Image Processing Reference
In-Depth Information
wake-up times for each neighbor of a node. This problem is particularly significant in broadcast
communications.
8.2.7 Power-Efficient and Delay-Aware MAC for Sensor
Networks
The Power-Efficient and Delay-Aware MAC for Sensor Networks (PEDAMACS) [Erg] is a TDMA-
based scheme that extends the single-hop TDMA to a multi-hop sensor network, using a high-
powered access point (AP) to synchronize the nodes and to schedule their transmissions and
receptions. he main assumption in PEDAMACS is that the AP has enough power to communicate
with all the sensor nodes in one hop, while packets from sensor nodes must travel over several hops
to reach the AP. Another assumption is that the nodes periodically generate data to be transmitted
totheAP.NetworktopologyisdiscoveredbyPEDAMACSautomatically.
PEDAMACS works according to four phases:
•
Topology learning
: Each node identifies its (local) topology, i.e., its neighbors, interferers,
and its parent node in the routing tree rooted at the AP, obtained according to some
routing metric.
•
Topology collection
: Each node sends its local topology information to the AP so that, at
the end of this phase, the AP knows the full network topology.
•
Scheduling
: he AP broadcasts a schedule which each node follows, going to sleep during
thetime-slotswhenitisnotsupposedtotransmitapacketortolistentoone.
•
Adjustment
: his phase is triggered as necessary to learn the local topology information
that was not discovered during the topology learning phase or to discover changes.
The PEDAMACS scheduling algorithm runs in polynomial time and guarantees a delay propor-
tional to the number of packets in the WSN to be transferred to the AP in each period. In [Erg]
PEDAMACSisshowntodramaticallyoutperformbothrandom-accessschemeswithoutsleepcycles
and S-MAC with two different duty cycles. he PEDAMACS framework can be generalized in many
ways and extended to the cases of event-driven data generation and existence of multiple APs.
8.2.8 Traffic-Adaptive Medium Access
The traffic-adaptive medium access (TRAMA) [Raj] [Raj] provides energy-efficient collision-
free channel access in WSNs exploiting transmission schedules to avoid collisions of data packets at
the receivers and an adaptive power switching policy to dynamically put nodes into low-power mode
whenever they are not transmitting or receiving.
TRAMA assumes a single, time-slotted channel for both data and signaling transmissions. Time
is divided into sections of random- and scheduled-access periods. Random-access slots are used for
signaling, while scheduled-access slots are used for transmission. During the random-access period,
nodes go through contention-based channel access and thus signaling packets may experience col-
lisions. On the contrary, transmission slots are used for collision-free data transmission and also for
schedule propagation. Given the low data rates of a typical WSN, the duration of time-slots is much
larger than typical clock drifts, so a timestamp mechanism is enough for node synchronization.
TRAMA consists of three components:
•
Neighbor Protocol (NP)
•
Schedule Exchange Protocol (SEP)
•
Adaptive election algorithm (AEA)
