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.
Sunday, December 9, 2007
Powerline Communications Systems
2.2.1 Historical Overview
PowerLine Communications is the usage of electrical power supply networks for communications
purposes. In this case, electrical distribution grids are additionally used as a
transmission medium for the transfer of various telecommunications services. The main
idea behind PLC is the reduction of cost and expenditure in the realization of new telecommunications
networks.
High- or middle-voltage power supply networks could be used to bridge a longer distance
to avoid building an extra communications network. Low-voltage supply networks
are available worldwide in a very large number of households and can be used for the
realization of PLC access networks to overcome the so-called telecommunications “last
mile”. Powerline communications can also be applied within buildings or houses, where
an internal electrical installation is used for the realization of in-home PLC networks.
The application of electrical supply networks in telecommunications has been known
since the beginning of the twentieth century. The first Carrier Frequency Systems (CFS)
had been operated in high-voltage electrical networks that were able to span distances over
500 km using 10-W signal transmission power [Dost97]. Such systems have been used
for internal communications of electrical utilities and realization of remote measuring
and control tasks. Also, the communications over medium- and low-voltage electrical
networks has been realized. Ripple Carrier Signaling (RCS) systems have been applied to
medium- and low-voltage networks for the realization of load management in electrical
supply systems.
Internal electrical networks have been mostly used for realization of various automation
services. Application of in-home PLC systems makes possible the management of numerous
electrical devices within a building or a private house from a central control position
without the installation of an extra communications network. Typical PLC-based building
automation systems are used for security observance, supervision of heating devices, light
control, and so on.
2.2.2 Power Supply Networks
The electrical supply systems consist of three network levels that can be used as a transmission
medium for the realization of PLC networks (Fig. 2.7):
• High-voltage (110–380 kV) networks connect the power stations with large supply
regions or big customers. They usually span very long distances, allowing power
exchange within a continent. High-voltage networks are usually realized with overhead
supply cables.
• Medium-voltage (MV) (10–30 kV) networks supply larger areas, cities and big industrial
or commercial customers. Spanned distances are significantly shorter than in the
high-voltage networks. The medium-voltage networks are realized as both overhead
and underground networks.
• Low-voltage (230/400 V, in the USA 110 V) networks supply the end users either as
individual customers or as single users of a bigger customer. Their length is usually
up to a few hundred meters. In urban areas, low-voltage networks are realized with
underground cables, whereas in rural areas they exist usually as overhead networks.
In-home electrical installations belong to the low-voltage network level. However, internal
installations are usually owned by the users. They are connected to the supply network
over a meter unit (M). On the other hand, the rest of the low-voltage network (outdoor)
belongs to the electrical supply utilities.
Low-voltage supply networks directly connect the end customers in a very large number
of households worldwide. Therefore, the application of PLC technology in low-voltage
networks seems to have a perspective regarding the number of connected customers. On
the other hand, low-voltage networks cover the last few hundreds of meters between the
customers and the transformer unit and offer an alternative solution using PLC technology
for the realization of the so-called “last mile” in the telecommunications access area.
2.2.3 Standards
The communications over the electrical power supply networks is specified in a European
standard CENELEC EN 50065, providing a frequency spectrum from 9 to 140 kHz
for powerline communications (Tab. 2.2). CENELEC norm significantly differs from
American and Japanese standards, which specify a frequency range up to 500 kHz for
the application of PLC services.
CENELEC norm makes possible data rates up to several thousand bits per second,
which are sufficient only for some metering functions (load management for an electrical
network, remote meter reading, etc.), data transmission with very low bit rates and the
realization of few numbers of transmission channels for voice connections. However, for
application in modern telecommunications networks, PLC systems have to provide much
higher data rates (beyond 2Mbps). Only in this case, PLC networks are able to compete
with other communications technologies, especially in the access area (Sec. 2.1).
For the realization of the higher data rates, PLC transmission systems have to operate
in a wider frequency spectrum (up to 30 MHz). However, there are no PLC standards that
specify the operation of PLC systems out of the frequency bands defined by the CENELEC
norm. Currently, there are several bodies that try to lead the way for standardization of
broadband PLC networks, such as the following:
• PLCforum [PLCforum] is an international organization with the aim to unify and represent
the interests of players engaged in PLC from all over the world. There are more
than 50 members in the PLCforum; manufacturer companies, electrical supply utilities,
network providers, research organizations, and so on. PLCforum is organized into four
working groups: Technology, Regulatory, Marketing and Inhouse working group.
• The HomePlug Powerline Alliance [HomePlug] is a not-for-profit corporation formed to
provide a forum for the creation of open specifications for high-speed home powerline
networking products and services. HomePlug is concentrated on in-home PLC solutions
and it works close to PLCforum as well.
Standardization activities for broadband PLC technology are also included in the work
of European Telecommunications Standards Institute (ETSI) and CENELEC.
2.2.4 Narrowband PLC
The narrowband PLC networks operate within the frequency range specified by the CENELEC
norm (Tab. 2.2). This frequency range is divided into three bands: A, to be used by
power supply utilities, and B and C, which are provided for private usage. The utilities use
narrowband PLC for the realization of the so-called energy-related services. Frequency
bands B and C are mainly used for the realization of building and home automation.
Nowadays, the narrowband PLC systems provide data rates up to a few thousand bits per
second (bps) [Dost01]. The maximum distance between two PLC modems can be up to
1 km. To overcome longer distances, it is necessary to apply a repeater technique.
The narrowband PLC systems apply both narrowband and broadband modulation
schemes. First narrowband PLC networks have been realized by the usage of Amplitude
Shift Keying (ASK) [Dost01]. The ASK is not robust against disturbances and, therefore,
is not suitable for application in PLC networks. On the other hand, Binary Phase Shift
Keying (BPSK) is a robust scheme and, therefore, is more suitable for application in PLC.
However, phase detection, which is necessary for the realization of BPSK, seems to be
complex and BPSK-based systems are not commonly used. Most recent narrowband PLC
systems apply Frequency Shift Keying (FSK), and it is expected that BPSK will be used
in future communications systems [Dost01].
Broadband modulation schemes are also used in narrowband PLC systems. The advantages
of broadband modulation, such as various variants of spread spectrum, are its
robustness against narrowband noise and the selective attenuation effect that exists in
the PLC networks [Dost01]. A further transmission scheme also used in narrowband PLC
system is Orthogonal Frequency Division Multiplexing (OFDM) [Bumi03].
A comprehensive description of various narrowband PLC systems, including their realization
and development, can be found in [Dost01]. The aim of this book is a presentation
of broadband PLC systems, and, therefore, the narrowband systems are not discussed in
detail. However, to sketch the possibilities of the narrowband PLC, we present several
examples for application of this technology in the description below.
A very important area for the application of narrowband PLC is building/home automation.
PLC-based automation systems are realized without the installation of additional
communications networks (Fig. 2.8). Thus, the high costs that are necessary for the installation
of new networks within existing buildings can be significantly decreased by the
usage of PLC technology. Automation systems realized by PLC can be applied to different
tasks to be carried out within buildings:
• Control of various devices that are connected to the internal electro installation, such
as illumination, heating, air-conditioning, elevators, and so on.
• Centralized control of various building systems, such as window technique (darkening)
and door control.
• Security tasks; observance, sensor interconnection, and so on.
