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Chapter 9
ADSL

Questions Answered in This Chapter

What is ADSL?

What is DSL?

What are the different flavors of DSL?

How is information coded into a DSL signal?

What are the DSL architectures?

What are POTS (and pans)?

How does ATM fit with ADSL?

What are the ADSL market hurdles?

What is ADSL?

What is ADSL? In terms of our OSI protocol model, ADSL is a physical layer transmission protocol for unshielded twisted pair media and is recognized as such by the ATM Forum. A word now about DSL to save, hopefully, some confusion later on. DSL is used to describe the general transmission technology of transporting data over twisted pair copper lines and it is also used to describe a specific flavor of DSL. So, there is a generic DSL that includes all flavors of DSL and there is the specific flavored DSL. We (folks in the industry) pray that the reader will realize which flavor is the topic by the usage in the literature. The usage of the term in this book is clear from the context and by specifically referring to the particular usage.

Asymmetrical Digital Subscriber Line (ADSL) technology—sounds kind of daunting. Well, if we look at each of the terms separately, perhaps we will not be overwhelmed by the seemingly incomprehensible terminology.

Symmetrical refers to the similarity of form or arrangement on either side of a dividing line or plane. If one side of the symmetrical arrangement is discarded by some means, then the resultant arrangement is called asymmetrical. So, asymmetrical describes some arrangement that had its origin in a process that generated a symmetrical arrangement. Subsequent processes produced the single asymmetrical arrangement. In the world of ADSL, asymmetrical refers to the fact that communications is primarily one way. That is, the bulk of ADSL communications is from the service provider, such as a video supplier or Internet service provider, to the user.

Digital refers to the practice of encoding information in digits. In this instance, we are encoding the information in a binary manner. That is, every possible value of the individual information elements, called bits, can assume only one of two values, either a zero or a one.

Subscriber is any person or company, or any other entity, that has telephone service provided by a telephone company. In other words, a subscriber is a telephone company customer.

Line is the twisted pair copper lines that are connected from the subscriber’s telephone to the local exchange. Now, for those few of you who already have fiber-to-the-curb, line refers to your fiber connection to the local exchange. A subscriber line is the telephone line connecting the telephone in your house to the switch at the local exchange.

Here is the confusing part. There really is no asymmetrical digital subscriber line. There is a twisted copper pair of lines that is physically connected from the customer premises to the local office. Only when an ADSL modem is connected to each end of the subscriber’s line is any such thing as an “asymmetrical digital subscriber line” magically pulled from the hat. Without the modems, it is just a humble, twisted pair of copper lines.

Okay, let’s get it out in the open now. No more hiding behind technological foo-foo dust. ADSL technology is modem technology. Whoopee! Big deal! Modems are a dime a dozen (figuratively speaking). We are accustomed to seeing and using subscriber line modems now. Everybody with dial-up Internet access must have a subscriber line modem. Even if you do not have Internet access you still probably have a subscriber line modem in your computer (or sitting next to it). So, what’s the big deal about Another Danged Subscriber Line (ADSL) modem? Speed and entertainment.


Figure 9-1.   Modern analog modem communications

The current modems are voice frequency analog modems. Voice frequency means they operate down in the frequency range of human voices, and at the low end of the range. Telephone companies band-limited telephone lines long ago from 600 Hz to 4 KHz yielding a 3.4 KHz bandpass on the PSTN. Therefore, current modem technology was developed to exploit the 3.4 KHz bandpass available in the voice frequency range. Now, only so much data can be transmitted in the 600 Hz to 4 KHz spectrum. With the latest analog technology available, analog modem manufacturers are pushing the envelope to 56.6 Kbps. Telephone companies treat voice frequency analog modems just like they treat your telephone conversation with your broker, or your granny, or anybody else you have a telephone conversation with. The analog modem signals are passed through the bowels of the telephone network without alteration. With a low price tag (now but we would not have made this claim several years ago), lots of phone lines to connect to, lots of slow-speed data to exchange, and the ability to connect them almost immediately anywhere in the world, voice frequency analog modems were Rulers of the Subscriber Line (RSL)!


Figure 9-2.   ADSL modem communications

But faster Internet access and video-on-demand are driving the desire to trash the slow analog modems and replace them with something that will bring computer monitors and television sets to life. ADSL modems will do just that. If you don’t replace your RSL modem with an ADSL modem PDQ, you will be SOL if you want VOD on your HDTV.

DSL Technology

Bellcore defined the basic DSL acronyms in 1989 when Bellcore engineers dreamed up ADSL. Originally, these engineers envisioned a digital data stream transmitted over the humble twisted pair lines that would be sufficient to transport video images. At the time, 1.5 Mbps upstream and 16 KHz downstream seemed sufficient to accommodate digital video traffic. Since its original conception, ADSL data rates have increased significantly as the companies compete to provide data rates sufficient to accommodate virtually every conceivable service subscribers may yearn for.


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