Integrated Services Digital Network (ISDN) is a legacy set of telecom standards for carrying digital voice and data over the telephone network. It came in two main sizes: the Basic Rate Interface (BRI), for small sites, and the Primary Rate Interface (PRI), used mainly to connect business phone systems to the carrier. ISDN is no longer a technology buyers choose for new projects. Most businesses meet it today as PRI circuits still feeding an older PBX, or as a carrier notice that ISDN service is being retired, and the practical question is how to replace it.
At a glance
- ISDN delivers digital voice channels plus a separate signaling channel over a circuit-switched connection.
- BRI carries two 64 kbps channels plus signaling; PRI carries 23 channels plus signaling on a North American T1, or 30 on a European E1.
- It is a legacy service: many carriers have withdrawn BRI and some are retiring ISDN altogether, on timelines that vary by country and carrier.
- SIP trunking and cloud phone systems are the usual replacements for voice; a gateway can keep an older ISDN-only PBX working during the transition.
- Phone numbers can usually be ported off ISDN to the replacement service.
What problem it solves
Before ISDN, business phone lines were mostly analog. Each line carried one call, and the signaling for that call, such as dialing and ringing, shared the same channel as the voice. ISDN gave the customer a digital connection into the telephone network, carried several calls on one physical connection, and moved signaling into its own channel. Calls could still reach ordinary analog lines at the far end; the network converted them as needed. That brought faster call setup, caller ID and direct inward dialing on one circuit, and for a period it was a common way to get dial-up data and backup links faster than an analog modem.
For today’s buyer, the problem ISDN poses is its replacement. As carriers retire circuit-switched networks and the copper behind them, ISDN lines become harder to repair and more expensive to keep. A planned move off ISDN avoids being forced into a rushed migration when a retirement date arrives.
How it works
Channels. ISDN splits a connection into bearer (B) channels that carry voice or data, each at 64 kbps, and a data (D) channel that carries signaling. The channels are delivered using time-division multiplexing (TDM), where each call gets a fixed time slot.
BRI. The Basic Rate Interface provides two B channels and one D channel. It was used for small offices, home workers, videoconferencing and backup data links. In many markets it has already been withdrawn.
PRI. The Primary Rate Interface provides 23 B channels plus one D channel over a T1 line in North America, or 30 B channels over an E1 in much of the rest of the world. It connects a PBX to the public switched telephone network and is the form of ISDN businesses most often still have.
Signaling. Because signaling travels on the D channel, ISDN can pass caller ID, dialed digits and call status quickly and separately from the voice path.
Physical delivery. ISDN usually runs over copper from the carrier, sometimes on fiber or multiplexed onto larger circuits. Copper retirement is one of the forces pushing carriers to end it.
Replacement path. SIP trunking replaces the PRI with voice channels delivered over an IP connection. If the PBX only has ISDN ports, a gateway or session border controller converts between ISDN and SIP until the PBX itself is replaced, often by a cloud phone system.
For help planning the move, see our PRI replacement solution page.
When it matters for buyers
- A carrier retirement notice. Find out the dates, what replacement the carrier proposes and whether pricing changes before then.
- Inheriting an older site or company. ISDN lines, especially BRI used for backup or alarm purposes, are easy to overlook in inventories.
- Replacing a PBX. Retiring the PBX and the ISDN together usually costs less effort than replacing them separately.
- Lines used for more than voice. Some ISDN lines carry broadcast audio, alarm, elevator or point-of-sale traffic; each needs its own replacement plan.
- Renewals. Avoid long new commitments on ISDN service that may be retired before the term ends.
Questions to ask vendors
- Which of our circuits are ISDN (BRI or PRI), and what are your retirement plans and dates for each?
- What replacement do you propose, and how does its price compare with what we pay now?
- Can we port every number on the circuit, including the billing number, to the new service?
- If we keep our PBX, which gateway or SBC do you support for converting ISDN to SIP?
- How will calls behave during cutover, and what is the rollback plan?
- Are any of our ISDN lines used for alarms, elevators, fax or data, and how will those be handled?
How it differs from PRI
ISDN is the family of standards; PRI is one interface within it. “ISDN” covers both the small Basic Rate Interface and the larger Primary Rate Interface, plus the signaling rules they share. When a business says it has “a PRI”, it has ISDN at the primary rate, usually delivered on a T1 or E1 to its PBX. Replacing a PRI is the most common ISDN replacement project.