PLC-based automation systems are not only used in large buildings but they are also
very often present in private households for the realization of similar automation tasks
(home automation). In this case, several authors talk about so-called smart homes.
A PLC variant of the EIB (European Installation BUS) standard is named Powernet-
EIB. PLC modems designed according to the Powernet-EIB can be easily connected to
any wall socket or integrated in any device connected to the electrical installation. This
ensures communications between all parts of an internal electrical network. Nowadays,
the PLC modems using FSK achieve data rates up to 1200 bps [Dost01].
As it is specified in CENELEC standard, power supply utilities can use band A for
the realization of so-called energy-related services. In this way, a power utility can use
PLC to realize internal communications between its control center and different devices,
ensuring remote control functions, without building extra telecommunications network or
buying network resources at a network provider (Fig. 2.9). Simultaneously, PLC can be
used for remote reading of a customer’s meter units, which additionally saves cost on the
personnel needed for manual meter reading. Finally, PLC can also be used by the utilities
for dynamic pricing (e.g. depending on the day time, total energy offer, etc.), as well
as for observation and control of energy consumption and production. In the last case,
especially, the utilities have been trying to integrate an increasing number of small power
plants; for example, small hydroelectric power stations, wind plants, and so on. However,
the small power plants are not completely reliable and their power production varies
depending on the current weather conditions. Therefore, the regions that are supplied by
the small plants should also be supplied from other sources if necessary. For this purpose,
the utilities need a permanent communication between their system entities, which can
be at least partly realized by PLC as well.
The building automation is a typical indoor application of the narrowband PLC systems,
whereas the energy-related services are mainly (not only) indoor applications. In [BumiPi03],
we find a very interesting example of an application of a PLC-based automation system in the
outdoor area. In this case, a PLC-based airfield ground–lighting automation system is used
for individual switching and monitoring of airfield lighting. The length of the airfields and
accordingly the necessary communications networks in a large airport is very long (several
kilometers). So, the narrowband PLC can be applied to save costs on building a separate
communications network. This is also an example of PLC usage for the realization of socalled
critical automation services with very high security requirements, such as the light
control of ground aircraft movement in the airports.
2.2.5 Broadband PLC
Broadband PLC systems provide significantly higher data rates (more than 2 Mbps) than
narrowband PLC systems. Where the narrowband networks can realize only a small number
of voice channels and data transmission with very low bit rates, broadband PLC
networks offer the realization of more sophisticated telecommunication services; multiple
voice connections, high-speed data transmission, transfer of video signals, and narrowband
services as well. Therefore, PLC broadband systems are also considered a capable
telecommunications technology.
The realization of broadband communications services over powerline grids offers a
great opportunity for cost-effective telecommunications networks without the laying of
new cables. However, electrical supply networks are not designed for information transfer
and there are some limiting factors in the application of broadband PLC technology.
Therefore, the distances that can be covered, as well as the data rates that can be realized
by PLC systems, are limited. A further very important aspect for application of broadband
PLC is its Electromagnetic Compatibility (EMC). For the realization of broadband PLC, a
significantly wider frequency spectrum is needed (up to 30MHz) than is provided within
CENELEC bands. On the other hand, a PLC network acts as an antenna becoming a
noise source for other communication systems working in the same frequency range (e.g.
various radio services). Because of this, broadband PLC systems have to operate with a
limited signal power, which decreases their performance (data rates, distances).
Current broadband PLC systems provide data rates beyond 2Mbps in the outdoor arena,
which includes medium- and low-voltage supply networks (Fig. 2.7), and up to 12 Mbps
in the in-home area. Some manufacturers have already developed product prototypes
providing much higher data rates (about 40 Mbps). Medium-voltage PLC technology is
usually used for the realization of point-to-point connections bridging distances up to several
hundred meters. Typical application areas of such systems is the connection of local
area networks (LAN) networks between buildings or within a campus and the connection
of antennas and base stations of cellular communication systems to their backbone
networks. Low-voltage PLC technology is used for the realization of the so-called “last
mile” of telecommunication access networks. Because of the importance of telecommunication
access, current development of broadband PLC technology is mostly directed
toward applications in access networks including the in-home area. In contrast to narrowband
PLC systems, there are no specified standards that apply to broadband PLC networks
PowerLine Communications is the usage of electrical power supply networks for communications
purposes. In this case, electrical distribution grids are additionally used as a
transmission medium for the transfer of various telecommunications services. The main
idea behind PLC is the reduction of cost and expenditure in the realization of new telecommunications
networks.
High- or middle-voltage power supply networks could be used to bridge a longer distance
to avoid building an extra communications network. Low-voltage supply networks
are available worldwide in a very large number of households and can be used for the
realization of PLC access networks to overcome the so-called telecommunications “last
mile”. Powerline communications can also be applied within buildings or houses, where
an internal electrical installation is used for the realization of in-home PLC networks.
The application of electrical supply networks in telecommunications has been known
since the beginning of the twentieth century. The first Carrier Frequency Systems (CFS)
had been operated in high-voltage electrical networks that were able to span distances over
500 km using 10-W signal transmission power [Dost97]. Such systems have been used
for internal communications of electrical utilities and realization of remote measuring
and control tasks. Also, the communications over medium- and low-voltage electrical
networks has been realized. Ripple Carrier Signaling (RCS) systems have been applied to
medium- and low-voltage networks for the realization of load management in electrical
supply systems.
Internal electrical networks have been mostly used for realization of various automation
services. Application of in-home PLC systems makes possible the management of numerous
electrical devices within a building or a private house from a central control position
without the installation of an extra communications network. Typical PLC-based building
automation systems are used for security observance, supervision of heating devices, light
control, and so on.
2.2.2 Power Supply Networks
The electrical supply systems consist of three network levels that can be used as a transmission
medium for the realization of PLC networks (Fig. 2.7):
• High-voltage (110–380 kV) networks connect the power stations with large supply
regions or big customers. They usually span very long distances, allowing power
exchange within a continent. High-voltage networks are usually realized with overhead
supply cables.
• Medium-voltage (MV) (10–30 kV) networks supply larger areas, cities and big industrial
or commercial customers. Spanned distances are significantly shorter than in the
high-voltage networks. The medium-voltage networks are realized as both overhead
and underground networks.
• Low-voltage (230/400 V, in the USA 110 V) networks supply the end users either as
individual customers or as single users of a bigger customer. Their length is usually
up to a few hundred meters. In urban areas, low-voltage networks are realized with
underground cables, whereas in rural areas they exist usually as overhead networks.
In-home electrical installations belong to the low-voltage network level. However, internal
installations are usually owned by the users. They are connected to the supply network
over a meter unit (M). On the other hand, the rest of the low-voltage network (outdoor)
belongs to the electrical supply utilities.
Low-voltage supply networks directly connect the end customers in a very large number
of households worldwide. Therefore, the application of PLC technology in low-voltage
networks seems to have a perspective regarding the number of connected customers. On
the other hand, low-voltage networks cover the last few hundreds of meters between the
customers and the transformer unit and offer an alternative solution using PLC technology
for the realization of the so-called “last mile” in the telecommunications access area.
2.2.3 Standards
The communications over the electrical power supply networks is specified in a European
standard CENELEC EN 50065, providing a frequency spectrum from 9 to 140 kHz
for powerline communications (Tab. 2.2). CENELEC norm significantly differs from
American and Japanese standards, which specify a frequency range up to 500 kHz for
the application of PLC services.
