The world is wired — wired for telephones, that is. DSL modems take advantage of this old, but common, technology to provide a high-speed Internet connection to consumers. The DSL option uses special equipment to pump much more data through the POTS lines than a traditional analog modem does.
Remember The telephone system is referred to as plain old telephone service (POTS) in the telecommunication industry.
DSL provides high-speed Internet connections by electronically converting your computer’s digital information into a form that can be transmitted from your home or business to the telephone company. When your data finds its way to the telephone company, it’s converted into another form and sent to your ISP.
Technical StuffDSL uses frequencies in the millions of cycles per second — the megahertz (MHz) range — compared to traditional analog modems, which work with signals in the thousands of cycles per second (KHz). You get much higher connection speeds when you use higher frequencies. The problem is, however, that the telephone system wasn’t designed to work with higher frequencies.
Fortunately, the brainiacs of the world have figured out how to get high-speed DSL connections from old, slow POTS wiring. They have designed new digital signal processing chips to overcome the POTS architecture. The result is that if you live close enough — roughly three to four miles — to your DSL provider’s equipment, you can use DSL to get connected to the Net.
This section describes the basic DSL modem-configuration issues. We take the time to give you an overview because you can easily get confused if you concentrate on just the details. Please check out the following list and get familiar with it. Getting your DSL modem working is easier after you do so.
Most consumer DSL providers now use the asymmetrical DSL (ADSL) type of connection. The following list describes the process for getting an ADSL connection working. (Please note that we use the generic acronym DSL interchangeably with ADSL. Most consumer DSL connections are really ADSL, and that’s the type of connection we describe in this chapter.) Follow these steps to set up DSL service:
Find a DSL provider.
You need to find out whether you live or work close enough to the DSL provider’s equipment to get a connection. DSL providers check your address and tell you whether they can take your business.
Connect your DSL modem to your telephone jack and your computer.
Your DSL modem acts as the intermediary between your computer and your DSL service provider. You must connect one side to the phone jack and the other to your computer’s Ethernet NIC.
Configure your Red Hat Linux computer to communicate with the DSL modem.
Your Red Hat Linux computer connects to the DSL modem via an Ethernet NIC. You must configure your Ethernet NIC to work with the modem.
Set up the DSL modem user and administrative passwords.
DSL modems provide a reasonable level of security. You should take advantage of this security by assigning your own password to the modem. That action prevents hackers from breaking into your modem and causing problems.
Set up your ISP PPP account name and password.
You must authenticate your DSL modem to your ISP. DSL connections get logged on to your ISP just like traditional analog modems do. You configure your DSL modem with your ISP username and password.
Configure the DSL modem’s internal (private) network interface.
Your DSL modem must be able to communicate with a Red Hat Linux computer over an Ethernet connection. You must configure the DSL modem so that it uses the same network parameters as your computer.
Configure the modem’s network address translation (NAT) settings.
Remember The Internet was designed to send — or route — information as quickly as possible to its destination. Internet Protocol (IP) addresses are used to designate where the information is coming from and where it’s going. IP addresses can be routable or nonroutable. Nonroutable addresses can be reused; you can use the same nonroutable addresses that your neighbor uses without interfering with one another.
NAT is used to convert nonroutable IP addresses into routable ones, which is useful when you’re connecting your private network to the Internet by translating your internal IP addresses into one of your ISP’s routable IP addresses. You need to configure your DSL modem to convert your computer’s private (for example, 192.168.1.1) and nonroutable address into an address assigned to your DSL connection by your ISP.
Save the settings to nonvolatile memory and reboot.
You need to save your DSL modem’s settings after you have them working. You don’t want to enter the configuration every time you turn on your modem.
You must obtain both DSL and ISP services to make your broadband connection. Some companies — notably, the regional Bell telephone companies — can provide both services. However, in our case, we preferred our ISP to the ISP that was aligned with the DSL provider. We were fortunate enough to retain our existing ISP when we purchased our DSL service.
