1112 lines
36 KiB
HTML
1112 lines
36 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Appendix: DSL Overview</TITLE
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>DSL HOWTO for Linux</TH
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="OVERVIEW">6. Appendix: DSL Overview</H1
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><P
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> DSL is a telephone loop technology that uses existing copper phones lines,
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and provides a dedicated, high speed Internet connection. One of the big
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advantages of some DSLs (notably ADSL), are that they can co-exist on the
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same line with a traditional voice service such as <SPAN
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CLASS="QUOTE"
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>"POTS"</SPAN
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> (Plain
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Old Telephone Service), and even ISDN. This is accomplished by utilizing
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different frequency ranges above the voice range (voice is up to 4KHz).
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Essentially, this gives two lines in one: one for voice, and one for Internet
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connectivity. When all is working normally, there should be no interference
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between the two <SPAN
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CLASS="QUOTE"
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>"lines"</SPAN
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>. This gives DSL a potentially broad
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consumer base, and helps minimize costs for service providers. </P
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><P
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> DSL is positioned for the Home and Small Office (SOHO) market that is
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looking for high speed Internet access at reasonable prices. Since it also
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typically provides dedicated, <SPAN
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CLASS="QUOTE"
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>"always on"</SPAN
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> access, it can be used
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for interconnecting low to mid range bandwidth servers, and provides a great
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access solution for small LANs. It is also great for those Linux power users
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that just want a fat pipe :-). </P
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><P
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> Phone companies, and other independent telecommunications providers (CLECs),
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are now deploying DSL to stay ahead of the Cable
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companies -- the main consumer and SOHO competition for DSL providers. This
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mad rush to get <SPAN
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CLASS="QUOTE"
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>"a piece of the pie"</SPAN
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>, is bringing much
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competition (a good thing!), much diversity, and some confusion, into the
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consumer market. The DSL provider (often, but not always, the phone company)
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will provide the DSL infrastructure. This would include your line, the DSLAM,
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and physical connection to the outside world. From there it is typically
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picked up by an ISP, who provides the traditional Internet services.
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</P
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><P
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> Consumer DSL plans are typically <SPAN
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CLASS="QUOTE"
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>"best effort"</SPAN
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> services. While
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boasting speeds approaching T1, and even surpassing that in some cases, it is
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not necessarily as reliable as T1 however. Business class DSL offers more
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reliability at a higher cost than consumer plans, and is a good compromise
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where both reliability and bandwidth are at a premium. All in all, the cost
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of DSL compared to traditional telco services, such as T1, is attractive and
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substantially more affordable for home and small business users. </P
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><P
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> DSL providers often do not have service contracts for home users,
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while business class DSL services typically do include similar SLAs (Service
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Level Agreements) to that offered for a T1 line. </P
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><P
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> The downside is that DSL is not available everywhere. Availability, and
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available bit rate (speed), are purely a function of where you live, where
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the telco has installed the prerequisite hardware, how far you are from the
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DSLAM/CO, and the quality of your phone line (loop). Not all loops are
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created equal, unfortunately. The primary limitation is distance. </P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="FAMILY">6.1. The DSL Family</H2
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><P
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> <P
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></P
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><UL
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN985">ADSL</H3
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><P
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> Asymmetric Digital Subscriber Loop currently supports downstream rates up
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to 8 Mbps, and upstream of 1024 Kbps, hence the <SPAN
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CLASS="QUOTE"
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>"asymmetric"</SPAN
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>.
