Abstract: In one general aspect, data transfer between the modem and a processor connected to the modem may be improved using a variable speed buffer. The variable speed buffer may include one or more first-in/first out buffers (FIFOs) and two interfaces. The FIFOs may be connected between the interfaces to form an input data path and an output data path. Each FIFO may store a complete message. Consequently, an entire message may be transferred from the processor at the processor's higher data rate without having to wait for a modulator to modulate the outgoing message at a slower data rate. The modem automatically may use characteristics of an incoming signal to detect which communications protocol is used to send a message, and perform protocol specific functions. The modem may perform system diagnostic functions to improve system performance.