Starting in the bottom right-hand corner, the data leaving the PC is converted into sounds by the modem. We now have data encapsulated in sounds. These sounds cross the PSTN network until they are encapsulated inside a bit stream by the CODEC at the boundary between the ISDN and the PSTN. This bit stream then passes through the ISDN network to the Terminal Adapter, which contains a CODEC that converts the bit stream back into sounds. These sounds are sent to the modem at the top left, that converts this back into the original data.

The process runs in the opposite direction to send data from the PC in the top left-hand corner to the PC in the bottom right-hand corner. This appears to be a lot of work: the data sent across the ISDN has been encapsulated twice. Click here to see why this is necessary.