CENELEC norm makes possible data rates up to several thousand bits per second,
which are sufficient only for some metering functions (load management for an electrical
network, remote meter reading, etc.), data transmission with very low bit rates and the
realization of few numbers of transmission channels for voice connections. However, for
application in modern telecommunications networks, PLC systems have to provide much
higher data rates (beyond 2Mbps). Only in this case, PLC networks are able to compete
with other communications technologies, especially in the access area (Sec. 2.1).
For the realization of the higher data rates, PLC transmission systems have to operate
in a wider frequency spectrum (up to 30 MHz). However, there are no PLC standards that
specify the operation of PLC systems out of the frequency bands defined by the CENELEC
norm. Currently, there are several bodies that try to lead the way for standardization of
broadband PLC networks, such as the following:
• PLCforum [PLCforum] is an international organization with the aim to unify and represent
the interests of players engaged in PLC from all over the world. There are more
than 50 members in the PLCforum; manufacturer companies, electrical supply utilities,
network providers, research organizations, and so on. PLCforum is organized into four
working groups: Technology, Regulatory, Marketing and Inhouse working group.
• The HomePlug Powerline Alliance [HomePlug] is a not-for-profit corporation formed to
provide a forum for the creation of open specifications for high-speed home powerline
networking products and services. HomePlug is concentrated on in-home PLC solutions
and it works close to PLCforum as well.
Standardization activities for broadband PLC technology are also included in the work
of European Telecommunications Standards Institute (ETSI) and CENELEC.
2.2.4 Narrowband PLC
The narrowband PLC networks operate within the frequency range specified by the CENELEC
norm (Tab. 2.2). This frequency range is divided into three bands: A, to be used by
power supply utilities, and B and C, which are provided for private usage. The utilities use
narrowband PLC for the realization of the so-called energy-related services. Frequency
bands B and C are mainly used for the realization of building and home automation.
Nowadays, the narrowband PLC systems provide data rates up to a few thousand bits per
second (bps) [Dost01]. The maximum distance between two PLC modems can be up to
1 km. To overcome longer distances, it is necessary to apply a repeater technique.
The narrowband PLC systems apply both narrowband and broadband modulation
schemes. First narrowband PLC networks have been realized by the usage of Amplitude
Shift Keying (ASK) [Dost01]. The ASK is not robust against disturbances and, therefore,
is not suitable for application in PLC networks. On the other hand, Binary Phase Shift
Keying (BPSK) is a robust scheme and, therefore, is more suitable for application in PLC.
However, phase detection, which is necessary for the realization of BPSK, seems to be
complex and BPSK-based systems are not commonly used. Most recent narrowband PLC
systems apply Frequency Shift Keying (FSK), and it is expected that BPSK will be used
in future communications systems [Dost01].
Broadband modulation schemes are also used in narrowband PLC systems. The advantages
of broadband modulation, such as various variants of spread spectrum, are its
robustness against narrowband noise and the selective attenuation effect that exists in
the PLC networks [Dost01]. A further transmission scheme also used in narrowband PLC
system is Orthogonal Frequency Division Multiplexing (OFDM) [Bumi03].
A comprehensive description of various narrowband PLC systems, including their realization
and development, can be found in [Dost01]. The aim of this book is a presentation
of broadband PLC systems, and, therefore, the narrowband systems are not discussed in
detail. However, to sketch the possibilities of the narrowband PLC, we present several
examples for application of this technology in the description below.
A very important area for the application of narrowband PLC is building/home automation.
PLC-based automation systems are realized without the installation of additional
communications networks (Fig. 2.8). Thus, the high costs that are necessary for the installation
of new networks within existing buildings can be significantly decreased by the
usage of PLC technology. Automation systems realized by PLC can be applied to different
tasks to be carried out within buildings:
• Control of various devices that are connected to the internal electro installation, such
as illumination, heating, air-conditioning, elevators, and so on.
• Centralized control of various building systems, such as window technique (darkening)
and door control.
• Security tasks; observance, sensor interconnection, and so on.
PLC-based automation systems are not only used in large buildings but they are also
very often present in private households for the realization of similar automation tasks
(home automation). In this case, several authors talk about so-called smart homes.
A PLC variant of the EIB (European Installation BUS) standard is named Powernet-
EIB. PLC modems designed according to the Powernet-EIB can be easily connected to
any wall socket or integrated in any device connected to the electrical installation. This
ensures communications between all parts of an internal electrical network. Nowadays,
the PLC modems using FSK achieve data rates up to 1200 bps [Dost01].
As it is specified in CENELEC standard, power supply utilities can use band A for
the realization of so-called energy-related services. In this way, a power utility can use
PLC to realize internal communications between its control center and different devices,
ensuring remote control functions, without building extra telecommunications network or
buying network resources at a network provider (Fig. 2.9). Simultaneously, PLC can be
used for remote reading of a customer’s meter units, which additionally saves cost on the
personnel needed for manual meter reading. Finally, PLC can also be used by the utilities
for dynamic pricing (e.g. depending on the day time, total energy offer, etc.), as well
as for observation and control of energy consumption and production. In the last case,
especially, the utilities have been trying to integrate an increasing number of small power
plants; for example, small hydroelectric power stations, wind plants, and so on. However,
the small power plants are not completely reliable and their power production varies
depending on the current weather conditions. Therefore, the regions that are supplied by
the small plants should also be supplied from other sources if necessary. For this purpose,
the utilities need a permanent communication between their system entities, which can
be at least partly realized by PLC as well.
The building automation is a typical indoor application of the narrowband PLC systems,
whereas the energy-related services are mainly (not only) indoor applications. In [BumiPi03],
we find a very interesting example of an application of a PLC-based automation system in the
outdoor area. In this case, a PLC-based airfield ground–lighting automation system is used
for individual switching and monitoring of airfield lighting. The length of the airfields and
accordingly the necessary communications networks in a large airport is very long (several
kilometers). So, the narrowband PLC can be applied to save costs on building a separate
communications network. This is also an example of PLC usage for the realization of socalled
critical automation services with very high security requirements, such as the light
control of ground aircraft movement in the airports.
2.2.5 Broadband PLC
Broadband PLC systems provide significantly higher data rates (more than 2 Mbps) than
narrowband PLC systems. Where the narrowband networks can realize only a small number
of voice channels and data transmission with very low bit rates, broadband PLC
networks offer the realization of more sophisticated telecommunication services; multiple
voice connections, high-speed data transmission, transfer of video signals, and narrowband
services as well. Therefore, PLC broadband systems are also considered a capable
telecommunications technology.
The realization of broadband communications services over powerline grids offers a
great opportunity for cost-effective telecommunications networks without the laying of
new cables. However, electrical supply networks are not designed for information transfer
and there are some limiting factors in the application of broadband PLC technology.
Therefore, the distances that can be covered, as well as the data rates that can be realized
by PLC systems, are limited. A further very important aspect for application of broadband
PLC is its Electromagnetic Compatibility (EMC). For the realization of broadband PLC, a
significantly wider frequency spectrum is needed (up to 30MHz) than is provided within
CENELEC bands. On the other hand, a PLC network acts as an antenna becoming a
noise source for other communication systems working in the same frequency range (e.g.
various radio services). Because of this, broadband PLC systems have to operate with a
limited signal power, which decreases their performance (data rates, distances).
