A limited
version of TMAC was implemented on the EYES hardware [13] with all the
97
Fig. 6. The early sleeping problem occurs at the node D.
nodes adopting the same schedule, and within the transmission range of each
other. TMAC shows excellent energy savings because nodes enter sleep mode
(rather idle listening) after TA seconds of no activity during the listen period.
4.3 SMAC with Coordinated Adaptive Sleeping
Periodic sleep introduces a serious latency problem in SMAC especially at very
low duty cycle. Figure 7 shows a three-hop communication scenario where a
packet is to be transmitted from A to D.We assume that the duration of listen
period is long enough to transmit only one packet. In order for the packet to
reach D, it requires three time frames. If an adaptive listening feature is added
to the SMAC protocol as proposed in [17], then node C will wake up when the
transmission from A to B is completed and B can start transmitting its data
to C as shown in Figure 8. However, node C should wait until the beginning
of the next frame to transmit its data to D, since D was not aware of B??™s
transmission to C (D was sleeping).
It is proven that in an N-linear hop network, SMAC with adaptive listening
can reduce the latency of traditional SMAC (without adaptive listening)
by almost 50% under light tra?±c conditions. This improvement can reach
more than 50%, since nodes two hops away from the transmitting node might
98 Ali Abu-el Humos et al.
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