Digital Subscriber Line

The biggest rival to the cable modem in the broadband Internet business is the digital subscriber line (DSL). DSL, like its predecessor ISDN, appeals to the telephone companies who might be able to use the existing POTS wiring to provide high-speed Internet access. Not every type of DSL is suitable for existing wiring; however, all but the fastest, most expensive types can sometimes be used with the existing POTS plant. DSL is also appealing to businesses that don't have access to cable modems but are looking for a high-performance, lower-cost alternative to the expensive ISDN services that top out at 128Kbps.

Note

Some technical discussions of DSL refer to xDSL. The x stands for the various versions of DSL being proposed and offered by local telephone companies and ISPs. DSL generally is used to refer to any type of digital subscriber line service.

One advantage of DSL compared to its most popular rivalcable modemsis that cable modems share common bandwidth, which means that a lot of simultaneous use by your neighbors can slow down your connection. If you use DSL you don't have this concern; whatever bandwidth speed you pay for is yoursperiod.

How DSL Works

DSL takes advantage of the broadband nature of the telephone system, using the system's capability to carry signals at multiple frequencies to allow both high-speed Internet traffic and phone calls at the same time. Two methods for sending and receiving signals are used by the most common type of DSL, Asymmetric DSL (ADSL):

Carrierless Amplitude/Phase (CAP)
Discrete Multitone (DMT)

Most early DSL installations used CAP, which splits the telephone line into three frequency bands. Exact frequency usage varies by system, but most typically, the divisions resemble the following:

Voice calls use frequencies from 30Hz to 4KHz. This frequency is also used by answering machines, fax machines, and alarm systems.
Upstream data such as web page requests and sent email uses frequencies between 25Hz and 160Hz.
Downstream data such as received web pages and email uses frequencies between 240KHz and 1.5MHz.

Some systems use the 300Hz700Hz range for downstream data and frequencies of 1MHz and above for upstream data.

Because voice, downstream, and upstream data use different frequencies, you can talk, surf, and send email at the same time.

DMT, the system used by most recent ADSL installations, divides the telephone line into 247 channels that are 4KHz wide. If a particular channel has problems, a different channel with better signal quality is used automatically. Unlike CAP, DMT uses some channels starting at around 8KHz to send and receive information.

Both types of signaling can have problems with interference from telephones and similar devices, so devices called low-pass filters are used to prevent telephone signals from interfering with signals above the 4KHz range, where DSL signals begin. The location of these filters depends on the type of DSL you use and whether you are installing DSL service yourself.

At the central switch, DSL data is transferred to a device called a DSL access multiplexer (DSLAM), which transfers outgoing signals to the Internet and sends incoming signals to the correct DSL transceiver (the correct name for the so-called "DSL modem" that connects to your computer).

Who Can Use DSLand Who Can't

DSL services have been slowly rolling out across the country for years, first to major cities and then to smaller cities and towns. As with 56Kbps modems, rural and small-town users are often the last to receive this service. If you live in an area without access to cable or Internet, you should consider satellite-based or fixed wireless Internet services where available for a faster-than-56Kbps experience.

Just as distance to a telephone company's central switch (CS) is an important consideration for people purchasing an ISDN connection, distance also affects who can use DSL in the markets offering it. For example, most DSL service types require that you be within about 18,000 feet (about 3 miles) wire distance to a telco offering DSL; some won't offer it if you're beyond 15,000 feet wire distance because the speed drops significantly at longer distances. Repeaters or a local loop that has been extended by the telco with fiber-optic line might provide longer distances. The speed of your DSL connection varies with distance: The closer you are to the telco, the faster your DSL access is. Many telcos that offer some type of DSL service provide websites that help you determine whether, and what type of, DSL is available to you.

If you want to locate DSL service providers in your area, compare rates, and see reviews from users of the hundreds of ISPs now providing DSL service, set your browser to http://www.dslreports.com. The site provides a verdict on many of the ISPs reviewed, summarizing users' experiences and ranking each ISP in five categories.

Note

If you want to connect DSL to your SOHO or office LAN, check first to see what the provider's attitude is. Some users report good cooperation, whereas others indicate they were told "we can't help you" or were told that DSL "couldn't be connected to a LAN." Fortunately, as home LANs continue to proliferate, this is becoming a less frequent problem. Again, check around for the best policies. Low-cost switch/router combinations from companies such as Linksys and D-Link and Microsoft's Internet Connection Sharing provide relatively easy ways to share both DSL and other types of high-speed connections.

