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The Basic Rate ISDN S/T-Interface is a 4-wire, full-duplex interface. When interconnecting multiple devices to a BRI line, this S-bus interface must be used. The S-bus is capable of terminating up to eight devices.
The S-bus is a 4-wire interface that transports full-duplex data streams of 192 KBPS with loss up to 6 dB. The interconnections used are 8-pin modular, RJ-45 jacks. A special line code, called ASI (Alternate Space Inversion) is employed on the line, resulting in a maximum line frequency of 96 KHz.
ASI is sometimes referred to as Modified Alternate Mark Inversion (MAMI). It is described in ITU Recommendation I.460.
Operation of the S-bus is full-duplex, 4-wire, at 192 KBPS.
Described in ITU Recommendation I.430, these frames (appropriately called "I.430 Frames") are 48 bits in length. The frame construction is different depending upon the direction of the signal (NT-1 to Terminal, or Terminal to NT-1).
A terminal device with nothing to send enters a "marking" state, presenting zero voltage on the line. This is one of the reasons that ASI coding is used on the S-bus interface.
Since the switch assigns B-Channel use, the only contention that occurs between terminals on the S-bus interface is for the D-Channel. The D-Channel transports the bits that make up the signaling and control messages for ISDN systems.
The D-Channel operates using CSMA/CR (Carrier Sense Multiple Access/Collision Resolution) techniques, a priority-based access system. The transmitting terminal checks for glare conditions by comparing transmitted D-Channel bits with the Echo-Channel bits from the NT-1.
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Last Revised on: Monday, 25-Oct-2004 19:46:22 EDT