Conventional wisdom would say that more bandwidth is actually a good thing for VoIP. But that is not the case when it comes to ADSL2+, according to this article in the CommsBusiness. The article explains that in classic ADSL scenario when a file is being transfered or a VoIP connection is made, the bandwidth allocated to that connection remains constant, till say the transfer of files is complete or say the call is finished.
From what I understand it is like having a dedicated port and every subscriber gets a certain amount of bandwidth. In the case of ADSL2+, subscribers share the bandwidth at the local exchange level, and the bandwidth allocation is more dynamic. Carriers like ADSL2+, as it allows them to over provision, though it is not such a good deal for DSL subscribers. (I am sure some of you might have a more eloquent description of how this technology works.)
“This new technology uses a rate adaption policy which effectively means it can (and does) steal bandwidth dynamically from subscribers as and when required. We have seen upstream capacity decrease to lower than 57k in peak working hours which is simply not viable for a VoIP application. Period.” Scott Dobson, Managing Director of Newbury Based distributor Vcomm.