Perhaps this helps, Mike: Will Statistical Multiplexing Kill ADSL?
(Part of an article Malaga, Northpoint CEO)
An ISP wants to offer customers what ADSL can deliver -1.5Mbit/s/s downstream signal with an upstream signal of 64Kbit/s or higher - must consider the impact on the backbone network. But the ISP that pays to upgrade that backbone so it can guarantee 1.5 mbits throughput, to its customers is going to want to charge more for the ADSL offering to cover its costs, Malaga says. " The first place where expectations may not be met is in regard to pricing" he says. "UUnet charges $2000 for a T1 link to the high speed backbone. An ISP isn't going to turn around and sell 1.5 Mbit/s service for $80 and guarantee that throughput."
More typically, smaller ISPs buy a connection to high-speed networks and oversell that link, counting on technical methods such as statistical multiplexing to get customers' traffic through--and hoping that not everyone is trying to get very high-speed access exactly the same moment.
At the same time, however, the company providing the ADSL transport - a telephone company or competitive carrier-- also has to oversell the backbone network, Malaga Says. Say, DS-3s (45Mbit/s) go for about $2.000. If I guarantee that bandwidth, I can only put 24 to 28 customers on that DS-3, and that's not economic," he says. So the telephone company also uses statistical multiplexing, which combines packet stream onto a backbone network, and oversell the DS-3. If the oversubscription is too high, everyone's traffic is delayed, prompting error messages and latency that is too high for services such as voice-over-the-internet Protocol.
"Thje customer thinks they are going to get 1.5Mbit/s to the internet and that's not going to happen," Malaga says. The ISP is oversubscribing and the local company is oversubscribing because they are managing their network as well. As more and more users start using this kind of network, the throughput for everyone drops. And ADSL gets a black eye. |
