Pole Position

By Team Digit Published Date
01 - Jan - 2006
| Last Updated
01 - Jan - 2006
Pole Position
Who needs 5 Mbps? Of course you do, so you can (illegally!) download a movie in 20 minutes. But who needs 54 Mbps? You'd need to buy a new hard disk every day if you kept downloading at that speed! But streaming video, teleconferencing and even HDTV become possible at 54 Mbps, which is the theoretical speed offered by VDSL-Very-high-bit-rate DSL. VDSL is the hybrid (fibre-cum-twisted pair copper wire) evolution of DSL (Digital Subscriber Line). OK, it's sort of cheating to call it DSL, but then, much of the last mile is over telephone lines.

If you've lost us, refer Turbocharging The Telephone, Digit July 2004. There, we talked about DSL and how it works. Back then, DSL wasn't really available in India except for a few select cities; now, corporations such as BSNL, MTNL, and Airtel offer ADSL (Asynchronous DSL) in several cities. The services are very expensive for those of us who like to download a lot of stuff (and that's most of us!), but it's there. Plans from the companies mentioned above are pretty competetive for casual surfers.

In any case, our purpose here is to talk about what VDSL is and whether it's possible in India. But before that, a recap about how DSL-specifically ADSL-works.

ADSL provides a bandwidth that differs from area to area, and between implementations; a typical figure would be 1.5 Mbps to 8 Mbps over a single copper pair, at distances up to 18,000 feet, and at a wire thickness of 0.5 mm.

You probably know that with ADSL, you can use your phone at the same time that you surf the Net. You're able to do this because the ADSL system divides the frequency spectrum into three parts-one for phone service, one for upstream data and one for downstream data. Phone service is at the lower end of the spectrum; data is at the higher end. To implement this frequency splitting, you need to install a splitter along with the DSL modem. Voice can typically be transmitted within the 4 KHz range.

Frequency Splitting In FDM And Echo Cancellation

In FDM (left), the frequency band is split into ranges for telephone service, upstream data, and downstream data, at the frequencies indicated above. In Echo Cancellation (right), the frequency bands for upstream and downstream data overlap

The actual splitting of the frequency range is done using one of two methods-Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM uses separate frequency bands for the transmitting and receiving channels. In Echo Cancellation, transmission and reception happens on the same frequencies-that is, the bands overlap.

The advantage of FDM is that it is cheaper to implement in the sense that the hardware needs to do less work to separate the transmitted and received signals; the advantage of Echo Cancellation is that it transmits at a relatively lower end of the spectrum, meaning less signal attenuation with distance. See figure Frequency Splitting In FDM And Echo Cancellation.

Actually, it's not quite as simple as that. There's modulation involved as well (refer this month's Fast Track for more on modulation). Modulation is required because of several factors, including the noise associated with transmitting data over the kinds of frequencies we're talking about. In ADSL, the signal from your home is modulated (most usually) using a technique called discrete multitone (DMT), which is also what has been approved by the standards authorities for VDSL.

DMT splits up the spectrum on which the downstream signal travels into 247 channels, each of these 4 KHz wide (and the downstream spectrum into 32 channels). The biggest advantage of splitting up the band into so many channels is the continuous quality check that is possible.

Each of the hundreds of channels is monitored, and if and when it is seen that quality is impaired on a particular channel, the data on that channel can be shifted onto another one. This happens continuously-the system is constantly re-assigning data to channels. What results is the "best" channels at any particular moment being chosen.

After all the data is switched onto the right place, your phone line transmits everything together to your service provider's Central Office (CO). At the CO, a DSL Access Multiplexer (DSLAM) aggregates all the ADSL lines coming in to it from all over the neighbourhood. Because of the frequency separation, the DSLAM can figure out what's voice and what's data. Voice is redirected to the PSTN (Public Switched Telephone Network), and Internet data is switched to the backbone connected to the ISP.

Data Rates And Distances

This graph is an indicative plot of connection speed versus distance from the service provider's CO. Because of the frequencies they use, the situation is quite different for ADSL and VDSL

Now, the problem remains of the signal dropping as distance from the CO increases. See figure Data Rates And Distances for how much this drop is, with ADSL compared against VDSL. It's clear that ADSL can carry data reliably over much longer distances.
Now here's the 54 Mbps we were talking about! VDSL offers such fibre-like speeds over copper wiring, but like we mentioned, some of the path from the home to the central office (full form: central telephone switching office) is indeed fibre.

