  Wireless Sensor Networks are basically used for gathering information needed
by smart environments but they are particularly useful in unattended situations
where terrain, climate and other environmental constraints may hinder in the
deployment of wired/conventional networks. Unlike traditional networks, these
sensor networks do not have a continuous power supply at their disposal. Rather
the individual sensors are battery operated and the lifetime of the individual
sensors and thus the overall network depend heavily on duty cycle of these
sensors. Analysis on WSNs shows that communication module is the main part
which consumes most of the sensor energy and that is why energy conservation is
the major optimization goal. Since routing protocols and MAC protocols directly
access the communication module therefore the design of protocols in these two
domains should take into account the energy conservation goal. In this paper,
we discuss different state-of-the-art protocols both in MAC and routing domains
that have been proposed for WSNs to achieve the overall goal of prolonging the
network lifetime. The routing protocols in WSNs are generally categorized into
three groups - data centric, hierarchical and location-based but we focus on
only the first two categories because location-based routing protocols
generally require a prior knowledge about sensors location which most of the
times is not available due to random deployment of the sensors. We then discuss
how schedule-based and contention-based MAC protocols can contribute to achieve
optimal utilization of the limited energy resource by avoiding or reducing the
chances of collisions and thus the need for retransmission.
