SDSL: Step forward or back for broadband?

SDSL may be the new kid on the broadband block but so far it hasn't exactly stunned the public or even attracted much attention. The cacophony of hype usually reserved for a new technology has been conspicuously absent - a fact that has got some people asking difficult questions about return on investment, hidden costs and availability.

But first of all, what exactly is SDSL and why is it important? SDSL stands for symmetric digital subscriber line and it is being targeted predominantly at small- to medium-sized enterprises (SMEs). 

It differs in several ways from its better known cousin, ADSL (asymmetric digital subscriber line), which is chiefly meant to be a residential offering. ADSL provides high-speed bandwidth if users are downloading data but not if they are uploading it. For example, a typical download speed might be 2Mb per second, while upload speeds will come in at a mere 256Kbps.

Moreover, ADSL has a 20:1 contention rate, which means that customers share the line with either 19 others. As a result, it is theoretically possible that if everyone used the line at the same time, data speeds could plummet to 100Kbps for downloading and 12.8Kbps for uploading -- not comparing well with the average 56Kbps modem.

Up and down
SDSL, on the other hand, has the same upload and download speeds, which currently range between 500Kbps and 2Mbps. It also boasts contention rates of only 10:1, which means that service is better because users share a line with only nine others.

Another advantage for business customers is that, depending on the vendor, SDSL comes with service level agreements (SLAs). These SLAs guarantee certain levels of availability and reliability and if service falls below what is promised, service credits are provided as compensation.

BT, for example, which launched its service in January, pledges a four-hour response time and a 24-hour fix time and will deduct charges from customers' bills if it fails to meet these standards.

Business replacement for ADSL
SDSL is really being positioned as a business replacement for ADSL and ISDN. It is aimed at those organisations that require high bandwidth but are unable to afford either dedicated, expensive leased lines or frame-relay services, which are currently only the remit of large enterprises.

Chris Lindsay, BT's new broadband propositions manager, explains: "SDSL is the next step along for businesses. It's almost a stepping stone to the big boy stuff. BTnet [leased lines] provides industrial-strength connections for those that can afford to go there, but SDSL is aimed at smaller companies where the amount of information going out and coming in is already meeting the restrictions of ADSL."