Image Processing Reference
In-Depth Information
ones, which need to be relayed with low latency. To achieve this requirement, the Sift protocol uses a
nonuniform probability distribution function of picking a slot within the slotted contention window.
If no node begins to transmit in the first slot of the window, then each node increases its transmission
probability exponentially for the next slot, assuming that the number of competing nodes is small.
Sift obtains a very low latency, but at the expense of increased energy consumption due to idle lis-
tening and overhearing, thus trading energy efficiency for latency. Moreover, it requires system-wide
time synchronization.
8.2.11 Lightweight Medium Access
The Lightweight Medium Access (LMAC) [Hoe] protocol aims to minimize the overhead of the
physical layer reducing the number of transceiver state switches and the energy waste due to the
preamble transmissions. LMAC uses TDMA to provide WSN nodes with a collision-free channel
access. Unlike traditional TDMA-based protocols, the time-slots in LMAC protocol are not assigned
to nodes by a central authority, but through a fully distributed slot assignment mechanism. Slots are
organized in frames. Each node is given one time-slot per frame, during which the node will transmit
a message consisting of two parts, a control message and a data unit.
The control message, which has a fixed size, carries:
•
ID of the time-slot controller
•
Distance (in hops) of the node to the gateway for routing purposes
•
Intended receiver(s)
•
Length of the data unit
The control message is also used to maintain synchronization between the nodes, as it contains the
sequence number of the time-slot in the frame.
All nodes listen to the control messages of their neighboring nodes. When a node is not the recip-
ient of that message and the message is not a broadcast one, the node will switch of its transceiver
only to wake at the next time-slot. Otherwise, if the node is the intended receiver, it will listen to
the data unit. If the latter does not fill up the remaining portion of the time-slot, after the message
transfer has completed, both transmitter and receiver(s) turn off their transceivers.
A short timeout interval ensures that nodes do not waste energy due to idle listening to free time-
slots. To limit the number of time-slots in the network, slot reusing is allowed at a noninterfering
distance. It is required that a slot is not reused within a two-hop neighborhood, so LMAC includes
a bitmap with all the slots assigned to a node's neighbors in the header. By combining the bitmaps of
all its neighbors, a node can assess which slots are free within a two-hop neighborhood.
Simulation results in [Hoe] show that LMAC significantly improves the network lifetime as
compared to S-MAC.
8.2.12 Z-MAC
Z-MAC [Rhe] is a hybrid MAC protocol for WSNs that tries to combine the positive features
of TDMA and CSMA scheme. In Z-MAC, nodes are assigned time-slots using a distributed algo-
rithm, but, unlike in typical TDMA mechanisms, a node can transmit in both its own time-slots
and slots assigned to other nodes. However, owners of the current time-slot always have priority in
accessing the channel over nonowners. The priority is implemented by adjusting the initial back-
off period, so that higher priority nodes have shorter back-off periods. he result is that during the
slots where owners have data to transmit, Z-MAC reduces their probability of collision, whereas
when owners do not have data to transmit, nonowners can steal the slot. This mechanism allows
