DSL Tutorial

Contents of this section:

Bandwidth Simulator
Watch this animation to see just how much faster a DSL connection is than 28.8K, 56K, and ISDN connections.

DSL: What is It?
DSL is short for Digital Subscriber Line, a generic name for a family of high-speed digital lines being provided by competitive local exchange carriers (CLECs) and local phone companies to provide access to their local subscribers. So, what does that mean to you? You've already discovered the benefits of the Internet, but sometimes it's like being stuck in a traffic jam-waiting for your modem to connect to the Internet, getting busy signals, waiting for pages to download, giving up use of the phone line while you're surfing, etc. What if you could bypass the traffic jam, and get to your destination using a different, faster road? That's exactly what DSL does.

Top of Page

Four Quick Facts About DSL
DSL is fast - DSL modems are much faster than analog modems. Each type of DSL provides different maximum speeds, from twice as fast to approximately 125 times faster than a 56.6K analog modem. The only speed limit with DSL is the speed of the Internet and all the different computers attached to it.

DSL doesn't tie up your phone line - DSL doesn't interfere with phone calls, even though it uses your regular phone line. What this means is that you can be on the Internet and you can pick up the phone and make a phone call on the same line. With DSL, you won't have to worry about missing calls, or logging off the Internet to order a pizza, and then logging back on when you're done with the call.

DSL is always on - Your DSL connection is always there. There's no need to dial up and listen to your modem squawk every time you want to do something online. And there's no frustration about the line dropping when you're in the middle of browsing or downloading. Want to check your e-mail? Set up your computer to check for new e-mail and notify you when you receive something instead of logging in and checking it yourself. Want to look at just one Web page? Just open your browser and look.

DSL is reliable - Phone company networks are among the most reliable in the world, experiencing only minutes of downtime each year.

Types of DSL
What is xDSL? - xDSL is used interchangeably with the acronym DSL to describe the different variations of the technology: ADSL, SDSL, HDSL and others. They all fall into one of two categories: asymmetric DSL and symmetric DSL.

Top of Page

DSL for Home and Telecommuters
Asymmetric DSL (ADSL) reserves more bandwidth going downstream to the user and less going to the Internet. Imagine you're caught in a traffic jam on a highway during rush hour. What if you could take some of the lanes that are not as congested and switch their direction to ease the traffic jam on your side? This is exactly how Asymmetric DSL works—using more bandwidth for downloads to the user and less bandwidth for uploads from the user to the Internet. It is most attractive to Internet surfers and users of remote LANs, because they typically download much more data than they send.

G.dmt ADSL - G.dmt ADSL (also known as Full-rate ADSL) is the other standard for home DSL service. The G.dmt variety can download data at up to 8 Megabits per second, and send data upstream at up to 1.5 Megabits per second, if the modem is located within 10,000-12,000 feet of the phone company's CO (central office). Up to 18,000 feet away from the CO, G.dmt ADSL can reach up to 1.5 Megabits per second downstream. This type of DSL may require the telephone company to install a device called a "splitter" on the phone line, requiring an installation visit to your home. The big difference between G.dmt and G.lite (discussed below) is speed.

G. lite ADSL - G.lite ADSL (also known as universal ADSL) is a new standard for DSL service that became available in mid-to-late 1999. The cost for equipment and service will be less than other varieties. It will also be easier to install than other varieties—you will be able to do it yourself. It is based on ADSL, and offers downstream speeds up to 1.5 Megabits per second and a maximum upstream data rate of 384 Kilobits per second. The major downside to G.lite ADSL is that if it is unacceptable for voice and entertainment applications.

RADSL (Rate Adaptive Asymmetric Digital Subscriber Line) - RADSL operates at the same bandwidths as ADSL—up to 7 Megabits per second downstream and up to 1.5 Megabits per second upstream, with the additional capability of adjusting bandwidth to the quality of the phone line during the data transmission, instead of just once at the start of the connection.

Top of Page

DSL for Business
Symmetric DSL provides the same rate both ways, and is suited more to Web servers, corporate networks, and those who send out large quantities of data.

