In the wakeup regime, at low tra?±c
load, they also are suboptimal. The energy e?±ciency of both is given by the
ratio of the active duration and the duty cycle period. Ideally under low tra?±c
load, the active duration should be the minimum possible to allow nodes to be
contacted, while the duty cycle is limited by the wakeup delay, as explained
in Section 1.3 and illustrated in Figure 8 as well. However, S-MAC has a fixed
active duration, which is tailored to the expected tra?±c load, while T-MAC
has a fixed duty cycle that is not necessarily determined by the maximum
wakeup delay (although it could be designed this way in principle).
Another class of synchronous MAC protocols for sensor networks is based
on time division multiple access (TDMA) [24] [25] [26]. In the TRAMA
protocol, time is divided into random access slots for signaling and scheduled
access slots for transmissions [26]. By vacating reserved slots and assigning
priorities to nodes for grabbing these vacated slots, fluctuating tra?±c demands
can be accommodated. Although this solution is very flexible and adaptive, it
su?®ers from increased implementation complexity as compared to S-MAC [10].
A TDMA-based protocol that explicitly includes provisions for device
wakeup is TDMA-W [25]. The basic operation is illustrated in Figure 9. Consider
node A. If there is no tra?±c, A is asleep and only wakes up in its
???wakeup??? slot. Another node, B, that wants to communicate with A contacts
it by sending a request in that wakeup slot.
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