Image Processing Reference
In-Depth Information
built. S-MAC aim is to reduce energy consumption from all the sources that cause energy waste, i.e.,
idle listening, collisions, overhearing, and protocol overhead. he S-MAC protocol consists of three
major components:
•
Periodic listen and sleep scheme
•
Collision and overhearing avoidance mechanism
•
Message passing
The idea behind periodic listen and sleep state is that in many WSN applications nodes may remain
idleforalongtimewaitingfortheoccurrenceofanevent,sotheydonothavetobelisteningallthe
time. he S-MAC protocol therefore exploits this feature by letting nodes go into periodic sleep mode.
While in the sleep mode, a node completely switches off its radio and sets a timer which is used to
turn it on aterwards. his significantly reduces the actual listening time and thus the relevant energy
consumption.
An entire cycle of listen and sleep is called a frame. he listening interval is normally fixed accord-
ing to the radio bandwidth and the contention window size. he duty cycle is given by the ratio of the
listening interval to the frame length. The sleep interval may change depending on the application
requirements. All the nodes have the same values as listen and sleep intervals and they are free to
choose their own listen/sleep schedules. However, to keep the protocol efficient while decreasing the
overhead, neighboring nodes coordinate their sleep schedules, so that they listen at the same time
andgotosleepatthesametime,ratherthanrandomlysleepontheirown.
Nodes exchange their listen/sleep schedules by broadcasting them to all the immediate neighbors
by periodic SYNC packets (including the address of the sender and the time of its next sleep) and
each node maintains a schedule table that stores the schedules of all its known neighbors. Although
this coordinated approach requires periodic clock synchronization among neighboring nodes, such
a synchronization is significantly looser than the one required by TDMA schemes with very short
time-slots, as in S-MAC the listening period is significantly longer than typical clock drift rates.
If multiple neighbors want to send DATA packets to a node, they go through a contention mecha-
nism similar to the one used in the IEEE . standard, including both virtual and physical carrier
sense and RTS/CTS (request to send/clear to send) exchange. he node whose RTS packet is received
first wins the contention and the receiver will reply with a CTS packet. Once their data transmission
starts, they do not follow their sleep schedules until the end of the transmission.
In order for a node to receive both SYNC packets and DATA packets, its listening interval is divided
into two parts. he irst part is for receiving SYNC packets, while the second one is for receiving DATA
packets. he S-MAC protocol tries to avoid overhearing by allowing interfering nodes to go to sleep
after they hear an RTS or CTS packet until the current transmission is over.
Since DATA packets are normally much longer than control packets, this approach prevents neigh-
boring nodes from overhearing long DATA packets and the following ACKs. Each node maintains
a network allocation vector (NAV) and an associated timer to keep track of the activity in its neigh-
borhood. When a node receives a packet destined to other nodes, it updates its NAV by the duration
field in the packet. As nonzero NAV value indicates that there is an active transmission in the neigh-
borhood, a node with a nonzero NAV has to sleep to avoid overhearing. he NAV value decrements
when the associated timer expires and a node can wake-up when its NAV reaches zero.
The message passing scheme proposed in S-MAC also aims to keep nodes in the sleep state as
long as possible to reduce their switching overhead. To this aim, all DATA packets have a duration
field, which represents the time needed for transmitting all the remaining data fragments and ACK
packets. If a neighboring node hears a RTS or CTS packet, it will go to sleep for the time needed to
transmit all the fragments.
Although S-MAC proposes the concept of virtual clusters that consist of synchronized nodes, there
is no real clustering. his means that S-MAC does not define hierarchical relations between nodes,
