The D version is the latest and greatest, according to the 3Com team. It has similar architecture to the previous models, but does have an additional feature. The D model includes an on board CSU/DSU. This device is actually known as a channel service unit/ data service unit. It functions to connect a router to a T-1 digital circuit. In simple terms, it cleans and aligns the signal. There was a time in the old wild west when digital circuits were not very trusty... they carried issues such as timing variance, as well as carrier loss issues. These times are mostly behind us now, and do not generally occur, from the majority of digital suppliers. There are cases in which a CSU/DSU is both necessary and desirable... but certainly not in every case. The D revision works well. The draw back is the price. The MSRP is $2850, at the time of this report. If you need a CSU/DSU, they are available in great profusion, from many sources. They could be free standing and added at a significant savings, if the lines you receive require this functional upgrading.
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