The dialup modem's in-built compression (V.44 or V.42bis) can only occur after the data has been serially transmitted to the modem, typically at a maximum rate of 115,200 bit/s.
The dial-up modem's internal compression (V.44 or V.42bis) is activated only after the data has been sent sequentially to the modem, with a maximum speed of 115,200 bits per second.