DSL has a secret
(IDG) -- Digital subscriber line (DSL) service that starts out at 1.5M bit/sec at your site isn't typically run at that bandwidth all the way through the DSL carrier's network. Rather, your line contends with other customers' DSL links for a shared pipe into the carrier network.
It is possible to buy a full bandwidth link all the way through the DSL carrier net, but be prepared to pay a premium and to negotiate service-level agreements (SLA) to ensure performance.
In the growing battle to sell inexpensive high-speed access pipes, DSL partisans claim their technology is better than cable modems because it offers dedicated bandwidth, while cable is a shared medium.
That is true, at least until the DSL line hits the first device in the service provider network, known as a DSL Access Multiplexer. There, DSL access lines are typically aggregated onto an oversubscribed backhaul trunk into the carrier's switched/routed network. That trunk represents a potential bottleneck that could make the service crawl.
"All networks are oversubscribed," explains Bill Southworth, chairman and CEO of Harvard.Net, an ISP in Boston that specializes in DSL access. "And it works as long as there is enough capacity so aggregated traffic doesn't get throttled."
When bandwidth through the service provider network is truly guaranteed, customers have to pay. For example, UUNET's UULink DSL Internet access costs $500, $600 and $900 per month for 128K, 364K and 768K bit/sec of guaranteed bandwidth, respectively.
Low-cost DSL services, such as the rock-bottom $39 per month SBC Communications charges for a service supporting 1.5M bit/ sec downloads, don't come with such guarantees.
What's a good number?
DSL provider NorthPoint Communications oversubscribes its trunks by a measure of 2 to 1, meaning the bandwidth coming into a network trunk is twice what the trunk can bear. That method works without degrading customer service because in practice, all customers aren't using their lines at the same time. "That ratio is very conservative. We are not even close to dropping a packet," says John Stormer, NorthPoint's vice president of marketing.
GTE, which has done some of the most thorough real-world DSL testing, says that 10-to-1 oversubscription 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.
With a group of casual Internet users as customers, the backhaul could be oversubscribed by 100 to 1 because not all the customers will be on at once, Veeneman says.
The ISP link
But there are other potential oversubscription bottlenecks, Veeneman warns.
Often an ISP offers DSL access but has another carrier set up the DSL links. Customer traffic is concentrated onto an oversubscribed link into the DSL provider's network, and then aggregated again onto an oversubscribed link to the ISP.
Customers need to be aware of aggregation practices into and out of the DSL carrier network, Veeneman says.
Harvard.Net's Southworth says he is always buying more bandwidth for Internet links to limit oversubscription and prevent service degradation.
Because there are multiple possible choke points, customers need to be clear about what part of the network the guarantees cover when they negotiate SLAs, says Beth Gage, an analyst with TeleChoice, a telecommunications consulting firm in Boston.
"Some SLAs don't start until the switching office, and some only cover the last-mile link," she says. "You need to go step by step through what is covered, from physical and logical links to outside events like acts of God."
Gage also notes that because DSL networks are still being built, links that are engineered to be oversubscribed may not be oversubscribed yet. What may work fine today might degrade tomorrow if service providers don't adjust as they add more.
Customers can check end-to-end DSL performance by pinging the mail server at the ISP being accessed. The ping will measure round-trip time and see if packets are dropped, says Mike Lutz, a former network manager who researches DSL for Avalon Networks, an ISP in Iowa City, Iowa.
If the customer can find an FTP server directly linked to the same ISP, downloading a file can indicate just how much bandwidth the DSL service provider is delivering, Lutz says.
Tim Greene is a senior editor for Network World. Network World Senior Editor Denise Pappalardo contributed to this story.
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