RLX-IH
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802.11b
Radio Configuration / Diagnostic Utility
Industrial Hotspot
Page 36 of 99
ProSoft Technology, Inc.
April 10, 2007
RSTP uses active communications between network devices to propagate
changes in the network and to cause transitions to occur much more quickly.
Because RSTP is an IEEE standard, IH radios work in conjunction with wired
Ethernet switches to form a redundant network.
Each RSTP device (IH radio or Ethernet switch) communicates with other RSTP
devices in the network via packets called Bridge Protocol Data Units (BPDUs).
BPDUs are sent out each of the devices ports. In a wired switch this would be
from each of the Ethernet ports. In an IH radio, in addition to the Ethernet port,
each wireless link is considered a port. These BPDUs are the communications
means to allow each RSTP device in the network to make sure that the proper
connections still exist.
In the following illustration, this IH radio has 4 RSTP "ports":
Ethernet port (1)
A port for its parent connection (2)
A port for each of its two child connections (11 and 12).
BPDUs are sent out the port at a rate called the "Hello Time". The accepted
standard value for this is 2 seconds. If a radio (or any other RSTP device) does
not get a BPDU for 2 Hello Times, it assumes the RSTP device that had been
there is no longer available. It can then open an alternate path if one is available.
This process is much like the STP process. If other devices on the network are
not operating in rapid spanning tree mode, the radio will revert to normal
spanning tree operation on a per-port basis.
RSTP provides a performance enhancement over STP operation. By
comparison, the radio using the STP algorithm would revert its port to the
listening state, and then to the learning state, before returning to the forwarding
state. Each of these states takes at least 15 seconds, during which the STP
devices are listening for BPDUs to re-negotiate the network topology. The
advantage of using the RSTP functionality is that is uses active handshaking
between adjacent RSTP devices to re-negotiate the network topology. This
process takes one to two seconds.
