DSL HOWTO for Linux
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7. Appendix: FAQ

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 a 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 is changing for the better. Xpeed now has drivers included in the kernel for source for their PCI IDSL and SDSL modems. This is good news! Alcatel has released drivers for the Alcatel SpeedTouch USB ADSL modem. IteX has also released drivers for their PCI ADSL modem. Hopefully more 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. I am told I am 20,000 ft from the CO. Isn't that too far? Will my speeds be really bad?
Not necessarily. This distance limitation is not where the CO is, but where the DSLAM is. These are often installed in CO's, but more and more are being installed in remote locations in order to expand the reach of DSL service.
Q. What are the speed tweaks for Linux?
This may not be 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 there might be a problem somewhere. See the Troubleshooting section for more. What you may 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). See the Tuning on how to mitigate this effect.
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 various factors. 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. Can DSL work with ISDN, and how is this different?
Yes, there have been DSL capable modems and service providers for some time. In fact, this is common in parts of Europe. So this is not an issue.
What makes ISDN different is the underlying signal on the line is fundamentally different than a POTS line. This means that any physical layer hardware has to be compatible with ISDN (and conversely it is incompatible with POTS lines). So this means the NT (modem), filters, etc, all have to be designed for ISDN. Other than these low level issues, the other aspects of DSL implementation are the same (e.g. network protocols).
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.
The impact of the disconnect problem can potentially be eased by adjusting the PPP LCP-echo settings to extend the period before the local end of the connection decides to terminate the session. Each end of the connection uses LCP echoes to make sure the other end is still "there". Nothing much can be done if the remote end decides to tear down a session (other than to do what you can to make sure you are responding to it's LCP echoes).
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 how 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 in most situations. 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. Now that I have a dedicated line, do I really need an ISP? Can't I be my own ISP?
Yes, and no. Linux has everything needed to run a small ISP. But even though the "line" is a dedicated connection, it is only dedicated to the telco end-point equipment. You still need someone to sell you bandwidth, and gateway access to the Internet. So, traditional ISPs still have their role. You might see if there is a local provider of some kind that will just sell you the bandwidth without all the frills (e.g. email and news). But this probably will not save any costs.
It is also technically possible to connect two DSL modems via a "dry" copper line. In some areas, a dry line (with no dial tone), is fairly inexpensive (but in others areas it's not). And then you need someone on the other end who is willing to provide the bandwidth and whatever services may be needed. Not all DSL modems support this (some common SDSL modems apparently do). This is also going to require dealing with the local phone company for something that is not a consumer type service (read: might be a real PITA). There is also a significant start up investment, that may not come with any telco guarantees for the intended use.
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 roll-out. 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 since many Operating Systems can't speak ATM natively, and the cost of ATM cards is more than ethernet. See http://linux-atm.sourceforge.net/ 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 higher performance signals like DSL. 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 ~15,000 ft) loops. The different bandwidths that you see advertised reflect various marketing battles 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 this does not fit the bill, then you can check with any local Business class DSL providers. This will cost more, but the Terms of Service, and guarantees, are generally much more suited to higher bandwidth usages.
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 January 2002. 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 and 5861, 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: Home == ethernet, 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 677 (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.

Modem manufacturers often ship modems to meet an ISP's specifications. Features are sometimes enabled or disabled as requested by the ISP. There are conceivably numerous, possible variations on each model. Something to consider if buying one second-hand.