Low-PowerWireless Sensor Network Platforms - Signal Processing Systems - page 404

energy consumption of router nodes forming a backbone network is higher. Both
protocol variations enable multihop networking with one or more sinks and share
a common hardware platform. The program memory usage of the low-energy
and low-latency TUTWSN protocols were 100 and 60 kB, respectively. The data
memory usage was less than 4 kB in both cases. Thus, the protocols represent
feasibility of the very resource constrained WSN hardware.
A TUTWSN node is controlled by a 8-bit Microchip PIC18LF8722 MCU and
Nordic Semiconductor nRF24L01 transceiver. The transceiver is used with 1 Mbit/s
data rate, which ensures short transmission and reception times, allowing node to
spend most of the time in low-power states. The node is powered by two AA
batteries.

8.1 Low-Energy TUTWSN

Low-energy TUTWSN uses a clustered topology, in which each cluster operates on
its own frequency referred to as a cluster channel. This increases scalability and
avoids collisions between clusters.
The experimented configuration of TUTWSN MAC utilize 2 s access cycle,
4 ALOHA slots, and 8 reserved slots. The frame size is 32 B due to hardware
limitations. As data is sent only in the reserved slots, the total throughput at MAC
layer is 1 kbit/s. The routing protocol uses cost-based approach while supporting
several sinks by maintaining a separate cost for each sink [ 27 ] . A node initially
searches its neighbors with a network scan. When a new neighbor is found, a node
sends a cost request to it. A node selects the next hop and sets its cost based on
the received replies. Additionally, a node periodically recalculates its costs and
broadcasts an advertisement to its neighbors. This way, nodes can react to the
changes in the network conditions that manifest as varying cost levels.

8.1.1 Scalability

In a low-duty cycle MAC protocol (such as IEEE 802.15.4 or TUTWSN MAC), the
maximum number of nodes ( ) in an interference area can be determined by the

access cycle length ( T AC ), the superframe length, the average number of member
nodes in each cluster, and the number of utilized non-interfering frequency channels
( n CH )as

α

T AC n CH ( 1 + n S )

α =

t guard ,

(2)

t SF +

where t SF is the length of a superframe, t guard is a short guard time between
consecutive superframes. is maximized by minimizing the superframe and guard

time lengths and by maximizing T AC , n CH ,and n S . It can be clearly seen in the
equation that by utilizing a high data-rate radio operating at a wide frequency band
provides the highest scalability.

α