Some Frequently Asked Questions about DSL and Linux.
Q. Does DSL work with Linux?
DSL is a technology, or more correctly, a group of related technologies.
This is akin to asking if Linux works with telephones. The technology
itself does not care. So, the short answer is "Yes, of
course!". The long answer is that if there are any impediments, they
are being imposed by the provider. There are things they may do, that can
make getting Linux up and running, more of a challenge than it needs to be.
Not having a compatible modem option available is one common gotcha. Also,
if the telco or ISP is doing the installation, they may require a Windows
or Mac system to be available. This saves them the costs of training their
techs on various alternative OSes. Buyer beware!
Basically all DSL does, is facilitate a high speed Internet connection. At
some point, it is all TCP/IP, and Linux, of course, handles TCP/IP quite
well.
Q. Where can I find drivers for my PCI (or USB) modem?
With few exceptions, you probably can't, because they are just not
available. Your best bet is an external, ethernet interfaced modem for all
intents and purposes. If your provider does not offer one, you will have
to find another provider, or buy your own modem outright. Just make sure
it is compatible with your provider's flavor of DSL.
The are exceptions to every rule. See the Modems
Section for a list of compatible modems as of this writing.
If an incompatible modem puts you in a bind, hopefully you will take the
time to politely harass the manufacturer ;-).
This situation may be changing. Xpeed now has drivers included in the
kernel for source for their PCI IDSL and SDSL modems. This is good news!
Alcatel has just recently
released drivers for the Alcatel SpeedTouch USB ADSL modem. Hopefully
others will follow suit. (Make sure you are reading the latest version of
this document, as I have intentions of keeping this situation updated as
needed.)
Q. How fast or good of a network card do I need?
Any card that is compatible with Linux should work fine. Remember even
low-end cards are 10 Mbps, and no consumer class DSL is near that at this
time. I would suggest a reasonably good quality card, just to help
eliminate the possibility of errors and premature failure.
Q. How can I find out when DSL will be available in my area?
Just where and when DSL gets deployed is totally in the hands of
your friendly local telco. They obviously can't do everyone at
once, so they probably are selecting areas based on competitive
factors. Getting a straight answer from a telco on this question
can also be a challenge. Probably so as not to tip their hand to
competitors. Unfortunately, it is a question only they can answer.
Q. I was disqualified because I am too far away. What can I do?
Move? Seriously, there isn't much you can do. If there are other providers,
get another opinion. You never know. Determining the loop length is an
inexact science, and there is room for errors. Many use databases for
this, and these databases routinely have some inaccuracies. Some providers
too, may be more aggressive in taking steps to help you out and clean up
the line. Also, some providers offer low-end speed services that have
greater reach. Maybe this will become available in your area. Or, the telco
may install, at some point, remote devices for customers who are now too
far away.
Q. What are the speed tweaks for Linux?
This probably is not necessary. Linux is pre-tweaked for the most part,
unlike some versions of Windows that really need some registry hacks to get
optimum performance. If you have a high latency connection, you may
benefit from increasing the TCP Receive Window. See the Tuning section.
Now if you are convinced you are not getting the performance you should
based on your distance and line conditions, then maybe there is a problem
somewhere. See the Troubleshooting section for
more. What you need is a fix, more than a tweak.
Q. My service is limited to 640K (for example). Can I get better speed by
getting a faster modem? Any way around this?
No, and no. The modem has little bearing on how fast your connection is for
all intents and purposes. The provider has a mechanism in place for
limiting your speed somewhere in the pipe before you hit the Internet.
There is no way to defeat this.
Q. Can I download and upload at the same time? Is one effected by the
other?
The upstream and downstream channels use separate frequency ranges within
the DSL signal, so simultaneous upload/download is not a problem and
available bandwidth is not normally impacted.
Where there may be somewhat of an adverse impact, is with asymmetric DSLs
like ADSL, and both the upstream and downstream are
simultaneously saturated. This is a TCP 'feature' and not DSL related
though. This can adversely effect the faster stream (i.e. the downstream).
How much of an impact depends on a slew of factors and is beyond the scope
of this document, but is more pronounced with higher ratios of downstream
to upstream (e.g. 640/90).