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ADSL is far and away the most widely deployed consumer DSL, and was
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specifically developed for the home and SOHO markets. The higher downstream
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rates lends itself to those not running serious servers -- at least
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anything more than a small, personal web site. ADSL is capable of sharing
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data with a POTS (or ISDN) voice line, so an additional line is not
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required. A big selling point. ADSL, like other DSLs, is limited by
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distance. 18,000 ft (5.5 km) is a typical cut-off point for telcos. ADSL
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does typically require either a splitter or filters to isolate the DSL
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signal from POTS. Sometimes referred to as <SPAN
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CLASS="QUOTE"
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>"full rate"</SPAN
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> ADSL in
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order to differentiate it from G.Lite DSL. There are two line encodings for
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ADSL: DMT and CAP. DMT (a.k.a. Alcatel compatible) has won the standards
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battle and is now the standard and the most common. Also, note that modems
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must be compatible with the encoding. In other words, a CAP modem will not
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work with a DMT service, and vice versa. Also, ISDN requires
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<SPAN
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CLASS="QUOTE"
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>"modems"</SPAN
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> (NTs), and related hardware such as filters, that are
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specific to that type of line since the signal on the line is very
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different for POTS and ISDN.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN991">G.Lite</H3
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><P
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>
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G.Lite is sometimes referred to as <SPAN
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CLASS="QUOTE"
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>"DSL Lite"</SPAN
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>,
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<SPAN
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CLASS="QUOTE"
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>"Universal DSL"</SPAN
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> or <SPAN
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CLASS="QUOTE"
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>"splitterless ADSL"</SPAN
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>, is a
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slower version of ADSL that requires no splitters <EM
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>or</EM
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>
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filters. G.lite uses a <SPAN
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CLASS="QUOTE"
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>"fast retrain"</SPAN
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> technique to negate the
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various signal disturbances caused by normal POTS usage. Currently G.Lite
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supports speeds up to 1.5 Mbps/512 Kbps, and at one time was expected to
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become the dominant consumer DSL service. As of this writing, it is not
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nearly as wide spread as <SPAN
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CLASS="QUOTE"
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>"full rate"</SPAN
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> ADSL however.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1000">SDSL</H3
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><P
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>
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Single-pair Digital Subscriber Loop, or also sometimes referred to
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as <SPAN
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CLASS="QUOTE"
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>"Symmetric Digital Subscriber Loop"</SPAN
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> since it is indeed
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symmetric with a current maximum rate of 1.5 Mbps/1.5 Mbps. SDSL requires a
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dedicated line, and thus true SDSL is not as readily adaptable to the
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consumer market as ADSL. SDSL also uses a 2B1Q encoding (same as ISDN and
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some T1) which is considered more robust than the DMT or CAP encoding of
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ADSL. True SDSL is generally considered more of a server quality DSL, and
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is typically marketed as a business class service. It is worth noting that
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some providers may be promoting a <SPAN
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CLASS="QUOTE"
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>"SDSL"</SPAN
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> service that is
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really ADSL pinched so that upstream/downstream are the same. Or vice
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versa, SDSL with asymmetrically allocated bandwidth. Wasn't all this
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confusing enough already?
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1005">IDSL</H3
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><P
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> ISDN Digital Subscriber Loop, 144 Kbps/144 Kbps is really a new and
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improved ISDN from Lucent Technologies and uses the same 2B1Q line encoding
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as ISDN, SDSL and others. IDSL does require a dedicated line however. The
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benefits are that it is an <SPAN
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CLASS="QUOTE"
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>"always on"</SPAN
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> technology, like other
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DSLs, and provides an additional 16 Kbps over traditional ISDN. It is being
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marketed by some DSL providers as a low end bit rate option, where line
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quality is not sufficient for higher speeds such as that of ADSL.
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Ironically, IDSL is generally priced significantly higher than ADSL.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1009">RADSL</H3
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><P
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> Rate Adaptive Digital Subscriber Loop was developed by Westell and has a
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potential of 2.2 Mbps downstream and 1.0 Mbps upstream. What makes RADSL
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more flexible is that the sync rate can be dynamically adjusted up or down
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as line conditions change. This makes it more of a viable alternative where
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line conditions are marginal due to distance or other factors. In many
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respects, RADSL is an enhanced ADSL to meet a more diverse set of line
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conditions. Like ADSL, RADSL can piggyback on the POTS line. RADSL does not
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require any splitters or filters.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1012">HDSL</H3
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><P
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> High bit-rate DSL was one of earliest versions of DSL. HDSL
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requires multiple, dedicated wire pairs, and is symmetric at 1.5
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Mbps/1.5 Mbps (the speed actually depends on number of wire pairs
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used). Not a viable alternative for the consumer or SOHO markets.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1015">VDSL</H3
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><P
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> Very high rate Digital Subscriber Loop is a DSL still in development
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with a current downstream capacity of 52.8 Mbps, and upstream of
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2.3 Mbps. At this time, VDSL is limited to very short loop lengths,
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and is not yet a viable alternative. It may find application where
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there is fiber to the neighborhood, and thus the copper loop
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segment is relatively short.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1018">UDSL</H3
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><P
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> Unidirectional Digital Subscriber Loop is a proposal from Europe that is
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not yet in use.
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</P
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></LI
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><LI
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1021">G.SHDSL</H3
|
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><P
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> The standards for G.SHDSL have just recently been finalized. SHDSL
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includes many enhancements, including better reach, better rate adaptation,
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and better upstream bandwidth. G.SHDSL is symmetric with speeds up to 2.3
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Mbps, and will more than likely be marketed as an SDSL alternative.
