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Broadband Powerline Communications Networks
the distributed allocation algorithm (Sec. 6.1.3), the station counts data slots to calculate
the start of the transmission. The polling procedure is realized by the base station that
sends so-called polling messages to each network station
(S
1
- S
n
) in accordance with
the round-robin procedure. Only the station receiving a polling message has the right to
send a transmission request. After a successful request, the rest of the signaling procedure
is carried out, such as in the case of ALOHA protocol, by using the distributed allocation
algorithm. The collisions are not possible, but a request can be disturbed and in this case,
it has to be retransmitted in the next dedicated request slot.
In the case of ALOHA protocol, it is possible to transmit exactly one transmission
request during a time slot. The acknowledgment from the base station is sent in the next
time slot, if there is no collision (Fig. 6.13). According to the polling protocol, the base
station can poll exactly one network station during a time slot, which also allows a request
per time slot. Acknowledgment is transmitted in the next time slot after the request, such
as in the ALOHA protocol.
Both ALOHA and polling protocols have the same procedural rules and a fair com-
parison can be made. Therefore, the base station has to be able to poll a network station
and to send an acknowledgment during the same time slot. A polling message in slot
i
addresses a network station to send a transmission request in slot
i
+ 1. At the same time,
an acknowledgment in slot
i confirms a request from slot i
- 1.
6.3.1.2 Network Utilization
Network utilization is observed as a ratio between used network capacity for the data
transmission and the common capacity of the PLC network. Only error-free segments
are taken into account for used network capacity. In this part of the investigation, a
simple packet retransmission method is implemented, in accordance with the send-and-
wait ARQ mechanism (Sec. 4.3.4). So, in the case of an erroneous data segment, all
segments of a user packet have to be retransmitted. Of course, the data segments that
had to be retransmitted are not counted as used network capacity. The simple packet
slot
i
- 1
slot
i
+ 1
slot
i
Request
Ack.
Uplink
t
Downlink
Uplink
t
Downlink
ALOHA
Ack.
Poll.
Ack.
Poll.
Ack.
Poll.
slot
i
- 1
slot
i
+ 1
slot
i
Request
Polling
Figure 6.13
Slot structure for ALOHA and polling-based protocols