In this paper, we propose a new packet scheduling algorithm to minimise the size of voids in optical packet switching. The effects of excess load, output utilisation, and packet loss probability are closely studied. Other contributing factors to the packet loss probability, which include the buffer depth and the numbers of wavelength channels, are also investigated. The proposed algorithm is of importance to next generation networks where broadband capabilities with end-to-end quality of service over all-IP optical networks is envisaged. IntroductionWhen optical packet switching (OPS) was first proposed in the research community, it was hoped that it would become a remedy to the shortcoming of electronic packet switching, handling large bandwidths with minimal intelligence [1,2]. The early proposals by RACE ATMOS, ACTS KEOPS, and others that followed, were based on the assumption that ATM would be the dominant technology at the time optical packet switches were deployed. However, as time passes, ATM has not been as popular as had been anticipated. Instead, the future of ATM is being challenged by the introduction of MPLS in the WAN and Ethernet in the LAN. Consequently, optical packet switching, with the capability to handle variable-length packets, is being introduced as an alternative to ATM technology. Processing variable length packets optically requires intelligence in the switching process in order to reduce packet loss probability [3]. In order to increase the network utilisation, it is necessary to decrease the size and frequency of voids by increasing the buffer granularity. To further decrease the void sizes, ACK packets are scheduled to fill in additional gaps.In this paper we consider the reasons for void formation and propose a buffering mechanism with a new ACK scheduling algorithm. Simulations of the new algorithm are presented and the results discussed. Void formingIn IP networks, having variable packet lengths (rather than fixed length packets) results in packet synchronisation processes at input ports becoming much more complicated and difficult. One of the problems with buffering variable-length IP packets is the setting of optical delay lines to match the sizes of the packets. Since the sizes of IP packets vary in a wide range from 40 bytes to around 1500 bytes, it is practically impossible to set up the delay lines to have the same granularity (the step increase of delay lines which are assumed to be uniform) and at the same time have a reasonable buffer depth (the number of delay lines). Failing to match the sizes of the IP packets to the delay lines will create unnecessary spaces at the output link, referred to as voids, and causes high excess utilisation (defined as a ratio of the generated voids in bytes over the total transmitted bytes [4]). The voids and excess utilisation, is a result of the delay line mismatch; however, such phenomena can also be caused by the difference in arrival times between packets from different sources, which is referred to as arrival time mismatch. Figure 1 il...
By eliminating optical synchronizers in optical IP routers, more complex scheduling algorithms are needed to schedule asynchronous packets. This will result in voids at the switch output thus reducing switch throughput. A novel scheduling algorithm had previously been proposed to reduce these excess losses by filling voids caused by asynchronous and variable length operation.Non-degenerate (i.e. non-uniform) buffer depth has previously been shown to improve the packet loss performance especially under bursty traffic. We investigated the performance of the void filling algorithm by combining nondegenerate and degenerate (uniform) fiber delay lines. Performance is studied for different threshold levels, i.e. the number of uniform delay lines in the feedback delay line before introducing the non-degenerate delay lines.The packet loss performance for combined non-degenerate and degenerate delay lines with void filling algorithm is presented for an optical router with a feedback delay lines buffer under self similar traffic. The recirculating delay lines buffer emulates a two-stage buffer where first stage buffers smaller packets whilst the second stage stores larger packets. This buffering mechanism is similar to SLOB (i.e. Switch With Large Optical Buffer).
No abstract
Semiconductor Optical Amplifiers (SOAs) are vital elements in future optical networks whether as amplifying elements to boost the optical signal. In addition to be used as amplifiers, SOAs can also be used as switching elements operating either as ON-OFF switches or as wavelength converters for wavelength routing switching. Additionally, their performance is compatible with nowadays and future transmission rates of 10, 40 and 80Gb/s and beyond. Hence, the impact they will have in all future optical networks is paramount and it is unimaginable that there will be any future optical network without them. With this knowledge a look into future optical networks reviles that the modulation format will be influenced by this development due to the physical impacts of the SOAs on the signal transmission. Hence the modulation format needs to be investigated. Here we pursue the investigation of different optical formats by means of optical simulation and do a comparison of the modulation formats with respect to the performance selected for this paper.
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