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Site Preparation and Planning
frequency greater than 4.9 GHz, making the Fresnel zone smaller. Every mesh deployment should consider
the Fresnel zone as part of its design, but in most cases, it is not expected that meeting the Fresnel clearance
requirement is an issue.
Hidden Nodes Interference
The mesh backhaul uses the same 802.11a channel for all nodes in that mesh, which can introduce hidden
nodes into the WLAN backhaul environment.
Figure 27: Hidden Nodes
Figure 27: Hidden Nodes, on page 65
• MAP X
• MAP Y
• MAP Z
If MAP X is the route back to the RAP for MAP Y and Z, both MAP X and MAP Z might be sending traffic
to MAP Y at the same time. MAP Y can see traffic from both MAP X and Z, but MAP X and Z cannot see
each other because of the RF environment, which means that the carrier sense multi-access (CSMA) mechanism
does not stop MAP X and Z from transmitting during the same time window; if either of these frames is
destined for a MAP, it is corrupted by the collision between frames and requires retransmission.
Although all WLANs at some time can expect some hidden node collisions, the fixed nature of the MAP
makes hidden node collisions a persistent feature of the mesh WLAN backhaul under some traffic conditions
such as heavy loads and large packet streams.
Both the hidden node problem and the exposed node problem are inherent to wireless mesh networks because
mesh access points share the same backhaul channel. Because these two problems can affect the overall
network performance, the Cisco mesh solution seeks to mitigate these two problems as much as possible. For
example, the AP1500s have at least two radios: one for backhaul access on a 5-GHz channel and the other
for 2.4-GHz client access. In addition, the radio resource management (RRM) feature, which operates on the
2.4-GHz radio, enables cell breathing and automatic channel change, which can effectively decrease the
collision domains in a mesh network.
There is an additional solution that can help to further mitigate these two problems. To reduce collisions and
to improve stability under high load conditions, the 802.11 MAC uses an exponential backoff algorithm,
where contending nodes back off exponentially and retransmit packets whenever a perceived collision occurs.
Theoretically, the more retries a node has, the smaller the collision probability will be. In practice, when there
are only two contending stations and they are not hidden stations, the collision probability becomes negligible
OL-27593-01
shows the following three MAPs:
Cisco Mesh Access Points, Design and Deployment Guide, Release 7.3
Hidden Nodes Interference
65
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