VDSL isn't new. It's been around for a while, but is being rolled out mostly in Japan and Korea. We know it hurts, but all we can do right now is sit and watch providers in these countries battle it out to deliver the fastest speeds to their customers!

As indicators, in Korea, we're looking at 50 Mbps downstream and 11 Mbps upstream being offered. In Japan, it's more like 70 Mbps downstream and 30 Mbps upstream-courtesy VDSL 2 (See Jargon Buster). In Japan, NTT charges a premium of just about $2 (Rs 90) per month over the cost of ADSL service!

So why doesn't everyone just move on to VDSL? The answer lies in what we've been talking about all along: fibre. (For example, markets in the United States have not been too responsive to VDSL.) To understand this, we need to understand what the last-mile problem is all about.

Fibre For The Last Mile
ISPs (In India, VSNL, Bharti and Reliance Infocomm) are connected to the Internet backbone. Telephone service providers (TSPs, such as MTNL) are connected to the ISP, and your home is connected to the TSP's exchange. In some cases, you might directly be connected to the ISP's central office (CO). If you're not connected directly to the ISP's CO, it's copper followed by fibre that carries data to the ISP.

It's the distance between the CO (or telephone exchange) and your home that is called the last mile, and here's where all the issues arise. "The last mile" typically means a couple of kilometres, but could be longer.

ADSL2 (See Jargon Buster), within a mile (1.6 kilometres) of the CO, can deliver about 25 Mbps. That should be enough, one might say, but faster is better, and there's no questioning that. VDSL can reach 100 Mbps within 1,000 feet of the CO, but as the distance from the CO increases to more than a mile, the speed begins to compare with ADSL2 's 25 Mbps.

Here's where fibre comes in. VDSL deployments use some length of fibre from the CO. How much, depends on the particular deployment. There's Fibre To The Neighbourhood (FTTN), Fibre To The Curb (FTTC), and Fibre To The Basement (FTTB). Of course, there's also Fibre To The Home (FTTH), which means fibre all the way with no telephone lines involved.

All these terms are somewhat self-explanatory, but to elaborate, FTTB is fibre all the way to the basement of a tall building, from where copper takes over to the individual offices in the building. FTTC means fibre would reach your street. And FTTN-the most practical possibility being talked about these days-involves the setting up of optical network units (ONUs) in a neighbourhood to which, on one side, fibre connects to the CO, and on the other side, copper connects to the home or office. The ONU's job is to convert the electronic signals from the copper to light signals, which are transmitted over the fibre.

The need (or the use) of fibre is what makes VDSL noteworthy as a technology. On the one hand, FTTN deployment may be difficult in some countries or areas; on the other hand, it's the fibre that enables VDSL to provide breakneck speeds over home telephone lines. And along the same lines, when it comes to ADSL2 vs VDSL, it's a question of how far from the CO the deployment will be-ADSL is suitable for long-reach broadband, and VDSL will be viable if fibre itself is viable.

Cable vs DSL 
DSL, of course, isn't the only choice if you're looking at broadband in India-at present, cable is the most visible alternative. Whereas the infrastructure needed for DSL-namely, telephone wiring-already exists, each new cable Internet installation requires cables to be laid.
How do they compare? They're pretty neck-and-neck in terms of how good they are and what they can deliver. Both are capable of very high bandwidths; it's up to the service provider how much he wants to give you at an affordable price! About last-mile connectivity, for cable, it depends on how the equipment in your neighbourhood is wired up; for DSL, it depends on the quality of copper you're connected to. With cable, as the number of users on the network increase, your speeds will go down; this is currently not the case with ADSL in India, where you're "guaranteed" dedicated bandwidths. Our guess, however, is that as ADSL penetration increases, the issue of sharing bandwidth will arise with ADSL as well, and you might not get dedicated bandwidth.
The situation, according to Sandeep Kolwadkar, deputy general manager (BD) at BSNL, is different for urban and rural areas: technicians in rural areas aren't proficient at setting up good cable networks, and that hampers speeds; in the case of DSL, it is poor copper quality that hampers speeds. Kolwadkar affirms that in urban areas, though, DSL does not suffer from the "poor copper" syndrome, and that connections are stable. That means we've only got to wait for prices to drop!
Jargon Buster 
ADSL2 is a standard approved by the ITU (International Telecommunications Union) in January of 2003. It doubles the maximum frequency for downstream data transmission from 1.1 MHz to 2.2 MHz. If you're within 2 km of the CO, downstream data rates can be up to 24 Mbps. There's also an optional mode that doubles upstream bandwidth.
VDSL 2 was ratified by the ITU in May 2005. The standard delivers up to 100 Mbps both upstream and downstream, and works over copper up to 12,000 feet. VDSL 2 uses about 30 MHz of spectrum, versus 12 MHz in VDSL.
HDTV is short for "high-definition television." It's digital TV wherein a wide-screen picture is transmitted with many times more detail than current analogue television. HDTV requires five times the bandwidth of current-generation TV signals.
Modulation is the term for making one signal ride on top of another. The first signal we're talking about is the one that is of importance; the second signal is called the carrier. The carrier signal can travel better through the medium than the first signal alone. For example, in radio, we have Frequency Modulation (FM); the carrier signal is of the frequencies you hear about-105 MHz and so on. The actual data signal is used to modify this signal to a small extent, and it is recovered by the receiving equipment.
A channel is a frequency band in which a specific signal is transmitted. So when one refers to DMT splitting the spectrum into 247 channels, it's 247 separate frequency bands, and a signal on one band or channel will be independent of, and will not interfere with, a signal on a different band or channel.

