SDSL stands for Symmetric Digital Subscriber Line. It is an Internet connection technology that fills the gap between cheap consumer Internet services and high-end services like T1 and Frame Relay. SDSL is the ideal Internet connectivity option for low-budget non-profit organisations that run their own servers. The challenge this group of users faces is that for running servers one needs something more dedicated and business-oriented than a consumer Internet service like ADSL or cable, yet traditional dedicated services like T1 and Frame Relay are still too expensive for a low-budget non-profit organisation.
SDSL is the solution. It's a symmetric fixed bit rate service that runs over a dedicated pair of wires (unlike ADSL which requires sharing a pair with a residential phone line, an insult to any self-respecting server operator), offers bandwidth options in the same range as fractional T1 and Frame Relay, but is only slightly more expensive than consumer DSL.
NOTE: our project has originally started with SDSL/2B1Q, but has now been expanded to include G.shdsl and IDSL as well.
As an apples-to-apples comparison of the two electrical signal formats (just the electrical signal formats themselves, no ecomonic or political factors), SDSL Flavor B is a more sensible electrical signal format than T1:
The T1 signal format revolves around one and only one fixed bit rate of 1544 kbps (8 kbps out of that is for framing); SDSL/2B1Q is not tied to any particular bit rate and supports anything from 144 to 2320 kbps. With SDSL changing the user data rate changes the actual electrical signaling rate along with it, not a gimmick like fractional T1.
SDSL uses a single pair of wires for full-duplex symmetric communication natively. Sure, there exists HDSL2 which accomplishes the same thing for T1, but it's an extra layer in the stack whereas with Flavor B a single DSU goes between V.35/EIA-530 and a pair of wires natively.
Traditional T1 requires repeaters every 6000 feet; SDSL/2B1Q running at 1.5 Mbps can go a good bit farther than that without repeaters. Need to go farther still? Although no one has done so to our knowledge, building a repeater for SDSL Flavor B is conceptually quite simple, and the spacing between such repeaters could be a good bit greater than 6000 feet.
With every ISP we have ever worked with who offers both SDSL and T1 services, SDSL is always significantly cheaper. Of course the details will vary with your ISP, geographic location and desired bandwidth option, but as one case in point, with one particular ISP we work with in one particular geographic location, SDSL at 1568 kbps costs exactly 4 times less than a fractional T1 at 768 kbps!
Believe it or not, some people don't need 1.5 Mbps.
Aside from the (big) difference in the recurring monthly cost, one other reason why we've been pursuing SDSL all these years instead of simply getting a T1 is because not only did we have no need for 1.5 Mbps of bandwidth when we first started our descent down the rabbit hole, but we in fact specifically wanted a lower bit rate: we wanted to use a 4.3BSD-Quasijarus gateway router with a DSV11 synchronous serial card as our WAN interface, and this old sync serial card can't handle 1.5 Mbps. (It is officially rated for 256 kbps, but it seems to be capable of going up to just a little under 1 Mbps.)
Well, we no longer use or intend to use a router based on that particular WAN card, but the sportive interest has remained. For all we know there do exist plenty of sync serial ports out there which can only handle lower speeds, and eccentric people who have absolutely no need for 1.5 Mbps and who are happy with lower speeds have rights too. We consider the SDSL technology to be a very good fit for such users because it actually slows down the electrical signaling rate on the line to match the user's subscription data rate; contrast with a fractional T1 in which the pipe still carries 1544 kbps at the electrical level, but the DSU then takes the 193-bit frame and throws most of those bits out.