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# Understanding Mobile Signal Strength: Why –81 dBm Is Good (and Why It’s Negative)
- URL: https://www.blog.nestict.com/understanding-mobile-signal-strength-why-81-dbm-is-good-and-why-its-negative/
- Published: 2026-03-08T15:36:04.000Z
- Updated: 2026-03-08T15:36:04.000Z
- Author: Nestict Infotech CSR
- Tags: Technology

# 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)

| Signal (dBm) | Quality   | Real-World Experience   |
| ------------ | --------- | ----------------------- |
| –50 to –70   | Excellent | Fast data, stable calls |
| –71 to –85   | **Good**  | Normal performance      |
| –86 to –100  | Fair      | Slower data, some drops |
| Below –100   | Poor      | Unreliable connection   |

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:

| ASU | dBm     | Result                  |
| --- | ------- | ----------------------- |
| 45  | –23 dBm | Extremely strong (rare) |
| 57  | –81 dBm | Good                    |
| 65  | –83 dBm | Slightly weaker         |
| 30  | –53 dBm | Excellent               |

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