VESDA Air-sampling Smoke Detectors work by:

• Continually drawing air into a pipe network attached to a detector unit.
• Passing the air through a dual stage filter to remove dirt
• Sending the clean air to a laser detection chamber for smoke detection.
• Measuring the light scatter caused by any smoke
• Processing the detector signal and presenting the smoke level graphically
• Communicating the information to a fire alarm control panel, a software management system or a building management system.

Here is a fantastic Quick Time Video from Xtralis on how VESDA works.