Design of 2B1Q transceiver for ISDN subscriber loops

The authors describe the design of 2B1Q transceivers based on echo cancellation techniques for integrated services digital network (ISDN) subscriber loops. The 2B1Q code has a finite spectrum at zero frequency, which results in DC wander caused by subscriber lines DC cutoff. For DC restoration, two types of transceivers are compared with regard to design and performance. One is based on a quantized feedback circuit and the other on a 1-D filter. The DC restoration circuit, an echo canceler, and other circuits are examined in sufficient detail to design efficient transceivers. Two prototype transceivers based on the above studies are fabricated, and total performance of the transceivers is measured using subscriber lines. Consequently, it is determined that transceivers using a quantized feedback circuit have better characteristics for sinusoidal cross talk (S/X) and near end cross talk (NEXT) margin