It is widely acknowledged that communication is the greatest
consumer of energy in many wireless sensor network
(WSN) applications. A large body of work has been produced
on increasing energy efficiency of Medium Access Control
(MAC) protocols to prolong the life time of sensor network
nodes operating on limited available energy. MAC protocols
can be loosely classified into two categories: asynchronous[1]
and synchronous protocols[2]. Other hybrid protocols[3] combine
elements of synchronous protocols to improve performance
in some areas whilst also supporting asynchronous
communications. Synchronous channel access can result in
lower power consumption than asynchronous protocols as it
tends to a TDMA schedule with sufficient synchronisation.
A common feature of all such MAC protocols is increased
latency caused by periods of lengthy inactivity to conserve
energy.
In this paper we propose an alternative method of wake up to
reduce latency introduced by low duty cycle MAC protocols.
We introduce a second lower power communication channel
using Vertical Cavity Surface Emmitting Lasers (VCSELs)
based Free Space Optics (FSO).
The design of a transceiver for FSO with an active receive
power consumption of less than 100μWis presented in Section
II. Section III analyses the performance of the transceiver and
the communication link. We discuss the practical considerations
of deploying optical sensor networks in Section IV.
In Section V we show how multi-hop communication is
performed using the dual channel mechanism and compare
the proposed system with example radio MAC protocols.
A. Application Scenarios
Traditional sensor applications such as environment monitoring
where data is generated by sensor nodes and routed to
sinks can work most efficiently with a synchronous protocol
as communication patterns are deterministic and latency can
be tolerated.
There are a number of applications to whichWireless Sensor
Network technology can be applied in built environments.
Nodes are deployed in fixed positions and require fully
connected networks, communication patterns are dynamic
and non-deterministic[4][5][6]. Such applications also require
much lower latency in communications as they may provide
services such as route finding to human users within a bounded
time. In the case of [4] and [5] the network may be inactive
for a very long period (measurable in years) but is required
to wake up instantly on discovery of a hazardous event. Both
classes of application require arbitrary paths of nodes (either
human navigable or paths of network connectivity) or for the
entire network to be woken up and to start communication in
a minimal time to react to an event.
As these applications are designed for use on a wireless sensor
network the standard consideration of power consumption
also applies. Therefore it is impractical to enable a standard
radio receiver at all times and use a simple CSMA protocol,
instead an energy conserving MAC protocol must be used.
If an asynchronous duty cycled, Low Power Listen (LPL),
protocol such as [1] is used, the average latency per hop is
half of the sleep period. Very low duty cycles (< 1%) will
result in intolerable latency along a path. If a synchronised
protocol such as [2] is used the average latency will also be
half of the sleep period (which may be longer to reduce power
consumption).
To eliminate the latency incurred through duty cycled MACs
and maintain the facility for asynchronous transmission we
require a receiver with sufficiently low power consumption
that it can be continuously maintained in active operation.