Remember The DSL provider market is fluid. Analyze the DSL service providers in your area carefully before choosing one, and remember that longevity is as important as a low price. Regional Bells are more likely to provide long-term service than many of their competitors.
TipOne advantage of DSL service is that you often don’t have to sign a service contract; you can reasonably switch providers if you’re not satisfied with the service.
Usually, you have to select an ISP after you choose a DSL provider. DSL providers either provide their own ISP or allow you to select from several independent ones (the DSL provider makes the arrangements and works directly with the third-party ISP).
Technical Stuff The many faces of DSL
DSL comes in a variety of flavors. Most consumers end up using ADSL because it offers inexpensive Internet connections at reasonably high speeds. ADSL serves an individual computer user’s Internet needs very well; it even provides a small business with adequate service. ADSL is, not surprisingly, the most available of all DSL flavors.
The other types are more suited for business use. Most locales probably have access to only two or three of these services. This list describes the DSL variations:
ADSL (Asymmetrical DSL): The ADSL download (downstream) speed isn’t the same as its upload (upstream) speed. (That’s why it’s asymmetrical.) The maximum ADSL speed is 8 Mbps, but it’s usually limited to less because of the POTS infrastructure limitations.
G.Lite: Also known as Universal DSL or splitterless ADSL, G.Lite is a low-speed version of ADSL that doesn’t require filtering out the POTS signal. It provides as much as 1.5 Mbps downstream and 512 Kbps upstream.
HDSL (high bit-rate DSL): HDSL is a symmetrical protocol with equal upstream and downstream speeds. You can use HDSL as a substitute for T1 connections because it provides the same data rates of 1.544 Mbps.
HDSL2 (high bit-rate DSL 2): HDSL2 provides the same specifications as HDSL but works over a single twisted-pair connection.
IDSL (ISDN Digital Subscriber Loop): IDSL, the successor to the current ISDN technology, uses the same line encoding (2B1Q) as ISDN and SDSL. IDSL is used mostly to provide DSL service in areas where the more popular forms, such as ADSL and SDSL, aren’t available. IDSL is capable of providing upstream and downstream rates of 144 Kbps.
SDSL (Single-line DSL): SDSL is commonly called Symmetric DSL because SDSL up-stream and downstream speeds are the same.
VDSL (Very high bit rate DSL): VDSL provides as much as 50 Mbps over distances up to 1,500 meters on short loops. VDSL is particularly useful for campus environments — universities and business parks. VDSL is now being introduced in market trials to deliver video services over existing phone lines. You can also configure VDSL in symmetric mode.
xDSL: xDSL is a generic term for all the DSL flavors.
Writing explicit configuration examples is always difficult and liable to leave some readers disappointed. But the DSL world is still young, and we’re not convinced that any standards have emerged. Therefore, we think that it’s better to provide the following instructions rather than none. (Chances are that many of you use the same DSL modems, and we hope that our instructions match your equipment.)
This section shows you how to use one of the more common DSL modems. Qwest, which is a “baby Bell” and one of the larger DSL providers, uses this equipment. Our Cisco 675 or 678 DSL modem-router is the Qwest-recommended equipment. Cisco is one the largest network equipment suppliers in the world, and many people use its DSL modems.
TipEven if you’re using different equipment, our instructions should still be useful in outlining the general process of configuring a DSL connection. The process goes like this:
Connect your Red Hat Linux computer to the modem so you can configure it.
Start Mozilla and open the modem’s IP address. DSL modems typically use public addresses by default, such as 192.168.1.1.
Set the modem’s passwords.
Tell the modem how to connect to your DSL provider.
Set up the modem’s firewall and NAT configuration, if it’s available.
Save the configuration.
The details of how to accomplish each step vary between manufacturers, but the idea is still the same. Consult your modem’s user guide for detailed configuration information.