Asymmetrical Rake-like Sdh Framing Structure

The frame-alignment monitoring procedures have been subject of extensive studies over the past decades, first to mention an excellent tutorial [1], together with the numerous theoretical analyses [2-6], with the last set of papers considering the time-variant channels [7-9]. When it seemed that nothing more could be contributed, a novel set of Recommendations for Synchronous Digital Hierarchy (SDH) [10] has appeared. The most important innovation is that Frame-Alignment Word (FAW) length need not be necessarily the same as the length of the inserted framing signal, as [10] emphasizes the possibility of splitting the framing signal into shorter FAWs. The procedure of splitting the frame-alignment signal into FAWs considerably improves synchronization holding time (time between the two consecutive false alarms), keeping out-of-frame (OOF) detection time (time between the real synchronization loss and its notification), if the FAWs are not too short, unchanged [12]. However, long framing signal has more possibilities for multiple checks within a single frame. But, it might cause infinite out-of-frame (OOF) detection time if non-optimal sequence is applied. This procedure cannot be applied straightforwardly to STM-M synchronization signal, as its repetitive structure prevents the detection of synchronization loss at certain slip positions. In this paper a method for avoiding this problem is proposed and applied to the existing SDH framing sequences – a rake-like method of synchronization signal splitting is proposed, enabling the usage of short FAWs and/or multiple checks within a single frame in the case of long framing signal, that guaranties longer synchronization holding time.