For A/T models:

The front sensor units consist of a gear-shaped sensor rotor and a sensor element. The element contains a magnet and IC. The front wheel sensors are installed on the front of the wheel knuckles. As the wheel rotates, the sensor generates a square-wave signal. The frequency increase as the wheel speed increase.

Fig 1: Identifying Anti-Lock Brake System Sensor (For A/T Models)

Courtesy of NISSAN MOTOR CO., U.S.A.