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Emerging wireless communication for smart grid applications Chapter 5 179

FIG. 5.2 Node topologies employed in IEEE 802.15.4 based WSNs.

in mesh topology in which whole devices of the network can communicate with
each other. Nonetheless, the devices aiming to communicate with others should
primarily contact with the PAN coordinator. The cluster-tree network scheme is
considered as an exclusive version of mesh (P2P) network topology in which a
large number of devices is the RFDs. Typically, communication range of the
IEEE 802.15.4 changes from 10 to 75 m because of the low-powered transmis-
sion characteristics. On the other hand, the coverage of the network may be
extended thanks to multi-hop and cooperative networking where some nodes
in the network can serve as a relay as in the cluster-tree topology [17].

5.3.1.1 PHY layer of IEEE 802.15.4 standard
The PHY layer of the standard is responsible for data processing transactions
regarding transmission and receiving like modulation, spreading and channel
selection [18]. Although the PHY layer defines several transmission mediums
in various bands, the most utilized operation bands in applications are 868 MHz,
915 MHz, and 2.4 GHz frequency bands. The ISM band (2.4 GHz frequency
band) is the most widely employed unlicensed frequency band all over the
world. Channel allocation of these frequency bands in the standard is illustrated
in Fig. 5.3. As the 868 MHz frequency channel is utilized through one channel
with 20 Kbps data rate in Europe, the 915 MHz frequency channel is employed
through 10 channel with 40 Kbps data rate in North America. The frequencies
between 2.4 and 2.483 GHz are employed by the last band to present 16 chan-
nels with 250 Kbps data rate. Each channel space is specified as 2 MHz in the
915 MHz frequency band whereas channel space is determined as 5 MHz in the
2.4 GHz frequency band as can be seen from the Fig. 5.3. In addition, other PHY
layer specifications of the standard is summarized in Table 5.1. In addition, the