From OpenSCADAWiki
The protocol supports the ability of the traffic compression. Only the Control Interface data command of the control interface and result of the command of the control interface are compressed. The fact of receipt of a compressed request or response is determined by the negative value of the req_size or the rez_size response. You can define the compression options on the configuration page of the module (Fig. 1). Response of the protocol input part is forced to the compression with level -1 (that is optimal) at the compressed requests.