Cisco BR1400 Deployment Manual page 27

When more then two buildings need to be connected together the installation gets slightly more
complicated. This is referred to as a Point to Multi-Point link.
Point to Multipoint Bridge Link
Figure 24.
When linking more then one building, like in the Point to Point link we have defined a Root Bridge
and all the other sites in the multi-point link take on the role as Non-Roots Bridges. A Non-Root
Bridge will always try to initiate a link to the Root.
Unlike the Point to Point link, the Root Bridge now needs to transmit in more then one direction in
order to establish a radio path with the other three Non-Root bridges. Directional antennas that we
described in the Point to Point links are not practical for the Root Bridge unless all three Non-Root
sites were in a straight line as the coverage pattern of a directional antenna only transmits a narrow
beam in a given direction.
In this application we could use a 1400 series Bridge that supports the use of remote antennas and
then use an Omni-Directional antenna on the Root Bridge which then transmits radio signals in all
directions. Since the radio signal would be going all directions this comes at a price since it is not
possible to transmit the energy nearly as far as it could if it was able to focus the energy in only one
direction. The remote Non-Root bridges can however focus their energy to the Root so directional
antennas may be used, therefore; performance degradation with respect to distance will occur
primarily at the Root.
Using the Cisco 9 dBi Omni hub antenna on the Root Bridge in communication with a Non-Root
sites using a 22.5 dBi integrated antenna a distance of 2 miles at 54 Mbps or 8 miles at 9 Mbps
could be achieved. If these distances were not long enough, each bridge has several non-
overlapping channels allowing for additional Bridges to be added creating a hybrid approach where
one or more Bridges could be used creating a Point to Point link while others could be configured
for Point to Multi-Point linking the closer sites.
Copyright © 2004 Cisco Systems, Inc. All rights reserved.
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