2. Round-trip time.
terministic, and they can be estimated by the packet size, transmission rate,
and speed-of-light.
In summary, the delays experienced when sending a message at real-time
t1 and receiving an acknowledgement (ACK) at real-time t4 are shown in
Figure 2. The message from node A incurs the software access, medium access,
transmission, and propagation times. These times are represented by S1, M1,
T1, and P12. Once the message is received by node B at t2, it will incur extra
delays through receiving and processing. After the message is processed, an
ACK is sent to node A at t3. The total delay at node B is the summation of
R2, S2, (1 ?± ??B)(t3 ??’ t2), S3, M3, and T3. After node B sends the ACK, the
ACK propagates through the wireless medium and arrives at node A. After
the reception, node A processes the ACK. The path delays for sending and
receiving the ACK from node B to A are P34, R4, and S4. The round-trip
time in real-time t for sending a message and receiving an ACK is calculated
by
t4 ??’ t1 = S1 +M1 + T1 + P12 + R2 + S2
+ (1 ?± ??B)(t3 ??’ t2) + S3 +M3
+ T3 + P34 + R4 + S4, (1)
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Weilian Su
where S,M, T, P, and R are the software access, medium access, transmission,
propagation, and reception times, respectively. In addition, ??B is the drift rate
at node B. The di?®erence (t3 ??’ t2) 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 ??A.
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