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Important Notes and Information Regarding EVM
EVM (error vector magnitude) is dependent on the modulation format and should be used as a
relative measurement of the link quality. A low EVM value indicates a better link quality than a
high value. Algorithmically, using QAM modulation, the transceiver calculates the value by
measuring the sample points of each "bit" and comparing it with the expected constellation based
on the modem type.
- EVM is a metric of the quality of the received signal. It is a dynamic value that is computed
only when data is received on the RF interface, and should be refreshed accordingly.
- Unlike RSSI which simply measures signal strength, EVM is a measurement of the
"correctness" of this signal. (This means how easily the received signal can be correctly
demodulated.)
- In general the lower the EVM the better the quality. A strong link will typically show an EVM
below 5.
The adaptive modulation mode (modulation automatic) allows directed traffic to adjust which
modem is used on a per-transmission basis. Adaptive modulation works in both upstream and
downstream mode. The mode selection varies between QPSK, 16QAM, and 64QAM. A signal
metric score is used to decide which modem selection to use. The score is determined based on
signal strength and packets received. Advanced configuration can be used to provide some
control over the adaptive modulation thresholds.
The primary use case for this feature is if an AP has some remotes that are close to the AP and
could support a higher data rate and some farther away (or obstructed) that can only support a
lower data rate. This mode allows the close remotes to take advantage of the higher data rate for
the directed messages, when otherwise the whole network would have had to be run at the lower
data rate. Note that broadcast or multicast data must always be transmitted at the lowest rate.
We recommend keeping adaptive modulation set for most installations.
Setting the security mode to EAP or PSK will enable device authentication. When enabled, the
remotes will authenticate with the AP (PSK) or a backend RADIUS server (EAP) before they are
allowed to pass data on the network. The authentication protocol is compliant with IEEE 802.1X.
If device authentication is enabled, over the air data encryption can also be enabled. This ensures
all over the air traffic is protected. When encryption is enabled, the device must occasionally
rotate the encryption keys. This rotation is logged in the event log with event type nx_auth.
These events can be suppressed in the event log configuration to prevent them from filling the
event log. See the MCR Technical Manual for instruction on controlling the event log.
76
Adaptive Modulation
Security
MDS™ Master Station
MDS 05-6399A01, Rev. F
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