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in the case of packet voice the continuous information flow provided in the classical
telephony does not exist and the voice packets can be delayed, especially if the number
of subscribers using the same transmission medium increases. Since the voice connections
are very sensitive to larger delays, making a telephony conservation less understandable
or even not possible, there are some limits for the maximum delay of the voice packets.
For example, for wireless networks applied in the access area, the maximum delays
are set to 2024 ms ([AlonAg00, KoutPa01]), or to 25 ms to avoid the usage of echo
cancellers [DaviBe96]. Note that, access networks, such as PLC, cover only a part of the
common transmission path between the participants of a voice connection. Therefore, the
delay limits for the telephony service in the access area are stronger than for the entire
transmission path.
In recent telecommunications networks, different kinds of services are transmitted over
the same transmission links, with a possible usage of same networks elements, such as
switching devices, routers, and so on. Nowadays, telecommunications networks worldwide
are based on the internet protocol (IP), originally developed for pure data transmission
and not designed for transmission of the voice. However, due to the trends for integration
of voice and data services, a solution for the realization of the voice service over IP
networks, so-called Voice over IP (VoIP), is seriously considered as a solution for so-
called integrated services networks. Therefore, VoIP is considered for applications in
broadband PLC access networks as well.
Recent experience with the VoIP service shows that the QoS requirements on the voice
service (e.g. delay, losses, etc.) can be well met in low-loaded networks. However, if
an IP network is highly loaded, the performance regarding VoIP decreases significantly.
Therefore, various mechanisms for the traffic control are considered to be applied in the
high-speed networks, ensuring a required QoS for the time-critical services also, such as
voice. On the other hand, the available data rates in PLC access networks are significantly
smaller than in the high-speed transport networks. Thus, the traffic control mechanisms
developed for providing data rates beyond 100 Mbps cannot be sufficient to ensure both
sufficient QoS for the voice service and a good network utilization in networks with
limited data rates (few Mbps). Therefore, it is necessary to provide additional mechanisms
within the PLC protocol stack, providing required QoS for the voice and other critical
telecommunications services and simultaneously ensuring a good network utilization.
4.4.2.2 Internet Access
The most used telecommunications service in recent PLC access network is data trans-
mission based on the internet access. Therefore, it is necessary to analyze internet data
traffic and outline the main characteristics of such traffic patterns, which are typical for
the access networks, such as broadband PLC networks. The traffic characterization (see
e.g. [F¨arbBo98]) is carried out with the help of numerous measurements in different net-
works, to achieve possibly more general results and to allow design of appropriate models,
representing possibly real traffic characteristics. However, during the last decade, the traf-
fic characteristics have been frequently changed, because of the rapid development of
new telecommunications services, a very intensive growth of wireline and wireless net-
works, increase in the number of subscribers and operators, and so on. Accordingly, the
changing nature of the traffic characteristics is also recognized in many research studies