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</P
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></LI
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></UL
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></P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN1023">6.2. The DSLAM</H2
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><P
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> This technology is made possible by the placement of DSLAMs, or Digital
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Subscriber Loop Access Multiplexers, from such suppliers as <A
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HREF="http://www.alcatel.com"
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TARGET="_top"
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>Alcatel</A
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> and
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<A
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HREF="http://www.cisco.com/warp/public/cc/pd/si/6000/prodlit/c6160_ds.htm"
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TARGET="_top"
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>Cisco</A
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>,
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in the telco's Central Office, or sometimes a suitable remote location.
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DSLAMs come in various shapes and sizes, and are the one, single complex and
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costly component of a DSL connection. When a qualified phone line is
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connected to a modem at the user's end of the loop, a high speed digital
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connection is established, typically over ATM, or sometimes frame relay. The
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DSLAM splits the signal back into separate voice and data channels. The voice
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channel stays within the telco network, whereas the data is picked up by an
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ISP (typically). </P
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><H3
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CLASS="BRIDGEHEAD"
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><A
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NAME="AEN1028"> Figure 4: A Typical DSL Connection Path</H3
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><P
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>
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<TT
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CLASS="LITERAL"
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> <P
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CLASS="LITERALLAYOUT"
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><br>
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Voice -+ +---> Voice <br>
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|<-- copper loop --> DSLAM/CO <--{ATM cloud}--->|<br>
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modem -+ | +---> Inet<br>
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| | |<br>
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ether..|..... DSL/ATM here ....|.... raw ATM here .....|.. TCP/IP ..<br>
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| |<br>
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SOHO...|............ telco (ILEC or CLEC) .............|.. ISP ..| NSP<br>
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<br>
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</P
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>
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</TT
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></P
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><DIV
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CLASS="SECT3"
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><H3
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CLASS="SECT3"
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><A
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NAME="AEN1033">6.2.1. Sync</H3
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><P
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>
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A good, working connection to the DSLAM is referred to as
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<SPAN
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CLASS="QUOTE"
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>"syncing"</SPAN
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>. This is typically indicated by LEDs on the modem.
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Without sync, nothing happens. The modem will establish a sync rate which is
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often throttled by the provider at a predefined limit. This limit, or
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<SPAN
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CLASS="QUOTE"
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>"cap"</SPAN
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>, is at the provider's discretion and is part of the
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service that is being provided. Your modem may well sync at a higher rate
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than the <SPAN
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CLASS="QUOTE"
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>"cap"</SPAN
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>, but your speed will be limited to whatever
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<SPAN
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CLASS="QUOTE"
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>"cap"</SPAN
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> the provider is enforcing. So while ADSL has an upward
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theoretical limit of 8 Mbps, you will not see that speed -- unless of course
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your provider is selling an 8 Mbps plan. Most plans are well below this. </P
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><P
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> Below is the status information from a SpeedStream 5660 modem/router via the
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built-in telnet interface. In this example, the customer is on a 1.5 Mbps/384
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Kbps service: </P
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><P
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> <TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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> Command-> show dslstatus
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--- Channel Info ATU-R ATU-C
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Current TX Rate - 384000 1500000
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Previous TX Rate - 0 0
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CRC Block Length - - -
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Interleave Delay - - -
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--- Physical Layer Info ATU-R ATU-C
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Current Attainable Rate - 448433 3890243
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Current SNR Margin - 10.5 17.0
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Current Attenuation - 54.5 31.5
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Current Output Power - 3.0 16.0
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Current Status:
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Defects detected - No No
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Loss of Framing - No Loss No Loss
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Loss of Signal - No Loss No Loss
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Loss of Power - No Loss No Loss
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Loss of Signal Quality - No Loss No Loss
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--- ATU-R Line Status
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Line Coding - DMT
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Line Type - Fast or Interleaved
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Command->
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></P
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><P
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> First notice the <SPAN
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CLASS="QUOTE"
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>"Current Attainable Rate"</SPAN
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> in the
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<SPAN
|
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CLASS="QUOTE"
|
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>"ATU-C"</SPAN
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> column. This is the downstream sync rate negotiated by
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the modem and DSLAM, which is over 3.5 Mbps. The actual speed is limited,
|
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however, to 1.5 Mbps/384 Kbps from the first row <SPAN
|
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CLASS="QUOTE"
|
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>"TX Rate"</SPAN
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>. This
|
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is the theoretical limit of this connection. This limit, or
|
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<SPAN
|
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CLASS="QUOTE"
|
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>"cap"</SPAN
|
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>, can be enforced at the DSLAM, as is the case the here, or
|
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further upstream. Had the first row <SPAN
|
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CLASS="QUOTE"
|
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>"TX Rate"</SPAN
|
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> been lower than
|
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the provider's imposed limit, then this would indicate some kind of problem
|
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with the connection, perhaps due to distance or some kind of line impairment.
