Low Duty Cycle Protocols and Wakeup Concepts in WSN

The central idea of low duty cycle protocols is to reduce the time a node is idle or spends overhearing an unnecessary activity by putting the node in the sleep state.
The most ideal condition of low duty cycle protocols is when a node is a sleep most of the time and wakes up only when to transmit or receive packets.
The concept of a low duty cycle is represented as a periodic wake-up scheme.
A node wakes up periodically to transmit or receive packets from other nodes.
Usually after a node wakes up, it listens to the channel for any activity before transmitting or receiving packets.
If no packet is to be transmitted or received, the node returns to the sleep state.
A whole cycle consisting of a sleep period and a listening period is called a sleep or wake-up period and is depicted in Fig.
Duty cycle is measured as the ratio of the listening period length to the wake-up period length which gives an indicator of how long a node spends in the listening period.
A small duty cycle means that a node is asleep most of the time in order to avoid idle listening and overhearing.
However, a balanced duty cycle size must be achieved in order to avoid higher latency and higher transient energy due to startup costs.
There are various low duty cycle protocols proposed for WSNs which differ in aspects of synchronization, the number of channels required, transmitter- or receiver-initiated operation.
The low duty cycle protocols can be categorized into two major classes, namely synchronous and asynchronous schemes.

The concept of synchronization is related with data exchanges in WSNs.
In asynchronous schemes, there are two basic approaches, namely
Transmitter initiated
Receiver-initiated.
Using a transmitter-initiated approach, a node sends frequent request packets (preamble, control or even data packet themselves) until one of them “hits” the listening period of the destination node.
The transmitter approach puts the energy cost on the transmitter.

On the other hand, the receiver-initiated approach is applicable when a node sends frequent packets (preamble, control, acknowledgment) to inform the neighbouring nodes about the willingness of the node to receive packets.
The receiver initiated schemes moves the cost to the receiver.