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Dated: Aug. 13, 2004

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What is DSL?

DSL (Digital Subscriber Line) is an always-on Internet connection. DSL is billed per month, usually for a fixed price, and for the majority of providers, for unlimited usage. The key advantage of DSL over modem is speed. DSL is from several, to dozens, of times faster than a modem connection. Connection speed, reliability, and the 'always-on' nature of DSL, are the main reasons it is so popular. For small businesses, DSL is also a great way to save money compared to pay per minute ISDN service, or expensive T1 lines.


How It Works

Traditional phone service (sometimes called "Plain Old Telephone Service" or POTS) connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signaling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information.

Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN, which one might think of as a limited precursor to DSL, you can receive up to 128 Kbps.) The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.

Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line and at the same time.


DSL Varieties

HDSL (High bit-rate DSL) - is the earliest variation of DSL to be used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).

IDSL (ISDN DSL) - is somewhat of a misnomer since it's really closer to ISDN data rates and service at 144 Kbps than to the much higher rates of ADSL. IDSL is usually not sold by ADSL providers, is symmetric ("duplex"), with the biggest advantage being the distance it can travel (5-6 miles). Similar to ISDN (128 Kbps) but uses the control channel to increase data rates to 144 Kpbs.

RADSL (Rate-Adaptive DSL) - is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.

SDSL (Single-line DSL) - is apparently the same thing as HDSL with a single line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It is considered to be the "business grade" DSL because of its symmetric speeds. SDSL is slower than ADSL but usually marketed with Service Level Agreement (SLA) such as the network will be guaranteed up for 99.5%, and there will be a 24-hour response time for every problem.

UDSL (Unidirectional DSL) - is a proposal from a European company. It's a unidirectional version of HDSL.

VDSL (Very high data rate DSL) - is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments are not yet determined. A number of standards organizations are working on it.

xDSL - Refers collectively to all types of Digital Subscriber Lines, the two main categories being ADSL and SDSL.

x2/DSL - is a planned modem from 3Com and US Robotics that supports 56 Kbps modem communication but is upgradable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."


DSL Varieties

With DSL Internet Service, pages that used to take excruciating minutes to load via dial-up now jump to life in merciful seconds. Trading stocks, planning vacations, and shopping online are no longer awkward novelties, but a fun, new way of life.

Speed Comparison

  50K Web Page 2MB Image 16MB Movie 72MB Movie
56 Kbps Modem 7.1 sec 4 min 48 sec 31 min 45 sec 2 hr 54 min
128 Kbps ISDN 3.1 sec 2 min 4 sec 8 min 45 sec 1 hr 28 min
1.5 Mbps ADSL 0.3 sec 21 sec 1 min 23 sec 6 min




1. What is DSL?
DSL (Digital Subscriber Line) is an always-on Internet connection. DSL is billed per month, usually for a fixed price, and for the majority of providers, for unlimited usage. The key advantage of DSL over modem is speed. DSL is from several, to dozens, of times faster than a modem connection. Connection speed, reliability, and the 'always-on' nature of DSL, are the main reasons it is so popular. For small businesses, DSL is also a great way to save money compared to pay per minute ISDN service, or expensive T1 lines.

2. DSL vs Dial-up
It's Actually Cheaper, When you factor in the cost of your current Internet Service Provider ($21.95) and the cost of a second phone line ($30.00), both of which you will no longer need. You get fast and unlimited connection time for a fix monthly fee, so you can shop, chat, or just window-shop the Web as long as you want, with no extra phone or access charges.

Cable modems are typically faster for downloads than most if not all DSL lines, when the cable infrastructure is new or well maintained. However, cable has a few disadvantages to DSL. Since cable is a shared media, there is a possibility that performance may degrade over time as additional households plug in, connect additional devices (videos, game machines) to the TV lines.

A cable company may react slowly to decreases in performance, as they never sell access by speed, or promise consistent speed or latency.

One of the largest disadvantages of cable over DSL is the upstream (return path). Cable companies are using a very narrow band for return signaling, below all the space allocated for TV channels. This band is prone to RF interference and is very limited in capacity. Upstream transmissions may therefore compete with others in the area, get delayed (suffer high latency) due to noise fighting techniques, and cable Terms Of Service typically prohibit any kind of constant upstream use. Internet use is shifting away from central servers broadcasting to many individuals and some interesting peer-to-peer applications are appearing (games, voice and video applications, communal libraries). These applications need a strong upstream channel.

4. What are ADSL and SDSL?
ADSL is designed for home user and generally available more cheaply than SDSL. However if you are going to be operating servers I would suggest SDSL if you can get it, not only because the maximum upstream speed is generally faster than ADSL, but because your DSL provider is more likely to understand your needs, give you a fixed IP and be more responsive to technical problems.

