We propose to apply elementary wave packets (EWP) series to investigate femtosecond (fs) optical pulse propagation through the single resonance Lorentz model dielectric. Ortho - normalized EWP- functions on confine segment are defined as real ones. So we use real functions solution of wave equation. Then we study well-known "precursory" for physically grounded samples of fs optical pulses. That enables us to propose formula for optical data transmission velocity (ODTV). Its value does not exceed light velocity in vacuum.