supply network.