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instead it is a flat peer-to-peer protocol. S-MAC is an energy-efficient protocol simple to implement,
butithasadrawback,i.e.,thelatencyincreasesduetotheperiodicsleepofthereceiver,whichmakes
the sender wait for the receiver to wake-up before it can send out data. Such a time is called a sleep
delay. This delay is particularly significant in multi-hop networks, as each hop introduces a sleep
schedule. he latency requirement of the application constraints the maximum sleep time. he adap-
tive listening technique proposed in [Ye] improves the sleep delay and thus the overall latency.
In this approach, a node that overhears the transmission of a neighbor wakes up for a short time at
the end of the transmission (which is known from the duration field of the transmitted packet). In
this way, if the node is the intended next-hop recipient, its neighbor can immediately send data to it.
Otherwise, it will go back to sleep until its next scheduled listening time.
S-MAC enables low-duty-cycle operation in a multi-hop network, achieving energy efficiency
and also good scalability and collision avoidance through a combination of schedule-based and
contention-based schemes.
However, the fixed duty cycle in S-MAC represents a limitation, as the active time mainly depends
on the message rate, which will usually vary over time and space in a WSN. If important messages
are not to be missed in any case, the active time has to be conservatively sized according to the worst-
case, i.e., in such a way that the highest expected load can be handled. However, if the load is lower
than that, which is often the case in WSNs, the active time is not optimally used and energy will be
wasted on idle listening. his observation is the motivation for another milestone in energy-efficient
MAC protocols for WSNs, which is described in Section ...
8.2.3 Timeout-MAC
The Timeout-MAC (T-MAC) protocol [Dam] reduces idle listening by transmitting all messages
in bursts of variable length and sleeping between bursts. Similarly to what happens with S-MAC,
T-MAC nodes form a virtual cluster to synchronize themselves on the beginning of a frame, but
instead of using a fixed-length active period, T-MAC dynamically adapts the duty cycle to the net-
work traffic. To determine the end of the active period T-MAC uses a timeout mechanism, in which
the timeout value at each node, TA, is set to span a small contention period and an RTS/CTS
exchange. If a node r does not detect any activity (i.e., an incoming message or a collision) within an
interval TA, it will conclude that no neighbor is going to communicate with it and enter into the sleep
mode. Otherwise, the node r will start a new timeout after the on-going communication finishes. A
comparison between the different approaches adopted by S-MAC and T-MAC is shown in Figure ..
In [Hal] T-MAC protocol has proven to adapt seamlessly to traffic fluctuations typical of sensor
network applications at the expense of a decreased maximum throughput. T-MAC performs slightly
better for variations over time (events) than for variations in location (periodic reporting). Imple-
mentation of the T-MAC protocol revealed that, under high workloads, nodes communicate without
sleeping, while under light workloads nodes use their radios for as little as .% of the time, saving
much more energy as compared to the S-MAC protocol.
A drawback of T-MAC is the early-sleeping problem, that occurs when a node r does not detect
any activity during the TA interval not because there is no node interested in communicating with it,
but because such a node, s , lost the contention with a third node n which is not a common neighbor
of the two. When this occurs, s remains silent and r goes to sleep. Ater n 's transmission finishes, s will
be able to send an RTS to r , but being the latter in the sleep state, s will not receive the matching CTS
and must wait until the next frame before retrying. To mitigate this problem, T-MAC provides two
mechanisms, the future request-to-send (FRTS) and the full-buffer priority options [Dam]. The
FRTS mechanism provides for sending an FTRS packet when a CTS destined for another node has
been overheard, so that its destination is advised to stay awake. he full-buffer priority solution gives
priority to a node with full bufer. his can be done by sending a new RTS to another node in response
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