There are two forms in which ISDN is supplied.

Basic Rate

• Access to the network is called Basic Rate Access (BRA).
• It is provided through a Basic Rate Interface (BRI).
• This kind of interface is also called an S₀ Interface.
• There are two channels that you can use.

Primary Rate

• Access to the network is called Primary Rate Access (PRA).
• It is provided through a Primary Rate Interface (PRI).
• This kind of interface is also called an S₂ Interface.
• There are either 30 channels (most of the world) or 23 channels (North America, Japan) that you can use.