| Summary: | 碩士 === 國立交通大學 === 電機資訊學院碩士在職專班 === 92 === ADSL has becomes one of the most widely broadband access technologies. But the traditional ADSL connector DSLAM, is built on the ATM environment. ADSL segments IP packets into ATM cells and an ATM cells which are reassembled back to IP packets. This study compares the performance of ADSL modems with ATM based DSLAMs or IP based DSLAMs, for assessing the influence of the overheads. We design some tests to compare the performance of two types of DSLAM. After test result be proved, IP DSLAM achieve better performance than ATM DSLAM. Transfer packet size more long the phenomenon more clearly. Additionally, experiments are also performed to demonstrate that the performance is affect by fast mode; interleave mode and trellis encode. The experimental results reveal that the interleave mode performs worse than the fast mode. Due to interleave mode frame interleaver and de-interleaver must be performed. However, the trellis code encoding does not affect the performance. On the other hand, ADSL has asymmetric links rate, downstream and upstream has difference link rate. A key issue is weather asymmetry affect the bi-directional traffic performance. This study uses three types of line rate combinations and selects the best one. This study found that when the upstream link rate approaches the downstream link rate in the bi-directional throughput performance is maximized. The optimum throughput performance thus was obtained 7616/800 kbps. Finally, three types of encapsulations are applied to a real ADSL network to re-confirm whether overheads influence performance. This study again confirms that IP DSLAM achieve superior performance to ATM DSLAM. Encapsulation protocol overheads directly affect the throughput performance. Summary above test result, the packets segmentation and reassemble is the most important factor about throughput performance. Packets encapsulation method is next effect factor. ADSL physical parameter changes will not huge effect throughput performance.
Keywords: ADSL, IP over ATM, IP over Ethernet, Testing
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