isdn offers a number of data channels, and a signalling channel. the data channels carry the actual service data. the two data channels are completely independent and can be used independently. the usual setup has two such data channels. that way, it is possible to send a fax through one channel, while doing a phone call over the other channel, for example. if the main focus is on a high quality voice connection, the two channels can be combined into one single channel carrying twice the amount of a single channel enhancing the quality dramatically.