Digital Signal Processing Reference
In-Depth Information
X-MAC
T-MAC
802.15.4
TUTWSN
Ideal MAC
X-MAC
T-MAC
802.15.4
TUTWSN
Ideal MAC
10
10
1
1
0.1
0.1
0.01
0.01
1
10
100
1000
1
10
100
1000
Data generation interval (s)
Data generation interval (s)
Fig. 9
Power consumption of MAC protocols, when maximum per-hop delay is 1 s
cycle saves energy, as beacons need to sent less frequently, while shortening the
access cycle gives more capacity. The active period length is long enough to allow
one data transmission. In the unsynchronized X-MAC, the channel listening interval
ranges from 0.1 to 3.3 s. Long listening intervals make LPL-based protocols energy
inefficient, because the transmission time of the preamble signal is relative to the
channel listening interval.
Clearly, the synchronized protocols are potentially the most energy efficient
choices for a WSN MAC. However, the energy efficiency has a trade-off with
latency. The maximum per hop forwarding delay, assuming no packet errors and
collisions, is the same as the access cycle length or channel listening interval. The
average per hop delay is half of the maximum delay.
Figure
9
shows the power consumption of the protocols with comparable delays.
On each case, the access cycle length is limited to 1 s. X-MAC uses 0.1-0.8 s
between 1-64 s data generation intervals, as it allows better energy-efficiency.
TUTWSN and IEEE 802.15.4 are the most energy-efficient choices for leaf
nodes, as they minimize idle listening. In T-MAC, the power consumption of a
leaf is high, because the protocol does not make a distinction between router and
leaf nodes. In this comparison, 802.15.4 has the highest has the highest router node
power consumption among synchronized MACs as its active period (CAP) length is
fixed, which causes a lot of idle listening. T-MAC and TUTWSN are more energy-
efficient because they have mechanisms to minimize idle listening. T-MAC adjusts
the active period length based on the traffic, whereas TUTWSN prefers contention
free channel access and adjusts the amount of reserved slots dynamically according
to the traffic.
The LPL-based X-MAC protocol is the least energy-efficient when network
traffic is high, but has the lowest power consumption when data generation interval
is low as a node uses energy only to transmit data and does not have to main-
tain synchronization. T-MAC is the most energy-efficient synchronized protocol
when data generation interval is long, because its energy-efficient synchronization