Current broadband PLC systems provide data rates beyond 2Mbps in the outdoor arena,
which includes medium- and low-voltage supply networks (Fig. 2.7), and up to 12 Mbps
in the in-home area. Some manufacturers have already developed product prototypes
providing much higher data rates (about 40 Mbps). Medium-voltage PLC technology is
usually used for the realization of point-to-point connections bridging distances up to several
hundred meters. Typical application areas of such systems is the connection of local
area networks (LAN) networks between buildings or within a campus and the connection
of antennas and base stations of cellular communication systems to their backbone
networks. Low-voltage PLC technology is used for the realization of the so-called “last
mile” of telecommunication access networks. Because of the importance of telecommunication
access, current development of broadband PLC technology is mostly directed
toward applications in access networks including the in-home area. In contrast to narrowband
PLC systems, there are no specified standards that apply to broadband PLC networks
PLC in the Telecommunications Access Area
2.1 Access Technologies
2.1.1 Importance of the Telecommunications Access Area
Access networks are very important for network providers because of their high costs and
the possibility of the realization of a direct access to the end users/subscribers. Lately,
about 50% of all investments in the telecommunications infrastructure is needed for the
realization of telecommunications access networks. However, an access network connects
a limited number of individual subscribers, as opposed to a transport communication
network (Fig. 2.1). Therefore, economic efficiency of the access networks is significantly
lower than in wide area networks (WAN).
In the case of the so-called big customers (business, governmental or industrial customers),
the access networks connect a higher number of subscribers who are concentrated
within a building or in a small region (e.g. campus). The big customers usually use various
telecommunication services intensively and bring high sales to the network providers.
Therefore, the realization of particular access networks for the big customers makes economical
sense.
As opposed to the big customers, individual subscribers (e.g. private subscribers,
Fig. 2.1) use the telecommunication services less intensively. Accordingly, realization
of the access networks for individual subscribers is also economically less efficient. On
the other hand, a direct access to the subscribers increases the opportunities for network
providers to offer a higher number of various services. This attracts the subscribers to
become contract-bound customers of a particular network provider, which increases the
usage of its transport network. Therefore, the access to the individual subscribers seems
to be important as well.
After the deregulation of the telecommunications market in a large number of countries,
the access networks are still the property of former monopolistic companies (incumbent
network providers). New network providers build up their transport networks (WAN), but
they still have to use the access infrastructure owned by an incumbent provider. Because
of this, new network providers try to find a solution to offer their own access network
to the subscribers. On the other hand, a rapid development of new telecommunications
services increases the demand for more transmission capacity in the transport networks
as well as in the access area. Therefore, there is a permanent need for an extension of the
access infrastructure. There are two possibilities for the expansion of the access networks:
• Building of new networks or
• Usage of the existing infrastructure.
Building of new access networks is the best way to implement the newest communications
technology, which allows realization of very attractive services. On the other hand,
building of new access networks is expensive. Thus, the usage of the existing infrastructure
for realization of the access networks is a more attractive solution for network
providers because of lower costs. However, the existing infrastructure has to be renewed
and equipped to be able to offer attractive telecommunications services as well.
2.1.2 Building of New Access Networks
Generally, the building of new access networks can be realized with the following
techniques:
• New cable or optical network
• Wireless access systems
• Satellite systems.
Nowadays, the optical telecommunications networks offer higher data rates than any
other communications technology. Frequent usage of optical transmission systems within
transport networks (WAN) reduces their costs. Therefore, the implementation of optical
communications networks also becomes economically efficient in the access area. This
allows realization of a sufficient transmission capacity and attractive services.
However, laying of new optical or cable networks is very costly because of the required
voluminous construction steps. Very often, it has to be carried out within urban areas
causing legal problems and additional costs. Finally, the building of new cable or optical
networks takes a long time. Because of these reasons, laying of new networks is mostly
done in new settlements and areas with a big subscriber concentration (business and
governmental centers, dense industrial areas, etc.).
To avoid realization of new cable or optical networks, various wireless transmission
systems can be applied in the access area. The two approaches that can be applied for
the realization of wireless access networks can be distinguished as follows [GargSn96]:
• Wireless mobile systems
• Fixed wireless systems.
Well-known wireless mobile systems are DECT, GSM/GPRS, and UMTS. Mobile networks
provide a large number of cells to cover a wide communication area, which ensures
a permanent connection for mobile subscribers in the area covered (cellular network,
Fig. 2.2). A frequency range is allocated to each cell allowing communication between
mobile terminals (MT) and base stations. Different frequencies (or codes for UMTS) are
allocated to neighbors’ cells to avoid interferences between them. Generally, a base station
covers a number of wireless communication cells connecting them to a WAN. The
wireless mobile systems offer sum transmission data rates up to 2 Mbps.
Fixed wireless systems, called WLL systems (Wireless Local Loop), are more suitable
for application in the access area than the mobile systems [GargSn96]. WLL systems
also provide base stations that connect a number of subscribers situated in a relatively
small area (Fig. 2.3). As opposed to mobile wireless systems, WLL subscribers have
a fixed position with antennas that are located on high posts on buildings or houses.
Therefore, WLL systems provide constant propagation paths between the base station and
the subscribers, and, accordingly, provide a better SNR (signal-to-noise ratio) behavior
than in the wireless mobile systems. The data rates are also higher than in the mobile
systems; up to 10 Mbps in the downlink transmission direction (from the base station to
the subscribers) and up to 256 kbps in the uplink (from the subscribers to the base station).
However, the data rates realized in different WLL systems are still increasing.
WLL systems realize connections between a base station and the appropriate customer
transreceiver station equipped with an antenna. A customer station usually covers
a building or a house with a number of individual subscribers using various communications
services. The connection between a customer station and its subscribers can be
realized in different ways, via a wireline communications infrastructure or as a home
wireless network.
Nowadays, the home wireless networks are realized as the so-called Wireless Local Area
Networks (WLAN). A WLAN operates usually within buildings and covers a relatively
small area, ensuring data rates beyond 20Mbps (see e.g. [Walke99]). WLAN systems are
used to cover a number of rooms within business premises or private households (e.g.
to cover a house with the belonging surroundings, garden, etc.). For this purpose, one or
more antennas are installed, which makes possible the usage of various communications
devices in the entire covered area, without a need for any kind of wireline connections.
The antennas are situated in the so-called access points (AP, Fig. 2.4), which are usually
connected to a wireline network. In this way, a WLAN is connected to the network servers
and to WAN. Thus, the mobile terminals of a WLAN are able to use various services and
access the global communication network.
Both mobile and fixed wireless systems are still expensive for application in access
networks. Furthermore, coverage of large areas with wireless systems needs a higher
number of base stations and antennas, which, additionally, increases the network costs.
Lastly, the maximum data rates reached in WLANs are significantly lower than the data
rates in optical networks.
The third possibility for the realization of the access networks are satellite systems,
which are nowadays mostly used for worldwide long-distance communications. The low
Earth orbit (LEO) and medium Earth orbit (MEO) satellites were developed for application
in the communications access area [Dixi99]. Such satellite systems, like the Iridium
system [HubbSa97], should extend the existing cellular systems in which the base stations
are replaced (or partly replaced) by the satellites. However, the satellite access
systems currently do not provide good economic efficiency and some satellite projects
have recently been canceled (e.g. Iridium).
