Wireless Sensor Networks : Technology, Protocols, and Applications

278 OPERATING SYSTEMS FOR WIRELESS SENSOR NETWORKS
10.3.3. MagnetOS
MagnetOS [10.4] is a distributed adaptive operating system designed specifically
for application adaptation and energy conservation. Other operation systems do
not provide a network-wide adaptation mechanism or policies for application to
effectively utilize the underlying node resources. The burden of creating adaptation
mechanisms (if any) is on the application itself. This approach is usually not energy
efficient. The goals of MagnetOS are (1) to adapt to the underlying resource and its
changes in a stable manner, (2) to be efficient with respect to energy conservation,
(3) to provide general abstraction for the applications, and (4) to be scalable for
large networks.
MagnetOS is a single system image (SSI) or a single unified Java virtual
machine that includes static and dynamic components. The static components
rewrite the application in byte-code level and add necessary instructions on
the semantics of the original applications. The dynamic components are used for
application monitoring, object creation, invocation, and migration. SSI abstraction
provides more freedom in object placement and simplifies application development.
MagnetOS provides an interface to programmers for explicit object placement and
override of the automatic object placement decisions. This OS also provides
two online power-aware algorithms (NetPull and NetCenter) for use in moving
application components within the entire network so as to reduce energy consumption
and extend network lifetime. Netpull works hop by hop at the physical layer,
and NetCenter runs multihop at the network level. The difference between
traditional ad hoc routing and NetPull (NetCenter) is that the communication
endpoints in ad hoc routing are fixed, whereas NetPull tries to move the
communication endpoints in order to conserve energy [10.4].
10.3.4 MANTIS
MANTIS [10.5] is a multithread embedded operating system, which with its
general single-board hardware enables flexible and fast deployment of applications.
With the key goal of ease for programmers, MANTIS uses classical layered
multithreaded structure and standard programming language. The layered structure
contains multithreading, preemptive scheduling with time slicing, I/O synchronization
via mutual exclusion, a network protocol stack, and device drivers. The
current MANTIS kernel realizes these functions in less than 500 bytes of RAM.
MANTIS uses standard C to implement the kernel and API.
In the current implementation of MANTIS, the RAM size allocated to each new
thread is fixed. The thread table stored in a global data structure has a capacity for of
items, each of which is 10 bytes and is used to store thread-related information. The
thread scheduler in MANTIS is priority based and round robin within each priority
level. The scheduler is triggered only by timer interrupts from hardware to perform
context switching. In MANTIS, other interrupts are handled by device drivers.
The network protocol stack in MANTIS has four layers: application, network,
MAC, and physical. MANTIS implements these as one or more user-level threads,