طول موج و زمان بهره برداری حوزه مدیریت QoS در سوئیچ های بسته نوری

This paper addresses the problem of congestion resolution and quality of service differentiation in optical packet switching. The paper shows that by designing congestion resolution algorithms that combine the use of the wavelength and the time domain it is possible to significantly reduce information loss phenomena and also to guarantee quality of service differentiation among traffic classes. In particular this is achieved by means of QoS algorithms specifically designed to exploit the characteristics of optical technology. The results are different from the QoS techniques typically implemented in electronic networks. Performance evaluation obtained by simulation shows the influence of the main system parameters on packet loss probability and delay for two service classes.

The combination of all-Optical Packet Switching (OPS) with the very high capacity of DWDM optical fibers promises to be the most powerful and flexible networking technology developed so far. OPS is still a medium-to-long term networking solution because of the still immature state of technology and of the current difficulties in the telecommunication market, but it represents an important research topic for the scientific community because of its huge potential. Several research projects, such as the UE funded KEOPS and DAVID [1], [2] and [3], demonstrated the feasibility of switching matrixes able to switch all-optical packet payloads. In these cases synchronous switching operation is considered in order to simplify the switching matrix design. The drawbacks of this approach are the need of all-optical synchronization, implying a substantial increase in hardware complexity, and the inefficient interworking with network protocols based on variable-length packets, such as IP. The adoption of asynchronous and variable-length optical packets overcomes these drawbacks at the cost of a more complex switching matrix design both from the physical and from the logical point of view [4]. In the last few years, several important studies have analyzed this scenario and investigated the related switching control problem and optical queuing performance [5], [6], [7] and [8]. In this context congestion resolution becomes one of the most critical issues. Optical memories are not available and congestion resolution in the time domain can usually be implemented by means of Fiber Delay Lines (FDLs), emulating a normal queue by means of lengthy fiber coils used to delay the signal. FDL buffers are not very effective in this scenario, both because of the very limited capacity and because of the mismatch between the variable packet sizes and the fixed length of the FDLs. In spite of these limitations, this paper shows how congestion can be effectively limited to tolerable values by designing suitable congestion resolution algorithms that combine the use of the time and the wavelength domain, and how such algorithms can also provide support for quality of service differentiation. Some heuristic congestion resolution algorithms proposed in previous works for connectionless operation [9] and for connection-oriented operation are reviewed here [10]. Then we propose extensions to these congestion resolution algorithms in order to manage the quality of service in an all-optical packet-switched network scenario, according to the Differentiated Service model [11]. In particular, it has to be taken into account that in an optical network scenario QoS management cannot be performed by means of queuing priorities or fair queuing scheduling algorithms [12], because of the very limited queuing space and, most of all, because queuing by delay lines does not allow a new incoming packet to overtake other packets already queued, a function that is necessary to implement conventional queuing priorities. Under these considerations we aim at showing that QoS differentiation among different traffic classes can be provided with good flexibility and very limited queuing resources, by means of resource reservation both in the time and in the wavelength domain. The paper reviews the overall problem of congestion resolution in OPS with variable length packets in Section 2, providing related results in Section 3. Then in Section 4 the problem of QoS management is addressed and results are presented in Section 5. In Section 6 some conclusions are drawn.

Congestion resolution algorithms able to support quality of service in DWDM optical packet-switched networks have been considered and analyzed in both connectionless and connection-oriented contexts. Good differentiation and QoS tuning capabilities with two classes of service have been shown, although performance of the two classes is correlated. QoS differentiation is achieved by means of two techniques, the former reserving resources to the high priority traffic in the delay queue (threshold-based) the latter reserving resources in the wavelength domain (wavelength-based). The threshold-based technique achieves good discrimination between the two classes in terms of packet loss while naturally limiting the delay of the low-priority class that, on the other hand, suffers the highest loss. The wavelength-based technique achieves similar performance and is also more effective for finer QoS tuning. Some effects of the algorithms on delay distribution have also been shown and discussed.