A communications system may be used for transmitting and/or receiving data over a network. The system needs to exchange capability information with other systems in the network, in order to determine which forms of communication can be mutually supported. The exchange of information includes logical expressions involving two or more parameters. This allows, for example, a system to indicate that a higher error rate will occur if the throughput is increased. In the preferred embodiment, each capability parameter is specified in terms of a range between a minimum and a maximum acceptable value.