Figure 2,19 illustrates the architecture for matched nodes between MS-SPRings. The two interconnecting nodes in each MS-SPRing are designated as primary and secondary interconnection nodes. The originating node sends the signal to the primary interconnection node, at which the signal is dropped and then continued on to the secondary interconnection node. Each interconnection node sends the signal to the other MS-SPRing. The secondary interconnection node sends the signal to the primary interconnection node, which is then able to select from two signals, one from the first MS-SPRing and one from the secondary interconnection node in the second MS-SPRing. Again in the case where the primary and the secondary nodes are not adjacent nodes on the ring, the secondary node needs to transmit the signal to the primary node making sure that the destination node is not on this path. Then the primary interconnection node sends the signal to the destination, avoiding the secondary interconnection node.
The case of ring architectures interconnecting with mesh DCS architectures via matched nodes has not been addressed by the standards bodies yet. In practice though, a ring interconnects with a mesh via matched nodes using the drop and continue feature again whereas on the mesh side the two nodes establish a link between them and then route the traffic according to the routing algorithm of the restoration mechanism of the mesh.
2.13 Mesh DCS Architectures vs Resilient Ring
Architectures
Comparisons of survivability and efficiency between resilient ring architectures and DCS mesh networks have been effected [53,107,108,109]. This followed the research on the definition and development of the two different SDH network architecture approaches [109]. Debates about the advantages and disadvantages of one or the other network solution have not yielded conclusive results. Depending on the network planning case, and a large number of network and traffic characteristics one or the
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other solution might be more appropriate. In most real network implementation though a combination of rings and meshes exist.
Resilient ring architecture network solutions can generally provide better availability than mesh DCS networks. The restoration ratio for single failures is always 100% which is not necessarily the case for mesh DCS networks. The restoration time in rings can be orders of magnitude faster than the one in mesh DCS networks, resulting in better availability figures. In rings, the restoration capacity is available on deployment and does not require any separate planning steps. An additional disadvantage of mesh DCS networks, is that protection is still an immature topic and is not standardised yet. Management, routing and operation functions are also much easier to perform in a ring networks than in mesh DCS networks. Less complexity exists, due to the relatively simple architecture of a ring. Mesh DCS network solutions are more complex and less compatible with the SDH add and drop concepts.
Network costs, one of the key issues in network planning, is an important parameter in the comparison between mesh DCS networks and rings. In rings less fibre and less ducts are usually required than in a mesh DCS network. Ring topology provides full connectivity with minimum fibre length. In terms of bandwidth costs though, mesh DCS networks, depending on the protection required and the traffic characteristics, can offer significant savings in capacity requirements. For ring architectures, traffic characteristics have a critical effect on their capacity utilisation performance. The development of Wavelength Division Multiplexing (WDM) [110] in SDH networks [111-115], with appropriate routing [116], could reduce the bandwidth costs and minimise the importance of these parameter in the choice between ring and mesh architectures. Particularly interesting is work which is carried out, mostly in France Telecom labs, regarding WDM-rings, based on the concepts of resilient rings, that are called Coloured Section Rings (CSRs). This work would allow for resilient ring architectures, based on WDM technology, with similar characteristics to resilient rings in terms of protection which achieve significant capacity savings [117-121].
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However, for current network planning cases, the differences in capacity utilisation performance, if any, can be critical in choosing among different network solutions. Therefore, traffic characteristics maybe the predominant issue in choosing the appropriate solution.
2.14 Summary
In this chapter an introduction to the SDH technology has been presented. We explained the need for evolution in telecommunication networks arising from the advent of SDH. The characteristics of the PDH and SDH multiplexing hierarchies have been discussed. A comparison between PDH and SDH and the benefits of SDH have been discussed.
A description of the SDH architectures has been presented and protection in telecommunication networks and how it applies to SDH networks has been discussed. Mesh DCS networks, their protection schemes and their characteristics have been presented and resilient ring architectures and their properties were explained in detail. The possible architectural choices in SDH network planning have been discussed and it has been noted that protection is something that will be increasingly required in all future telecommunication networks. The choice between the candidate survivable architectures though is a very difficult (and usually case dependent) task.
The advantages and disadvantages of choosing one or the other SDH network solution were discussed. One of the key factors in the capacity utilisation performance of resilient ring architectures, that depends on the characteristics of each single network planning case, is the characteristics of traffic. Therefore it is essential to understand some traffic issues in telecommunication networks and how these affect the performance of SDH networks. Having established this background of SDH network technology, the next chapter presents the work of modelling the traffic birth and traffic distribution analysis process.