IP over ATM效能問題探討在ADSL網路上

Abstract

非對稱數位迴路(ADSL)成為目前最普遍的上網方式之一，但傳統的ADSL DSLAM建立於ATM環境下，導致IP封包必須切成ATM cell或ATM cell必須重組成IP封包才能進行傳送。本論文將利用ADSL modem與ATM based DSLAM和IP based DSLAM間的效能比較測試，來證明overhead將影響傳輸效能。實驗證明，在IP DSLAM下傳送效能確實優於ATM DSLAM，封包越大情況越明顯。此外，並針對ADSL的fast mode、interleave mode和trellis code encode是否影響傳輸效能，做實驗來驗證。Interleave mode須要將資料打散和重組，其效能會略低於fast mode。Trellis coding的影響不容易從這實驗中觀察到。另外ADSL其非對稱的特性是否也會影響其傳輸效能，本論文會從三種不同組合的連線速率上，找出最佳的雙向傳輸連線速率。當上傳的連線速率越接近下傳時，其雙向傳輸的效能將會最大。所以7616/800得到最佳的雙向傳輸效能。最後，利用三種不同封裝方式在實際的網路上做效能測試，再次證明overhead將影響傳輸效能。且證明經過IP DSLAM的效能優於經過ATM DSLAM封裝協定的overhead越多也直接影響傳輸的效能。總結上述結果，封包的分割與重組為影響效能之最主要原因，其次才為封包的封裝方式。ADSL內其它參數的改變，影響效能最小。

關鍵詞：非對稱用戶數位回路、IP over非同步傳輸模式、IP over乙太網路、測試

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