What Is a Security System and How Does it Work?

Security systems detect entry into your home. After a brief delay to allow you to disarm the system, a siren blares and a professional monitor—if you subscribe to such a service—calls to make sure it's not a false alarm. 

Let's take a look at common security sensors, monitoring options, communication protocols, and more.

See our top picks: The Best Home Security Systems

Read more: 

• Essential Home Security Systems Guide
• What Are the Common Components of a Security System?
• What Are the Best Features to Include in a Security System?
• How Do Security Systems Protect Against Fire and Flood?
• What Are the Smart Home or Home Automation Features of Security Systems?

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Security components communicate their status to the control panel, and the control panel communicates to the monitoring station (if applicable—more on that below) and to your smartphone. 

Security communication protocols differ from system to system. Hardwired systems use electrical wires to connect all components to the control panel. Wireless security components communicate to the control panel through radio frequencies or a mesh network.

Control panels use Wi-Fi or cellular data to connect to your smartphone app and the monitoring center. Landline-based security systems are rare.

Read more:

• Can I Control My Home Security Through My Phone?
• If the Power Goes Out, Does My Security System Still Work?
• If the Internet Goes Out, Does My Wireless Security System Still Work?

The control panel is the computer that arms and disarms the security systems, communicates with each installed component, sounds the alarm when a security zone is breached, and communicates with an alarm monitoring company.

They typically feature a touchpad for easy programming and interaction. You'll enter your PIN to arm and disarm the system, but some can be armed with a voice command. You can also use a key fob to arm and disarm the system quickly, or use a smartphone app to control the system from anywhere in the world—literally.

Door and window sensors—also called contact or entry sensors—are comprised of two parts. They're installed right next to each other on a closed door and door frame (or closed window and window frame). One part of the sensor detects a magnet in the second part. As soon as the magnet is moved out of range (i.e., the door or window opens), the sensor communicates a breach to the control panel.

Read more:

• How Does a Door Sensor Work?
• How Does a Window Sensor Work?

When armed, motion sensors protect a given space by creating an invisible zone that cannot be breached without sounding an alarm. Common motion sensor technology includes ultrasonic waves and passive infrared technology.

Motion sensors are typically used to protect hallways, rooms with multiple windows or entry points, and areas with an open floor plan. 

Read more:

• How Do Motion Detectors Work?
• What Are Pet-Immune Motion Detectors and How Do They Work?
• The Beginner's Guide to Motion Sensors

Glass break sensors protect rooms with multiple windows. They work by sensing either the vibration or sound of breaking glass. Sound-based glass break sensors are more common. 

Read more:

• How Does a Glass Break Detector Work?

Available in both wired and wireless configurations, the typical security camera only records footage when motion is detected. 

Most security cameras connect to a smartphone app via Wi-Fi so you can view live footage at any time. Recorded footage is stored in the cloud or on a microSD card. 

Read more: 

• How Do Wireless Security Cameras Work?
• Wired vs. Wireless Security Cameras
• How Do Doorbell Cameras Work?

What your security systems does in the event of an intrusion depends on whether you have professional monitoring or not. 

If your security system is professionally monitored by an alarm company, they are alerted when a security problem arises in your home. A trained security expert might attempt to communicate with the homeowner via the control panel if it’s set up for 2-way voice communication, or will call the emergency contact number listed on the account.

These types of security systems communicate with the monitoring company in one of several ways, including:

• Over existing home phone lines, which continue to work during power outages when battery backup is in use.
• Wirelessly through cellular radio frequencies like cell phones use, which also continue to work during power outages when battery backup is in use.
• Voice over Internet Protocol (VoIP), which typically doesn’t work in a power outage.
• Via the Internet, which also typically does not work in a power outage.

In the event of an actual emergency, the monitoring company will notify the appropriate emergency response personnel in your area. 

Read more:

• What Is Alarm Monitoring and How Does It Work?
• How Does the Police Response Work?
• How Long Does It Take My Security System to Notify My Provider of an Intrusion?

There are plenty of DIY security systems available today that don’t include professionally monitored services. In the event of a home intrusion when this type of security system is installed, a high-decibel alarm sounds (provided one is installed). Contacting police, fire, or other emergency response personnel must be initiated by the homeowner by dialing 911.

Read more:

• What Is the Difference Between Monitored and Unmonitored Security Systems?
• Pros and Cons of Self-Monitoring Security Systems

• Do I Really Need a Security System?
• Are Security Systems and Alarm Monitoring Worth It?
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• Home Safety and Security Checklist
• The Apartment Security Guide for Renters