They are more likely to meet the majority of their subscriber locations with a uniform 640 kb/s or slighly lower, is my guess, and that is why they chose a mid-point number.
DSLs can be delivered from 128 kb/s to upwards of 12 Mb/s and 52 Mb/s in the case of VDSL designs. The speeds are variable, depending on which architectural variant of DSL is chosen. With increasing distances from the central office, or from a field pedestal, the speeds always decrease.
If they had published 1.5 Mb/s across the board, they could only meet the requirements for this specification in a limited number of situations. At the same time, they would have increased the levels of expectations on the parts of users, which, again, could only be met in those limited situations.
A tradeoff, in my opinion, must have been reached to provide a relatively uniform speed. One which was much higher than the normal POTS and ISDN, but at the same time one that was lower than the max. And in the process, one that would not vary much from user to user. It's got to do with ease of service administration, IMO.
If there is one thing that characterizes an ILEC of Bell Lineage, it's uniformity of look at feel to all users for the same service. In fact, that is a regulatory mandate that they have in certain situations.
Hope this helps, Frank Coluccio |