The Situation In India

VDSL in India? Far-fetched? We don't even have good ADSL yet! Anyway, we were wondering what the scene is like and what it will be, so we asked someone in the know- Jagbir Singh, group CTO, Infotel, Bharti Tele-Ventures Ltd.

First off, Mr Singh says DSL is the "global trend" and that it will remain the dominant technology for a long time. He also says things are "moving towards DSL, with around 44 million landline connections in the country and both the incumbent operators becoming aggressive." 44 million isn't too large a figure when you compare it with the penetration in the United States or even China. Still, it's there, whereas cable is virtually non-existent in rural India.

Broadband is definitely getting cheaper, with, for example, Airtel offering a wide range of affordable plans. However, Mr Singh says, "What we need today are policies that would replicate the mobile telephony growth success in the Internet arena. Unbundling of the local loop, peering for all at NIXI (National Internet eXchange of India), and Open Sky policy for DTH providers are a couple of steps that would allow better and cost-effective reach for any service provider to its target market segments."

Some explanation is warranted here. "Unbundling of the local loop" refers to privatisation of the last mile; currently, the government has a stronghold on what happens to the phone line between your home and the telephone exchange. We're not sure how long this will take.

The "Open Sky policy" refers to allowing DTH (Direct To Home) operators to have satellite uplinks from wherever they please; contrast that with the situation now, where they are only allowed to uplink from certain areas. Again, here, we don't know when the government will take positive steps in this direction.

"Peering over the NIXI" is when two ISPs agree to exchange traffic over the NIXI for free. An Internet Exchange, such as the NIXI, is a facility that allows ISPs to "meet" and exchange traffic. The purpose of establishing the NIXI was that Internet traffic that originates in India and has a destination in India should stay within the country; the advantage, of course, would be cost savings because of a reduction in reliance on international bandwidth. It would also increase speeds for obvious reasons-Indian traffic would not unnecessarily be routed all the way to, say, the UK.

We then asked about where the future of broadband in India lies-DSL, fibre, wireless, cable, what? Mr Singh is of the opinion that given the kind of content available and the needs of the Indian market as Bharti sees it, broadband connectivity on copper is "here to stay." Cable, according to Mr Singh, is still a long way from becoming a viable proposition-this, ostensibly because of the relative lack of an existing infrastructure.

Then, of course, we wondered about VDSL. We asked if Fibre To The Neighbourhood was feasible anytime soon. What Mr Singh said was a total surprise: he went beyond VDSL, and said that it was Fibre To The Home (FTTH) that holds the most potential as the technology for broadband services! Imagine an India with fibre running all the way to homes!

Of course, "rolling out FTTH quickly and in a cost-effective manner to cover rural as well as urban areas is a challenge," according to Mr Singh. "Deploying this technology means permissions from every residential block; there are administrative costs involved, and it takes time to carry fibre to every household.

"At present, ADSL delivers the best value proposition, both for service providers as well as customers."

So is it ADSL for now and FTTH for later? That's the way it would seem.
Some, however, are of the opinion that VDSL is the way to go for India. At the Convergence India 2005 International Exhibition and Conference, Moti Shulak, senior director, marketing, Infineon Technologies, spoke about the latest advancements in VDSL technology, and how they "fit perfectly to India's booming market."

In any case, we can conclude that the broadband situation is not as dismal as it might seem right now! Pricing, however, might make you gloomy again!

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