The small-office and home form of a digital telephone interface. The line is part of an Integrated Services Digital Network (ISDN). Two 64 kbps bearer channels and one 16 kbps signaling channel make up the line, 144 kbps in total.
One bearer can carry a call while the other carries data or a fax. The larger carrier and business interface is the Primary Rate Interface (PRI).
Channel Layout and Rates
Each channel has a fixed rate. Adding the three rates gives the 144 kbps total for the whole line, not for a single bearer.
- Bearer channels: Two channels, each 64 kbps. Either one can hold a voice call, a data session, or a fax.
- Signaling channel: One 16 kbps channel that sets up and clears the connections on the bearer channels.
- Combined rate: 64 kbps plus 64 kbps plus 16 kbps is 144 kbps.
How a Call Shares the Line
A small office or a home can keep a conversation on one bearer and put data or a fax on the other at the same time. The signaling channel places and clears those connections and does not take the place of either bearer.
When both bearers are already in use, this interface has no third bearer. That limit is why the form fits a small office or a home rather than a carrier or a larger business.
Basic Rate and Primary Rate
Both are digital telephone interfaces. The basic-rate form is the small-office and home line, with two bearer channels. The primary-rate form is the larger carrier and business interface for sites that need more than that pair.
| Trait | Basic-Rate Interface | Primary-Rate Interface |
| Setting | Small office and home | Carrier and larger business |
| Bearer channels | Two, each 64 kbps | A larger set than the basic-rate pair |
| Signaling channel | One at 16 kbps | Handled inside the larger interface |
| Combined rate | 144 kbps | More capacity than the 144 kbps basic-rate total |
| Simultaneous use | One bearer on a call while the other carries data or a fax | Sized for a carrier or a larger business, not a two-bearer desk line |