Cancelation of pointer adjustment jitter in SDH networks

Abstract

The timing jitter induced by pointer adjustments in the basic STM-1 frame represents a serious technical problem in SDH-based networks. This paper describes two jitter reduction techniques to cope with this phenomenon. The first technique is based on digital phase-lock loop (PLL) theory, and obtained through two structural modifications of a previously proposed desynchronizer. The second technique is entirely novel, and it avoids the generation of the random noise with uniform probability density, which is required in PLL-type desynchronizers to smooth the 1-b phase steps at the output of the first stage. We describe two different methods to adapt the speed of this desynchronizer to the incoming pointer adjustment statistics. The performance of both jitter reduction techniques is investigated in both the normal mode and the degraded mode of operation of the network. Using a design example, it is shown that the peak-to-peak jitter in the presence of isolated pointer adjustments that characterize the normal operation mode is kept below 0.1 b. It is also shown that with frequency offsets up to 4.6 ppm in the degraded mode, the peak-to-peak jitter does not exceed 0.6 b. Lower jitter values are achievable if the complexity and memory requirements of the desynchronizer are allowed to increase. >