11 Distributed Coordination Function (DCF) protocol. At light tra?±c,
SMAC outperforms IEEE 802.11 because idle listening is the major source of
energy consumption in IEEE 802.11 at light load. SMAC reduces the energy
consumption during idle listening through periodic sleep. However, in heavy
tra?±c, idle listening is not expected to happen frequently and SMAC achieves
its energy savings through avoiding message overhearing and transmitting
long massages using message passing technique. On the other hand, some
intermediate nodes which are involved in packet routing may show better
energy savings using the IEEE 802.11 than using SMAC during heavy tra?±c.
This is because of the synchronization overheard (SYNC packets) associated
with SMAC and due to the latency introduced by SMAC due to the periodic
sleep.
4.2 Timeout-MAC (TMAC) Protocol
TMAC protocol [13] tries to enhance energy savings in SMAC by further
reducing the idle listening. A node in the listen mode will go back to sleep
after time TA, as shown in Figure 5, if there is no activation event (start of
96 Ali Abu-el Humos et al.
Chapter 4 Medium Access Control Protocols for Wireless Sensor Networks
Fig. 5. Every node turns its transceiver o?® if there is no activity for a time duration
TA.
listen period, receiving any packet, etc). The choice of TA is critical for the
performance of TMAC. The following equation defines the minimum value of
TA:
TA > CW + TxRTS + T
where CW is the contention window size, TxRTS is the the transmission time
of the RTS packet and T is the turn-around time after the end of the RTS
packet and the arrival of the CTS packet.
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