Image Processing Reference
In-Depth Information
Receive RTS/ Send CTS**
Receive RTS/ Send CTS**
No CTS or busy tone or
unrelated RTS
Queue > 0
send RTS to
destination if it is not transmitting
No packet
or noise
New RTS
Send CTS**
Await CTS
(1 time step)
Await packet
(1 time step)
BEB
Idle
Time expired and destination is
not transmitting/send RTS
Receive RTS
Send CTS**
Packet received
Receive CTS
Packet starts
arriving/transmit
busy tone
Transmission ends
Transmit
packet
Receive
packet
If data channel is idle and no noise
over control channel
Ignore RTS/CTS transmissions
Receive other RTS/transmit busy tone
Ignore all CTSs received
FIGURE .
PAMAS protocol. (Redrawn from Singh, S. and Raghavendra, C.S.,
Comput. Commun. Rev.
,(),,
.)
PAMAS adopts an out-of-band signaling scheme with two separate channels, one used for signaling
and one for data transmission. An RTS-CTS message exchange takes place over the signaling channel
and any node hearing the signaling destined to another node is able to determine when and for how
long it can turn off its radio. A diagram that resumes the protocol operations is shown in Figure ..
A node is aware of the fact that a neighbor is transmitting, because it can hear the transmission
over the data channel. Likewise, a node which has data to transmit knows if one or more of its neigh-
bors is receiving, as the receivers transmit a busy tone over the signaling channel when they begin
receiving a packet and in response to RTS transmissions. For these reasons, any node in the system
can independently decide when to switch off.
To determine for how long the node should be switched off, there are two cases. If a packet trans-
missionintheneighborhoodbeginswhilethenodeisstillswitchedon,thenodeknowstheduration
of that transmission thanks to the RTS/CTS exchange. If there is an on-going transmission when the
nodeisbackandswitchesontheradio,thenodewilluseamechanismbasedonprobepacketsto
assess the remaining transmission time. he probe mechanism enables nodes to estimate the length
of time that the radio can be switched of. Although the conservative approach proposed in [Sin]
might underestimate the length of this period, thus resulting in suboptimal power saving, it ensures
that the delay/throughput of PAMAS remains unchanged.
In [Sin] simulation results proved that PAMAS achieves significant energy saving. However,
there are two drawbacks. First, collisions may still occur between probe messages or RTS/CTS pack-
ets. Second, out-of-band signaling requires a complex radio which may be unfeasible to be embedded
in low-cost sensor nodes.
8.2.2 Sensor MAC
The Sensor MAC (S-MAC) protocol proposed in [Ye] [Ye] represents a milestone among energy-
efficient MAC protocols for WSNs, as it provides the insights upon which many other protocols were