2.1.3 Usage of the Existing Infrastructure in the Access Area
The building of expensive new communications networks can be avoided by the usage
of the existing infrastructure for the realization of access networks. In this case, already
existing wireline networks are used to connect the subscribers to the transport telecommunications
networks. The following networks can be used for this purpose:
• Classical telephone networks
• TV cable networks (CATV)
• Electrical power supply networks.
Nowadays, the classic telephone networks are equipped by Digital Subscriber Line
(DSL) systems to provide higher data rates in the access area. Asymmetric Digital Subscriber
Line (ADSL) is a variant of DSL technology, mostly applied in the access networks
(e.g. operated by the Deutsche Telekom – German Telecom) [OrthPo99]. The ADSL technique
can ensure up to 8Mbps in downlink transmission direction and up to 640 kbps in
the uplink [Ims99] under optimal conditions (length, transmission features of lines, etc.).
The subscribers using DSL access systems are connected to a central switching node
(e.g. local exchange office) over a star formed network, which allows each DSL subscriber
to use the full data rates (Fig. 2.5). The central nodes are usually connected to the
backbone network (WAN) over a distribution system using high-speed optical transmission
technology.
For the realization of DSL access networks, appropriate equipment is needed on the
subscriber side (e.g. ADSL modem) as well as within the central node. Generally, ADSL
modems on the subscriber side connect various communications devices to the transmission
line. Nowadays, the most applied communications service usingDSLtechnique is broadband
Internet access. However, there is a possibility of the realization of classical telephone
service as well as advanced services providing transmission of various video signals
pay and broadcast TV, interactive video, etc.). The central node provides a number of
modems connecting the individual subscribers and acts as a concentrator, so-called DSL
access multiplexer, connecting DSL end user to the backbone communications network.
So, for the realization of DSL-based access networks, it is only necessary to install
the appropriate modems on both the subscriber and the central node side. However, in
some cases there is a need for a partial reconstruction and improvement of the subscribers’
lines, if the physical network features do not fulfill requirements for the realization of DSL
access. The maximum data rates in DSL systems depend on the length of the subscribers’
lines and their transmission characteristics. Table 2.1 presents an overview of different
DSL techniques and their features.
CATV networks are designed for the broadcasting of TV programs to homes, but they
are also very often used for the realization of other telecommunications services. In some
regions, CATV networks are widely available and connect a very large number of end
users. Also, cabling technique used for CATV wire infrastructure has to ensure higher data
rates providing transmission of multiple TV channels with a certain quality. Therefore,
CATV networks seem to be an alternative solution for the realization of access networks
too. The access systems realized over CATV networks offer up to 50 Mbps in downlink
and up to 5 Mbps in uplink transmission direction [Ims99, Hern97]. However, on average
there are about 600 subscribers connected to a CATV access network who have to share
the common network capacity – shared medium (Fig. 2.6).
The subscribers of a CATV access network are connected to a central node, similar to
DSL access networks. The appropriate modems, so-called cable modems, are also needed
on both the subscriber and the central node side. The subscribers of a CATV system
equipped to serve as a general access network are able to use various communication
services as well. However, within the network there are amplifiers that usually operate
only in the downlink direction, because the original purpose of CATV networks is to
transmit TV signals from a central antenna to the subscribers. Therefore, the amplifiers
have to be modified to operate in both transmission directions, allowing bidirectional data
transmission, which is needed for the realization of access networks.
Telephone networks usually belong to former monopolistic companies (incumbent
providers) and this is a big disadvantage for the new network providers to use them
to offer services like ADSL. It is also very often the case with the CATV networks.
Additionally, the CATV networks have to be made capable of bidirectional transmission,
which results in extra costs. In some cases, the subscriber lines have to be modified to
ensure application of DSL technology, which increases the cost as well. Because of these
reasons, the usage of power supply systems for communication seems to be a reasonable
solution for the realization of alternative access networks. However, PowerLine Communications
(PLC) technology should provide an economically efficient solution and should
offer a big palette of the telecommunications services with a certain quality to be able to
compete with other access technologies.
2.1.1 Importance of the Telecommunications Access Area
Access networks are very important for network providers because of their high costs and
the possibility of the realization of a direct access to the end users/subscribers. Lately,
about 50% of all investments in the telecommunications infrastructure is needed for the
realization of telecommunications access networks. However, an access network connects
a limited number of individual subscribers, as opposed to a transport communication
network (Fig. 2.1). Therefore, economic efficiency of the access networks is significantly
lower than in wide area networks (WAN).
In the case of the so-called big customers (business, governmental or industrial customers),
the access networks connect a higher number of subscribers who are concentrated
within a building or in a small region (e.g. campus). The big customers usually use various
telecommunication services intensively and bring high sales to the network providers.
Therefore, the realization of particular access networks for the big customers makes economical
sense.
As opposed to the big customers, individual subscribers (e.g. private subscribers,
Fig. 2.1) use the telecommunication services less intensively. Accordingly, realization
of the access networks for individual subscribers is also economically less efficient. On
the other hand, a direct access to the subscribers increases the opportunities for network
providers to offer a higher number of various services. This attracts the subscribers to
become contract-bound customers of a particular network provider, which increases the
usage of its transport network. Therefore, the access to the individual subscribers seems
to be important as well.
After the deregulation of the telecommunications market in a large number of countries,
the access networks are still the property of former monopolistic companies (incumbent
network providers). New network providers build up their transport networks (WAN), but
they still have to use the access infrastructure owned by an incumbent provider. Because
of this, new network providers try to find a solution to offer their own access network
to the subscribers. On the other hand, a rapid development of new telecommunications
services increases the demand for more transmission capacity in the transport networks
as well as in the access area. Therefore, there is a permanent need for an extension of the
access infrastructure. There are two possibilities for the expansion of the access networks:
• Building of new networks or
• Usage of the existing infrastructure.
Building of new access networks is the best way to implement the newest communications
technology, which allows realization of very attractive services. On the other hand,
building of new access networks is expensive. Thus, the usage of the existing infrastructure
for realization of the access networks is a more attractive solution for network
providers because of lower costs. However, the existing infrastructure has to be renewed
and equipped to be able to offer attractive telecommunications services as well.
2.1.2 Building of New Access Networks
Generally, the building of new access networks can be realized with the following
techniques:
• New cable or optical network
• Wireless access systems
• Satellite systems.
Nowadays, the optical telecommunications networks offer higher data rates than any
other communications technology. Frequent usage of optical transmission systems within
transport networks (WAN) reduces their costs. Therefore, the implementation of optical
communications networks also becomes economically efficient in the access area. This
allows realization of a sufficient transmission capacity and attractive services.
However, laying of new optical or cable networks is very costly because of the required
voluminous construction steps. Very often, it has to be carried out within urban areas
causing legal problems and additional costs. Finally, the building of new cable or optical
networks takes a long time. Because of these reasons, laying of new networks is mostly
done in new settlements and areas with a big subscriber concentration (business and
governmental centers, dense industrial areas, etc.).
To avoid realization of new cable or optical networks, various wireless transmission
systems can be applied in the access area. The two approaches that can be applied for
the realization of wireless access networks can be distinguished as follows [GargSn96]:
• Wireless mobile systems
• Fixed wireless systems.
