ing of two "B" channels of 64kbps each and one packet data or "D" channel of 16kbps added for framing, error detection, and other overhead functions. The ISDN line of "U" interface operates at a raw data rate of 160kbps. In the mid 1980's the T1 committee in the United States created a standard U interface using a four-level line code referred to as 2B1Q for two binary bits per symbol carried by a quaternary symbol design. 2B1Q line code was designed to support ISDN transmission through loops of 18000ft or less, meeting voltage pulses of +/- 875V and +/- 2.625V. The symbol rate is 80000 baud and the energy spectrum used by ISDN peaks at 40000Hz. The ISDN signal is transmitted in full duplex mode, bidirectionally on the same pair of wires. In order to accomplish this, transceivers must contain a hybrid function to separate the two directions of transmission. To help the receiver differentiate between far-end transmission and reflections of near-end transmission from irregularities in the twisted pair transmission line due to wire gauge changes and bridged taps, echo cancellation tecniquess are used. The range of operation of ISDN is dictated by both attenuation and self near-end crosstalk (NEXT) from adjacent 2B1Q ISDN signals. The 2B1Q line code is sometimes referred to as a baseband signal because it uses energy in frequencies down to zero, overlapping with the voice frequency band. In order to carry voice through a DSL, the voice signal is digitized using PCM techniques and carried in one of the B channels. In ISDN applications the D channel is reserverd for data packets that are primarily used for call processing. In carrying simultaneous voice and data the ISDN basic rate line carries a maximum of 64kbps of data. In the absence of voice, both B channels may be bonded together to increase the data capacity to 128kbps. Both ends of an ISDN connection must use the same bonding protocol. ISDN connections are made by dialed access thoug...
