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August 08, 2005

TECH TALK: Next-Generation Networks: VDSL

In this week’s columns, we will look at some network technologies: VDSL, WiMax, 3G and 4G, and Broadband over power lines. We’ll start by taking a look at VDSL.

Very high bit-rate DSL (VDSL) can be thought of as the successor to ADSL (Asymmetric digital subscriber line) technology. Both work on regular telephone lines and as much are one of the most important wired broadband technologies. In India, BSNL and MTNL have launched DSL services across the country. Give the fact that unbundling of the local loop doesn’t seem likely for the foreseeable future, DSL offerings from the incumbent telcos is going to be the best bet for getting high-speed connectivity into homes and enterprises in the near-term.

HowStuffWorks writes about VDSL:


VDSL operates over the copper wires in your phone line in much the same way that ADSL does, but there are a couple of distinctions. VDSL can achieve incredible speeds, as high as 52 Mbps downstream (to your home) and 16 Mbps upstream (from your home). That is much faster than ADSL, which provides up to 8 Mbps downstream and 800 Kbps (kilobits per second) upstream. But VDSL's amazing performance comes at a price: It can only operate over the copper line for a short distance, about 4,000 feet (1,200 m)…The key to VDSL is that the telephone companies are replacing many of their main feeds with fiber-optic cable.

transceiver in your home and a VDSL gateway in the junction box, the distance limitation is neatly overcome. The gateway takes care of the analog-digital-analog conversion problem that disables ADSL over fiber-optic lines. It converts the data received from the transceiver into pulses of light that can be transmitted over the fiber-optic system to the central office, where the data is routed to the appropriate network to reach its final destination. When data is sent back to your computer, the VDSL gateway converts the signal from the fiber-optic cable and sends it to the transceiver. All of this happens millions of times each second!

Dave Burstein discusses about the use of VDSL by Bell South in the US: “SBC is selling satellite to 50% of their users -a fancy TIVO style set top and a slow DSL connection, and upgrading the rest to low profile VDSL2 they call fiber to the node. From the projected 2,000-5,000 feet, low profile VDSL2 is maybe 20 meg down, 1-3 meg up, most of which will be used for their video…BellSouth has 13 million lines, a million of which have fiber to the curb from a quiet build begun years ago, yes. Those are the lucky ones, because they will be upgraded to 100 meg symmetric VDSL over the next few years. Think 60 megs in practice, but still pretty good. BellSouth has just picked that build up to 200,000 lines for 2005 after slowing down for a few; unfortunately, at that rate it will take them fifty years to complete their rollout. ..Nominally ADSL2+, will morph into VDSL2 low profile soon. But VDSL2 low profile really is a slightly improved ADSL2+ (2-5 meg faster at these distances), not the 100 meg ‘high profile’ that only works 500-1000 feet they are using for the lucky fiber to the curb types.”

A July 2004 News.com report about South Korea discussed its VDSL adoption: “In Korea, large apartment buildings make it relatively simple for a telecommunications company to draw a fiber line to the basement and then provide VDSL (very high speed digital subscriber line). VDSL can offer as much as 50 to 100 megabits of service over short copper lines, so it is well-suited to these buildings. But the technology doesn't work so well in the United States, where the distance between homes and the telephone company's central offices are often large. As a result, the big phone companies say they are avoiding VDSL for the most part and looking instead to install fiber optics as a next-generation technology.”

In this context, it is also interesting to read the view of UGO Online (December 2004) about Fibre-to-the-home (FTTH): “An ideal implementation of the service will eliminate any need for dedicated telephone lines, satellites, TV cabling etc, as everything will be delivered on the one high speed optical line straight to your house. The exact details of such a system are not set in stone, but generally existing telephone exchanges will act as the hub to which the fiber is connected to, inserting all of the available services into the line to each house with high reliability and low maintenance…Besides the questionable reach of FTTH, there is also the matter of equipment costs. Laying cable is never cheap, which is why Cable Internet has failed to provide a widespread broadband solution. But the real costs come with the end user equipment…Whilst FTTH is by far the most impressive and feature-filled technology on display here, the likeliness of it ever reaching a wide audience isn't very high, at least not in the near future.”

Given that a lot of fibre backbones exist in India, what the telcos should be looking to do is to upgrade the last-mile infrastructure to offer higher speeds into Indian homes and enterprises with VDSL.

Tomorrow: WiMax

Posted by dymaxion at August 8, 2005 09:40 PM



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