ISDN service suppliers have the opportunity to supply interfaces where not all the channels are active. In most countries, when you take a Primary Rate Interface, you are charged a rental per channel. If you don't need all the channels that are available, you can ask to have some of these channels deactivated. This is known as fractional Primary Rate. The number of channels that you may request will vary according to the marketing policy of your service supplier.

What happens if you try to use more channels than your subscription permits? In ISDN, the network is the arbitrator of everything. When you want to place a call through an ISDN network, you send a request in your D channel to the network. The network will either attempt to satisfy your request or it will refuse it.

This user has only one B channel active in his subscription. He will be successful in making the first call. If he requests a second call while the first is still active, the network will reject the request.

Fractional BRI also exists, but is less common.