PLC MAC Layer
153
in detail one of its extended implementation variants, IEEE 802.11 MAC protocol. A
comprehensive performance evaluation of the reservation protocols for PLC is separately
presented in Chapter 6.
5.3.1 Classification of MAC Protocols
MAC protocols can be divided into two main groups: protocols with a fixed or a dynamic
access. The fixed access schemes assign a predetermined fixed capacity to each subscriber
for the entire duration of a connection, as is the case in classical telephony. The assigned
network capacity is allocated for a subscriber independent of its current need for a certain
data rate. Thus, if internet access is used, the allocated network capacity remains unused
during viewing phase (Sec. 4.4.2), when no data is transmitted over the network caus-
ing so-called "transmission gaps", as shown in Fig. 5.27. On the other hand, the bursty
characteristic of a data stream can cause so-called "transmission peaks", when capacity
of the allocated channel is not enough to serve the data burst, causing additional delays
and decreasing data throughput. For these reasons, the fixed strategies are suitable only
for continuous traffic, but not for bursts of data traffic (bursty traffic) [AkyiMc99], typi-
cal for different kinds of data transfer that are expected in the access networks, such as
broadband PLC networks.
Unlike fixed access methods, dynamic access protocols are adequate for data transmis-
sion, and in some cases, it is also possible to ensure realization of QoS guarantees for
various telecommunications services. The dynamic protocols are divided into two sub-
groups; contention and arbitration protocols (Fig. 5.28). In accordance with the contention
access principle, the network stations access the transmission medium randomly, which
can cause collisions between data units of different network users. Note that a network
station does not have knowledge about transmission needs of other stations. So, if two
or more stations start to transmit their data packets at the same time, a collision will
occur. On the other hand, the arbitration protocols provide a coordination between the
network stations, ensuring a dedicated access to the medium. In this case, the network
stations access the medium in a determined manner, avoiding the collisions. However,
the arbitration procedure takes an additional time, causing longer transmission delays in
the network.
Basic protocol solutions, such as ALOHA and CSMA random access methods, as
well as token-passing and polling arbitration protocols, can be extended to improve their
performance. Thus, the random protocols can be extended by implementation of vari-
ous mechanisms for collision resolution to reduce number of collisions in the network,
t
f
Data burst
Transmission gap
Allocated
channel
bandwidth
Figure 5.27
Bursty data traffic and fixed access strategy