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.