PRI Circuit
A PRI circuit, or Primary Rate Interface circuit, is a digital telephone connection based on ISDN technology that allows multiple voice or data calls to travel over a single line. In North America, an ISDN PRI circuit typically uses a T1 connection with 23 B channels and one D channel, often described as 23B+D. Each B channel operates at 64 Kbps, while the D channel handles signaling for the PRI line.
Businesses have traditionally used PRI circuits to connect a PBX or PRI phone system to the public telephone network. A single PRI can support multiple simultaneous incoming and outgoing calls along with features such as Direct Inward Dialing, caller identification, and call routing.
What Is a B Channel?
A B channel, short for Bearer Channel, carries the actual voice or data traffic on a PRI circuit. A standard North American PRI provides 23 B channels, with each channel operating at 64 Kbps. In a typical voice configuration, each B channel can support one active call, allowing up to 23 simultaneous calls on a full PRI circuit.
What Is a D Channel?
The D channel, or Delta Channel, carries the signaling information that controls the B channels. Rather than carrying the conversation itself, the ISDN D channel communicates information needed to establish, route, manage, and disconnect calls.
On a North American PRI, the D channel typically operates at 64 Kbps and uses the 24th T1 timeslot. One shared D channel manages signaling for the circuit’s B channels.
What Is NI2?
NI2, or National ISDN 2, is a North American ISDN signaling standard used with PRI circuits. It was created to provide a more standardized way for PRI equipment and telephone company switches to communicate, reducing differences between carrier-specific ISDN implementations.
When a PRI uses the National ISDN switch type, NI2 applies to PRI service, while NI1 is associated with Basic Rate Interface, or BRI, service.
PRI circuits may also be called PRI lines, ISDN PRI, PRI T1s, PRI phone lines, or Primary Rate Interface circuits.
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