|
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</P
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><P
|
|
> The attainable sync rate is the result of a number of factors, including wire
|
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distance to the DSLAM, quality of both inside and outside wiring, the loop
|
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wire gauge and various other factors within the loop. Actual measurable,
|
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real world throughput, on the other hand, is first of all dependent on sync
|
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rate. Low sync rate means low throughput. In the above example, had the sync
|
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rate been lower, say 500 Kbps, then that would be the maximum for that
|
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connection, even though the customer is paying for a 1.5 Mbps service.</P
|
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><P
|
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>
|
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Secondarily, throughput will depend also on the ISP's network, and then the
|
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ISP's upstream provider. You will lose approximately 10-20% of potential
|
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throughput to networking overhead. In the above example where the connection
|
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is throttled at 1.5 Mbps, the actual, real-world maximum throughput would be
|
|
somewhere around 1.2-1.3 Mbps when overhead is taken into account. Moreover,
|
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once you hit the Internet proper, all bets are off as there are any number of
|
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factors that may impact throughput. A overloaded or busy server is likely to
|
|
be slow no matter how fast your DSL connection is. </P
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></DIV
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></DIV
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><DIV
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CLASS="SECT2"
|
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><H2
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CLASS="SECT2"
|
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><A
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NAME="DSLMODEMS">6.3. DSL Modems</H2
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><P
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> The <SPAN
|
|
CLASS="QUOTE"
|
|
>"modem"</SPAN
|
|
> is the last piece of the connection. The modem is
|
|
connected directly to the DSLAM via the copper loop on the telco end, and
|
|
plugs into a wall jack on your end. When all is well, the modem
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"syncs"</SPAN
|
|
> with the DSLAM, and then makes an IP connection to the
|
|
ISP, and off we go! </P
|
|
><P
|
|
> For Linux users, <EM
|
|
>the modem is a very important
|
|
consideration</EM
|
|
>! There are many modems supplied by ISPs that are not
|
|
Linux compatible. Your best bet is an external, ethernet interfaced modem (or
|
|
modem/router combo) that connects via a standard ethernet NIC, since many
|
|
other modem options (PCI, USB, onboard) will not work due to a lack of
|
|
drivers at this time! All ethernet based modems will work fine. (See the
|
|
<A
|
|
HREF="appendix.html#MODEMS"
|
|
>Modems Section</A
|
|
> for an up-to-date list of
|
|
compatible modems.) ISDN users will need a modem (NT) designed specifically
|
|
for DSL over ISDN.
|
|
</P
|
|
><P
|
|
>
|
|
With ethernet modems, the only potential compatibility issue is the Network
|
|
Card (NIC). (And really any compatible ethernet NIC should do just fine --
|
|
100 Mbps is not even necessary.) You are probably better off anyway, since PCI and
|
|
USB modems can be more problem prone. If your chosen provider does not
|
|
offer a compatible modem as an option, then you either need to look
|
|
elsewhere, or you will have to buy one outright from a third party. </P
|
|
><P
|
|
> As always, there are exceptions. <A
|
|
HREF="http://www.xpeed.com"
|
|
TARGET="_top"
|
|
>Xpeed</A
|
|
>
|
|
now has drivers for two PCI modems included with the kernel drivers (as of
|
|
2.2.18, not in 2.4 yet though AFAIK). These are the first open source Linux DSL modem
|
|
drivers, and is welcomed news. <A
|
|
HREF="http://www.alcateldsl.com"
|
|
TARGET="_top"
|
|
>Alcatel's</A
|
|
> ADSL SpeedTouch USB modem
|
|
now has Linux drivers. And more recently, the Eci Hi Focus ADSL USB Modem
|
|
has drivers (and some related chipsets are supported as well, see
|
|
<A
|
|
HREF="http://eciadsl.sourceforge.net/"
|
|
TARGET="_top"
|
|
>http://eciadsl.sourceforge.net/</A
|
|
>).
|
|
IteX PCI ADSL modems, based on the Apollo chipset, have Linux drivers.
|
|
(Modems using this chipset are sold under a number of various brand names.)
|
|
Diamond also makes [made?] an internal PCI modem which has binary-only
|
|
drivers, but it is not in widespread use, and seems to be discontinued at
|
|
this point. It is also possible to make a direct ATM connection using a modem
|
|
plus an ATM network card, though this delivery system is not used in the U.S.
