A State-Based Programming Model for Wireless Sensor Networks

20 Chapter 2. Wireless Sensor Networks
segment, and runtime stack) is constrained to this amount. The few designs
with a larger memories space, organize their data memory into multiple pages
of 64 Kbytes. Then one page serves as runtime memory (which holds the runtime
stack and program variables) while the others can be used as unstructured
data buffers. Current SOC designs typically feature RAM in the order of a few
Kbytes.
Memory occupies a significant fraction of the chip real-estate. Since the die
area is a dominating cost factor in chip design, memories contribute significantly
to sensor-node costs. This is true for COTS microcontroller designs as
well as custom-designed sensor-node SOCs. For example, 3 Kbytes of RAM of
the Spec sensor-node SOC occupy 20-30% of the total die area of 1mm 2 , not including
the memory controller [60]. For comparison: the analog RF transmitter
circuitry in the Spec mote requires approximately the same area as one Kbyte of
SRAM, as can bee seen in Fig. 2.1 (a). In general, FLASH memory has a significant
density advantage over SRAM. Modern FLASH technology produces storage
densities higher than 150 Kbytes per square millimeter against the record of
60 Kbytes per square millimeter for optimized SRAM dedicated to performing
specific functions [60]. Fig. 2.1 (b) shows a comparison of the occupied die sizes
for 60 Kbytes of FLASH memory against 4 byte of SRAM in a commercial 8-bit
microcontroller design.
Unpaged RAM beyond 64 Kbytes also requires an address bus larger than the
standard 16-bit (and potentionally memory-management units), which results
in a disproportional increase in system complexity. Because of the aforementioned
reasons we expect that even in the future a significant amount of all cost
and size constrained sensor nodes (which we belive will be a significant amount
of all sensor nodes) will not posses data memories exceeding 64 Kbytes.
Wireless Communication Subsystems
Common to all wireless sensor networks is that they communicate wirelessly.
For wireless communication among the nodes of the network, most sensor networks
employ radio frequency (RF) communication, although light and sound
have also been utilized as physical communication medium.
Radio Frequency Communication. RF communication has some desirable properties
for wireless sensor networks. For example, RF communication does not
require a line of sight between communication partners. The omnidirectional
propagation of radio waves from the sender’s antenna is frequently used to implement
a local broadcast communication scheme, where nodes communicate
with other nodes in their vicinity. The downside of omnidirectional propagation
is, however, that the signal is diminished as its energy spreads geometrically
(known as free space loss).
Various wireless transceivers have been used in sensor-node designs. Some
transceivers provide an interface for adjusting the transmit power, which gives
coarse-grained control over the transmission radius, if the radio propagation
characteristics are known. Very simple RF transceivers, such as the TR 1000
from RF Monolithics, Inc merely provide modulation and demodulation of bits
on the physical medium (the “Ether”). Bit-level timing while sending bit strings
as well as timing recovery when receiving has to be performed by a host proces-