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The PRI U-Interface

Primary Rate ISDN systems always interconnect at the U-Interface. North American systems transmit a full-duplex signal of 1.544 MBPS. This includes 23 B-Channels (1472 KBPS), 1 B-Channel (64 KBPS), and a framing/synchronization pattern (8 KBPS).

International ISDN systems transmit a full-duplex signal of 2.048 MBPS. This includes 30 B-Channels (1920 KBPS), 1 B-Channel (64 KBPS), and a framing/synchronization pattern (64 KBPS).

North American T1 PRI Line Coding

In North American Primary Rate ISDN systems, transmission of T1 signals normally occurs over Twisted-Pair cable using one of two types of line coding - AMI or B8ZS. B8ZS is more commonly used in this application. At the DSX-1 (Digital Cross-Connect Level 1) interconnection, a typical pulse amplitude of +/- 3V Peak-Base is transmitted. A termination impedance of 100 ohms is used. Line coding is defined in ITU Recommendation G.703 and a variety of US standards.

International E1 PRI Line Coding

International E1 systems utilize HDB3 (High Density Bipolar 3) line coding. There are two kinds of transmission media used to transmit E1 signals - Coaxial cable and Twisted-Pair cable.

With Coaxial transmission, a termination impedance of 75 ohms is used. The transmission is unbalanced, and the nominal pulse amplitude is 2.37 V Peak-Base. With Twisted-Pair cable, the transmission is balanced and terminated with a 120 ohm impedance. The nominal pulse amplitude with Twisted-Pair transmission is 3 V Peak-Base. Both transmission schemes can accomodate loss up to 6 dB.

Both transmission schemes are defined in ITU Recommendaton G.703. When interfacing to E1 systems, it is important to identify the type of transmission being used (balanced/unbalanced) and the type of connectors and cables required for the equipment interconnection.