In [4] has shown that the energy consumption of contention based MAC protocols is too huge and is not appropriate in wireless sensor networks due to the energy wastage caused by idle listening. In a contention based MAC protocol, a node has to keep sensing the medium to beware of a possible transmission. One benefit of contention based MAC protocols is that the MAC protocols can operate well under the unaware condition of topology and environment setups. Contention based MAC protocols have no knowledge of the incoming rate of data. Instead, they will just keep listening to the medium all the time or in a certain period of a duty cycle.
SMAC uses the wake-up/sleep duty cycle scheme to reduce energy consumption. It replaces the state of idle listening with sleeping in general wireless networks to achieve energy efficiency. IEEE802.11 has a similar scheme called power saving mode. In this kind of scheme, a sensor node will wake up for a certain period and listen to the medium. After this listening period, the sensor node will turn off the communication radio. In this radio-off period, called sleeping, this sensor node will not beware of any incoming data. In SMAC, the sensor nodes do not have to acknowledge the network topology. Instead, a sensor node will only try to forward the data packets it received in the wake-up state to the next node closer to the sink. There are no incoming data packet prediction actions in SMAC. The only thing SMAC can do to adapt to the network topology is adjusting the wake-up/sleep duty cycle. A lower duty cycle (sleep more often) will save more energy but it will increase the data latency and the extra energy
overload for sensor nodes that try to transmit data packets to sleeping sensor nodes. A higher duty cycle (wake up more often) will reduce the data latency and prevent the buffer from overflowing. However, the energy wastage caused by idle listening will be increased at the same time. The adaption of duty cycle is one of the largest unsolved problems in SMAC.
Timeout-MAC (TMAC) [10] inherits the duty cycle scheme from SMAC. It tries to improve SMAC by using an adaptive duty cycle. It uses a time out scheme to further reduce the idle listening wastage. When a sensor node wakes up and listens to the medium, it will only listen in a certain time length TA called timeout. If there is no active transmission within this TA period, a sensor node will turn off the antenna and go to sleep earlier than usual duty cycle. Although TMAC has the same performance as SMAC under constant traffic load, it saves more energy under variable traffic. With this timeout scheme, TMAC suffers from latency penalty due to that every sensor node will go back to sleep earlier than usual. SMAC and TMAC also have problems under high traffic load because they group the transmission in a small time period. Although the lower the duty cycle is, the more the energy is saved in SMAC and TMAC, the possibility of buffer overflowing will also increase and cause the protocol to collapse.
Data-gathering MAC (DMAC) [2] is another MAC protocol inherits SMAC.
DMAC focuses on improving the data latency of SMAC caused by waiting for the next node to wake up, i.e., sleeping latency. Sleeping latency is one of the major latency sources in wireless networks. When a sensor node wakes up and receives data, its next sensor node might just enter the sleep state at the same time. Because a sensor node can only receive data packets in the wake-up state in SMAC-like protocols, this sensor node must wait for the entire sleep state length in order to send out the data packets. In order to prevent latency being greatly increased in this situation, DMAC uses an improved
staggered duty cycle. When a sensor node overhears its children’s CTS or ACK packet, it will remain in the wake-up state for an additional time slot. In this setup, a packet can be forwarded two hops only because sensor nodes that are next to this overheard sensor node do not overhear the CTS or ACK packet. They will go to sleep directly. DMAC introduces a staggered wake-up schedule that staggers the sending and receiving time slots. When a sensor node receives a packet from its children nodes, it will send it out immediately and predict that there are more packets waiting for transmission. Every sensor node will wait and listen for a certain time length after each packet was sent out.
If no packets arrive during this time length, this sensor node will go to sleep directly.
The advantage of DMAC is that it can greatly reduce the packet latency via minimizing the sleeping latency which costs only an additional time slot in each duty cycle.
Pattern MAC (PMAC) is a MAC protocol uses adaptive sleep schedule according to the traffic load of every sensor node and its neighbors. PMAC will predict the following data arrival within a time period and generate a pattern for every sensor node via network traffic history. The most special feature of PMAC is that the generated pattern does not imply that the operation will be performed in the future. Instead, the real schedule for a sensor node will adjust the pattern with the real traffic load. For example, a sensor node is assigned to a time slot for sending data in original pattern but there is no data in buffer to be sent at that time slot. In such case, the sensor node will stay in sleep. A sensor node in PMAC will fall into a deeper sleep if there is no active transmission after a few short sleep periods. It will tune the generated pattern and try to save more energy. This method adjusts the duty cycle to fit the network traffic load and improves the energy-efficiency. But when the network traffic is not constant, prediction of PMAC might miss and cause the energy wastage or increase the latency. That is to say, a sensor node will remain in sleep for a long time while there is a packet waiting in
queue, or it may keep waking up to listen to the not existing transmission and check the buffer which contains no data packet.