0.0
0.2
0.4
0.6
0.8
1.0
300 400 500
CDF
Time (microseconds)
(a) Sleep operation over-head
0.0
0.2
0.4
0.6
0.8
1.0
1600 1800 2000
CDF
Time (microseconds)
(b) Wake up operation over-head
Figure 4.2: Sleep and wake up operations
With more power modes than previous 802.11 standards, 802.11n provides an opportunity to
design a novel energy management mechanism for clients, which combines micro-sleep schedul-ing
(putting the client NIC in sleep mode) with configuration management (setting the client’s
antenna configuration in an energy-efficient state). But, before designing such a mechanism, it
is important to quantify what performance benefits are achievable with these mechanisms for re-alistic
traffic scenarios. We now discuss the results of several experiments designed to answer
this question. The methodology we use for these experiments is the same as that used for our
evaluations (Section 4.4), so we defer the details to that section but explain enough here to help
the reader understand our results.
4.1.2 Micro-sleep Scheduling
In a WLAN, clients can exploit at least two kinds of sleep opportunities. First, when an AP is
transmitting to client A, another client B can be put to sleep. Second, interframe gaps can be
exploited by putting the client to sleep for the duration of the gap. This is, of course, idealized,
since it assumes that interframe gaps are known before-hand, and that the client can be put to
sleep for arbitrarily small intervals (a capability sometimes called micro-sleep).
Micro-sleeping is possible on most 802.11n NICs, by using the same driver functionality used
by 802.11 Power SaveMode (PSM) to put NICs in sleep mode. However, there are practical limits
73