HDSL (High Bit-Rate Digital Subscriber Line) - HDSL is the most established of the DSL technologies. It is symmetric, with a maximum 1.5 Megabits per second traveling both ways over two copper phone lines, or 2 Megabits per second over three phone lines. It is often utilized as an alternative to T1 connections. (A T1 connection is a high speed, dedicated telephone line offering 1.54 Megabits per second of data transfer.) HDSL is limited to a distance of 12,000-15,000 feet. This range can be extended with the use of signal repeaters.

HDSL II (High Bit-Rate Digital Subscriber Line II) - HDSL II offers the same performance as HDSL, but over a single phone line.

SDSL (Symmetric Digital Subscriber Line) - SDSL offers a symmetric transmission of data at the same speed as HDSL, with two important differences: it can be done using only one phone line and the user must be no more than 10,000 feet from the phone company's central office. SDSL is the forerunner to HDSL II.

IDSL (ISDN Digital Subscriber Line) - IDSL is a hybrid of DSL and ISDN technologies. It uses the same data encoding technique of ISDN devices and delivers up to 144 Kilobits per second bandwidth. The difference between the two is that IDSL bypasses the congested phone network and uses the data network instead. Also, there is no call setup delay like you experience with ISDN connections.

VDSL (Very High Bit-Rate Digital Subscriber Line) - Currently in its experimental phase, VDSL is the fastest DSL technology, with rates from 13 to 52 Megabits per second downstream and 1.5 to 2.3 Megabits per second upstream. The tradeoff for this speed is that the maximum distance from the central office to the user must be between 1,000 and 4,500 feet.

Top of Page

Choosing a Service Provider
Because DSL uses phone lines, many of the same companies that provide Internet access with an analog modem will offer DSL service. There are three types of providers: 1) traditional telephone companies, 2) new, competitive telephone companies and, 3) ISPs (Internet service providers).

Traditional telephone companies include all the companies that used to provide telephone service in one area on a monopoly basis (also known as Incubent Local Exchange Carriers or ILECs). These include huge companies such as Southwestern Bell, Pacific Bell, Bell Atlantic, BellSouth, USWest, Ameritech and GTE, as well as very small companies that serve a single service area. The larger phone companies, as well as many of the smaller ones, offer DSL in at least part of their service area, with coverage areas increasing every month.

The new, competitive telephone companies (created after The Telecommunications Act of 1996 was passed) that compete to offer local phone services will also offer DSL service (also known as Competitive Local Exchange Carriers or CLECs).

Internet service providers (ISPs) provide access to the Internet. The ISPs that offer DSL usually don't own the equipment that makes the service possible. Instead, they buy the service from a traditional phone company or one of the newer competitive ones. The distinctions between telephone companies and Internet service providers are already blurred because ISPs can also be telephone companies. Also, many telephone companies sell Internet access. The terms NSP (network service provider) and USP (universal service provider) are coming into use to describe these companies that sell many different communication services.

Top of Page

It is important to make sure that the DSL modem (sometimes referred to as a "terminal adapter") works with the DSL provider's equipment. The DSL modem should be included in the package with the high-bandwidth service sold to you by the DSL provider. This will probably change in the future as more manufacturers produce equipment based on universal standards. If you buy a modem off the shelf, ask if it is compatible before you order service from a DSL provider.

If you have more than one computer at home, they can all be connected to one DSL modem using a home network. One option is to buy an Ethernet hub and connect all your computers to it, much like a small-office local area network (LAN). You can then connect the hub to your DSL modem and all the computers can access the DSL connection. There are some DSL modems that include an Ethernet hub. One disadvantage of this approach is that you would need to install special wiring throughout your home to connect the computers. Another option is to use one computer as a "gateway" to other computers in the home via home networking technology. There are three types of home networking that don't require any new wiring in the house: powerline, phone line, and wireless. Powerline technology uses the electrical wiring and outlets of your home to create a network. Phoneline networking does the same thing using the telephone wiring and outlets—and it does not interfere with phone calls on the same wires. Wireless technology accomplishes the task using two-way radio waves transmitted through the house. Overall, using a PC as a gateway has the disadvantages of: requiring some technical expertise, requiring the gateway PC to be turned on for other PCs and networked devices to use the Internet connection, and lacking reliability as PCs often crash or lock up.

