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, bi-directional 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 techniques are used. The range of operation of ISDN is dictated by both attenuation and self near-end cross talk (NEXT) from adjacent 2B1Q ISDN signals. The 2B1Q line code is sometimes referred to as a base band 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 reserved 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 though a local digital switch that also terminates voice lines. QAM Quadrature Amplitude Modulation (QAM) utilizes amplitude and phase modulation to transmit multiple bits per baud. Unmodulated signal exhibits only two possible states allowing us only to transmit a zero or a one. With QAM, it is possible to transmit many more bits per state, as there are many more states. This scheme utilizes a sign...
