Low-PowerWireless Sensor Network Platforms - Signal Processing Systems - page 390

Fig. 6 Operation of synchronized low duty-cycle protocols

The drawback of the basic LPL mechanism is that the transmission and reception
of long preamble increases energy consumption significantly. Therefore, several
variations are proposed to reduce the preamble energy. For example, in X-MAC [ 4 ] ,
a sender transmits multiple short preambles with the address of the intended
receiver. Each preamble is followed by a short reception period. Upon receiving
a preamble, the destination node sends an acknowledgment (ACK) between the
preambles. Other nodes can enter early a sleep mode for reducing overhearing. After
receiving the ACK, the source node begins the transmission of a data frame.
The preamble can be eliminated completely by utilizing an additional transceiver
referred to as a wake-up radio [ 13 ] . The wake-up radio mechanism is based on
the assumption that the listen mode of the wake-up radio is ultra low power and
it can be active constantly. At the same time, the normal data radio is in the sleep
mode as long as packet transmission or reception is not required. The wake-up radio
protocols are successful in avoiding overhearing and idle listening in the data radio.
Their major problems are the energy consumption and cost of the wake-up radio. In
addition, the difference in the transmission ranges between data and wake-up radio
may pose significant problems.
Unsynchronized protocols are relatively simple and robust, and require a small
amount of memory compared to synchronized protocols. A general drawback
is rather high overhearing, since each node must receive at least the beginning
of each frame transmitted within radio range. Thus, they suit best for relatively
simple WSNs utilizing very low data rates. Unsynchronized protocols tolerate
dynamics in networks, but their energy-efficiency is limited by the channel sampling
mechanism [ 58 ] .

5.3 Synchronized Low Duty-Cycle MAC Protocols

Synchronized low duty-cycle MAC protocols utilize scheduling to ensure that nodes
agree on the data exchange times. Due to the synchronized operation, nodes know
the exact moments of active periods in advance, thus eliminating the need of long
preambles. As a global synchronization is very difficult in large networks, the
synchronization is often realized by receiving beacon frame from one or more
neighbor nodes, as shown in Fig. 6 . The beacon frame includes synchronization and