SMAC : Sensor MAC

SMAC stands for Sensor MAC . This protocol tries to reduce energy consumption due to overhearing, idle listening and collision. In this protocol also every node has two states, sleep state and active state. Unlike STEM, SMAC does not use two channels. A node can receive and transmit data during its listen period.

SMAC adopts a periodic wake up scheme. SMAC tries to synchronize the listen periods of neighboring nodes. The listen period of a node is divided into three phases as shown below. The listen period is the time during which a node is awake, rest of the time node is sleeping. The listen and sleep periods in the S-MAC are fixed intervals.



Three phases of listen period

In sync phase the neighboring synchronize their listen periods, a table is maintained regarding neighbors schedules, in RTS phase all nodes wishing to communicate to a particular node send RTS in CSMA mode with additional back off and in CTS the node acknowledges a particular RTS and communication between the two nodes starts and proceeds even in their sleep periods. The neighbors synchronize periodically.
SYNC packet is used to synchronize periodically. The SYNC packet contains senders address and time of its next sleep. The next sleep time is according to the sender, the receiver will adjust its timers after it receives the SYNC packet and updates the neighbor’s schedule.
In SMAC long data messages are fragmented and sent form transmitter to receiver. The receiver has to acknowledge for every fragment, else it is retransmitted. A series of fragments are sent with only one CTS and RTS message. This method is called as message passing. A protocol called T-MAC is proposed which is similar to S-MAC but with variable Listen and Sleep periods, this will help to suit the listen and sleep periods according to the load in the network.
The main concept in SMAC is that, all the neighboring nodes form virtual clusters and synchronize their sleep and listen periods. They communicate during their listen periods and sleep rest of the time. The immediate neighbors of nodes, which are transmitting and receiving, sleep until the communication is completed. A long message is divided into many fragments and all the fragments are sent as burst.
S-MAC contributes in these ways; reduction of idle listening(as nodes sleep and not stay in idle state),collision and overhearing avoidance by using RTS and CTS, and saving energy and time, by sending a series of fragments of a long message together, rather than going for contention after sending every fragment.