Q. I am paying for 768 Kbps service, and the best I ever get is 640 Kbps or
so. Why? Is the service oversold? I am not getting what I pay for.
You will lose 10-20% of the rated capacity due to the overhead inherent
in the various protocols utilized. Most of us will probably fall closer to
20%. This is just a fact of life for everybody. Just how much is lost here
depends on a number of factors, and may vary from provider to provider. You
seem to be close to your maximum when this is taken into consideration.
Also, if you read the fine print, many ISPs are advertising speeds
"up to" such and such. Check your service agreement and see if
there are any guarantees. If there are, they may be well below the
advertised maximum speed, and may be based on sync rate instead of actual
throughput. Though this may vary from provider to provider as well.
Also, be careful how you test this. Some of the so-called test sites can be
pretty unreliable. There can be many factors between you and that site that
can impact your throughput and skew results -- not the least of which is
how many people might be trying that same test at the same time. The best
test is via FTP download from a known good, close, not too busy site.
Q. Why does PPPoX have such a bad rap?
The occasional disconnects is one of the biggest gripes. PPP seems to be
sensitive to any interruptions in the connection. Generally a disconnect
means a new IP. And there are those that say PPP, by its very nature, was
never meant to be an "always on" protocol. PPP is a session
management protocol at heart, that requires a user to initiate a
connection and authenticate him or herself. PPPoE/A are not yet
particularly mature protocols either. They do not have much of a history
or track record. Some would say the telcos and hardware manufacturers have
rushed this out the door. PPPoE also requires an additional layer of
software just to maintain the connection. This is one more layer of code
and one more potential point of failure. Also, more system overhead is
utilized to manage the connection.
Q. Why PPPoX? This seems like a bad idea!
PPP gives several advantages to the provider: they can use their existing
infrastructure and hardware that they now use for their (larger) dialup
customer base. It is easier to control user authentication and potential
abuse situations, and easier to manage their network and related issues. In
fact, it most boils down to its just easier for them. Easier, means saves
man hours, and therefore saves costs (at least from their perspective).
It is not a conspiracy to conserve IP addresses, or thwart heavy users. IP
address costs are insignificant in the overall scheme of things.
Q. The only provider in my area does not support Linux. What can I do?
Will I have to use Windows?
NO! "Support" here is support as in "tech
support". They are just saying that they will not give you tech
support when and if you have problems. This does not mean you cannot use
Linux on their network. Just that you may have to fend for yourself when
and if a problem does arise. Anything that is forbidden will be in their
Acceptable Use Policy (AUP), or Terms of Service (TOS) agreement.
I have heard stories where a new tech or installer has misinterpreted their
own company's policy on this and told someone "you can't use Linux
here". Same with NT server. But this is almost always a misinformed
individual.
But -- if a provider does not support Linux, they may balk at installing
onto a Linux box. Hopefully, they will have a self-install option to get
around this annoyance. YMMV.
Q. My fax software does not work with my DSL modem. Why is that?
Faxes are normally transmitted over typical analog phone lines by dialing
the fax machine on the other end. Analog modems can handle this, but
DSL modems have no dialing capability. Don't throw out that 56K yet!
Q. What does "FastPath" mean? Is it better? Faster? What is
interleaving? How can I get better ping times?
Interleaving is a feature of DMT line encoding. Essentially it is a form
of error correction that is configurable at the DSLAM. The side effects are
a slower connection, especially higher latency. With FastPath (or sometimes
called non-interleaved) DMT, gateway pings can be in the 10-25 ms range. With
interleaving, this is more likely to be in the 40-75 ms range depending on
the degree of interleaving that has been enabled.
On the positive side, a marginal line is more stable and less prone to
errors with interleaving. Many telcos have interleaving on by default since
increased stability would seem to be a good thing. But this is only
beneficial for marginal lines, and everyone else is paying a latency tax
for this. Some telcos may be amenable to turning this feature on/off. YMMV.
Q. How fast and powerful of a computer do I need for DSL? My ISP says I
need at least a Pentium 200. Why?
At the most basic level, a 386 will work fine. In most situations, you are
connected to what is essentially an ethernet based network. So
theoretically anything that can handle a very slow ethernet connection
would work. No comment on well Netscape will run on a 386 though ;-) But as
far as just managing a raw connection, a 386 is indeed workable. What else
you can do with it, is another matter.
