Abstract

Clock rate adaptation in digital transmission networks such as synchronous optical network (SONET) and synchronous digital hierarchy (SDH), is necessary for two reasons. Firstly, it provides timing transparency for tributaries that are multiplexed at different levels of PDH (pulse stuffing); secondly, it provides timing transparency for transporting the tributaries within the STM frame of the SDH (pointer adjustment). The clock rate adaptation circuits of network synchronizers work by controlling the output data rate by pulse stuffing. The technique of stuff threshold modulation (ST-Mod) is usually used to reduce the waiting time jitter resulting from to pulse stuffing. For a given set of data transmission conditions, the selection of a suitable ST-Mod dither sequence is addressed. It has been shown previously that if the ST-Mod sequence is selected as a sawtooth sequence then it is possible to choose a dither sequence which is optimum for the given operating condition. It is shown that using either a random sequence or a modified wave sequence it is possible to obtain a near optimal filter performance for given synchronizer-desynchronizer (SYN-DES) characteristics.

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