In this paper, we derive the optimal packet length for automatic repeat request (ARQ) protocols and hybrid ARQ I (HARQ I) protocols with chase combining. Chase combining consists of a maximum ratio combination of received packets. The average signal-to-noise ratio is measured at the receiver and sent back to the transmitter to adapt packet length. The study is valid for any M-QAM (quadrature amplitude modulation). HARQ I protocol uses a convolutional encoder. Simulation results have shown that the proposed adaptive link layer protocol using optimal packet length allows 0.7–4 dB gains with respect to fixed packet length.