Even if your telco's central switch is well within wire distance range of your location, that's no guarantee that you qualify for DSL service. The design and condition of the wiring plant connecting your location with the central switch can prevent you from qualifying for DSL service. Because DSL service depends on successful sending and receiving of high-frequency data, a telephone wiring plant that blocks high-frequency signals can't be used for DSL service. Some of the typical issues with telephone lines that aren't DSL-friendly include

Loading coils. These amplifiers boost voice signals and are sometimes called voice coils. Unfortunately, these block the high-frequency signals needed by DSL service.
Bridge taps. Used to extend service to new customers without running separate lines all the way back to the central switch. Bridge taps can create a circuit that's too long for DSL service.
Fiber-optic cables. Used to carry a lot of signals in a small physical space, fiber-optic cables use analog-to-digital (A/D) and digital-to-analog (D/A) converters where they connect to copper telephone lines. A/D and D/A converters can't pass DSL signals through to their destinations.

Major Types of DSL

Although the term DSL is used in advertising and popular discussions to refer to any form of DSL, many, many variations of DSL are used in different markets and for different situations. This section discusses the most common forms of DSL and provides a table that compares the various types of DSL service. Although many types of DSL service exist, you can choose only from the service types offered by your DSL provider:

ADSL (Asymmetrical DSL). The type of DSL used most often, especially in residential installations. Asymmetrical means that downstream (download) speeds are much faster than upstream (upload) speeds. For most users, this is no problem because downloads of web pages, graphics, and files are the major use of Internet connections. Maximum downstream speeds are up to 9Mbps, with up to 640Kbps upstream. Most vendors who offer ADSL provide varying levels of service at lower speeds and prices, as well. Voice calls are routed over the same wire using a small amount of bandwidth, making a single-line service that carries voice and data possible. ADSL is more expensive to set up than some other forms of DSL because a splitter must be installed at the customer site, meaning that you must pay for a service call (also called a truck roll) as part of the initial setup charge.
CDSL (Consumer DSL). A slower (1Mbps downstream) form of DSL that was developed by modem chipset maker Rockwell. It doesn't require a service call because no splitter is required at the customer site.
G.Lite (Universal DSL, and also called DSL Lite or Splitterless DSL). Another version that splits the line at the telco end rather than at the consumer end. Downstream speeds range from 1.544Mbps to 6.0Mbps, and upstream speeds can be from 128Kbps to 384Kbps. This is becoming one of the most popular forms of DSL because it enables consumers to use self-install kits. Note that the DSL vendor might cap the service at rates lower than those listed earlier in the chapter; check with the vendor for details.
SDSL (Symmetrical DSL). This type of DSL service provides the same speed for upstream as for downstream service. Generally, SDSL is offered to business rather than residential customers because it requires new cabling (rather than reusing existing phone lines). A long-term contract frequently is required.

Table 17.2 summarizes the various types of DSL.

Table 17.2. DSL Type Comparison
DSL Type	Description	Data Rate Downstream; Upstream	Distance Limit	Application
IDSL	ISDN Digital Subscriber Line	128Kbps	18,000 feet on 24-gauge wire	Similar to the ISDN BRI service but data only (no voice on the same line)
CDSL	Consumer DSL from Rockwell	1Mbps downstream; less upstream	18,000 feet on 24-gauge wire	Splitterless home and small business service; similar to DSL Lite
DSL Lite (same as G.Lite)	Splitterless DSL	From 1.544Mbps to 6Mbps downstream, depending on the subscribed service	18,000 feet on 24-gauge wire	Sacrifices speed for not having to install splitters
HDSL	High bit-rate Digital Subscriber Line	1.544Mbps duplex on two twisted-pair lines; 2.048Mbps duplex on three twisted-pair lines	12,000 feet on 24-gauge wire	T-1/E1 service between server and phone company or within a company
SDSL	Symmetric DSL	1.544Mbps duplex (U.S. and Canada); 2.048Mbps (Europe) on a single-duplex line downstream and upstream	12,000 feet on 24-gauge wire	Same as for HDSL but requiring only one line of twisted pair
ADSL	Asymmetric Digital Subscriber Line	1.544Mbps to 8.448Mbps downstream; 16Kbps to 640Kbps upstream	1.544Mbps at 18,000 feet; 2.048Mbps at 16,000 feet; 6.312Mbps at 12,000 feet; 8.448Mbps at 9,000 feet	The most common DSL used for Internet access
VDSL	Very High Subscriber Line Digital	12.9Mbps to 52.8Mbps downstream; 1.6Mbps to 2.3Mbps upstream	4,500 feet at 12.96Mbps; 3,000 feet at 25.82Mbps; 1,000 feet at 51.84Mbps	ATM networks; Fiber to the Neighborhood

With any type of DSL, an external device called a DSL modem is attached to the computer through either of the following:

A crossover cable running to an Ethernet card or port in the computer
A USB cable running to a USB port in the computer

An RJ-11 (standard telephone) cable is attached between the DSL modem and the RJ-11 port that has been set up for DSL service.

