Broadband Calculations, Part 2: VDSL

11/11/2005 7:00 PM Eastern

If there’s one thing that’s wild about telephone companies, it’s how much their technology strategies differ, one to the next.

SBC Communications Inc., for instance, is taking a path that combines a little bit more fiber with a lot more work on its digital-subscriber-line techniques. Verizon Communications plans to go long with fiber, and barely bother with DSL.


This week’s translation focuses on the theoretical and practical broadband max speeds for SBC, which detailed its technology progress to analysts in New York on Nov. 3. (It posted the information on; search “IPTV.”)

During the briefing, SBC plainly stated its max speed: “Using VDSL in conjunction with gigabit-Ethernet technology, bandwidth of 20 Mbps to 25 Mbps was achieved, sufficient to provide four streams of high-quality video (including one HD stream) per line, high-speed Internet access, and, in the future, consumer [voice-over-Internet protocol] service.”

But it never hurts to go beyond the obvious. “VDSL,” for starters, stands for “very high bit rate DSL.” VDSL is one technology in the offspring of ADSL (asymmetrical DSL), or what consumers know as just plain DSL.

VDSL is different because it’s faster (duh). It’s faster, in part, because it reaches higher up the wire, spectrally, than “regular” DSL. Specifically, it runs up as high as 30 MHz — versus the 1.1 MHz range of today’s DSL. Rule of thumb No. 1: The higher the frequency, the harder stuff is to manage, technically.

Then, there’s the matter of “loop length,” or the actual footage of the twisted-pair phone wires that loop from your house to the central office or neighborhood node. Rule of thumb No. 2: The longer the loop, the slower the speeds, for all flavors of DSL.

(A reference point: People who study loop lengths say that 12,000 feet is the most prevalent in the U.S. —- about half of us live in houses with that footage.)

SBC plans to shorten its copper loops, to 3,000 feet, for 18 million homes within its footprint. Deadline: June 2008. Decreasing loop lengths means increasing fiber lengths — in SBC’s case, to neighborhood nodes of 300 to 500 homes.

The relationship between VDSL and loop length is an intimate one. On short loops, with low service penetration, VDSL sings. On long loops, piled with services and customers, it wimpers. This explains the plan to get to a 3,000-foot loop length as quickly as possible. (It doesn’t explain why only a portion of SBC’s footprint gets the fiber booster.)

There are other variables that impact DSL speeds, not mentioned in the SBC update, but closely watched by bandwidth aficionados. One is the number of copper pairs in the wire bundle connecting homes to a central office.


Rule of thumb No. 3: The more twisted pairs there are in a wire bundle, the more potential there is for “cross talk,” which happens when signals leak out and crash into each other. This is especially true of digital services, riding in that higher spectral zone.

That’s some of the behind-the-release details about SBC’s broadband plans. A downstream speed of 25 Mbps, using VDSL and deeper fiber, is their deployable broadband max. VDSL can go higher, theoretically — up to 100 Mbps, the vendor community says — but that’s with very short loops and pristine conditions.

Short of stretching fiber all the way to homes (which SBC plans for new housing areas), the telco’s deployable broadband max will likely remain 25 Mbps downstream.

Next time: How to do the math on broadband max speeds for fiber-to-the-home networks, like Verizon’s “FiOS.”

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