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ADSL Lite: The Plot Lightens By CommWeb
We've spent the last several lessons working through some important DSL technologies and issues. We started off with HDSL (High-Bit-Rate DSL), and then examined some of the common characteristics and limitations of DSL in a lesson I dubbed DSL: Do You Qualify? (Initially, I didn't qualify, then I qualified on a limited basis, and now I qualify fully.)
In the last lesson, we explored ADSL (Asymmetrical Digital Subscriber Line), the original ADSL version. Now we're ready to dig into ADSL Lite. That's the version with which many of us are familiar, the one I'm having installed, and the one that will serve most of us into the foreseeable future.
Background
ADSL, as you'll recall from the last lesson, runs over a UTP (Unshielded Twisted Pair) local loop provided by the ILEC (Incumbent Local Exchange Carrier). At distances up to 18,000 feet, a qualified loop can support simultaneous voice, data and entertainment TV transmission.
The TV capability was offered in test markets, but hasn't been offered commercially to the general market as it is so difficult to support over all but the most exceptional local loops. So, that leaves us with voice and data.
Voice is supported in its native analog format via a 4 kHz channel. Data transmission is supported at various signaling rates, with the maximum rates being sensitive to the specifics of the local loop. Those specifics include wire gauge(s), number of splices, loop length, bridged taps, and crosstalk, as we discussed in DSL: Do You Qualify?
ADSL is asymmetric in nature, at least with respect to data transmission. Significantly more bandwidth is provided downstream than upstream, for technical reasons of signal attenuation and crosstalk.
Fortunately, this approach matches most user requirements in terms of Internet access, as most of the action happens on the downstream side. If you're running a Web-based business with a server on your premises, your transmission requirements are exactly the reverse, with most of the action happening on the upstream side, and ADSL is not for you.
The analog voice channel, of course, is symmetric.
So, What's ADSL Lite?
ADSL Lite, also known as G.lite and Splitterless ADSL, is an interoperable extension of full-rate ADSL, also known as G.dmt after the DMT (Discrete MultiTone) modulation technique employed. (The G comes from the ITU-T standards designation G.922.1 for ADSL and G.922.2 for ADSL Lite.)
ADSL Lite works just like ADSL in virtually every respect. The same DMT modulation technique is used, the same asymmetry applies, the same local loop qualification issues arise, and the same speed ratings apply. The big difference is that ADSL Lite requires no centralized splitter (also known as a modem or filter) at the customer premises.
That means that no truck roll is required on the part of the telco to install an ATU-R (ADSL Terminating Unit-Remote). In my case, rather, the telco mailed me a DSL modem, some software, and five (5) microfilters for my telephone sets.
The attached figure that I borrowed from my book, Communications Systems & Networks, illustrates a typical ADSL Lite configuration.
The ADSL modem plugs into the RJ-11 wall jack on one side and into the workstation on the other. As the inside wiring, of course, is shared between the workstation and the telephone sets, the effect is that of multiple bridged taps on the line -- they're just inside the premises rather than down the street.
So, you plug microfilters between the telephone jacks and the sets. The microfilters serve to filter out any electrical and RF anomalies when phones ring, or one or more go off-hook to answer a call or on-hook to terminate a call.
That's about it. If everything goes just right, ADSL Lite works just great. The natural expectation, of course, is that the inside wire bridged taps will have an impact on performance -- and they will. Usually, they're not that much of a problem, and it's a fair enough tradeoff for the reduced costs of installation and service.
Remember that you install ADSL yourself, at least on the premises side of things. The telco does its thing at the CO (Central Office), where an ATU-C (ADSL Terminating Unit-Centralized) resides, just as was the case with the original full-rate ADSL.
So, while ADSL Lite really isn't completely splitterless, it certainly is splitterless at the premises. The ATU-C terminates in a DSLAM (DSL Access Multiplexer) collocated in the CO. If the ILEC is also your ISP, the telco-owned DSLAM connects to the Internet backbone via a high speed circuit such as a T1, T3, or SONET fiber optic facility.
If you use another ISP, its collocated DSLAM connects to its offices via a high speed facility. Traffic destined for the Internet backbone connects over another high speed facility, perhaps looping right back through the same CO.
What about Speed?
ADSL Lite can run at the same speeds as full rate ADSL, which can be as high as 6.144 Mbps downstream and 640 Kbps upstream, although that is very unusual due to the demands such high speeds place on the local loop cable plant. As a practical matter, ADSL Lite more typically tops out at 1.536 Mbps (T1) downstream, and 256 Kbps upstream. In my case, through Verizon, my options are as follows:
So, that's the essentials of ADSL Lite in 1,000 words, or so. As my more loyal readers know, I'm currently waiting, as I write this column, with bated breath for my ADSL Lite service to be installed.
(For more on Professor Ray's ongoing DSL ordeal, check out his threaded messaging postings, titled "True Lies" and "Professor Ray's DSL Saga" in both the DSL: Do You Qualify? column and ADSL.)
This afternoon, I'm installing my new 802.11 WLAN (Wireless Local Area Network) to connect my laptop and desktop, and Margaret's (She's my drop-dead gorgeous bride.) two laptops through a hub/switch. I'll give that network a day or two to settle down.
On Thursday, October 17, I'm planning to install and program the ADSL Lite modem, connect it to my 802.11 hub/switch, plug the microfilters in front of the telephones and fax machine that share the same ADSL line, and surf the Web at T1 speeds.
So, by the time you read this column, I am planning to be one very happy guy. That assumes, of course, that my friends at Verizon don't let me down, which means that my loop test was accurate and that the thing will really work as promised.
I'll keep you posted.
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