5. How long does it take to install DSL?
This can vary from a week, in some parts of the country, to up to 2 months, or more, depending on what kind of problems there are between the DSL provider, Telco and the ISP. In some areas with straightforward ADSL installs, the whole setup can be done inside a week.

6. Does anyone have to visit my house?
Generally, yes. However some DSL provider are already experimenting with user-installed ADSL, where no visit is necessary.

7. I live in an apartment. Can I get DSL?
You should be able to get the DSL service provided by the local phone company assuming your lines support it. Whether you can get DSL that needs to be run over a separate copper pair depends on if the apartment building landlord/manager will allow a new copper pair to be installed into the phone box along with the wiring up to your apartment.

8. Can I use a Notebook with DSL?
You can use a Notebook with DSL almost as easily as a desktop. Instead of an internal modem, or Ethernet card, you need to buy a PCMCIA card supporting 10/100 Ethernet (if your Notebook has no Ethernet port on it). The other option is to select a USB DSL modem when getting a line (if your laptop has a USB port on it). Apart from that, everything else is the same as a desktop DSL connection.

9. Can I keep my existing email address?
Almost any popular mail account (hotmail, MSN, AOL) can be accessed from anywhere, with the appropriate mail client setup, or even via the web. AOL has access option for external use of AOL DSL provider. Earthlink email can continue to be used without even any reconfiguration, as can many other ISPs including MSN. So once you get your DSL line, you can continue to use and maintain your old email account! if you continue to pay them.

Although rules for different mail systems differ, you may find you can purchase a minimum (MSN or AOL or Earthlink etc) plan that still allows full use of your mailbox, or email could be set to forward to your new address.

10. Can I get DSL with no phone service?
It may be hard to order DSL, whether it is ADSL from the Telco, or SDSL, ADSL or IDSL from an ISP, without existing phone service.

This is because the phone company may be reluctant to deliver a clean copper loop to a premise that they do not have "in their systems" because they are providing phone service already.

11. What is the fastest speed that DSL connections are capable of?
Typical residential offerings usually have a maximum of 1.5Mbps (1.5 megabits per second), but special connections for home and office can be obtained to go well above that. There are two limiting factors that are in place for most connections: the modem and the wiring used to transfer the signal from the modem to the computer. Most home use DSL modems are limited to 10Mbps on the user (LAN) side. The wiring from the modem to the computers is normally Category 5 wiring, or thin Ethernet, and that is limited to 100Mbps speed.

12. Will my old PC be fast enough for DSL?
Yes. Although if you start to use the DSL line to view video on demand or other memory and processor hungry functions you may well decide your PC is too slow. For general surfing, majority of sites can quite happily be seen and used by slower PCs.

13. Will my Macintosh work with DSL?
The Macintosh computing platform accesses the Internet through the same protocols that the Windows computing platform does. This means that theoretically, any type of Internet service Windows computers can use, the Macintosh can as well.

14. Can DSL allow me to be on the phone & on the internet at the same time?
Yes, the DSL service operates at a different set of frequencies. Voice at the lower frequencies and your DSL at the higher frequencies. For the most common DSL (ADSL), the two can co-exist on the same wire. You are unable to hear the high frequencies even if your telephone was able to play them as sounds. The DSL signal should not be disturbed by the lower voice frequencies you use when talking on the phone.

15. How much bandwidth I really need?
You can never have enough. The same advice people give for PC processor speed can be given for bandwidth: buy as much as you can reasonably afford, since nobody ever has "too much" bandwidth.

16. Why is a 1.5mbs DSL more expensive than 2mbs cable connection?
Your DSL line is a guaranteed 1.5mbs. Where as cable is a 10mbs-shared line between people located on your node with no guarantee of speed.

17. What do I look for in a DSL contract if my ISP require?
Terms and conditions need to be read carefully. Understand that most special offers require you to sign up for 1 or 2 years! So in particular, you should watch for any clauses that allow the provider or ISP to charge you for early termination, or for moving from one location to another. In addition, the contract may specify what is not allowed, and this list, apart from the usual things like "no Spam" and so on, may include more widely applicable terms, for instance, they reserve the right to terminate your line and charge you disconnection costs if you do anything they believe is "destabilizing" to the system. Without a clear definition of what "destabilizing" is, they are basically free to terminate your line anytime they want, and you have no right to argue about it.

Not really a subject for terms & conditions, but, also check what optional service costs are, like additional IP addresses and DNS service etc.Also be careful to check for any traffic restrictions both up and down, or limits on supported configurations, or running servers at home. On the plus side, you should hope to see guarantees from them on committed information rates. And what your rights are when minimum service levels are not met.

