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Figure 8. Tow-hop network scenario
gin similar to that of Z-MAC. This performance
similarity is explained by the fact that H-MAC
uses the same medium access scheme as Z-MAC,
and because all nodes are within one-hop distance
from the destination, so the senders can be easily
synchronized with each other.
Two-hop scenario
: this scenario is used to
measure the performance of the different protocols
when hidden terminals are present. We organized
nodes into two clusters as illustrated in Figure 8.
The two clusters are placed approximately 5
meters apart. A receiver node (or routing node)
is placed in the middle of the two clusters. We
ensure that all senders find the receiver as a one-
hop neighbor and all nodes are reachable by two
hop communications. We also reduced the trans-
mission power of senders to 1 dBm (1.3 mW) to
control the number of hidden terminals.
In the tow-hop scenario, we measured the data
throughput when hidden terminals are present.
We varied the number of senders while fixing the
number of neighbors. As in the one-hop bench-
mark, all senders have always data to send. Each
additional sender is chosen from the alternating
clusters.
For H-MAC tests, we set the frame size to 20
slots. In this test, we run H-MAC with the local
clock synchronization protocol in which each
sender sends one synchronization packet in every
100 packets transmitted. The data throughput
reported by H-MAC includes the overhead of the
clock synchronization and ECN.
The Figure 9 shows the two-hop tests results.
With the ns-2 simulator, we verified that the two
node clusters do not sense each other to maximize
the number of hidden terminals. We noticed that
despite using the RTS/CTS mechanism in H-MAC
during the control period, H-MAC maintains the
same good performance but with slightly deg-
radation in channel utilization to 73%. Z-MAC
has suffered from performance degradation that
undergo until 68%. This performance degradation
is caused by the presence of the hidden terminals,
and by the overhead of ECN messages.
Multi-hop scenario: in this scenario, we cre-
ated a network of 20 nodes, placed randomly in
a 100*100m surface area. The maximum two-hop
neighborhood size of all nodes is 19 and the
maximum local frame size is set to 20 slots. We
used fixed routing paths for all tests. The purpose
behind this scenario is to measure the total network
throughput in the multi-hop environment (See the
Figure 10).
In the multi-hop scenario, each node has always
data to send. All senders are transmitting at their
full transmission power. The number of channels
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