To prevent telephone signals from interfering with DSL frequencies, splitters or microfilters must be installed on a DSL line. If you choose a technician-installed form of DSL, a device called a splitter is used at your location to prevent interference. Splitter-based DSL allows faster speeds than splitterless DSL installations, but the wait for a technician to show up and add the splitter can add days or weeks to your installation process.

If you self-install DSL, you will install small devices called microfilters to block interference from telephones, answering machines, and similar devices. These devices might fit behind the faceplate of the wall outlet used for DSL service or inline between the phone, answering machine, or fax machine and the wall outlet (see Figure 17.2).

Figure 17.2. Two types of DSL self-installations; if a splitter is used to set up a separate DSL line, the microfilters shown here are not necessary.

Tip

If you have a security system attached to your telephone line, watch out for problems if you select DSL as your preferred broadband access method. Security systems are often designed to seize the line, interrupting a phone call in progress to send an alarm to the security company. This feature won't work with normal microfilters, so you should purchase a special DSL Alarm filter to allow your alarm system to coexist with your DSL installation. Get more information about the alarm microfilter and alternative DSL installation options from www.hometech.com/learn/dsl.html.

DSL Pricing

DSL pricing varies widely, with different telephone companies offering different speeds of DSL and different rates. One thing that's true about the most commonly used flavors of DSL is that they are usually an asymmetrical servicewith download speeds faster than upload speeds. ADSL installations can typically be run over existing copper wires, whereas SDSL installations usually require that new high-quality copper wires be installed between the CO and the subscriber's location.

For unlimited use, typical residential DSL pricing ranges anywhere from $15 to $80 a month depending on whether you want a static or dynamic IP address and the download speed, which ranges from 256Kbps to 1.5Mbps. Business DSL pricing ranges from $50 to as high as $500 per month.

The wide variance is partly due to the upload speeds permitted. The lower-cost plans typically use a lower upload speed (some variation on ADSL or G.Lite); in contrast, the more expensive plans often use SDSL. Check carefully with your vendor because your traditional telephone company might not be the only DSL game in town. Some major cities might have as many as half a dozen vendors selling various flavors of DSL.

DSL Security Issues

Unlike other types of broadband access, DSL is a direct one-to-one connection that isn't shared; you have no digital "neighbors" who could casually snoop on your activities. However, as with any broadband "always-on" connection, intrusion from the Internet to your computer is a very real possibility and you should safeguard your system behind a router with firewall capabilities. Using an additional software firewall, such as the one included with Windows XP, is also a good idea.

	For more information on securing any type of Internet access, see "Securing Your Internet Connection," p. 1075.

Technical Problems with DSL

Telecommunications has always had its share of difficulties, starting with the incredibly slow and trouble-plagued 300bps modems used on early PCs, but as speed increases, so do problems. DSL connections are often very difficult to get working correctly because DSL, as you've seen, combines the problems of adding high-speed data access to the telephone line with network configuration using TCP/IP (the most powerful and most complex network protocol in widespread use; see Chapter 18, "Local Area Networking," for details).

A review of comments from DSL users in various forums, such as DSLReports.com and others, shows that the most common problems include the following:

Poor coordination between the DSL sales department of the telco or third-party provider and the installers. This can lead to broken or very late appointments for installation; if possible, contact the installer company to verify the appointment. If possible, opt for a self-install version of DSL to avoid problems with late or missing appointments.
Installers who install the hardware and software and then leave without verifying it works properly. Ask whether the installer carries a notebook computer that can test the line; don't let the installer leave until the line is working.
Poor technical support before and after installation. Record the IP address and other information used during the installation; read reviews and tips from sources listed earlier in the chapter to help you find better DSL providers and solutions you can apply yourself or ask your telco or provider to perform.
Lower speeds than anticipated. This can be due to a poor-quality connection to the telco from your home or business or problems at the central switch; ask the installer to test the line for you during initial installation and tell you the top DSL speed the line can reach. On a healthy line, the problem is often traceable to a very low value for the Windows Registry key called RWIN (receive window), which should be adjusted from its default of 8192 (8KB) to a value as high as 32768 (32KB) or even 65535 (64KB). If your system previously was used with a dialup modem, the value for RWIN can be as low as 2144; low values force your DSL connection to receive data at rates hardly faster than those for a dialup modem connection. For interactive tests that will help you find the best value to use for RWIN or other Registry options, find line problems, and adjust your configuration, go to http://www.dslreports.com and follow the Tools link from the home page.

Because of the problems with trying to retrofit an aging voice-oriented telephone network with high-speed Internet service, many pure DSL companies are having financial problems. Some once-prominent DSL ISPs went out of business in 20002001, leading to service cancellations in some cases. Before you sign a long-term contract for DSL service, you should determine what your options are if your telco, DSL line provider, or ISP drops DSL service.