If you like what the sales people say, make sure you get it in writing, and ask if they apply for the duration of the contract, because terms and conditions have a habit of changing after you signed up.

So here is a final checklist:

  • What are any system requirements the DSL provider/ISP expect?
  • Is rental an option for the CPE (premise equipment)
  • Does the ISP offer any phone number for dial-up access backups? What about roaming.
  • How long does it take for service to be activated?
  • What promised speeds are there, what happens generally at peak periods?
  • Is there any recourse if promised rates are not met?
  • Are all charges included in the quoted monthly fee?
  • Are there are any data-transfer limits on download or upload side?
  • Does the installer care about the PC or Operating system you have
  • Will additional interior or exterior wiring be necessary?
  • Who pays for any installation expenses?
  • Do accounts come with any home directory, ftp or web space?
  • What's the limit on e-mail addresses provided?


  • Does the ISP monitor the state of the line for you?
  • Is there a 24hour number for network operations center?
  • Is routed IP available, and at what extra cost?
  • Is this a PPPoE setup; are their plans to switch to PPPoE?
  • What will the routing be from you to local internet sites?
  • Will you be allowed to host a Web site?
  • Does the ISP support reverse DNS lookups?
  • What's the limit on IP address provided?
  • What's the cost of extra IP addresses?
  • If you have a home network, can you share the connection over it?
  • If shared access is allowed, is any hardware or software support provided for it?

18. What if I move houses?
If you plan to move, you should certainly ask exactly what the costs are of early termination of your current contract and whether you can take your equipment with you (if it is rented) to the new location, assuming it is serviced by the same DSL provider, and whether there are any discounts over a new install price given for such a situation. Also how much notice you have to give them. If you are constantly moving, look for DSL provider that are not require you to sign any contract.

19. I want to operate my own server with DSL, am I allowed?
Servers like web? Ftp? mp3 streaming? Sure, although read your contract first. Some DSL services will forbid servers.

20. Would it slow down my speed if I ran a web server?
Not unless your website is very busy, or contains large files that people frequently wish to download. There is no traffic, and no speed impact, by just having a web server.

21. Can I share my single DSL line amongst more than one computer?
Yes. Exactly how you share your single DSL line depends on the operating system on your computer.

22. My Win98 PC wants a login every time I boot my PC. Can I bypass this annoyance?
Yes, go to Start / Settings / Control Panel / Network, for Primary Logon select Windows Logon and select OK. Go to Passwords enter your old password and leave the new password box empty. Restart your computer and it should be gone.

23. What type of security does a DSL line provide?
None. It is up to the end user to supply that security.

24. Should I turn off my DSL when I am not using it?
No. Why? It doesn't save you any money. If you concerned about security, though, and don't mind shutting your computer down, it may be prudent. A windows product called ZoneAlarm is also neat. ZoneAlarm offers a button, click this when you leave your computer, and it freezes network traffic, unclick when you get back to continue.

25. If I am on the net 24hrs a day, will I get hacked?
Short answer: probably not. Longer answer probably not because you are one of millions of computers and there are not enough hackers to go around. Anyway, any spare ones are not interested in your digital family photo album or excel expense worksheets, but they could get to your files if you don't care to learn about security.

26. Is my line DSL line private, or can my hacker neighbor listen in?
Unlike cable modems, your DSL is totally private, up to your ISP. It is possible that your ISP can monitor your use of the web, but no more likely than if you use a dialup modem. Note: this does not mean you cannot be hacked by someone else on the internet, it just means that you have a separate line from you to your ISP, so no local neighbors can listen to your data.

27. Do I Need a Firewall?
"Good fences make good neighbors." With your new broadband (DSL or cable) connection, your computer is potentially visible to others on the Internet every moment it has power applied to it. The general consensus is that cable users are more vulnerable than DSL subscribers. But no one without a firewall is without risk. A good firewall makes your computer invisible, or at least less visible, to others on the Internet. There are other users that will try to probe your machine looking for ways to see what is on your computer's hard drive. Some may only be curious, but others may be trying to be very malicious - even to the point of erasing files on your system. They could potentially gain access to your passwords and other private information you may have stored in files on the computer. You may ask 'which is the best firewall'. There are special setups that actually involve hardware and these hardware firewalls are considered the best choice. The down side of the hardware solution is cost. Many users therefore choose to install a software firewall. These have the advantages of low cost and easy setup. Which solution is best for you is a question you have to answer for yourself.

28. What can I do to ensure clean house lines for my DSL connection?
The wiring inside your home or business is your responsibility. Normally, your ISP will run wiring from the demarcation point (box where the phone wires are located within your building) to your pc. Make sure these wires are not close to fluorescent lights or devices that emit magnetic fields or high voltage.



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