internship.ppt

Editor's Notes

1. SDH Synchronization Method Master-slave synchronization uses a hierarchy of clocks in which each level of the hierarchy is synchronized with reference to a higher level, the highest level being the primary reference clock (PRC). Clock reference signals are distributed between levels of the hierarchy via a distribution network which may use the facilities of the transport network. The hierarchical levels are shown in the slide. The distribution of timing between hierarchical node clocks must be done by a method that avoids intermediate pointer processing. Two possible methods are recommended : Recover timing from a received STM-N signal. This avoids the unpredictable effect of a pointer justification on the downstream slave clock. Derive timing from a synchronization link that is not supported by an SDH network, for example, a PDH network. The slave clock determines the synchronization link to be used as its reference and changes to an alternative if the original link fails.
2. Synchronization network reference chain The node clocks are interconnected via N network elements each having clocks compliant with Recommendation G.813. The longest chain should not exceed K slave clocks compliant with Recommendation G.812. The quality of timing will deteriorate as the number of synchronization links increases. The value of N will be limited by the quality of timing required by the last network element in the chain. This ensures the short-term stability requirements .To determine synchronization clock specifications, the values for the worst-case synchronization reference chain are: K=10, N=20 with the total number of SDH network element clocks limited to 60. Because jitter can rise on the signal after it has passed many nodes, in some NEs down the transmission link, the synchronization signal must be recovered and reshaped. In this case a Synchronization Supply Unit (SSU) will be used. In addition to recovering and reshaping, the SSU will also able to run on its own when the reference signal (e.g. coming from the PRC) is lost, in this case, the SSU will be able to maintain the quality of the reference signal it produces by means of a holdover mode: the frequency and phase of each reference signal are stored in a memory and used to generate a reference signal locally
3. Multiplex Section Shared Protection RING The-two-fiber bidirectional multiplex section protection ring requires only two fibers for each span of the ring. Each fiber carries both working channels and protection channels. On each fiber, half the channels are defined as working channels and half are defined as protection channels. The normal traffic carried on working channels in one fiber are protected by the protection channels in another fiber traveling in the opposite direction around the ring. This permits the bidirectional transport of normal traffic. Two-fiber MS shared protection rings support ring switching only. When a ring switch is invoked, the normal traffic is switched from the working channels to the protection channels in the opposite direction. From 1 to N according to the order that they appear in the multiplex. AU-4s numbered from 1 to N/2 shall be assigned as working channels, and AU-4s numbered from (N/2) + 1 to N shall be assigned as protection channels. The normal traffic carried on working channel “m” is protected by protection channel (N/2) +m.