The data stream on an analogue modem is intermittent because the traffic is asynchronous - that is to say, it starts and stops.

If you watch the lights on a modem when connected to an ISP, for example, you will notice that the transmit and receive data lights are off for much of the time.

On the other hand, B channel activity on an ISDN line never stops. This is because it is synchronous. This means that there is always data flowing.

There are two approaches that are used to handle the transition between these different types of data:

• encapsulation - the asynchronous data is carried across the digital network, and is then converted back to asynchronous data on the other side of the network.
• conversion - the asynchronous data is irretrievably converted to synchronous data. This is appropriate where the destination device can handle synchronous data; this method depends on an appropriate higher level protocol such as PPP. (Strictly speaking, this is not Rate Adaptation. However, Rate Adaptation is normally performed by terminal adapters, that are often also capable of performing protocol conversion.)