However, a MAC protocol
typically also incorporates a lot of extra functionality that is not needed in
this case, such as supporting a certain throughput and ensuring fairness. After
a node becomes awake, it needs to select the MAC protocol and its parameters
to actually participate in communication [12]. The MAC protocol could learn
the tra?±c behavior or even use advanced knowledge of the application to
optimally tune its behavior. These aspects are issues related to MAC layer
design and fall outside the scope of this chapter.
1.3 The Energy-Delay Tradeo?®
Most wakeup protocols allow the designer to trade o?® the wakeup delay
Dwakeup versus the power consumption in the quasi-dormant state. This behavior
can be understood by considering again Figure 2. If the wakeup protocol
involves a periodic active-sleep pattern, as is the case in most synchronous and
asynchronous schemes (See sections 4 and 5), the worst-case wakeup delay can
be approximated as:
Dwakeup ?? Tperiod ??’ Tactive. (5)
This delay can be normalized with respect to the length of the active
period, which should be chosen as small as possible:
Dnorm =
Dwakeup
Tactive
??
Tperiod
Tactive
??’ 1. (6)
199
Curt Schurgers
The energy consumption, on the other hand, depends on the fraction of
time the node is in the active versus the sleep state. It is instructive to assume
that the power in the idle, transmit and receive mode are approximately equal,
and that the power in the sleep mode is negligible.
Pages:
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331