The larger digital telephone interface used by organizations and carriers on an Integrated Services Digital Network (ISDN). Many calls share one circuit instead of each call taking a line of its own.
In North America it has 23 bearer channels of 64 kbps plus one signaling channel. In Europe it has 30 bearer channels plus one signaling channel. The two-bearer form for a home or small office is Basic Rate Interface (BRI).
How the Channels Are Split
Each bearer channel carries one voice call, or a data stream, at 64 kbps. The signaling channel does not carry the conversation. It carries the digits dialed and the messages that start and end the call, so setup does not occupy a bearer channel.
- Bearer channel: Holds one phone call or a data stream at 64 kbps.
- Signaling channel: Handles dialing, ringing, and release beside the bearer channels.
- Regional count: 23 bearer channels in North America and 30 bearer channels in Europe, plus one signaling channel in either case.
Where the Larger Trunk Is Placed
Organizations and carriers use this larger trunk when a site must handle many calls at once. A main number can take any free bearer channel, and a busy channel does not block the calls already connected. When every bearer channel is busy, a new call cannot connect until one frees. A home or small office, which needs only two calls active together, uses the two-bearer form instead.
Primary Rate Compared with Basic Rate
Both forms pair bearer channels with a single signaling channel. They differ in how many bearer channels they provide and in the site they serve.
| Interface | Typical Use | Bearer Channels | Signaling |
| Primary rate | Organizations and carriers | 23 in North America, each 64 kbps, or 30 in Europe | One signaling channel |
| Basic rate | A home or small office | Two | One signaling channel |