|
|
as far as I know, and should not be considered as a viable option. This would
|
|
also require a 2.4 kernel. </P
|
|
><P
|
|
> The most common type of modem in use today is actually a combination
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"bridge"</SPAN
|
|
> and modem device. The bridge is a simple device,
|
|
typically with little configuration. Network traffic passes blindly across
|
|
the ATM to ethernet bridge in either direction. Your point of exposure is the
|
|
interface (typically a NIC) that is connected to the modem/bridge.
|
|
</P
|
|
><P
|
|
> Some ISPs are also offering <SPAN
|
|
CLASS="QUOTE"
|
|
>"routers"</SPAN
|
|
>. These are basically
|
|
combination modem/routers that can handle NAT, and may have other feature
|
|
enhancements such as port forwarding, a built in hub, etc. These are all
|
|
external, so should work too. But probably not a big deal for Linux users,
|
|
since Linux can do anything these do, and more. A locked down Linux box makes
|
|
a most excellent firewall/gateway/proxy! </P
|
|
><P
|
|
> To confuse things even more, there are also all-in-one devices: combo
|
|
bridge+router+modem, sometimes called <SPAN
|
|
CLASS="QUOTE"
|
|
>"brouters"</SPAN
|
|
>. In this case,
|
|
the modem can be configured for either bridged or routed modes -- but it
|
|
can't be both at the same time. </P
|
|
><P
|
|
> All providers should make available a modem of some sort. Many ISPs will have
|
|
more than one modem option. Some may give away the modem at no additional
|
|
charge. Some may offer a free base model, and charge the difference for the
|
|
better models with more features. Many of the modems that ISPs supply are not
|
|
available through normal retail channels. Should you want to buy one
|
|
yourself, this leaves used equipment outlets (e.g. ebay), or possibly buying
|
|
a modem that your ISP may not support (i.e. a possibility of no tech support
|
|
if you have a problem). </P
|
|
><P
|
|
> While some ISPs provide modems that are not readily available through normal
|
|
retail channels, there are a number of manufacturers that are getting on the
|
|
DSL modem bandwagon, and offering a good selection. Most have a
|
|
number of enhancements. At this time Alcatel (now owned by Thomson), Intel,
|
|
Zyxel, Cisco, 3Com, and Cayman have products available. Depending on model
|
|
and feature set, prices range from a little over $100 US to $800 and up. Many
|
|
of these handle their own authentication and encapsulation (DHCP, PPPoE,
|
|
etc).
|
|
</P
|
|
><P
|
|
> Are some modems better than others? Short, easy answer: no. Modems are not
|
|
much of a factor in speed in most cases. But some do have enhanced features,
|
|
such as diagnostics or the combo modem/routers. Ethernet modems are
|
|
generally considered the most reliable. Fewer IRQ hassles, no buggy drivers,
|
|
etc. So the fact that Linux users are mostly relegated to ethernet modems is
|
|
a blessing in disguise really. Are any of these better than others? Hard to
|
|
say since most of this is so new there is not enough of a track record to
|
|
compare brands and models with any degree of assurance. In other words, any
|
|
old external, ethernet modem should do -- provided it matches your
|
|
provider's DSL, and is configured for that service. Remember, there can be
|
|
differences here. </P
|
|
><DIV
|
|
CLASS="WARNING"
|
|
><P
|
|
></P
|
|
><TABLE
|
|
CLASS="WARNING"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="25"
|
|
ALIGN="CENTER"
|
|
VALIGN="TOP"
|
|
><IMG
|
|
SRC="../images/warning.gif"
|
|
HSPACE="5"
|
|
ALT="Warning"></TD
|
|
><TD
|
|
ALIGN="LEFT"
|
|
VALIGN="TOP"
|
|
><P
|
|
> Make sure any third party modem or router you may purchase is compatible with
|
|
your DSL provider. There are two major line encodings for ADSL (CAP and DMT
|
|
a.k.a. Alcatel compatible), and several options for IP encapsulation. And
|
|
different DSLs (SDSL, IDSL, etc) will require their own modems too, as will
|
|
ISDN lines. Your provider should have a list of compatible options. It may
|
|
well have to be configured for your ISP's service too. Don't expect it to
|
|
work right out of the box either (unless it does indeed come from your
|
|
provider). Many are accessible via telnet, or a web browser, where the
|
|
configuration options are available. See the owner's manual for this.
