What factors affect modem signal strength and signal quality?

Beverly McRae

The measure of a connection depends on both the Signal Strength and Signal Quality. RSSI is a measure of Signal Strength, while CINR, SINR, and Ec/Io are measures of Signal Quality for various technologies. The values we report are specific to different technologies (e.g. LTE) and carriers (e.g. Verizon).

NOTE: The following values are approximations; they are solely CradlePoint’s interpretations. Each of these values will vary for different technologies and carriers: contact your carrier for more authoritative information. CradlePoint does not guarantee adequate performance, regardless of the Signal Strength and Signal Quality. An adequate connection depends on many different factors, including:

  • Proximity to the cellular tower
  • Tower load
  • Physical barriers (mountains, buildings, trains, etc.)
  • Competing signals
  • Weather
  • Signal going through a cellular repeater

Signal Strength and Signal Quality numbers do not incorporate all of the relevant factors. In particular, measurements of Signal Strength and Signal Quality for a specific moment do not reflect on the STABILITY of a connection, as these values will vary as conditions change.

Signal Strength

RSSI = Received signal strength indicator (measured in dBm)

Excellentabove −65 (closer to 0 is better)
Good−65 to −75
Fair−75 to −85
Poorbelow −85

Signal Quality

Ec/Io (several technologies including HSPA+ and EVDO; measured in dB)
Excellentabove −2 (closer to 0 is better)
Good−2 to −5
Fair−5 to −10
Poorbelow −10

SINR = Signal to Interference plus Noise Ratio (LTE only; measured in dB)

Excellentabove 12.5
Good10 to 12.5
Fair7 to 10
Poorbelow 7

CINR = Carrier to Interference plus Noise Ratio (WiMAX only; measured in dB)

Excellentabove 22
Good16 to 22
Fair9 to 16
Poorbelow 9

Additional Disclaimers: 

There is no black and white answer to what constitutes a successful connection. It is possible to disconnect with green values – or connect with red values – for several reasons, including:

1) Modems may vary.

We support hundreds of modems, and not all of them have the same ranges of acceptable values. For example: in our experience the Pantech UML290 and Novatel 551L require minimum RSSI values of −64 for acceptable connections.

2) You must factor in BOTH Signal Strength and Signal Quality.

It is possible to have excellent RSSI but disconnect because of poor quality (and vice versa).

3) Signal Strength and Signal Quality values do not hold constant. 

The variance of a signal is a significant factor in the success of a connection. A particular reading represents one moment, but these values may vary dramatically over time. A stable connection requires consistency. This is a sample graph of RSSI values that shows how much the values jump (from https://radiomobile.pe1mew.nl/):
Sample RSSI graph

4) Other factors: tower load, signal going through a repeater/booster, etc.

Some factors that don’t show up in Signal Strength/Signal Quality values can greatly impact a connection.
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