11 MAC
protocol. The interference range typically extends beyond two wireless hops and is
dependent on the communication signal rate??™s required SNR. The medium access algorithm
involves random back off times likely resulting in unnecessary medium idle times [20].
Furthermore, the hidden station phenomenon may cause collisions if no RTS/CTS scheme
is employed, resulting in additional transmission back offs and retransmissions. Two-radio
mesh networks realize a somewhat improved aggregate throughput as they eliminate the
interference caused by transmissions on the AP side, yet they still must manage contention
on the mesh.
Figure 13.4: A linear five hop mesh topology with one user per mesh AP. In real-world
deployments, the effective interference range, shown here for mesh AP3, will extend over two
or more wireless mesh hops.
Figure 13.5 shows the measured throughput degradation over five mesh hops using
the Sensoria dual radio ER500 mesh router. The results confirm the 1/n relationship
between throughput and hop count cited by [23] and [24] for a single client. A latency test
demonstrated 5ms over five hops on a lightly loaded network, increasing to 35ms when the
network is fully loaded [34].
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