radios use fixed modulation schemes whereby any degradation in the wave
propagation conditions (for example, due to adverse weather conditions such as
heavy fog or rain) led to complete loss of the signal and a disruption of
traffic. In a fixed modulation scheme, the microwave radio link had a binary
state of either “'available” (on) or “unavailable” (off).
advanced microwave radios use an adaptive modulation scheme. In an adaptive
modulation scheme, when the microwave link degrades due to adverse weather
conditions, the radio changes its modulation scheme to a more robust scheme.
The radio continues to broadcast but with less capacity. As a result, the radio
can be in several capacity or bandwidth states, and not just on or off.
In the case of
microwave links with adaptive modulation, the control Operation,
Administration, and Maintenance (OAM) protocols are unable to make best use of
the available bandwidth due of the following OAM characteristics:
If the protocol
used for failure detection is tagged as low-priority traffic, then momentary
congestion over the native Ethernet (that is, the nonmicrowave) links could
lead to loss of continuity and spurious protection switching.
Even though the
network topology must be provisioned with enough redundant bandwidth to handle
a complete failure, in certain situations where the service committed
information rate (CIR) is very low, forwarding as much excess traffic (above
the CIR) as possible is important. Therefore, for those situations, treating
bandwidth degradation as a complete failure is not desirable.