The invention relates to a digital data transmission system with a circuit-switched data connection (V.110) between a transmitting party (A) and a receiving party (B) the nominal data transmission rate of which may vary during the call. A sliding-window data flow control protocol is used on the data connection, in which a number of transmitted data frames to which the transmitting party (A) has not received an acknowledgement from the receiving party (B) is not allowed to exceed the size of the sliding window. In accordance with the invention, a unit (MSC) in the data transmission system that changes the nominal data transmission capacity (data transmission rate) of a non-transparent data connection notifies the change to the transmitting party (A) and to the receiving party (B). This information enables the receiving and transmitting parties to change the size of the sliding window in a controlled manner.