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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="OVERVIEW">6. Appendix: DSL Overview</H1
><P
> DSL is a telephone loop technology that uses existing copper phones lines,
and provides a dedicated, high speed Internet connection. One of the big
advantages of some DSLs (notably ADSL), are that they can co-exist on the
same line with a traditional voice service such as <SPAN
CLASS="QUOTE"
>"POTS"</SPAN
> (Plain
Old Telephone Service), and even ISDN. This is accomplished by utilizing
different frequency ranges above the voice range (voice is up to 4KHz).
Essentially, this gives two lines in one: one for voice, and one for Internet
connectivity. When all is working normally, there should be no interference
between the two <SPAN
CLASS="QUOTE"
>"lines"</SPAN
>. This gives DSL a potentially broad
consumer base, and helps minimize costs for service providers. </P
><P
> DSL is positioned for the Home and Small Office (SOHO) market that is
looking for high speed Internet access at reasonable prices. Since it also
typically provides dedicated, <SPAN
CLASS="QUOTE"
>"always on"</SPAN
> access, it can be used
for interconnecting low to mid range bandwidth servers, and provides a great
access solution for small LANs. It is also great for those Linux power users
that just want a fat pipe :-). </P
><P
> Phone companies, and other independent telecommunications providers (CLECs),
are now deploying DSL to stay ahead of the Cable
companies -- the main consumer and SOHO competition for DSL providers. This
mad rush to get <SPAN
CLASS="QUOTE"
>"a piece of the pie"</SPAN
>, is bringing much
competition (a good thing!), much diversity, and some confusion, into the
consumer market. The DSL provider (often, but not always, the phone company)
will provide the DSL infrastructure. This would include your line, the DSLAM,
and physical connection to the outside world. From there it is typically
picked up by an ISP, who provides the traditional Internet services.
</P
><P
> Consumer DSL plans are typically <SPAN
CLASS="QUOTE"
>"best effort"</SPAN
> services. While
boasting speeds approaching T1, and even surpassing that in some cases, it is
not necessarily as reliable as T1 however. Business class DSL offers more
reliability at a higher cost than consumer plans, and is a good compromise
where both reliability and bandwidth are at a premium. All in all, the cost
of DSL compared to traditional telco services, such as T1, is attractive and
substantially more affordable for home and small business users. </P
><P
> DSL providers often do not have service contracts for home users,
while business class DSL services typically do include similar SLAs (Service
Level Agreements) to that offered for a T1 line. </P
><P
> The downside is that DSL is not available everywhere. Availability, and
available bit rate (speed), are purely a function of where you live, where
the telco has installed the prerequisite hardware, how far you are from the
DSLAM/CO, and the quality of your phone line (loop). Not all loops are
created equal, unfortunately. The primary limitation is distance. </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FAMILY">6.1. The DSL Family</H2
><P
> <P
></P
><UL
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN985">ADSL</H3
><P
> Asymmetric Digital Subscriber Loop currently supports downstream rates up
to 8 Mbps, and upstream of 1024 Kbps, hence the <SPAN
CLASS="QUOTE"
>"asymmetric"</SPAN
>.
ADSL is far and away the most widely deployed consumer DSL, and was
specifically developed for the home and SOHO markets. The higher downstream
rates lends itself to those not running serious servers -- at least
anything more than a small, personal web site. ADSL is capable of sharing
data with a POTS (or ISDN) voice line, so an additional line is not
required. A big selling point. ADSL, like other DSLs, is limited by
distance. 18,000 ft (5.5 km) is a typical cut-off point for telcos. ADSL
does typically require either a splitter or filters to isolate the DSL
signal from POTS. Sometimes referred to as <SPAN
CLASS="QUOTE"
>"full rate"</SPAN
> ADSL in
order to differentiate it from G.Lite DSL. There are two line encodings for
ADSL: DMT and CAP. DMT (a.k.a. Alcatel compatible) has won the standards
battle and is now the standard and the most common. Also, note that modems
must be compatible with the encoding. In other words, a CAP modem will not
work with a DMT service, and vice versa. Also, ISDN requires
<SPAN
CLASS="QUOTE"
>"modems"</SPAN
> (NTs), and related hardware such as filters, that are
specific to that type of line since the signal on the line is very