Where this gets a little more complicated is the modem, and the client
that the ISP may require. Any PCI or USB modem is going to require
drivers, which means more CPU and system resources. Also, PPPoE does even
more processing, so again the potential CPU load is increased. Windows
tends to be not so efficient with all this going on, hence the requirement
for mid range Pentiums by some ISPs.
With Linux it will depend on what you are going to do. A low end Pentium
should be fine for most uses. A 386/486 should do nicely for just a
firewall/gateway box. Just remember if you are running PPPoE, you may take
a performance hit on low end hardware.
Q. I just got my DSL installed, and my speed sucks, and/or my connection
constantly drops. What is the problem?
Not enough information to say, really. There are many, many things that
can cause a poor connection. The list is too long to mention them all.
One of DSL's weaknesses is that the signal can be fairly fragile. Many
things can degrade the signal, making for poor connections, and thus
speed. This can be caused by poor or substandard inside wiring, a wiring
problem outside (like bad splice), RFI from any number of sources, AM
radio signal interference from a nearby station, bridge taps on your
line, excessive distance from the DSLAM and so on. Not to mention possible
hardware problems with your modem, NIC, or the telco's DSLAM, etc. Not
always easy to sort out.
Your provider should be able to assist you. First, make sure the problem
isn't with your setup as they likely won't help solve a Linux problem. Then
be persistent, and don't hesitate to go over someone's head if the help is
not forthcoming. Most problems are solvable. The trick is isolating it. A
good telco tech, trained for DSL, can find all kinds of obscure wiring
problems.
Q. My provider's tech support staff is clueless. What can I do?
Common complaint. Seems to be the nature of the beast. First line tech
support is an entry level position, and mostly filled by young people
with little technical or networking knowledge. Grin and bear it, or try
calling back.
Q: Are there ADSL Standards?
Sort of. The U.S. Bell Operating Companies have standardized on Discrete
Multi-Tone (DMT) (ANSI T1.413) in their current rollout. Most others
should follow their lead in the states. There are other types of modems, most
notably Carrier-less Amplitude Phase Modulation (CAP), which of course, is
incompatible with DMT.
A biased comparison from an DMT-based vendor on this subject can be found at
the http://www.aware.com. Still,
it provides the best detail on this issue I have seen so far.
A rather expensive copy of the ANSI standard can be ordered at: American
National Standards Institute ANSI Home
Page
Asymmetric Digital Subscriber Loop (ADSL) Metallic Interface
ANSI TI.413-1995
Note: ANSI TI.413 Issue 2 was released September 26, 1997
Q: Can I use ATM to connect to DSL?
Technically speaking, you can. Some DSL modems (at least the Alcatel
version) has a ATM Forum 25Mbps interface, which connects to a PCI ATM
card. But this is rarely done in practice. See http://lrcwww.epfl.ch/linux-atm/
for more details.
Q: Why does DSL have all these bit rates (384/1.5/7.1M/20M/etc) options?
The basic problem is the 100 year old design of the copper loop. It works
great for analog phone, but it presents a real challenge for a digital
signal. Remember that the distance of a loop is inversely proportional to the
data rate that it can carry. Rate adaptive technologies are great for making
a digital signal work in many situations, but it can't provide a consistent
bandwidth for all applications, especially for very long (over 18,000 ft)
loops. The different bandwidths that you see advertised reflect various
marketing wars of vendors equipment, and the telco struggle to finalize on a
standard set of data rates. The bottom line is for the telco to be able
to reach as broad a customer base as possible.
Check out the next question on the loop impairments that cause this to
happen.
Q: What are all these loop impairments (bridge taps, load coils, DLCs) that
could disqualify my line from DSL? (thanks to Bruce Ediger)
Load coils: in-line inductances that improve voice-frequency transmission
characteristics of a telephone circuit. Essentially, a "load" steals energy
from high frequencies and gives it to lower frequencies. Typically only used
in very long (> 9,000 ft) phone lines.
By "bridges" I assume you mean "bridged taps". In older neighborhoods, the
phone wiring will have been used by more than one customer. Perhaps these
customers lived at different (though near-by) addresses. The unconnected
"spur" of wiring is a "bridged tab" on the currently connected circuit.
