A method is provided for more efficiently transmitting data from a server computer to a client computer over a communications network, the nodes of which are capable of providing two classes of transmission quality. Data is transmitted using the high priority class until a local cache at the client computer is filled to a certain upper threshold, at which point the data is transmitted using the low priority class. If the local cache at the client computer subsequently reaches a lower threshold then the transmitted reverts to the use of the high priority class. The communications network may be connection-based (e.g. ATM) or connection-less (e.g. the Internet).