A new category of ADSL equipment known as residential gateways is emerging. These dedicated devices act as a bridge between the Internet and the home LAN. It is a specific purpose, stand-alone box that acts as a DSL modem and a home networking hub for multiple PCs. The main advantages of a residential gateway will be their ease of use and reliability. More intelligent residential gateways, called multi-service residential gateways, will provide additional capabilities, such as enhanced telephone features and entertainment services. 2Wire's Home Portal is a multi-service residential gateway. For more information on the HomePortal, click here.

Top of Page

Competing Technologies and Comparisons
Great thing about all these new technologies is the competition among them. You should expect competition to lead to higher quality and lower prices. You are the one who should benefit, but how do you make the right choice? What do all the acronyms mean? Can a cable modem really deliver 30 megabits per second? How reliable is an ISDN line?

What you need are clear, simple facts about all your options. There’s no use trying to hide the fact that we’re in the DSL residential gateway business. But we figure the best way for you to make the right choice is to know all the facts. If you’d like to know how DSL stacks up against the others, read on.

vs. Analog dial-up modems

Analog modems have reached their technical limit in V.90 technology that delivers up to 56.6 Kilobits per second. Attractive pricing and compatibility with most phone lines have made them the communication technology of choice for individual PC users. It is widely believed that a 56K, V.90 modem squeezes close to the maximum amount of data that can be fit on an analog phone line, and you can only get that bandwidth if your ISP has V.90-compatible equipment on their end. In addition, the modem only achieves its top speed if the signal only goes through one analog-digital conversion and the quality of the line is good, which means that most V.90 modems rarely deliver the full 56 Kilobits per second.

The only analog trick left is to bond two 56K modems together for more bandwidth, sometimes referred to as the "shotgun" setup. But, this increases your Internet access costs, and can be frustrating to set up. If you want to try this, you'll have to find an ISP that can support this method—and you'll need two telephone lines. Even if you can get 112K out of this arrangement, it still has drawbacks when compared to the other technologies. If you're more than a casual Internet user, a 56K modem is not good enough. For the near future, you can expect to deal with the problems caused by congestion of the public phone network, which was not designed to carry Internet traffic.

vs. Cable modems

Cable modems are the primary competitor to DSL. They offer high-bandwidth Internet access over cable TV lines, the service is reasonably reliable, and the service costs approximately $40-60/month (plus a $100-175 installation fee). Cable modems are used primarily for residential Internet access because office buildings and business parks aren't usually wired for cable television service.

There are two types of cable modems. The most prevalent is the hybrid fiber/coax (HFC) modem, which runs over HFC cable networks and offers theoretical download speeds from 3 to 30 megabits per second; however, real-world data indicate that you can expect speeds from 400 to 1440 Kilobits per second.

The other, older type is a one-way modem that runs over standard cable coaxial networks. These modems offer up to 2 Megabits per second download speeds, but they offer no upload capacity because cable networks were not originally designed for two-way communication. With the older system, if you need to send out information over the Internet, you will likely need a separate phone line, modem, and ISP. This one-way approach is being dumped as cable companies upgrade to a two-way infrastructure.

In order to offer the two-way service, the cable company must have already upgraded your neighborhood to hybrid fiber/coax cable lines. As of early 1999, only about 30% of the total cable lines in the United States had been converted to HFC. One reason for the spotty availability of cable modem service is that upgrading lines is very expensive. In addition, cable lines are not likely to be located around office buildings and business parks, so high-bandwidth options for businesses probably won't include cable.

Cable networks differ from DSL networks in their basic structure. With a cable modem, you are sharing access to the Internet, unlike DSL which runs on a dedicated connection. Cable modem service is set up like a local area network (LAN), making it possible for many users to share the same bandwidth. The downside of shared access is security—experienced hackers may be able to break into other computers on the same cable network in the neighborhood. Also, as more users in a neighborhood send and receive information, the available bandwidth for individual users could shrink, slowing speeds similar to the rush hours on a city highway.