Well-known wireless mobile systems are DECT, GSM/GPRS, and UMTS. Mobile networks
provide a large number of cells to cover a wide communication area, which ensures
a permanent connection for mobile subscribers in the area covered (cellular network,
Fig. 2.2). A frequency range is allocated to each cell allowing communication between
mobile terminals (MT) and base stations. Different frequencies (or codes for UMTS) are
allocated to neighbors’ cells to avoid interferences between them. Generally, a base station
covers a number of wireless communication cells connecting them to a WAN. The
wireless mobile systems offer sum transmission data rates up to 2 Mbps.
Fixed wireless systems, called WLL systems (Wireless Local Loop), are more suitable
for application in the access area than the mobile systems [GargSn96]. WLL systems
also provide base stations that connect a number of subscribers situated in a relatively
small area (Fig. 2.3). As opposed to mobile wireless systems, WLL subscribers have
a fixed position with antennas that are located on high posts on buildings or houses.
Therefore, WLL systems provide constant propagation paths between the base station and
the subscribers, and, accordingly, provide a better SNR (signal-to-noise ratio) behavior
than in the wireless mobile systems. The data rates are also higher than in the mobile
systems; up to 10 Mbps in the downlink transmission direction (from the base station to
the subscribers) and up to 256 kbps in the uplink (from the subscribers to the base station).
However, the data rates realized in different WLL systems are still increasing.
WLL systems realize connections between a base station and the appropriate customer
transreceiver station equipped with an antenna. A customer station usually covers
a building or a house with a number of individual subscribers using various communications
services. The connection between a customer station and its subscribers can be
realized in different ways, via a wireline communications infrastructure or as a home
wireless network.
Nowadays, the home wireless networks are realized as the so-called Wireless Local Area
Networks (WLAN). A WLAN operates usually within buildings and covers a relatively
small area, ensuring data rates beyond 20Mbps (see e.g. [Walke99]). WLAN systems are
used to cover a number of rooms within business premises or private households (e.g.
to cover a house with the belonging surroundings, garden, etc.). For this purpose, one or
more antennas are installed, which makes possible the usage of various communications
devices in the entire covered area, without a need for any kind of wireline connections.
The antennas are situated in the so-called access points (AP, Fig. 2.4), which are usually
connected to a wireline network. In this way, a WLAN is connected to the network servers
and to WAN. Thus, the mobile terminals of a WLAN are able to use various services and
access the global communication network.
Both mobile and fixed wireless systems are still expensive for application in access
networks. Furthermore, coverage of large areas with wireless systems needs a higher
number of base stations and antennas, which, additionally, increases the network costs.
Lastly, the maximum data rates reached in WLANs are significantly lower than the data
rates in optical networks.
The third possibility for the realization of the access networks are satellite systems,
which are nowadays mostly used for worldwide long-distance communications. The low
Earth orbit (LEO) and medium Earth orbit (MEO) satellites were developed for application
in the communications access area [Dixi99]. Such satellite systems, like the Iridium
system [HubbSa97], should extend the existing cellular systems in which the base stations
are replaced (or partly replaced) by the satellites. However, the satellite access
systems currently do not provide good economic efficiency and some satellite projects
have recently been canceled (e.g. Iridium).
2.1.3 Usage of the Existing Infrastructure in the Access Area
The building of expensive new communications networks can be avoided by the usage
of the existing infrastructure for the realization of access networks. In this case, already
existing wireline networks are used to connect the subscribers to the transport telecommunications
networks. The following networks can be used for this purpose:
• Classical telephone networks
• TV cable networks (CATV)
• Electrical power supply networks.
Nowadays, the classic telephone networks are equipped by Digital Subscriber Line
(DSL) systems to provide higher data rates in the access area. Asymmetric Digital Subscriber
Line (ADSL) is a variant of DSL technology, mostly applied in the access networks
(e.g. operated by the Deutsche Telekom – German Telecom) [OrthPo99]. The ADSL technique
can ensure up to 8Mbps in downlink transmission direction and up to 640 kbps in
the uplink [Ims99] under optimal conditions (length, transmission features of lines, etc.).
The subscribers using DSL access systems are connected to a central switching node
(e.g. local exchange office) over a star formed network, which allows each DSL subscriber
to use the full data rates (Fig. 2.5). The central nodes are usually connected to the
backbone network (WAN) over a distribution system using high-speed optical transmission
technology.
For the realization of DSL access networks, appropriate equipment is needed on the
subscriber side (e.g. ADSL modem) as well as within the central node. Generally, ADSL
modems on the subscriber side connect various communications devices to the transmission
line. Nowadays, the most applied communications service usingDSLtechnique is broadband
Internet access. However, there is a possibility of the realization of classical telephone
service as well as advanced services providing transmission of various video signals
pay and broadcast TV, interactive video, etc.). The central node provides a number of
modems connecting the individual subscribers and acts as a concentrator, so-called DSL
access multiplexer, connecting DSL end user to the backbone communications network.
So, for the realization of DSL-based access networks, it is only necessary to install
the appropriate modems on both the subscriber and the central node side. However, in
some cases there is a need for a partial reconstruction and improvement of the subscribers’
lines, if the physical network features do not fulfill requirements for the realization of DSL
access. The maximum data rates in DSL systems depend on the length of the subscribers’
lines and their transmission characteristics. Table 2.1 presents an overview of different
DSL techniques and their features.
CATV networks are designed for the broadcasting of TV programs to homes, but they
are also very often used for the realization of other telecommunications services. In some
regions, CATV networks are widely available and connect a very large number of end
users. Also, cabling technique used for CATV wire infrastructure has to ensure higher data
rates providing transmission of multiple TV channels with a certain quality. Therefore,
CATV networks seem to be an alternative solution for the realization of access networks
too. The access systems realized over CATV networks offer up to 50 Mbps in downlink
and up to 5 Mbps in uplink transmission direction [Ims99, Hern97]. However, on average
there are about 600 subscribers connected to a CATV access network who have to share
the common network capacity – shared medium (Fig. 2.6).
The subscribers of a CATV access network are connected to a central node, similar to
DSL access networks. The appropriate modems, so-called cable modems, are also needed
on both the subscriber and the central node side. The subscribers of a CATV system
equipped to serve as a general access network are able to use various communication
services as well. However, within the network there are amplifiers that usually operate
only in the downlink direction, because the original purpose of CATV networks is to
transmit TV signals from a central antenna to the subscribers. Therefore, the amplifiers
have to be modified to operate in both transmission directions, allowing bidirectional data
transmission, which is needed for the realization of access networks.
Telephone networks usually belong to former monopolistic companies (incumbent
providers) and this is a big disadvantage for the new network providers to use them
to offer services like ADSL. It is also very often the case with the CATV networks.
Additionally, the CATV networks have to be made capable of bidirectional transmission,
which results in extra costs. In some cases, the subscriber lines have to be modified to
ensure application of DSL technology, which increases the cost as well. Because of these
reasons, the usage of power supply systems for communication seems to be a reasonable
solution for the realization of alternative access networks. However, PowerLine Communications
(PLC) technology should provide an economically efficient solution and should
offer a big palette of the telecommunications services with a certain quality to be able to
compete with other access technologies.
Introduction
During the last decades, the usage of telecommunications systems has increased rapidly.
