Understanding Mobile Signal Strength: Why –81 dBm Is Good (and Why It’s Negative)

When checking mobile network diagnostics on a phone, LTE router, or modem, you may encounter signal readings such as:

Signal strength: –81 dBm (57 ASU)

At first glance, this can be confusing—especially the negative number. Why is the signal shown as –81 and not +81? And what does it mean when the number increases or decreases?

This article breaks it down in practical, non-abstract terms.

1. What Is dBm?

dBm stands for decibels relative to one milliwatt. It is a logarithmic unit used to measure received radio signal power.

Key points:

• It is an absolute measurement, not a percentage
• It is measured relative to 1 milliwatt (mW)
• In wireless communications, received signals are almost always below 1 mW

Because of this, the resulting values are negative.

2. Why Signal Strength Is Negative (–81 dBm)

A signal strength of –81 dBm means:

The received signal power is much weaker than 1 milliwatt

In fact:

• 0 dBm = 1 milliwatt
• –30 dBm = 0.001 milliwatt
• –60 dBm = 0.000001 milliwatt
• –81 dBm = extremely small power, but still very usable

Why not +81 dBm?

A signal of +81 dBm would represent:

• Over 12,000 watts
• Far stronger than radio towers themselves
• Physically impossible for a mobile device to receive

So:

• Negative does not mean bad
• It simply reflects how radio power is measured

3. Understanding “Increasing” vs “Decreasing” Signal Strength

This is where most confusion happens.

In dBm:

• –60 dBm is stronger than –81 dBm
• –90 dBm is weaker than –81 dBm

Even though –90 looks like a “bigger” number, it is worse.

Rule of thumb:

Closer to zero = stronger signal

4. Signal Strength Quality Scale (dBm)

Your value:

–81 dBm → Good signal

5. What Is ASU and Why 57 ASU Matches –81 dBm

ASU (Arbitrary Strength Unit) is a device-friendly representation of signal strength.

For LTE (4G) networks, the conversion is typically:

dBm = (ASU × 2) − 113

Using your reading:

(57 × 2) − 113 = –81 dBm

This confirms:

• The modem/phone is reporting consistently
• The signal is genuinely in the good range

6. Does Increasing ASU Improve Signal?

Yes—but only if it results in less negative dBm.

Examples:

So:

• Higher ASU usually means better signal
• But the real authority is dBm

7. What Causes Signal to Improve or Degrade?

Signal Improves When:

• You move closer to the cell tower
• You install an external LTE antenna
• Line-of-sight obstruction is reduced
• Network load is low

Signal Degrades When:

• You move indoors or underground
• Buildings, metal, or terrain block RF paths
• Weather conditions affect propagation
• The tower is congested or distant

8. Practical Impact of –81 dBm

At –81 dBm, you can expect:

• Stable voice calls
• Reliable LTE data
• Usable hotspot/tethering
• Minimal battery drain from signal searching

Slow speeds at this level are usually due to congestion, not signal strength.

9. Final Takeaway

• Negative dBm values are normal
• –81 dBm is good, not weak
• Signal quality improves as the number moves closer to zero
• ASU is just another way of expressing the same information
• Good signal does not always equal fast internet—network load matters

If the community would like, follow-up topics can include:

• RSRP vs RSRQ vs SINR
• Diagnosing slow LTE despite strong signal
• External antenna tuning and placement
• LTE vs 5G signal interpretation