|
|
|
|
</P
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></DIV
|
|
><DIV
|
|
CLASS="SECT2"
|
|
><H2
|
|
CLASS="SECT2"
|
|
><A
|
|
NAME="ISPCONN">6.4. The ISP Connection</H2
|
|
><P
|
|
> The modem connects to the DSLAM, and then the DSLAM is connected to the
|
|
telco's ATM network (or frame relay), where it is picked up by the ISP. The
|
|
ISP typically will take over at what we <SPAN
|
|
CLASS="QUOTE"
|
|
>"see"</SPAN
|
|
> as the first hop on a
|
|
<B
|
|
CLASS="COMMAND"
|
|
>traceroute</B
|
|
>. Everything up to that point is in the hands
|
|
of the telco/DSL provider. The ISP will connect to the telco's ATM network
|
|
via a high-speed data connection, usually ATM over a DS3 (45 Mbps) or
|
|
possibly an OC-3 (155 Mbps). The important thing here is that an ISP must
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"subscribe"</SPAN
|
|
> with your telco to provide this connection. The ISP
|
|
will provide traditional ISP type services: email, DNS, news, etc. It is
|
|
really a two step connection -- DSL from one provider, Internet from a second
|
|
-- even though these may be combined into one billing. </P
|
|
><P
|
|
> The Baby Bells (RBOCs) in the U.S. all own ISPs. These, of course, are
|
|
connected to their DSLAMs, and are providing Internet services via the
|
|
telco's ISP subsidiary. Many independent ISPs are availing
|
|
themselves of the ILEC's DSL services, and in essence
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"reselling"</SPAN
|
|
> the DSL services of the ILEC. While the underlying
|
|
infrastructure is the same in this case, having more than one ISP working out
|
|
of a CO may mean a better selection of features and prices for the consumer.
|
|
</P
|
|
><P
|
|
> CLECs (independent telcos) are now installing their own DSLAMs in many U.S.
|
|
markets. This makes them a direct competitor to the ILEC. In this scenario,
|
|
there would be two (or more) DSL providers in the same CO, each with their
|
|
own DSLAM(s), and each competing against each other. This complicates the ISP
|
|
situation even further, as each DSL provider will be <SPAN
|
|
CLASS="QUOTE"
|
|
>"partnered"</SPAN
|
|
>
|
|
with one or more ISPs. If you are lucky here, you will have many choices of
|
|
plans and pricing structures. </P
|
|
><P
|
|
> At this time, there is a shake out going on in the U.S. market. The
|
|
independents are all struggling to match the deep pockets of the regional
|
|
phone companies. The independents that have survived are now focusing more
|
|
on small business and higher-end consumer customers. This means, it will
|
|
cost more, but you should also expect to get more. Especially, in the
|
|
quality department.</P
|
|
><P
|
|
> Typically, your service agreement is with the ISP, and not the DSL
|
|
provider. This makes the actual DSL provider a <SPAN
|
|
CLASS="QUOTE"
|
|
>"behind the
|
|
scenes"</SPAN
|
|
> player. This may vary, and in some cases, you may wind up
|
|
with a separate service agreement for both the DSL provider and the ISP. </P
|
|
></DIV
|
|
><DIV
|
|
CLASS="SECT2"
|
|
><H2
|
|
CLASS="SECT2"
|
|
><A
|
|
NAME="AEN1088">6.5. Availability</H2
|
|
><P
|
|
> Who can get DSL? The first requirement is that a telco has installed the
|
|
necessary hardware somewhere on their end. Typically this is in the CO. You
|
|
have no choice on which CO is yours -- it is wherever your loop terminates.
|
|
If your CO has a DSLAM, and the necessary other components, then DSL may be
|
|
available to you. This is often known as <SPAN
|
|
CLASS="QUOTE"
|
|
>"pre-qualifying"</SPAN
|
|
>, and
|
|
is Step One in getting service. </P
|
|
><P
|
|
> More and more <SPAN
|
|
CLASS="QUOTE"
|
|
>"remote terminals (aka DSLAMs)"</SPAN
|
|
> are being
|
|
deployed. This is certainly good news for those that are a long way from
|
|
the CO. CO distance is not the limiting factor it once was.</P
|
|
><DIV
|
|
CLASS="SECT3"
|
|
><H3
|
|
CLASS="SECT3"
|
|
><A
|
|
NAME="AEN1094">6.5.1. Ordering</H3
|
|
><P
|
|
> Before ordering service, check to see what providers there are in your area.
