Asynchronous transfer mode (ATM) is now well recognised as the fundamental switching and multiplexing technique for future broadband ISDN's.As these networks will be increasingly relied upon for providing a multitude of integrated voice,data and video services, network reliability is a key concern.
In high-speed networks, a network or even a node failure can cause a large loss of data even in a short outage . It is hence imperative to make the service interruption time as short as possible. Self-healing algorithms have been proposed to achieve fast restoration from a failure , but their success greatly depends on how traffic is distributed and how spare capacity is dimensioned over the network when the failure happens . In order to offer better network survivability, it is crucial that a network manager realises a restorable traffic assignment in response to changing traffic demand and facility network configuration.
Virtual path concept
A virtual path route is established by the virtual path identifier(VPI) and the path routing table(RT).The VPI is a number contained in the cell header that identifies the assigned path of the cell.Path restoration in ATM networks is realised by redirecting cells on the failed VP to a backup VP. The VP has some unique characteristics.The most striking characteristic is the independence of route and bandwidth establishment,allowing a VP route to be established without assigning its bandwith along the path. This is not the case in STM networks where a digital path is established by assigning a time slot of the TDM frame at each cross-connect on the path, allowing only fixed bandwidth digital paths to be established.
In the following sections of the article we address the problem of an optimal
VP routing for survivable ATM networks. The objective is to find a VP
configuration and bandwidth assignment in response to a dynamic change
of network environment so that a self-healing algorithm can succeed.A higher
level of survivability is achieved by minimising the expected amount of
lost flow upon restoration from a link failure.Since the amount of lost flow
is calculated based on distributed self-healing algorithm , the solution
gives an optimal traffic distribution with the minimum service interruption.
The survivability functions are embedded at the VP and higher layers , considering the fact that path level recovery enables rapid and efficient restoration and considerably reduces the complexity of traffic management. Given a VP-level traffic demand satisfying call level QoS, the VP manager configures virtual paths so the survivability measure is optimally enchanced.The VP manager also performs fast VP restoration when a network failure happens.If the VP manager cannot maintain a survivability measure at a desired level due to a growth of traffic demand , the FN layer must initiate a facility network planning process.
A good measure of survivability should express the actual amount of damage experienced in the network , instead of using traditional reliability criteria such as global availability.A number of lost calls at the call layer of a telecommunications network or the amount of lost flow are measures that have been proposed to express the damage to the VP layer.
The article discusses the effect of virtual path routing control for survivable ATM networks that minimizes the expected amount of lost flow upon restoration from a network failure.The VP reconfiguration could be used not only to achieve a high level of network survivability after a change in the facility networks, but also to guarantee the quality of service of the lower layers in response to varying traffic demand patterns while maintaining the survivability level.
A lot of research is still being done into modelling a survivable virtual
path routing problem , taking into account several optimisation parameters,
such as the step size, and their effect in attaining the desirable survivability level.