ISDN Interfaces (BRI/PRI)

ISDN Access Interfaces

B and D channels describe the transport and signaling functions of ISDN. In practice, user access was defined through two main interfaces: Basic Rate Interface (BRI) for a smaller number of channels and Primary Rate Interface (PRI) for a larger number of channels. They differ in channel structure, physical layer, and device connection.

Basic Rate Interface (BRI)

BRI was designed as a digital successor to a single analog telephone line. It defines a structure of two B channels and one D16 channel. On the U section, this structure is transported over one copper pair, while the user-side S/T interface uses two pairs.

2B + D₁₆

Structure: two 64 kbit/s bearer channels and one 16 kbit/s D channel

Physical BRI configurations

In an installation with NT2, TE terminals connect to NT2 through the S interface, and NT2 connects to NT1 through the T interface. When NT2 is absent, the S and T points coincide at NT1. Each configuration has its own cable-length limits:

Point-to-Point: In this setup, only one terminal (TE) is connected to the NT1. This configuration allows for the longest cable runs, up to 1 kilometer, as signal quality degradation is minimal.
Short Passive Bus: This is a multipoint configuration where up to 8 terminals can be connected in parallel at different points along the bus. Because of signal reflections, the total length is usually 100-200 meters: about 100 m for 75 Ω cable or 200 m for 150 Ω cable. The bus is terminated at both ends with 100 Ω resistors to limit reflections.
Extended Passive Bus: A variation where typically up to 4 terminals are grouped at the far end of the line. The objective is a total length of at least 500 meters, with connection points arranged at intervals of about 25-50 m. The administration specifies the exact combination of length, spacing, and terminal count.

Primary Rate Interface (PRI)

PRI was designed to serve as a digital trunk line, connecting large corporate phone systems () or data networks to the public network. Its structure is based on the pre-existing digital transmission hierarchies used in different parts of the world.

European Standard (E1-based)

30B + D₆₄

Thirty 64 kbps B channels.
One 64 kbps D channel for signaling.
Total Bit Rate: $2.048 \text{ Mbit/s}$ .

North American and Japanese standard (T1-based)

23B + D₆₄

Twenty-three 64 kbps B channels.
One 64 kbps D channel for signaling.
Total Bit Rate: $1.544 \text{ Mbit/s}$ .

Physical connection

PRI is a point-to-point interface. It uses a four-wire connection between the network, for example a central office, and one NT2 device such as a PBX or router. This dedicated link carries multiple B channels and one D channel for telephony and data services.

BRI and PRI compared

Feature	BRI	PRI (Europe/E1)	PRI (North America/T1)
User Type	Home / Small Office	Medium to Large Enterprise
Structure	2B + D16	30B + D64	23B + D64
Total Data Rate	128 kbps	1.920 Mbps	1.472 Mbps
Total Bit Rate	144 kbps	2.048 Mbps	1.544 Mbps
Physical Config	Point-to-point or Multipoint Bus	Point-to-Point Only