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.