DLCs, Digital Loop Carriers: there's a bunch of systems for carrying more
than one voice transmission on a single pair of wires. You can shift the
frequencies up or down, or you can digitize the voice transmissions and
divide the telephone circuit by time or code or something. The more
general term is "pair gain".
These things cause different problems for high-frequency communications.
Load coils will completely mess up things by filtering high frequencies and
passing low frequencies. They probably also change the "delay envelope",
allowing some frequencies to arrive before others. One byte's tones will
interfere with the next byte's.
Bridged taps act as shunt capacitances if they're long in relation to the
signals wavelength, and they'll actually act as band pass filters if they're
about 1/4 wavelength of the signal. That is, they'll pass particular
frequencies freely. Particular tones of a DMT modem might get shunted back,
rather than passed along to the receiving modem, reducing bandwidth for that
telephone line.
Pair gain, digital or analog, limit the bandwidth available to one
transmission in order to multiplex several on one wire. High and low tones
of a DMT transmission get filtered out by the apparatus.
The book "Subscriber Loop Signaling and Transmission Handbook", by Whitham D.
Reeve, , IEEE Press 1992, ISBN 0-87942-274-2 covers the math of how to
calculate the effect of line length, bridged tap, etc on the transmission
characteristics of a telephone line. It's pretty expensive, however.
Q. Can I run a web server with my DSL connection?
Sure. You are connected to a TCP/IP network, so theoretically you can run
any service that the protocols allow -- mail, ftp, ssh, irc, etc. Where
there may be problems, is with the ISP's TOS (Terms of Service). Some ISPs
are pretty open on this, while others forbid any type of server, and may
even block certain ports. You should research this, or ask the ISP before
making any plans. ISPs that are selling a consumer service are not
going to allow any high volume servers -- just personal, or low traffic
services at best.
If you do not have a static IP, you can get around this with one of the
many Dynamic DNS services that are out there for just this purpose. See the
links section.
Q: Do you have examples of DSL Modems?
Short Answer: Yes. Real Answer: The evolution of this technology is
moving too rapidly for anyone to keep up to date in a HOWTO.
Check http://dslreports.com/information/equiprated/all for up to date information.
However, below is a list of some of the current modem offerings as of
March 2001. All are ADSL modems with DMT encoding (a.k.a. Alcatel
compatible), unless specified otherwise. [Note: Some items retained from
original list dated June 1998.]
Router/Modems with 10/100baseT Ethernet Interface:
Examples: Flowpoint 2000 DSL(CAP), 3COM Viper-DSL (CAP), Westell
ATU-R-Flexcap (CAP), Aware x200, Zyxel P641, Efficient Networks
SpeedStream 5660, Cayman 3220H, Cisco 673 (SDSL), Cisco 675 (ADSL/CAP),
Cisco 677 (ADSL/DMT), Alcatel SpeedTouch Pro
Bridge/Modems with 10/100baseT Ethernet Interface:
Examples: Alcatel 1000, Alcatel SpeedTouch Home [note: there are also USB
and PCI SpeedTouch versions!], Westell ATU-R-Flexcap2 (CAP), Efficient
Networks SpeedStream 5260, Efficient Networks SpeedStream 5251 (SDSL),
Westell WireSpeed.
Modems with ATMF Interface:
Examples: Alcatel 1000, Alcatel SpeedTouch Home, Cisco 627 (DMT), Ariel
Horizon II
Bridge/Modems with V.35 Serial Interface (T1, Serial Router)
Examples: Westell ATU-R
Modems with USB Interface:
Efficient Networks SpeedStream 4060, Intel 3100, Alcatel SpeedTouch USB
PCI Modems:
Examples: Cisco 605, Efficient Networks SpeedStream 3060/3061, Intel 2100,
Xpeed X200 (IDSL), Xpeed X300 (SDSL), Alcatel SpeedTouch PCI
Wireless Modems (IEEE 802.11b):
Examples: Alcatel SpeedTouch Wireless
Dedicated Router (no built in modem) with 10/100baseT Ethernet Interface:
Examples: Netgear RT311, SMC 7004BR, Linksys BEFSR11
This is but a very small sampling and should not be construed as
endorsements of the products listed. It is just a simple illustration
of a few of the available products.
