Two alternative communication link modes, unbalanced or balanced mode, are available for the communication between the client (controlling station) and the server (controlled station). In this case, since the IEC 60870-5-104 client establishes separate TCP (point-to-point) connections towards each IEC 60870-5-104 server, the balanced mode is the most commonly used.
In the balanced link mode, both the client and the server can initiate message transfers. In the unbalanced mode, the client initiates the transmission with a request to which the server responds.