Page 169 - From Smart Grid to Internet of Energy

146 From smart grid to internet of energy

FIG. 4.7 PHY layer transmitter of ITU-T Rec. G.9903 (G3-PLC). (From Narrowband orthogonal
frequency division multiplexing power line communication transceivers for G3-PLC networks, ITU-
T Rec. G.9903, August 2017. [Online] Available: http://www.itu.int/rec/T-REC-G.9903 (Fig. 7-1),
with permission.)

beginning of next contention period (CP) considering the message priority, and
then random period starts. In the event of the channel is detected idle following
the random back-off time, the node can convey its data. On the other hand, if the
channel is busy following the random back off, the device has to wait for the
next CP with respect to message priority, and then new random period is initi-
ated for channel access process. The random back-off mechanism expands the
time over which stations wishes to send through an additive decrease multipli-
cative increase (ADMI) back-off mechanism.
Two priority levels (high and normal) and two contention time windows that
are depicted in Fig. 4.8 are enabled to define channel access prioritization.
Three different slots are identified during the CP. The first slot is called
contention-free slot (CFS) that is employed to convey next segments of
MAC frame without exploiting back-off mechanism to avoid possible interrup-
tion from other nodes. The other slots are related to high-priority contention
window (HPCW) and normal-priority contention window (NPCW). The mes-
sages are assigned by MAC layer into the one of these contention windows with
respect to their priority.
In the standard, cyclic redundancy check (CRC) and automatic repeat
request (ARQ) mechanism are used to provide delivery and integrity of mes-
sages. If a node obtains a MAC frame, it firstly controls the CRC of the message
and then accepts the frame in the event of an accurate reception. After the accu-
rate reception of a frame, an acknowledge (ACK) frame is transmitted. The
accuracy of the received frame is analyzed by controlling the CRC. When a cor-
ruption is found, the receiver unit transmits a negative ACK (NACK) frame. In