Performance analysis of Multi-channel Aloha (MCA) and Multi-channel Slotted Aloha (MCSA) schemes for optical fiber network

多頻道純 Aloha 及分段 Aloha 應用於光纖網絡時的表現分析

Student thesis: Master's Thesis

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  • Man Hon YUEN

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Awarding Institution
Award date15 Feb 2011


Advances in optical fiber technology have facilitated transmission of large volumes of data at different wavelengths in the same fiber network using wavelength division multiplexing (WDM). We are interested in analyzing the performance of a Multi-channel Multiple Access Scheme for WDM optical fiber networks. In this thesis, we propose to use the Aloha scheme as the multiaccess protocol because optical fiber provides huge volumes of bandwidth that are often far larger than user requirements. The advantage of low efficiency Aloha schemes is their low cost and simple structure. Performance of single-channel and multi-channel Aloha and Slotted Aloha schemes with variable packet length is analyzed using a simple closed-loop model. The analytical model assumes a perfect environment where transmission can fail only if packet collision occurs. Analysis parameters include collision probability, throughput, packet rate, and end-to-end delay. Analysis results are verified by some discrete C event simulation results supported at 95% confidence interval (C.I.). Most of the analysis results are very close to those produced by simulation programs employing binary exponential backoff (16 windows size) as the retransmission scheme. Analysis and simulation results have provided many interesting findings. The external packet arrival rate or packet rate is proportional to the number of available channels with the same collision probability. Collision probability decreases as the number of available channels increases, if external packet arrival rate remains constant. Several states are defined such as steep period, turning period and flat period to describe the shape of these curves. The middle point of the turning period is recommended as the most effective choice because the performance improvement is not great after this point and the required number of channels is not large. Although these values vary in different situations, most of them are about 0:15, a little bit smaller than the "threshold collision probability" of 0:2. Throughput increases along with increase in external packet arrival rate until it reaches the maximum point. After the maximum point, the trend changes from increasing to decreasing and finally it becomes zero. Throughput is proportional to the number of available channels while the utilization improves slightly or remains the same after the number of available channels is increased. End-to-end delay increases slowly until the external packet arrival rate reaches the corresponding arrival rate of "threshold collision probability" and then there is a steep upward slope after the arrival rate exceeds the threshold value. Binary exponential backoff schemes have been shown to have better performance than fixed retransmission probability approach and random selection of one interval for a fixed period approach. Either "BEB16" or "BEB32" are suggested to be used as retransmission schemes of multi-channel Aloha and Slotted Aloha schemes because their overall performance in simulation are the best and close to analysis results. In addition, Aloha scheme is proposed to be applied in short average packet length distribution while long average packet length distribution must choose Slotted Aloha scheme. For medium average packet length distribution, both schemes are applicable; the selection criterion depends on the cost or the performance.

    Research areas

  • Optical fiber communication