ISDN2 and ISDN30

Integrated Services for Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network

The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI),

ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s

Primary Rate Interface

The other ISDN access available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 ‘B’ channels of 64 kbit/s, one ‘D’ channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s.

In North America PRI service is delivered on one or more T1 carriers (often referred to as 23B+D) of 1544 kbit/s (24 channels). A PRI has 23 ‘B’ channels and 1 ‘D’ channel for signalling (Japan uses a circuit called a J1, which is similar to a T1). Inter-changeably but incorrectly, a PRI is referred to as T1 because it uses the T1 carrier format. A true T1 (commonly called “Analog T1” to avoid confusion) uses 24 channels of 64 kbit/s of in-band signaling. Each channel uses 56 kb for data and voice and 8 kb for signaling and messaging. PRI uses out of band signaling which provides the 23 B channels with clear 64 kb for voice and data and one 64 kb ‘D’ channel for signaling and messaging. In North America, Non-Facility Associated Signalling allows two or more PRIs to be controlled by a single D channel, and is sometimes called “23B+D + n*24B”. D-channel backup allows for a second D channel in case the primary fails. NFAS is commonly used on aT3.

PRI-ISDN is popular throughout the world, especially for connecting PBXs to PSTN.

While the North American PSTN can use PRI or Analog T1 format from PBX to PBX, the POTS or BRI can be delivered to a business or residence. North American PSTN can connect from PBX to PBX via Analog T1, T3, PRI, OC3, etc…

Even though many network professionals use the term “ISDN” to refer to the lower-bandwidth BRI circuit, in North America BRI is relatively uncommon whilst PRI circuits serving PBXs are commonplace.

Bearer channels

The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.

Each one of these channels is known as a DS0.

Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was commonplace in the 20th century, but has since become less so.

Signaling channel

The signaling channel (D) uses Q.931 for signaling with the other side of the link.

United Kingdom

In the United KingdomBritish Telecom (BT) provides ISDN2 (BRI) as well as ISDN30 (PRI). 

In ISDN, there are two types of channels, B (for “bearer”) and D (for “data”). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).

There are two ISDN implementations. Basic Rate Interface (BRI), also called basic rate access (BRA) — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary Rate Interface (PRI), also called primary rate access (PRA) in Europe — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1).



  • ISDN2 is also called Basic Rate Interface or BRI for short.
  • ISDN30 is also called Primary Rate Interface or PRI for short.

ISDN2 has a minimum installation of 2 channels
ISDN30 has a minimum installation of 8 channels.

ISDN2 doesn’t have a maximum number of channels, but rarely goes over 8. This is because at 8 channels, ISDN30 becomes cheaper. The only exception might be in more remote locations where ISDN30 isn’t available.

So typically if you want 2/4/6/8 channels you’ll go for ISDN2, if you want 8 or more channels you’ll go for ISDN30. Simple*.

The functionality of ISDN2e and ISDN30 is very similar. ISDN2e is supplied in multiples of two lines. You can expand further in multiples of two, but each expansion requires a visit from BT (two week lead-time) plus system programming. If you require 6 lines or less on your telephone system and it is likely that this figure will not increase, then ISDN2e is usually adequate. For larger organisations, the correct solution is ISDN30. ISDN30 is delivered as ‘a site connection’, you merely specify how many lines you want (eight is the minimum). One advantage of ISDN30 is the use of fibre optic cable, which is more reliable than traditional copper wire. (note ISDN30 is quite often now supplied on copper).


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