HP 3551A Transmission Test Set is designed for use with North American Bell standards and is capable of measuring attenuation distortion, loss, message circuit noise, noise-with-tone, noise-to-ground, & single frequency Interference while simultaneously sending tone. Both level and frequency are fully auto-ranging. The HP3551A is a rugged, portable test set that is ideally suited for basic analog tests on voice, program, and data circuits up to 50kbps.