different for POTS and ISDN.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN991">G.Lite</H3
><P
>
G.Lite is sometimes referred to as <SPAN
CLASS="QUOTE"
>"DSL Lite"</SPAN
>,
<SPAN
CLASS="QUOTE"
>"Universal DSL"</SPAN
> or <SPAN
CLASS="QUOTE"
>"splitterless ADSL"</SPAN
>, is a
slower version of ADSL that requires no splitters <EM
>or</EM
>
filters. G.lite uses a <SPAN
CLASS="QUOTE"
>"fast retrain"</SPAN
> technique to negate the
various signal disturbances caused by normal POTS usage. Currently G.Lite
supports speeds up to 1.5 Mbps/512 Kbps, and at one time was expected to
become the dominant consumer DSL service. As of this writing, it is not
nearly as wide spread as <SPAN
CLASS="QUOTE"
>"full rate"</SPAN
> ADSL however.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1000">SDSL</H3
><P
>
Single-pair Digital Subscriber Loop, or also sometimes referred to
as <SPAN
CLASS="QUOTE"
>"Symmetric Digital Subscriber Loop"</SPAN
> since it is indeed
symmetric with a current maximum rate of 1.5 Mbps/1.5 Mbps. SDSL requires a
dedicated line, and thus true SDSL is not as readily adaptable to the
consumer market as ADSL. SDSL also uses a 2B1Q encoding (same as ISDN and
some T1) which is considered more robust than the DMT or CAP encoding of
ADSL. True SDSL is generally considered more of a server quality DSL, and
is typically marketed as a business class service. It is worth noting that
some providers may be promoting a <SPAN
CLASS="QUOTE"
>"SDSL"</SPAN
> service that is
really ADSL pinched so that upstream/downstream are the same. Or vice
versa, SDSL with asymmetrically allocated bandwidth. Wasn't all this
confusing enough already?
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1005">IDSL</H3
><P
> ISDN Digital Subscriber Loop, 144 Kbps/144 Kbps is really a new and
improved ISDN from Lucent Technologies and uses the same 2B1Q line encoding
as ISDN, SDSL and others. IDSL does require a dedicated line however. The
benefits are that it is an <SPAN
CLASS="QUOTE"
>"always on"</SPAN
> technology, like other
DSLs, and provides an additional 16 Kbps over traditional ISDN. It is being
marketed by some DSL providers as a low end bit rate option, where line
quality is not sufficient for higher speeds such as that of ADSL.
Ironically, IDSL is generally priced significantly higher than ADSL.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1009">RADSL</H3
><P
> Rate Adaptive Digital Subscriber Loop was developed by Westell and has a
potential of 2.2 Mbps downstream and 1.0 Mbps upstream. What makes RADSL
more flexible is that the sync rate can be dynamically adjusted up or down
as line conditions change. This makes it more of a viable alternative where
line conditions are marginal due to distance or other factors. In many
respects, RADSL is an enhanced ADSL to meet a more diverse set of line
conditions. Like ADSL, RADSL can piggyback on the POTS line. RADSL does not
require any splitters or filters.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1012">HDSL</H3
><P
> High bit-rate DSL was one of earliest versions of DSL. HDSL
requires multiple, dedicated wire pairs, and is symmetric at 1.5
Mbps/1.5 Mbps (the speed actually depends on number of wire pairs
used). Not a viable alternative for the consumer or SOHO markets.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1015">VDSL</H3
><P
> Very high rate Digital Subscriber Loop is a DSL still in development
with a current downstream capacity of 52.8 Mbps, and upstream of
2.3 Mbps. At this time, VDSL is limited to very short loop lengths,
and is not yet a viable alternative. It may find application where
there is fiber to the neighborhood, and thus the copper loop
segment is relatively short.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1018">UDSL</H3
><P
> Unidirectional Digital Subscriber Loop is a proposal from Europe that is
not yet in use.
</P
></LI
><LI
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1021">G.SHDSL</H3
><P
> The standards for G.SHDSL have just recently been finalized. SHDSL
includes many enhancements, including better reach, better rate adaptation,
and better upstream bandwidth. G.SHDSL is symmetric with speeds up to 2.3
Mbps, and will more than likely be marketed as an SDSL alternative.
</P
></LI
></UL
></P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1023">6.2. The DSLAM</H2
><P
> This technology is made possible by the placement of DSLAMs, or Digital
Subscriber Loop Access Multiplexers, from such suppliers as <A
HREF="http://www.alcatel.com"
TARGET="_top"
>Alcatel</A
> and
<A
HREF="http://www.cisco.com/warp/public/cc/pd/si/6000/prodlit/c6160_ds.htm"
TARGET="_top"
>Cisco</A
>,
in the telco's Central Office, or sometimes a suitable remote location.
