wireless ad hoc networking

74 Wireless Ad Hoc Networking
3.3.1.4 Description of RT-Link
RT-Link employs an out-of-band time-synchronization mechanism, which
also globally synchronizes all nodes and is less vulnerable than the above
schemes. We believe that hardware-based time synchronization adds new
enabling capabilities to wireless sensor networks and warrants extensive
exploration and usage in practical large-scale environments.
RT-Link supports two node types: fixed and mobile. Both node
types include a microcontroller, 802.15.4 transceiver, and multiple sensors
as described in Section 3.2. The fixed nodes have an add-on timesynchronization
module, which is normally a low-power radio receiver
designed to detect a periodic out-of-band global signal. In our implementation,
we use an AM/FM time-synchronization module for indoor operation
and an atomic clock receiver for outdoors. For indoors, we use a carriercurrent
AM transmitter, 3 which plugs into the power outlet in a building
and uses the building’s power grid as an AM antenna to radiate the timesynchronization
pulse. We can also feed an atomic clock pulse as the input
to the AM transmitter to provide the same synchronization regime for use
both indoors and outdoors across a wide coverage area.
The time-synchronization module detects the periodic synchronization
pulse and triggers an input pin in the microcontroller which updates the
local time. As shown in Figure 3.4, this marks the beginning of a finely
slotted data communication period. The communication period is defined
as a fixed-length cycle and is composed of multiple frames. The synchronization
pulse serves as an indicator of the beginning of the cycle and the
first frame. Each frame is divided into multiple slots, where a slot duration
is the time required to transmit a maximum-sized packet. RT-Link supports
two kinds of slots:
scheduled slots (SS) within which nodes are assigned specific
transmit and receive time slots, and
a series of unscheduled or contention slots (CS) where nodes, which
are not assigned slots in the SS, select a transmit slot at random as
in slotted-Aloha.
TDMA cycle
Synchronization
pulse
Time-synchronization
cycle
Scheduled slots
Contention slots
Figure 3.4
RT-Link time slot allocation with out-of-band synchronization pulses.