The diagram below shows asynchronous traffic being transmitted across the ISDN network to a device that understands asynchronous data (e.g. an asynchronous port on a router providing Internet access at an ISP).

In order to allow the asynchronous traffic to pass across the ISDN, the terminal adapter (TA) between the PC's serial port and the network must fill all the empty gaps in the asynchronous data stream with something.

In addition, it must fill in the difference between the speed of the serial port and the speed of the B channel. This process is sometimes called bit stuffing because extra bits are inserted into the data stream to fill it out to 64kbps.