In PRTG Network Monitor, a sensor is the most fundamental monitoring component that measures a specific metric of a monitored device or service. In other words, a sensor is a measurement point that collects, processes, and reports numerical data describing what is happening across your network, servers, applications, or services.

Within the PRTG Network Monitor architecture developed by Paessler, all licensing, alerting, and reporting logic is built entirely around sensors. For this reason, understanding the sensor concept is essential for deploying and operating PRTG efficiently and correctly.

What Does a Sensor Do?

A PRTG sensor performs three core functions:

1. Data Collection

The sensor retrieves data from the target system using supported protocols such as SNMP, WMI, HTTP, Ping, Flow technologies (NetFlow, sFlow, IPFIX), and SSH.

2. Channel-Based Data Structuring

A single sensor can consist of multiple channels, each representing a distinct sub-metric.

For example, a CPU Load sensor may include multiple channels such as:

• CPU Core 1

• CPU Core 2

• Average CPU utilization

This channel-based structure enables granular and precise performance analysis.

3. Status Evaluation and Alerting

Collected data is continuously evaluated against defined thresholds. Based on these evaluations, the sensor transitions into one of the following states:
Up, Warning, Down, Paused, Unusual, or Unknown, triggering alerts when necessary.

Sensor Logic in PRTG

PRTG uses a hierarchical monitoring model structured as follows:

Core Server / Probe → Group → Device → Sensor → Channel

• Device: The monitored object, such as a server, switch, firewall, or service

• Sensor: A single monitoring task assigned to a device

• Channel: An individual metric measured by the sensor

📌 Important note:
PRTG licensing is based on the number of sensors, not the number of devices.

Examples of Sensor Types

PRTG includes more than 300 predefined sensor types. Commonly used examples include:

Basic Sensors

• Ping Sensor – Verifies device availability

• Uptime Sensor – Measures system runtime

• Port Sensor – Checks TCP/UDP port availability

SNMP Sensors

• SNMP Traffic – Interface traffic monitoring

• SNMP CPU Load – Processor utilization

• SNMP Memory – RAM usage

Server & System Sensors

• WMI CPU / Disk / Memory

• Windows Service Sensor

• Linux SSH Load Sensor

Traffic & Bandwidth Monitoring

• NetFlow v5 / v9 Sensor

• sFlow Sensor

• Packet Sniffer Sensor

Application & Service Monitoring

• HTTP / HTTPS Sensor

• DNS Sensor

• SMTP / POP3 / IMAP Sensor

• SQL Sensor

What Do Sensor States Mean?

Each sensor in PRTG operates in one of the following states:

• Up (Green)
  The sensor is functioning normally and values are within acceptable thresholds.

• Warning (Yellow)
  Thresholds are being approached, indicating potential risk.

• Down (Red)
  A critical threshold has been exceeded or the sensor cannot retrieve data.

• Paused (Gray)
  Monitoring is temporarily suspended due to manual action, scheduling, or dependency rules.

• Unusual (Blue)
  Values deviate significantly from historical trends.

• Unknown (Gray/Purple)
  The sensor cannot determine its status due to missing or invalid data.

One Sensor = One Monitoring Responsibility

A key principle in PRTG’s design philosophy is

One sensor monitors one specific aspect.

For example:

• CPU usage is monitored by a dedicated sensor

• Disk space is monitored by a separate sensor

• Network traffic is monitored by another sensor

  
  

This approach:

• Enables faster root-cause analysis

• Simplifies alerting and reporting

• Supports scalable and maintainable monitoring architectures

Why Does the Number of Sensors Matter?

• Performance: Unnecessary sensors increase Core Server load

• Licensing: Each sensor consumes a license unit

• Alert Noise: Excessive sensors often result in unnecessary alerts

Best Practices

• Monitor only metrics that provide real business value

• Use longer scanning intervals for non-critical sensors

• Limit the use of NetFlow and Packet Sniffer sensors to essential traffic

Conclusion

In PRTG, the sensor is the heart of monitoring, the source of alerts, and the foundation of reporting.
Using the right sensors, in the right place, and at the right polling intervals results in:

• Reduced alert noise

• Faster incident detection

• Lower resource consumption

• More meaningful and actionable reports

In short:
What makes PRTG powerful is not the number of sensors deployed, but how well those sensors are designed and implemented.