Page 207 - From Smart Grid to Internet of Energy

Emerging wireless communication for smart grid applications Chapter 5 183

conveys beacon frames to whole end-devices existing in the network. The bea-
cons are responsible for three significant purposes that can be sorted as ensuring
synchronization among devices, identifying PAN infrastructure and defining
structure of superframes. A superframe is employed in beacon-enabled mode
to manage communication in the wireless channel. The duration between
two beacons is defined as a beacon interval (BI) that is composed of an active
period and an optional inactive period. The nodes can preserve their power
sources by activating sleeping mode (low-power mode) during the inactive
period. The active period of the superframes is called as superframe duration
(SD). Every SD is composed of 16 time periods with equal lengths. Further-
more, Contention Access Period (CAP) and Contention Free Period (CFP) sec-
tions are also defined in the active periods. The CFP that is managed by a PAN
coordinator is generally exploited in low-latency applications, and contains up
to seven GTSs. The nodes can contact with each other by employing a slotted
CSMA/CA or ALOHA in the CAP. The operational modes of IEEE 802.15.4
MAC layer are illustrated in Fig. 5.5.
A PAN coordinator describes the superframe scheme which is defined on
the basis of macBeaconOrder (BO) and macSuperframeOrder (SO) values.
The BO defines the time period whereas the SO identifies the active period
and beacon frame. A general superframe scheme is illustrated in Fig. 5.6 where
the beacon-enabled mode is taken into account. The relationship between BI
and BO can be given as follows.

BI ¼ aBaseSuperframeDuration 2 BO symbols, for 0 BO 14 (5.1)
In the event of the value of BO is 15, the value of SO is ignored and beacon
frames are not sent unless a special request is available. Furthermore, the
relationship between SO and SD can be given as follows.

FIG. 5.5 Operational modes of IEEE 802.15.4 MAC layer.