|
|
Look for options on both the telco/DSL side and the ISP side. You may have
|
|
several options, including the large phone companies, as well as smaller,
|
|
local ISPs. Once an order is placed, you must wait for the qualification
|
|
process before a provider will agree to provide service. </P
|
|
></DIV
|
|
><DIV
|
|
CLASS="SECT3"
|
|
><H3
|
|
CLASS="SECT3"
|
|
><A
|
|
NAME="QUALIFY">6.5.2. Qualifying</H3
|
|
><P
|
|
> Once local availability is established, the next step is
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"qualifying"</SPAN
|
|
> your loop. The provider will run various tests to
|
|
make sure that your loop can handle the DSL signal. This is to determine how
|
|
suitable your line is for DSL, and maybe what level of service will be
|
|
available to you. You probably will have to order service just to find out
|
|
this much. It can be a fairly involved process, with a variety of different
|
|
tests being run. There are a number of things that may
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"disqualify"</SPAN
|
|
> a line. The most common limitation is distance. </P
|
|
><P
|
|
>
|
|
All DSLs have distance limitations. ADSL is limited
|
|
to a loop length of roughly 18,000 ft (5.5 km), but the actual cut off point
|
|
will vary from provider to provider. The further away you are, the weaker the
|
|
signal, and the potential for poor connections is greater. With ADSL, if you
|
|
are within approximately 12,000 ft (3.7 km), you should be able to get at
|
|
least 1.5 Mbps -- all other things being equal. IDSL has even greater reach,
|
|
mainly because the maximum speed for IDSL is considerably lower at 144
|
|
Kbps/144 Kbps. </P
|
|
><P
|
|
> Still even if you're close enough, there are a number of potential
|
|
impediments that may disqualify a line. Two such common impediments
|
|
are load coils and bridge taps. These are aspects of the old telco
|
|
infrastructure that once were deemed beneficial, but now are getting
|
|
in the way of the newer, digital technologies.
|
|
Whether you hit a snag like this or not, is pretty much hit or miss. Fiber
|
|
anywhere in the loop is also a disqualifier. The provider may take steps to
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"clean"</SPAN
|
|
> the line. Just how far they are willing to go will vary
|
|
from provider to provider, and this will likely add additional time to the
|
|
installation process. </P
|
|
><P
|
|
> Once the line is <SPAN
|
|
CLASS="QUOTE"
|
|
>"qualified"</SPAN
|
|
>, the next step is deciding on which
|
|
plan is suitable for your situation. The provider may have differing plans
|
|
available depending on how strong a signal they think your line can handle.
|
|
If you are marginal, you will not be qualified for the higher speed plans.
|
|
And if price is a factor, having a tiered pricing structure is good also
|
|
since the lower end plans are obviously less expensive. How this is
|
|
structured also varies wildly from provider to provider. Since, DSL is a new
|
|
service, and providers are trying to find the right price/feature
|
|
combinations that will attract the most users and thus gain a competitive
|
|
edge.
|
|
</P
|
|
><P
|
|
> Some common data rates: </P
|
|
><P
|
|
> <A
|
|
NAME="AEN1110"><BLOCKQUOTE
|
|
CLASS="BLOCKQUOTE"
|
|
><P
|
|
CLASS="LITERALLAYOUT"
|
|
> <br>
|
|
Downstream/Upstream <br>
|
|
<br>
|
|
128 Kbps/128 Kbps<br>
|
|
<br>
|
|
256 Kbps/256 Kbps <br>
|
|
<br>
|
|
384 Kbps/128 Kbps <br>
|
|
<br>
|
|
640 Kbps/90 Kbps<br>
|
|
<br>
|
|
1.5 Mbps/384 Kbps <br>
|
|
<br>
|
|
2.0 Mbps/512 Kbps<br>
|
|
<br>
|
|
7.1 Mbps/1024 Kbps<br>
|
|
<br>
|
|
</P
|
|
></BLOCKQUOTE
|
|
></P
|
|
><P
|
|
> and a near infinite number of other possibilities. The cost of different
|
|
plans generally goes up with their speed.</P
|
|
><P
|
|
> Should you be disqualified, and have other options, get a second opinion.
