scope:

This report describes the multiplexing of ISDN Basic Rate Access signals (two 64-kb/s B-Channels and one 16-kb/s D-channel) for transmission over a DSI facility. The multiplexing method utilizes any three of 24 DS0 (64 kb/s) time slots on a channelized, byte-oriented DS1 (1.544 Mb/s) facility. These three DS0s do not need to be contiguous. The DS1 facilities must not alter the frame relationship among the three bytes being transported. This report provides bit assignments within the three time slots, labeled B1, B2, and D+ byte is of particular interest, as it not only transports the 16-kb/s D-channel, but also guarantees that DS1 requirements will not be violated by the transport of the ISDN BRA data and provides an 8-kb/s channel to carry maintenance and synchronization information. The D+ byte can convey zero byte substitution (ZBS) indications for the B1 and B2 bytes, thereby enabling ISDN clear channel transport over restricted DS1 facilities.