DSLAMs come in various shapes and sizes, and are the one, single complex and
costly component of a DSL connection. When a qualified phone line is
connected to a modem at the user's end of the loop, a high speed digital
connection is established, typically over ATM, or sometimes frame relay. The
DSLAM splits the signal back into separate voice and data channels. The voice
channel stays within the telco network, whereas the data is picked up by an
ISP (typically). </P
><H3
CLASS="BRIDGEHEAD"
><A
NAME="AEN1028"> Figure 4: A Typical DSL Connection Path</H3
><P
>
<TT
CLASS="LITERAL"
> <P
CLASS="LITERALLAYOUT"
><br>
Voice -+ +---> Voice <br>
|<-- copper loop --> DSLAM/CO <--{ATM cloud}--->|<br>
modem -+ | +---> Inet<br>
| | |<br>
ether..|..... DSL/ATM here ....|.... raw ATM here .....|.. TCP/IP ..<br>
| |<br>
SOHO...|............ telco (ILEC or CLEC) .............|.. ISP ..| NSP<br>
<br>
</P
>
</TT
></P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN1033">6.2.1. Sync</H3
><P
>
A good, working connection to the DSLAM is referred to as
<SPAN
CLASS="QUOTE"
>"syncing"</SPAN
>. This is typically indicated by LEDs on the modem.
Without sync, nothing happens. The modem will establish a sync rate which is
often throttled by the provider at a predefined limit. This limit, or
<SPAN
CLASS="QUOTE"
>"cap"</SPAN
>, is at the provider's discretion and is part of the
service that is being provided. Your modem may well sync at a higher rate
than the <SPAN
CLASS="QUOTE"
>"cap"</SPAN
>, but your speed will be limited to whatever
<SPAN
CLASS="QUOTE"
>"cap"</SPAN
> the provider is enforcing. So while ADSL has an upward
theoretical limit of 8 Mbps, you will not see that speed -- unless of course
your provider is selling an 8 Mbps plan. Most plans are well below this. </P
><P
> Below is the status information from a SpeedStream 5660 modem/router via the
built-in telnet interface. In this example, the customer is on a 1.5 Mbps/384
Kbps service: </P
><P
> <TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
> Command-> show dslstatus
--- Channel Info ATU-R ATU-C
Current TX Rate - 384000 1500000
Previous TX Rate - 0 0
CRC Block Length - - -
Interleave Delay - - -
--- Physical Layer Info ATU-R ATU-C
Current Attainable Rate - 448433 3890243
Current SNR Margin - 10.5 17.0
Current Attenuation - 54.5 31.5
Current Output Power - 3.0 16.0
Current Status:
Defects detected - No No
Loss of Framing - No Loss No Loss
Loss of Signal - No Loss No Loss
Loss of Power - No Loss No Loss
Loss of Signal Quality - No Loss No Loss
--- ATU-R Line Status
Line Coding - DMT
Line Type - Fast or Interleaved
Command->
</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
> First notice the <SPAN
CLASS="QUOTE"
>"Current Attainable Rate"</SPAN
> in the
<SPAN
CLASS="QUOTE"
>"ATU-C"</SPAN
> column. This is the downstream sync rate negotiated by
the modem and DSLAM, which is over 3.5 Mbps. The actual speed is limited,
however, to 1.5 Mbps/384 Kbps from the first row <SPAN
CLASS="QUOTE"
>"TX Rate"</SPAN
>. This
is the theoretical limit of this connection. This limit, or
<SPAN
CLASS="QUOTE"
>"cap"</SPAN
>, can be enforced at the DSLAM, as is the case the here, or
further upstream. Had the first row <SPAN
CLASS="QUOTE"
>"TX Rate"</SPAN
> been lower than
the provider's imposed limit, then this would indicate some kind of problem
with the connection, perhaps due to distance or some kind of line impairment.
</P
><P
> The attainable sync rate is the result of a number of factors, including wire
distance to the DSLAM, quality of both inside and outside wiring, the loop
wire gauge and various other factors within the loop. Actual measurable,
real world throughput, on the other hand, is first of all dependent on sync
rate. Low sync rate means low throughput. In the above example, had the sync
rate been lower, say 500 Kbps, then that would be the maximum for that
connection, even though the customer is paying for a 1.5 Mbps service.</P
><P
>
Secondarily, throughput will depend also on the ISP's network, and then the
ISP's upstream provider. You will lose approximately 10-20% of potential
throughput to networking overhead. In the above example where the connection
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,
once you hit the Internet proper, all bets are off as there are any number of
factors that may impact throughput. A overloaded or busy server is likely to
be slow no matter how fast your DSL connection is. </P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DSLMODEMS">6.3. DSL Modems</H2
><P
> 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
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