When the tra?±c rate is low, they essentially behave as a wakeup
protocol. However, their operation is not specifically optimized for the low
throughput conditions that characterize the quasi-dormant state, and they
typically contain more than the strictly needed functionality in this case.
One early proposal was the S-MAC protocol [8]. The design philosophy
behind it is to improve the energy e?±ciency of the carrier sense multiple access
(CSMA) mechanism in a contention-based MAC such as IEEE 802.11. Nodes
are synchronized within a virtual cluster and operate in a duty cycled fashion
which is similar to the basic scheme of Figure 2 and also the power saving
mode of Figure 6. Each period starts with a time to exchange synchronization
packets, as shown in Figure 8. This is followed by an active duration, which
occupies a fraction of the period. The CSMA collision avoidance handshake
(RTS/CTS) is executed in this active duration, and nodes remain awake if
they need to exchange data. The length of the active duration is fixed, and
the duty cycle period can be chosen based on the expected tra?±c demands. An
enhancement to S-MAC, called T-MAC, introduces an adaptive duty cycle [7].
In this case, the active duration is controlled by a timeout mechanism, while
the duty cycle period is fixed. Both S-MAC and T-MAC have the disadvantage
that they concentrate the tra?±c to a fraction of the available time, which
results in ine?±ciencies at high tra?±c load.
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