DSL HOWTO for Linux
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6. Appendix: DSL Overview

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 "POTS" (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 "lines". This gives DSL a potentially broad consumer base, and helps minimize costs for service providers.

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, "always on" 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 :-).

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 "a piece of the pie", 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.

Consumer DSL plans are typically "best effort" 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.

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.

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.

6.1. The DSL Family

ADSL

Asymmetric Digital Subscriber Loop currently supports downstream rates up to 8 Mbps, and upstream of 1024 Kbps, hence the "asymmetric". 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 "full rate" 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 "modems" (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.
G.Lite

G.Lite is sometimes referred to as "DSL Lite", "Universal DSL" or "splitterless ADSL", is a slower version of ADSL that requires no splitters or filters. G.lite uses a "fast retrain" 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 "full rate" ADSL however.
SDSL

Single-pair Digital Subscriber Loop, or also sometimes referred to as "Symmetric Digital Subscriber Loop" 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 "SDSL" 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?
IDSL

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 "always on" 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.
RADSL

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.
HDSL

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.
VDSL

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.
UDSL

Unidirectional Digital Subscriber Loop is a proposal from Europe that is not yet in use.
G.SHDSL

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.

6.2. The DSLAM

This technology is made possible by the placement of DSLAMs, or Digital Subscriber Loop Access Multiplexers, from such suppliers as Alcatel and Cisco, 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).

Figure 4: A Typical DSL Connection Path

Voice -+                                               +---> Voice
        |<-- copper loop --> DSLAM/CO <--{ATM cloud}--->|
modem -+                       |                       +---> Inet
        |                       |                       |
ether..|..... DSL/ATM here ....|.... raw ATM here .....|.. TCP/IP ..
        |                                               |
SOHO...|............ telco (ILEC or CLEC) .............|.. ISP ..| NSP

6.2.1. Sync

A good, working connection to the DSLAM is referred to as "syncing". 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 "cap", 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 "cap", but your speed will be limited to whatever "cap" 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.

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:

 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->

First notice the "Current Attainable Rate" in the "ATU-C" 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 "TX Rate". This is the theoretical limit of this connection. This limit, or "cap", can be enforced at the DSLAM, as is the case the here, or further upstream. Had the first row "TX Rate" 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.

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.

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.

6.3. DSL Modems

The "modem" 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 "syncs" with the DSLAM, and then makes an IP connection to the ISP, and off we go!

For Linux users, the modem is a very important consideration! 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 Modems Section for an up-to-date list of compatible modems.) ISDN users will need a modem (NT) designed specifically for DSL over ISDN.

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.

As always, there are exceptions. Xpeed 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. Alcatel's 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 http://eciadsl.sourceforge.net/). 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.

The most common type of modem in use today is actually a combination "bridge" 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.

Some ISPs are also offering "routers". 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!

To confuse things even more, there are also all-in-one devices: combo bridge+router+modem, sometimes called "brouters". In this case, the modem can be configured for either bridged or routed modes -- but it can't be both at the same time.

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).

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).

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.

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.

6.4. The ISP Connection

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 "see" as the first hop on a traceroute. 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 "subscribe" 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.

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 "reselling" 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.

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 "partnered" with one or more ISPs. If you are lucky here, you will have many choices of plans and pricing structures.

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.

Typically, your service agreement is with the ISP, and not the DSL provider. This makes the actual DSL provider a "behind the scenes" 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.

6.5. Availability

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 "pre-qualifying", and is Step One in getting service.

More and more "remote terminals (aka DSLAMs)" 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.

6.5.1. Ordering

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.

6.5.2. Qualifying

Once local availability is established, the next step is "qualifying" 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 "disqualify" a line. The most common limitation is distance.

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.

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 "clean" 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.

Once the line is "qualified", 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.

Some common data rates:

Downstream/Upstream

  128 Kbps/128 Kbps

  256 Kbps/256 Kbps

  384 Kbps/128 Kbps

  640 Kbps/90  Kbps

  1.5 Mbps/384 Kbps

  2.0 Mbps/512 Kbps

  7.1 Mbps/1024 Kbps

and a near infinite number of other possibilities. The cost of different plans generally goes up with their speed.

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 "clean" 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.

6.6. Choosing Providers

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.

A compatible modem. For now with Linux (or any alternative OS) this essentially means an ethernet interface. (There are rare exceptions.) "Routers" (i.e. combo modem/routers) should be OK too since these seem to be all ethernet devices.
Installation. 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.
Static vs Dynamic IP Address. 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 "invisible", or a least harder to keep track of.
Encapsulation. Is the connection "Bridged" or "PPP". PPPoX has the reputation of being not as stable a connection, and not "always on". PPPoE requires client software to manage the connection, so one more layer of code.
Server Policy. 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.
Contract. Is there a contract, and what are the out clauses? Cancellation fees?
Connection Limits. Is it "always on" (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!)?
Linux Support. 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. ("Supported" means like "tech support".) If they say "we don't care", you should be good to go.
Free Dialup Account. A nice thing to have if the connection is down, or you just need to check mail from another location.
Setup program. 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.
Reliability and Quality of Service. 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?

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.

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