• MANOU MARZBAN ON ADSL - THE PATHWAY TO BROADBAND?
Basically, ADSL makes your Internet experience faster…and that is something we all want.

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What is all this talk about ADSL in the UK?
You probably have read or heard something about ADSL by now. It’s the sort of thing that makes grown people with a background in technology, giggle excitedly like school kids. Advocates of ADSL claim it will change how we use the Internet and therefore, they claim, it will change how we communicate in general. My perception is a bit more cautious, only because technology moves so quickly that before we can even understand the basics of ADSL, a new acronym will be introduced that promises to be better, faster and cheaper than ADSL. Nonetheless, you need to know about ADSL, as it is about to be plastered over everything Internet in the UK.
For those new to ADSL, the acronym stands for Asymmetric Digital Subscriber Line. What ADSL basically does is to transform existing twisted copper wires between a local telephone exchange and a customer's telephone socket into a high speed digital line. It is called "asymmetric" because it moves data more quickly from exchange to customer than from customer to exchange. This makes it particularly suitable for applications where customers expect to receive more data than they transmit such as use of the World Wide Web, corporate intranets, and reception of digital audio-visual material.

How does ADSL work?
Although ADSL operates over the same twisted pair copper loop used to provide voice telephony, it utilises the frequency band from 20kHz to 1.1MHz. The scheme, first implemented by AT&T, is called Carrierless Amplitude Phase Modulation (CAP). However, before AT&T could deliver significant quantities of chipsets to roll out ADSL in North America, the American National Standards Institution (ANSI) created a standard (T1.413) for ADSL based on a technique known as Discrete Multitone (DMT). Are you bored to tears yet? Wait…it gets better.
DMT operates by dividing the available bandwidth into 32kHz wide "bins" each capable of carrying upto 64kbits of information. This allows a theoretical maximum transmission rate of approximately 16Mbps, but current chipsets have a ceiling of 12Mbps. This is split into two parts, "upstream" to the telephone exchange and "downstream" from the exchange. Because of the way bins are allocated, the maximum upstream data rate is approximately 640kbps, while downstream performance is approximately 8Mbps. Hence the technology is suitable for applications such as the Internet, where you expect to receive more information than transmit.

As I mentioned, ADSL operates over a traditional telephone line. This means that an existing telephone line can be used to send and receive high-speed digital data. The ADSL signal is carried by two ADSL modems - one at the user end and one in the local exchange. These ADSL modems are designed to exploit the physical transmission capabilities of the copper line, to achieve higher data rates over the line than ever before. A 'splitter' (which is a filter), one at the user end and one at the exchange end, separates the telephony signal from the ADSL signal. This means that telephone calls can be made at the same time that data is being sent or received (i.e. a customer can surf the Internet and still make telephone calls). This is a true bonus - for those of you with teenage children - you know what I mean. The benefits of ADSL are simple: 1) Speed - its high speed data rates provide great potential for both commercial and domestic use, making video-on-demand a reality, 2) Always On - customers no longer have to waste valuable time obtaining a connection via dial up, and 3) Flexibility - it does not affect the normal telephone, so customers can make and receive telephone calls or faxes whilst they are on line.

So, with all the technical jibberish out of the way, what does this mean to the punter? Basically, ADSL makes your Internet experience faster…and that is something we all want. Depending on line length between the exchange and the customer, it is possible to transfer data downstream at up to 6Mbps. This is 10 to 40 times faster than today's typical modems. What this all means is that it may be possible that ISDN, lately offered as an Internet and remote access solution, may be rejected in favour of ADSL. There are some issues. For example, data transmission performance reduces with increases in the distance over which the information is transmitted. In short, this means that only subscribers close to the telephone exchange (central office) will receive data at rates sufficient to carry MPEG-1 or 2 video, but almost every subscriber will be able to obtain data transmission rates of 1.536Mbps, which I'm sure you will agree affords excellent internet performance. Certainly better than what we currently have on the books anyway.

What is going to happen with ADSL?
As the years, or, more likely months pass, there will be a multitude of services striving to use bandwidth on the Internet. We're all familiar with analogue modems. They've been around in one flavour or another for many years, for example, dial-up or leased lines. Everyone also knows that even 56Kbps (even if your phone line and ISP can actually achieve it) is not going to be enough bandwidth to fully take advantage of the Internet. Many new sites contain multimedia performances (Java, Flash) which are just too painful to wait for if you have to download them at 56kbps, or even worse....33.6kbps! So what are our options as consumers?

Urge your network operator to install optical fibre to your home as soon as possible. This may be to no avail…as the most probable answer you will receive is that fibre is too expensive to deploy on a countrywide basis. Once you have been told this - you can say 'fair enough - how about ADSL?'
Network operators have too much investment tied up in the copper loops which already join their exchanges to our homes and offices to give it up easily. However, since ADSL technology incorporates the existing copper infrastructure - network operators will be keen to adapt to ADSL because it extends the life of their investment by many years. Their problem is that if they give us a quantum leap in performance now, it will be a very short time before we ask for the next step. However, increasing use of the Internet for all manner of tasks mean that we should demand ADSL now, because, with the best will in the world (take ISDN as an example) deployment will take at least two years. So there is no harm in contacting your local operators and asking about ADSL connectivity. What will it cost? Who knows…initially, I would say in the £300-£400 per year range - as in current ISDN rates. Call BT and see what they charge, always a decent benchmark.
There are hurdles. No authority has, as yet, published a document which describes the filtering arrangement necessary to separate the voice band signals from the ADSL band frequencies. The filtering arrangement, known as a POTS splitter will need to be addressed shortly if a delay in the commercial roll-out of ADSL in Europe is to be avoided. The reasons for this are quite simple.
Firstly, in the US and Canada, a 600 ohm termination of the telephone line is the norm. This permits the use of a very simple, passive splitter. Because of the complex termination arrangements in Europe, such a splitter cannot be implemented in splitter designs for ADSL in Europe. Secondly, the European standards for telephone line use do not anticipate use of the frequency bandwidth above 100kHz. Add the complication of inserting another device into the telephone network in series with the customers branch system and the situation becomes even less clear. The ADSL Forum is taking steps to address this issue, but no hard and fast measures have so far been taken to remove these obstacles of confusion which must confound the system engineers in companies such as BT and Kingston Communications.

Finally, until the standardisation issues are solved - we will have to make do with ISDN or worse. But watch this space, for it is only a matter of time before ADSL becomes a reality here - as in the United States. Once that happens, your entire perception of the Internet, surfing, ecommerce and Web communications in general will change. That is a promise… for we all know one thing…time is money and speed is everything.

Manou Marzban is Head of Content and Channels at BiblioTech, 50 Carnwath Road, London SW6 3EG, 0171 384 6900, he can be emailed at manou@bibliotech.net.

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