It is worth asking about content restrictions before you purchase cable modem service. If you want to watch video, some cable systems would prefer that you use their cable TV service for that, so they restrict the amount of streaming video you can access with your cable modem! One company is trying to require cable modem subscribers to take its proprietary content. In other words, you would see what the company wants you to see, and you wouldn't be able to venture beyond that area to the whole Internet. The Consumers Union and the Consumer Federation of America are attempting to stop these practices by convincing the Federal Communications Commission that cable systems are common carriers like phone networks. If they are successful, the FCC could legally prevent cable companies from putting any restrictions on your access to the Internet.

Top of Page

vs. Satellite access

The most widely available high-bandwidth Internet access technology in a geographic sense is Direct Broadcast Satellite (DBS), also known as Digital Satellite Service (DSS)—a competitor to cable television that 6.6 million Americans were using in 1998. As long as you have a clear line of sight to the southern sky, you can order a special type of Internet access through your DBS service.

DBS service requires a small dish (usually 18 to 21 inches across), mounted outdoors to receive data sent from a stationary satellite. A recent test showed that DBS service providers can deliver download speeds in the neighborhood of 350 kilobits per second.

The downside is that the competitively-priced services are receive-only, much like the one-way cable modem setup. You still need a phone line and modem to request information from the Internet. Also, satellites are slow for Internet usage. Let's say you have a DBS Internet access provider and you click on a link to pull up a Web page. Clicking on that link is actually a request for information that travels out on your phone line, through the ISP, through the normal paths on the Internet, and is finally answered. The information you requested is then sent 22,300 miles up to the satellite, and then back down 22,300 more miles to your DBS dish. Even at the speed of light, this method of communication is slow enough to cause a noticeable lag between click and response. This is not a big problem for downloads, but if you're a chat junkie, or you use the Internet for a lot of live, two-way communication, DBS may not be the best choice for you.

Setup is a bit more of a hassle with DBS, too. Getting the DBS hardware to "talk" to your computer is an issue, and satellites can have technical problems. Historically, they have been very reliable, but when they quit working, there is no quick fix. It's possible that you could be without service for a while, or you may be asked to switch to another satellite, possibly requiring you to reposition your dish and/or retune your satellite receiver.

One of the most well-known companies in DBS Internet access services is charging $179 - $229 for installation, and the dish and satellite modem together cost about $350. Monthly service rates range from $20 for 25 hours per month of online usage, up to $110 for 200 hours per month. Additional hours are billed at $1.99.

vs. ISDN

ISDN is a widely available, faster alternative to analog modems. If your area does not offer any of the other high-bandwidth options, ISDN is the way to go. ISDN works over standard copper phone wiring, like DSL, and gives you three data channels to work with. Two channels operate at 56 Kilobits per second or 64 Kilobits per second, depending on how your phone company has configured them. These two channels can be "bonded" together to double the speed. The third channel is used to connect your calls and uses only 16 Kilobits per second of bandwidth.

With the right hardware however, 9.6 Kilobits per second of this bandwidth can be used as an always-on Internet connection. This feature is called Always-On/Dynamic ISDN (AO/DI). It is useful for low-bandwidth applications such as e-mail and stock quotes. In addition, when the capacity hits the 9.6 Kilobits per second limit, the hardware can bring up another channel to accommodate it. When the capacity need falls back down, the channel is dropped. Always-On/Dynamic ISDN is just catching on. If you are interested, you will have to buy an AO/DI modem, buy a Dial-Up Networking Patch for Windows, and find an ISP that can support it.

Depending on how much competition there is in your high-speed access market, installation and setup charges for ISDN range from $125 to $400, including a good adapter to connect your computer to the network. Monthly service costs range from around $20 to $80.

Conclusion

Which technology do you choose? Well, let's put it this way—if you have a choice, you're lucky. In many parts of the United States, only one or two of these technologies are available. As with most electronic media, bigger cities make more choices possible. If you do live in an area that has more than one option, then we hope we've made the job of choosing which is best easier for you.

Whichever way you go, high-speed Internet promises to be much easier to use than the dial-up method you're probably accustomed to. Imagine the rich content of the Internet at your fingertips, ready to be accessed with a few clicks and no wait! Think of how much more fun surfing the Internet will be—graphics popping into place instead of slowly rendering, CD-quality sound, high-quality video, and blazing fast downloads. The options are endless! There's just one question that we can't answer—What will you do with DSL?

This information came from http://www.2wire.com/learning_center/dsl.asp

Return to previous menu