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Fiber to the home not necessary for future multiple HD services

One of the great myths of IPTV is that fiber will solve all its bandwidth problems by running all the way to the home. While this may be true in some urban areas and densely populated regions, for 80 percent of the population in Europe, North America and Asia, direct access to fiber will not be happening anytime soon, despite the promises set out in various national broadband plans. It is just too expensive, and the cost escalates the closer the fiber gets to the home. The problem is not the last mile; it is the final leg from the end of the street to the home that will be hardest to jump.

But, this is not actually a problem at all because the copper transmission industry has the matter in hand and will be able to support sufficiently high bit rates to deliver multichannel HD services by the time they are widely consumed. The point to remember is that until TVs connect directly to optical fiber, which is not in prospect for a decade or more (if ever), the final link will be electrical over cable anyway, even if this is just the last few feet. The question then becomes how short that link must become to deliver the bandwidth required.

It is worth pausing over that figure, because that defines what needs to happen within the copper access network. At present, 20Mb/s is often considered the minimum acceptable bit rate for a broadband service delivering IPTV, whether over copper or FTTH (fiber to the home). But, this will soon be inadequate given that cable operators, for example, are already offering 50Mb/s or even 100Mb/s, which for now may be headline figures to aid marketing but will eventually be needed to deliver multichannel HD services. In fact, based on the assumption that HD will move forward to 1080p, compared with the 720p or 1080i used for most services today, each channel would require 16Mb/s even with H.264 encoding, perhaps somewhat less with the most advanced codecs. This really means that to have headroom to serve a household watching several HD streams at once, as well as to provide access to broadband Internet services, 100Mb/s should be the target.

At present, this is well beyond most DSL-based broadband services, but this is a function of the copper loop length as well as the technology. Improvements in the technology combined with shortening the loop length should achieve the 100Mb/s target comfortably, at least according to the developers of the technology such as Alcatel-Lucent.

The current state of play is that most broadband services accessed over copper use ADSl, at speeds around 4Mb/s; ADSL 2+, in the 10Mb/s-20Mb/s range; or VDSL2, which achieves 20Mb/s-30Mb/s speeds over distances up to 1km from the nearest DSLAM (DSL Access Multiplexer).

VDSL increases bit rates by using higher frequencies up to 30MHz, but this runs into the major bottleneck of dense DSL deployments, which is crosstalk between neighboring pairs. This becomes worse at higher frequencies, but the impact can be mitigated by a further enhancement known as vectoring. Vectored DSL uses advanced signal-processing techniques to mitigate or even completely eliminate crosstalk, with different techniques used for downstream and upstream transmission. The key in both cases, however, is that signal processing is no longer performed singly on individual pairs but jointly among a group of pairs at the DSLAM.

A further improvement where two pairs are available, as is often the case as a legacy from the days many homes had two separate telephone lines for voice, is to use bonding. This doubles speed again by combining two pairs in a single connection. With both vectoring and bonding, the 100Mb/s target can be reached, but only over the shorter distance of 400m. For many IPTV operators, this will involve building fiber out further, but this is what they should be doing anyway.

But even this may not leave sufficient spare capacity, especially because DSL lines have a habit of falling well below their theoretical headline speeds. A further push was needed, and this is being providing by, among others, Alcatel-Lucent’s Bell Laboratories. This is through an old technique, dating back over a century, which exploits the fundamental idea of the twisted-pair cable itself through which a positive signal is sent down one wire of the pair and a negative signal is sent down the other, so the interference cancels out. Alcatel-Lucent realized that if you take two pairs together, you can then superimpose a further positive signal on top of one whole pair and a negative one on the other pair, making a total of three positive/negative signals over the pairs. This new third signal is called the phantom signal, so the new mode is called DSL Phantom Mode.

In theory, this should enable triple the bit rate over two pairs, but the problem is that the addition of the third signal creates new interference that mitigates the gain. Here again the vectoring technology can come to the rescue, as Bell Labs has discovered, eliminating this additional noise. The result is that Bell Labs has demonstrated at least 300Mb/s transmission over two copper pairs at 400m. This will bring relief to beleaguered telcos wondering if they will ever be able to close the gap on their cable TV competitors moving up with the help of node splitting, DOCSIS 3.0 channel bonding and other techniques.