Time Synchronization for Wireless Sensor Networks - Adaptive Signal Processing in Wireless Communications - page 374

13.3 Fundamental Approaches

to Time Synchronization

Time synchronization in wireless sensor networks can be achieved by transferring a
group of timing messages to the target sensors. The timing messages contain the infor-
mation about the time stamps measured by the transmitting sensors. There exist two
well-known approaches for time synchronization in wireless sensor networks, which are
categorized as sender-receiver synchronization (SRS) and receiver-receiver synchroniza-
tion (RRS). SRS is based on the traditional model of two-way message exchanges between
a pair of nodes. For RRS, the nodes to be synchronized first receive a beacon packet from
a common sender, then compare their receiving time readings of the beacon packet to
compute the relative clock offset. Most of the existing time synchronization protocols
rely on one of these two approaches. For instance, the Network Time Protocol (NTP) [3]
and the Timing-sync Protocol for Sensor Networks (TPSN) [17] adopt SRS since they
depend on a series of pairwise synchronizations that assume two-way timing message
exchanges. Notice also that the Reference Broadcast Synchronization (RBS) protocol
[18] relies on RRS since it requires pairs of message exchanges among children nodes
(except the reference) to compensate their relative clock offsets.
Recently, a new approach for time synchronization, called receiver-only synchroniza-
tion (ROS), was proposed. It aims at minimizing the number of required timing messages
and energy consumption during synchronization while preserving a high level of accu-
racy [19]. This approach can be used to achieve network-wide synchronization with much
less timing messages than other well-known existing protocols such as TPSN and RBS.
Next we will present and analyze each of these synchronization approaches and
illustrate how the general design considerations can be resolved in them. For all these
approaches, we only present the underlying signaling mechanisms for performing pair-
wise synchronization, i.e., synchronizing a pair of nodes, since network-wide synchro-
nization can be simply achieved by performing a group of pairwise synchronizations.

13.3.1 Sender-Receiver Synchronization

This approach is based on the classical two-way timing message exchange mechanism
between two adjacent nodes. Consider a parent node P and one of its children nodes,
node A, in Figure 13.1 . The clock model for the two-way message exchange is depicted in
Figure 13.2 , where θ ( AP ) denotes the clock offset between node A and node P and timing
messages are assumed to be exchanged multiple ( N ) times [4, 17]. Here, the time stamps
made during the i t h message exchange T 1, i

( A ) and T 4, i ( A ) are measured by the local clock of

( P ) are measured by the local clock of node P, respectively. Node
A transmits a synchronization packet, containing the value of time stamp T 1, i

( P ) and T 3, i

node A, and T 2, i

( A ) to node

( P ) and transmits an acknowledgment packet to node A

P. Node P receives it at time T 2, i

( P ) . This packet contains the value of time stamps T 1, i ( A ) , T 2, i ( P ) , and T 3, i ( P ) . hen, node A

at T 3, i

( A ) .
As discussed before, packet delays can be characterized into several distinct compo-
nents: send, access, transmission, propagation, and receive times. These delay components

finally receives the packet at T 4, i