DSL oversubscription
 
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Last week, Genuity - formerly GTE Internetworking - launched its first digital subscriber line Internet access service for business users.
The Enterprise DSL Internet Access Service is available now in 45 markets and offers users competitive service rates, ranging from $90 to $300 per month. But there is one important item that Genuity doesn't want to talk about: the ISP's oversubscription rate.
When an ISP sets up DSL services it deploys gear, usually a digital subscriber line access multiplexer (DSLAM), within a local exchange carrier's central office switch site. These DSLAMs are essentially setting up a link between the Internet and a user at the other end of that copper line that leads to their office or home.
DSL is an always-on service, which means that most service providers oversubscribe the ports on the DSLAMs because most users will not be on at the very same time. This is a common practice, but it is also one that users need to keep an eye on.
UUNET, which has been offering DSL services for almost four years, says that it oversubscribes its ports at a 4-to-1 ratio. Cable & Wireless, which launched its DSL service nearly seven months ago, also oversubscribes its ports at a 4-to-1 ratio.
But Genuity will not disclose its oversubscription ratio. (Of course, it's only fair to point out that AT&T also is keeping its lips sealed on the same information.)
When service providers take this approach, it's easy to believe that there is information that they don't want the public to know. And in this case, it could mean that Genuity's ratio is probably higher than that of most other ISPs.
GTE, Genuity's parent, has said that much higher oversubscription ratios are fine.
"GTE, which has done some of the most thorough real-world DSL testing, says that 10-to-1 over subscription still provides full throughput to customers 95% of the time, even if they are all surfing the Internet simultaneously. That's because even during heavy Web surfing, the link is idle much of the time, according to Dale Veeneman, senior principal member of technical staff at GTE Labs."
- from Network World, 03/01/00
While it is not clear at this point what Genuity's oversubscription ratio is, business users should not sign DSL service contracts without having that information in writing. If an ISP is jamming too many users onto a single port, service performance will suffer.
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Denise Pappalardo is a senior editor for Network World, covering ISPs, VPNs and related topics. Reach her at denisep@nww.com.
Internet Services archive
Past newsletters.
Network World, 05/01/00.
Genuity to offer single-source DSL services
Network World, 05/16/00.
Who is really selling that DSL service?
Network World, 11/22/99.
