Image Processing Reference
In-Depth Information
Node A
Node B
S
1
M
1
T
1
Sending message at
t
1
P
12
R
2
S
2
T
3
M
3
S
3
Receiving ACK at
t
4
P
34
Receiving message at
t
2
Sending message at
t
3
S
4
R
4
S
= Software access time
P
= Propagation time
M
= Medium access time
R
= Reception time
T
= Transmission time
FIGURE .
Round-trip time.
the software access, medium access, transmission, and propagation times. Theses times are repre-
sented by
S
,
M
,
T
,and
P
. Once the message is received by node B at
t
, it will incur extra delays
through receiving and processing. After the message is processed, an ACK is sent to node A at
t
.
The total delay at node B is the summation of
R
,
S
,
,
S
,
M
,and
T
.AternodeB
sendstheACK,theACKpropagatesthroughthewirelessmediumandarrivesatnodeA.Aterwards,
node A processes the ACK. he path delays for sending and receiving the ACK from node B to A are
P
,
R
,and
S
. The round-trip time in real-time
t
for sending a message and receiving an ACK is
calculated by
(
±
ρ
B
)(
t
−
t
)
t
−
t
=
S
+
M
+
T
+
P
+
R
+
S
+(
±
ρ
B
)(
t
−
t
)+
S
+
M
+
T
+
P
+
R
+
S
(.)
where
S
,
M
,
T
,
P
,and
R
are the software access, medium access, transmission, propagation, and
reception times, respectively. In addition, ρ
B
isthedritrateatnodeB.hediference
(
t
−
t
)
is to
account for the waiting time before an ACK is sent to node A by node B.
If the round-trip time is measured using the hardware clock of node A, it has to be adjusted by the
drift rate of node A ρ
A
. If the granularity of the hardware clock is coarse, the error δ contributed by
the granularity should be accounted for. As a result, the round-trip time measured with the hardware
clock is bounded by an error associated with the clock drift and granularity as determined by
(
−
ρ
A
)(
t
−
t
)≤
H
(
t
)−
H
(
t
)<(
+
ρ
A
)(
t
−
t
)+
δ
(.)
heboundfortheround-triptimeluctuateswithrespecttotimesincethesotwareandmedium
access fluctuate according to the load at the node and in the channel. Although the transmission,
propagation, and reception times may be deterministic, they may contribute to the asymmetric delay
that can cause time offset between nodes A and B.
In the following section, different types of time synchronization protocols are described. Each of
them tries to minimize the effect of the nondeterministic and asymmetric delays. For sensor net-
works, it is best to minimize the propagation delay variation. For example, the delays and jitters
