STEM: Sparse Topology and Energy Management

STEM stands for Sparse Topology and Energy Management. This protocol tries to save energy due to idle listening. This protocol does not provide a complete MAC protocol, however a MAC protocol can be used along with it to give a complete MAC protocol. This protocol proposes to use two channels, wake up channel and data channel. Wake up channel is used to inform the receiver that a transmitter wants to transmit data to it. Data channel is used to transmit data, underlying MAC protocol is used for this data transmission. STEM is designed for applications which wait for an event and report that event, when the event takes place. In other words STEM is applicable where nodes have two states, monitor sate, where nodes monitor and no event takes place, and transfer state, where event is detected and data has to be transmitted.
On the Wake up channel time is divided into sleep period and listen period, these together are called wake up period. This can be seen in the diagram below



Channels in STEM

There will be two transceivers in every sensor node. One is for wake up channel and other is for data channel. The transceiver of the data channel will always be in sleep mode until some has to received or transmitted by the node and the transceiver of the wake up channel will be sleep in sleep period and be active in listen period. During the listen period the wake up channel receiver is switched on and the node waits to check if any data is to be received if so the data channel transceiver is switched on or else the wake up channel transceiver goes to sleep.

The STEM protocol has two flavors; they are STEM-B and STEM-T. In STEM-B a node which wishes to transmit to another node, sends beacons periodically on the wake up channel. This beacon contains the address of transmitter and receiver. The receiver detects the beacons during its listen period and acknowledges the transmitter, and then both shift to data channel and exchange data. In STEM-T the transmitter sends busy tone on wake up channel for a long enough time to hit the receivers listen period. As there is no address of the receiver in the busy tone all neighboring nodes which hear busy shift to data channel, however on receiving the data, only the node for which the data was intended will reply and all others go back to sleep.