ISDN Interfaces (BRI/PRI)

Basic Rate Interface (BRI) and Primary Rate Interface (PRI): structures, configurations, and applications.

Beyond the Basics: How Interfaces are Delivered

While the concept of B and D channels defines what ISDN offers, the real-world implementation depends on two distinct access interfaces: the Basic Rate Interface (BRI) for small-scale needs and the Primary Rate Interface (PRI) for high-capacity corporate and enterprise use. These interfaces differ not only in their channel capacity but also in their physical configuration and intended application.

Basic Rate Interface (BRI) – Digital Access for Homes and Small Offices

BRI was designed to be the digital upgrade to the single analog telephone line, providing significantly more capability over the same copper pair.

2B + D₁₆

Structure: Two 64 kbps Bearer channels and one 16 kbps Delta channel

Physical Configurations of BRI

The connection between the NT1 (network termination at the customer's premise) and the terminals (TE) over the S/T interface can be arranged in several ways, each with different distance limitations:

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 anywhere along the bus. Because of signal reflections, the total length is limited to 100-200 meters. The bus must be terminated at both ends with 100 Ω resistors to prevent echoes.
Extended Passive Bus: A variation where up to 4 terminals are clustered together at the far end of the line. This allows for a longer total bus length of up to 500 meters, but the terminals must be within a short distance (20-25m) of each other.

Primary Rate Interface (PRI) – The High-Capacity Enterprise Solution

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/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

Unlike the flexible BRI configurations, a PRI connection is always point-to-point. It uses a four-wire connection between the network (e.g., central office) and a single NT2 device (like a large PBX or router). This dedicated, high-capacity link is the foundation for corporate telephony and data services.

BRI vs. PRI at a Glance

Feature	BRI	PRI (Europe/E1)	PRI (North America/T1)
Target User	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