|
|
Calculating the effective loop length is by no means an exact science. There
|
|
is plenty of room for errors. Also, some providers may go to greater lengths
|
|
to <SPAN
|
|
CLASS="QUOTE"
|
|
>"clean"</SPAN
|
|
> the loop than others. And, if you have more than one
|
|
phone line, and are disqualified, then try the other line. Just because they
|
|
both terminate at your location, does not necessarily mean they are the same
|
|
length! The telco network is full of surprises. </P
|
|
></DIV
|
|
></DIV
|
|
><DIV
|
|
CLASS="SECT2"
|
|
><H2
|
|
CLASS="SECT2"
|
|
><A
|
|
NAME="CPROVIDERS">6.6. Choosing Providers</H2
|
|
><P
|
|
> Should you have more than one choice, here are some things to keep in mind
|
|
when comparing services from different providers. If you are in a populous
|
|
area, chances are you do have a number of choices. There is a dizzying array
|
|
of possibilities at this time. Remember too, that it is a two step
|
|
connection: DSL provider and ISP. You may have choices for each. </P
|
|
><P
|
|
> <P
|
|
></P
|
|
><UL
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>A compatible modem</EM
|
|
>. For now with Linux (or any
|
|
alternative OS) this essentially means an ethernet interface. (There are
|
|
rare exceptions.) <SPAN
|
|
CLASS="QUOTE"
|
|
>"Routers"</SPAN
|
|
> (i.e. combo modem/routers) should
|
|
be OK too since these seem to be all ethernet devices.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Installation</EM
|
|
>. A self-install option, of course, let's
|
|
anyone get up and running, and is less expensive. But if there is no
|
|
self-install available, will the the provider install onto a Linux only
|
|
site? Many will not! Having a Windows (or Mac) box temporarily available
|
|
is a work around here. Even a laptop may be enough.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Static vs Dynamic IP Address</EM
|
|
>. If wanting to run
|
|
servers, or hosting your own domain, static is the way to go. A dynamic
|
|
IP, on the other hand, makes you a little harder to find should you wish
|
|
to remain <SPAN
|
|
CLASS="QUOTE"
|
|
>"invisible"</SPAN
|
|
>, or a least harder to keep track of.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Encapsulation</EM
|
|
>. Is the connection
|
|
<SPAN
|
|
CLASS="QUOTE"
|
|
>"Bridged"</SPAN
|
|
> or <SPAN
|
|
CLASS="QUOTE"
|
|
>"PPP"</SPAN
|
|
>. PPPoX has the reputation of
|
|
being not as stable a connection, and not <SPAN
|
|
CLASS="QUOTE"
|
|
>"always on"</SPAN
|
|
>. PPPoE
|
|
requires client software to manage the connection, so one more layer of
|
|
code.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Server Policy</EM
|
|
>. Some ISPs are fairly open about this,
|
|
while others forbid any servers -- even personal web sites. Others may even
|
|
go so far as to block certain ports.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Contract</EM
|
|
>. Is there a contract, and what are the out
|
|
clauses? Cancellation fees?
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Connection Limits</EM
|
|
>. Is it <SPAN
|
|
CLASS="QUOTE"
|
|
>"always on"</SPAN
|
|
> (at
|
|
least theoretically :-)? Are there session limits, or idle timeouts? Is
|
|
bandwidth metered and limited to so much per month? Do they forbid a LAN
|
|
behind the connection (dumb!)?
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Linux Support</EM
|
|
>. A few ISPs may offer some degree of
|
|
tech support for Linux, but most will not. This isn't so bad, as long as
|
|
they don't go overboard and refuse to help with anything just because you
|
|
run a non-supported OS. (<SPAN
|
|
CLASS="QUOTE"
|
|
>"Supported"</SPAN
|
|
> means like <SPAN
|
|
CLASS="QUOTE"
|
|
>"tech
|
|
support"</SPAN
|
|
>.) If they say <SPAN
|
|
CLASS="QUOTE"
|
|
>"we don't care"</SPAN
|
|
>, you should be
|
|
good to go.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Free Dialup Account</EM
|
|
>. A nice thing to have if the
|
|
connection is down, or you just need to check mail from another location.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Setup program</EM
|
|
>. A few ISPs may have a setup program you
|
|
are required to run the first time you connect in order to setup your
|
|
account. This will likely not have a Linux version. (BellAtlantic.net was
|
|
doing this at last report.) Other than this, there is nothing proprietary
|
|
about DSL, and related protocols.
|
|
|
|
</P
|
|
></LI
|
|
><LI
|
|
><P
|
|
> <EM
|
|
>Reliability and Quality of Service</EM
|
|
>. Ask around in your
|
|
local area from those that have the same DSL provider and ISP. A local LUG
|
|
is a good place to get this kind of info. How much down time (hopefully not
|
|
much)? Are mail and news services good? Backbone routing? Tech support?
|
|
|
|
</P
|
|
></LI
|
|
></UL
|
|
></P
|
|
><P
|
|
> There are a number of other options and features that might be worth looking
|
|
at too: multiple IPs, domain hosting (DNS), free web space, number of
|
|
email accounts, web mail, etc. All things considered, the better plans
|
|
are probably going to cost more for a reason.
|
|
</P
|
|
></DIV
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
SUMMARY="Footer navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
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CELLPADDING="0"
|
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CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="tuning.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="index.html"
|
|
ACCESSKEY="H"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="faq.html"
|
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ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>Performance Tuning and Troubleshooting</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
> </TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Appendix: FAQ</TD
|
|
></TR
|
|
></TABLE
|
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|
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></BODY
|
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> |