Руководство По Проектированию для Cisco Cisco Aironet 1522 Lightweight Outdoor Mesh Access Point
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Cisco Mesh Access Points, Design and Deployment Guide, 7.2
OL-21848-03
Site Preparation and Planning
Figure 47
Removing Obstructions in a Fresnel Zone
Fresnel Zone Size in Wireless Mesh Deployments
To give an approximation of size of the maximum Fresnel zone to be considered, at a possible minimum
frequency of 4.9 GHz, the minimum value changes depending on the regulatory domain. The minimum
figure quoted is a possible band allocated for public safety in the USA, and a maximum distance of one
mile gives a Fresnel zone of clearance requirement of 9.78 ft = 43.3 x SQR(1/(4*4.9)). This clearance is
relatively easy to achieve in most situations. In most deployments, distances are expected to be less than
one mile, and the 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.
frequency of 4.9 GHz, the minimum value changes depending on the regulatory domain. The minimum
figure quoted is a possible band allocated for public safety in the USA, and a maximum distance of one
mile gives a Fresnel zone of clearance requirement of 9.78 ft = 43.3 x SQR(1/(4*4.9)). This clearance is
relatively easy to achieve in most situations. In most deployments, distances are expected to be less than
one mile, and the 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, as shown in
nodes into the WLAN backhaul environment, as shown in
.
Figure 48
Hidden Nodes
shows the following three MAPs:
•
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
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
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MAP X
MAP Y
MAP Z