In one aspect there is provided a host device having: a modem interface arranged to transmit transmission units between the host device and a modem; a communication function configured to generate primitives to establish a communication event between the host device and a remote device; a client agent connected to receive control primitives from the communication function and operable to convert the control primitives to data transmission units; a host routing interface operable to route data transmission units from the client agent according to a predetermined route option which is set based on whether a communication event control function for processing the data transmission units is located on the host device or the modem.