Because of a permanent necessity for new telecommunications services and additional
transmission capacities, there is also a need for the development of new telecommunications
networks and transmission technologies. From the economic point of view,
telecommunications promise big revenues, motivating large investments in this area.
Therefore, there are a large number of communications enterprises that are building up
high-speed networks, ensuring the realization of various telecommunications services that
can be used worldwide. However, the investments are mainly provided for transport networks
that connect various communications nodes of different network providers, but do
not reach the end customers. The connection of the end customers to a transport network,
as part of a global communications system, is realized over distribution and access networks
(Fig. 1.1). The distribution networks cover larger geographical areas and realize
connection between access and transport networks, whereas the access networks cover
relatively smaller areas.
The direct connection of the customers/subscribers is realized over the access networks,
realizing access of a number of subscribers situated within a radius of several hundreds
of meters. However, the costs for realization, installation and maintenance of the access
networks are very high. It is usually calculated that about 50% of all network investments
belongs to the access area. On the other hand, a longer time is needed for paying back the
invested capital because of the relatively high costs of the access networks, calculated per
connected subscriber. Therefore, the network providers try to realize the access network
with possibly low costs.
After the deregulation of the telecommunications market in a large number of countries,
the access networks are still the property of incumbent network providers (former
monopolistic telephone companies). Because of this, the new network providers try to find
a solution to offer their own access network. An alternative solution for the realization
of the access networks is offered by the PLC (PowerLine Communications) technology
using the power supply grids for communications. Thus, for the realization of the PLC
networks, there is no need for the laying of new communications cables. Therefore, application
of PLC in low-voltage supply networks seems to be a cost-effective solution for
so-called “last mile” communications networks, belonging to the access area. Nowadays,
network subscribers use various telecommunications services with higher data rates and
QoS (Quality of Service) requirements. PLC systems applied in the access area that ensure
realization of telecommunications services with the higher QoS requirements are called
“broadband PLC access networks”. The contribution of this book is directed to give a set
of information that is necessary to be considered for the design of the broadband PLC
access systems and their network components.
To make communications in a power supply network possible, it is necessary to install
so-called PLC modems, which ensure transmission of data signals over the power grids
(Fig. 1.2). A PLC modem converts a data signal received from conventional communications
devices, such as computers, telephones, and so on, in a form that is suitable for
transmission over powerlines. In the other transmission direction, the modem receives a
data signal from the power grids and after conversion delivers it to the communications
devices. Thus, the PLC modems, representing PLC-specific communications equipment,
provide a necessary interface for interconnection of various communications devices over
power supply networks. The PLC-specific communications devices, such as PLC modems,
have to be designed to ensure an efficient network operation under transmission conditions,
typical for power supply networks and their environment.
However, power supply networks are not designed for communications and they do not
present a favorable transmission medium. Thus, the PLC transmission channel is characterized
by a large, and frequency-dependent attenuation, changing impedance and fading
as well as unfavorable noise conditions. Various noise sources, acting from the supply
network, due to different electric devices connected to the network, and from the network
environment, can negatively influence a PLC system, causing disturbances in an error-free
data transmission. On the other hand, to provide higher data rates, PLC networks have
to operate in a frequency spectrum of up to 30 MHz, which is also used by various radio
services. Unfortunately, a PLC network acts as an antenna producing electromagnetic
radiation in its environment and disturbs other services operating in the same frequency
range. Therefore, the regulatory bodies specify very strong limits regarding the electromagnetic
emission from the PLC networks, with a consequence that PLC networks have
to operate with a limited signal power. This causes a reduction of network distances and
data rates and increases sensitivity to disturbances.
The reduction of the data rates is particularly disadvantageous because of the fact that
PLC access networks operate in a shared transmission medium, in which a number of
subscribers compete to use the same transmission resources (Fig. 1.3). In the case of PLC
access networks, the transmission medium provided by a low-voltage supply network is
used for communication between the subscribers and a so-called PLC base station, which
connects the access network to a wide area network (WAN) realized by conventional
communications technology.
To reduce the negative impact of powerline transmission medium, PLC systems have
to apply efficient modulation, such as spread spectrum and Orthogonal Frequency Division
Multiplexing (OFDM). The problem of disturbances can also be solved by wellknown
error-handling mechanisms (e.g. forward error correction (FEC), Automatic Repeat
reQuest (ARQ)). However, their application consumes a certain portion of the PLC network
capacity because of overhead and retransmission. On the other hand, a PLC access
network has to be economically efficient, serving possibly a large number of subscribers.
This can be ensured only by a good utilization of the limited network capacity. Simultaneously,
PLC systems have to compete with other access technologies (e.g. digital subscriber
line (DSL), cable television (CATV)) and to offer different telecommunications services
with a satisfactory QoS. Both good network utilization and provision of QoS guarantees
can be achieved by an efficient Medium Access Control (MAC) layer.
Nowadays, there are no existing standards or specifications considering physical and
MAC layers for PLC access networks. The manufacturers of the PLC equipment developed
proprietary solutions for the MAC layer that are incompatible with each other.
Therefore, we consider various solutions for realization of both physical and MAC layers
in broadband PLC access networks to be implemented in PLC-specific communications
equipment, such as PLC modems (Fig. 1.3). Detailed description of the PLC physical
layer, including consideration of the PLC network characteristics, such as transmission
features and noise behavior, and consideration of modulation schemes for PLC, can also
be found in another available book on this topic, “Powerline Communications”, written
by Prof. Dostert [Dost01], in which both the narrowband and broadband PLC systems
are considered. In this book, we focus on the broadband access networks and describe
characteristics of the physical layer and applied modulation schemes for the broadband
PLC systems, and introduce an investigation of PLC MAC layer. Nowadays, the issue of
the PLC MAC layer is only considered in a few scientific publications (e.g. [Hras03]).
Therefore, in this book we emphasize a consideration of the MAC layer and its protocols
to be applied in the broadband PLC access networks.
The book is organized as follows: in Chapter 2, we discuss the role of PLC in telecommunications
access area and present basics about narrowband and broadband PLC systems,
network structure with its elements and PLC-specific performance problems that have to
be overcome for realization of broadband access networks. The characteristics of the PLC
transmission medium are presented in Chapter 3, which includes a topology analysis of
the low-voltage supply networks, description of the electromagnetic compatibility issue
(EMC) in broadband PLC, noise characterization and disturbance modeling, as well as a
description of the PLC transmission channel and its features. In Chapter 4, we present a
protocol architecture for PLC networks and define PLC-specific network layers. Later, we
describe spread spectrum and OFDM modulation schemes, which are outlined as favorable
solutions for PLC. Furthermore, various possibilities for realization of error handling
in PLC systems are considered. Finally, in Chapter 4, we analyze telecommunications
services to be used in PLC networks and specify traffic models for their representation
in investigations of the PLC networks. The MAC layer, as a part of the common PLC
protocol architecture, is separately analyzed in Chapter 5. We introduce different solutions
of multiple-access schemes and consider various MAC protocols for their application in
PLC. Furthermore, several solutions for traffic control in PLC networks are discussed.
Finally, in Chapter 6, we present a comprehensive performance evaluation of reservation
MAC protocols, which are outlined as a suitable solution for application in broadband
PLC access networks. In this investigation, we compare various signaling MAC protocols
under different traffic and disturbance conditions, representing a typical user and noise
behavior expected in broadband PLC access networks.
