A computer workstation runs an application 12 which generates data for transmission over a network. The workstation includes a communications subsystem 14 which is responsible for actually sending data generated by the application onto the network. The application therefore passes data 22 for sending to the communications subsystem, along with the identification 24 of the channel over which that data is to be sent, and compression information 26. In a first embodiment the communications subsystem contains a variety of compression routines 40, and the compression information indicates which of these routines is most appropriate for the data in question. In a second embodiment, the compression information is a callback function, whereby the communications subsystem can call back to the application program to perform the compression itself.