Because of a permanent necessity for new telecommunications services and additional
transmission capacities, there is also a need for the development of new telecommunications
networks and transmission technologies. From the economic point of view,
telecommunications promise big revenues, motivating large investments in this area.
Therefore, there are a large number of communications enterprises that are building up
high-speed networks, ensuring the realization of various telecommunications services that
can be used worldwide. However, the investments are mainly provided for transport networks
that connect various communications nodes of different network providers, but do
not reach the end customers. The connection of the end customers to a transport network,
as part of a global communications system, is realized over distribution and access networks
(Fig. 1.1). The distribution networks cover larger geographical areas and realize
connection between access and transport networks, whereas the access networks cover
relatively smaller areas.
The direct connection of the customers/subscribers is realized over the access networks,
realizing access of a number of subscribers situated within a radius of several hundreds
of meters. However, the costs for realization, installation and maintenance of the access
networks are very high. It is usually calculated that about 50% of all network investments
belongs to the access area. On the other hand, a longer time is needed for paying back the
invested capital because of the relatively high costs of the access networks, calculated per
connected subscriber. Therefore, the network providers try to realize the access network
with possibly low costs.
After the deregulation of the telecommunications market in a large number of countries,
the access networks are still the property of incumbent network providers (former
monopolistic telephone companies). Because of this, the new network providers try to find
a solution to offer their own access network. An alternative solution for the realization
of the access networks is offered by the PLC (PowerLine Communications) technology
using the power supply grids for communications. Thus, for the realization of the PLC
networks, there is no need for the laying of new communications cables. Therefore, application
of PLC in low-voltage supply networks seems to be a cost-effective solution for
so-called “last mile” communications networks, belonging to the access area. Nowadays,
network subscribers use various telecommunications services with higher data rates and
QoS (Quality of Service) requirements. PLC systems applied in the access area that ensure
realization of telecommunications services with the higher QoS requirements are called
“broadband PLC access networks”. The contribution of this book is directed to give a set
of information that is necessary to be considered for the design of the broadband PLC
access systems and their network components.
To make communications in a power supply network possible, it is necessary to install
so-called PLC modems, which ensure transmission of data signals over the power grids
(Fig. 1.2). A PLC modem converts a data signal received from conventional communications
devices, such as computers, telephones, and so on, in a form that is suitable for
transmission over powerlines. In the other transmission direction, the modem receives a
data signal from the power grids and after conversion delivers it to the communications
devices. Thus, the PLC modems, representing PLC-specific communications equipment,
provide a necessary interface for interconnection of various communications devices over
power supply networks. The PLC-specific communications devices, such as PLC modems,
have to be designed to ensure an efficient network operation under transmission conditions,
typical for power supply networks and their environment.
However, power supply networks are not designed for communications and they do not
present a favorable transmission medium. Thus, the PLC transmission channel is characterized
by a large, and frequency-dependent attenuation, changing impedance and fading
as well as unfavorable noise conditions. Various noise sources, acting from the supply
network, due to different electric devices connected to the network, and from the network
environment, can negatively influence a PLC system, causing disturbances in an error-free
data transmission. On the other hand, to provide higher data rates, PLC networks have
to operate in a frequency spectrum of up to 30 MHz, which is also used by various radio
services. Unfortunately, a PLC network acts as an antenna producing electromagnetic
radiation in its environment and disturbs other services operating in the same frequency
range. Therefore, the regulatory bodies specify very strong limits regarding the electromagnetic
emission from the PLC networks, with a consequence that PLC networks have
to operate with a limited signal power. This causes a reduction of network distances and
data rates and increases sensitivity to disturbances.
The reduction of the data rates is particularly disadvantageous because of the fact that
PLC access networks operate in a shared transmission medium, in which a number of
subscribers compete to use the same transmission resources (Fig. 1.3). In the case of PLC
access networks, the transmission medium provided by a low-voltage supply network is
used for communication between the subscribers and a so-called PLC base station, which
connects the access network to a wide area network (WAN) realized by conventional
communications technology.
To reduce the negative impact of powerline transmission medium, PLC systems have
to apply efficient modulation, such as spread spectrum and Orthogonal Frequency Division
Multiplexing (OFDM). The problem of disturbances can also be solved by wellknown
error-handling mechanisms (e.g. forward error correction (FEC), Automatic Repeat
reQuest (ARQ)). However, their application consumes a certain portion of the PLC network
capacity because of overhead and retransmission. On the other hand, a PLC access
network has to be economically efficient, serving possibly a large number of subscribers.
This can be ensured only by a good utilization of the limited network capacity. Simultaneously,
PLC systems have to compete with other access technologies (e.g. digital subscriber
line (DSL), cable television (CATV)) and to offer different telecommunications services
with a satisfactory QoS. Both good network utilization and provision of QoS guarantees
can be achieved by an efficient Medium Access Control (MAC) layer.
Nowadays, there are no existing standards or specifications considering physical and
MAC layers for PLC access networks. The manufacturers of the PLC equipment developed
proprietary solutions for the MAC layer that are incompatible with each other.
Therefore, we consider various solutions for realization of both physical and MAC layers
in broadband PLC access networks to be implemented in PLC-specific communications
equipment, such as PLC modems (Fig. 1.3). Detailed description of the PLC physical
layer, including consideration of the PLC network characteristics, such as transmission
features and noise behavior, and consideration of modulation schemes for PLC, can also
be found in another available book on this topic, “Powerline Communications”, written
by Prof. Dostert [Dost01], in which both the narrowband and broadband PLC systems
are considered. In this book, we focus on the broadband access networks and describe
characteristics of the physical layer and applied modulation schemes for the broadband
PLC systems, and introduce an investigation of PLC MAC layer. Nowadays, the issue of
the PLC MAC layer is only considered in a few scientific publications (e.g. [Hras03]).
Therefore, in this book we emphasize a consideration of the MAC layer and its protocols
to be applied in the broadband PLC access networks.
The book is organized as follows: in Chapter 2, we discuss the role of PLC in telecommunications
access area and present basics about narrowband and broadband PLC systems,
network structure with its elements and PLC-specific performance problems that have to
be overcome for realization of broadband access networks. The characteristics of the PLC
transmission medium are presented in Chapter 3, which includes a topology analysis of
the low-voltage supply networks, description of the electromagnetic compatibility issue
(EMC) in broadband PLC, noise characterization and disturbance modeling, as well as a
description of the PLC transmission channel and its features. In Chapter 4, we present a
protocol architecture for PLC networks and define PLC-specific network layers. Later, we
describe spread spectrum and OFDM modulation schemes, which are outlined as favorable
solutions for PLC. Furthermore, various possibilities for realization of error handling
in PLC systems are considered. Finally, in Chapter 4, we analyze telecommunications
services to be used in PLC networks and specify traffic models for their representation
in investigations of the PLC networks. The MAC layer, as a part of the common PLC
protocol architecture, is separately analyzed in Chapter 5. We introduce different solutions
of multiple-access schemes and consider various MAC protocols for their application in
PLC. Furthermore, several solutions for traffic control in PLC networks are discussed.
Finally, in Chapter 6, we present a comprehensive performance evaluation of reservation
MAC protocols, which are outlined as a suitable solution for application in broadband
PLC access networks. In this investigation, we compare various signaling MAC protocols
under different traffic and disturbance conditions, representing a typical user and noise
behavior expected in broadband PLC access networks.
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