ADSL, Back to the Future

Just setup a DSL line to a home in a remote community near Branson, Missouri - the only affordable option was CenturyLink DSL. This is that Odyssey.

First came the sign-up through the CenturyLink (CL) online Chat service. Took a little over an hour, not that there are a lot of options, but a lot of backend checking, coordination and swivel chair searching for the obligatory "Compatible Modem". -- In this case an xDSL modem.

DSL - "Digital Subscribe Line" is an evolved version of online Internet Dial-up, evolved such that it is (always) online. There are have been many generations of it beginning shortly after I left the world of Trumpet Winsock and Windows 3.11 way back in the early 1990s.

Essentially back then you had limited options if you wanted to connect to the Internet. You could get to your ISP (Internet Service Provider) over a direct Ethernet connection (from school or work), or over a T-1 (Terrestrial One) Digital Subscriber line (these were more commonly use through a CSU/DSU to carry voice or data traffic) or you could get a "pure" ISDN line, which again was a voice or data technology which used TDM - Time Division Multiplexing, to "share" the line between packets intended to carry voice or data traffic.

Current DSL was an evolution of the acoustic dial-up modem and ISDN. Basically its not a T-1. ISDN was "symmetric" in that upload and download speeds were the same and traffic was dedicated to either voice or data but not both, later versions would make this more flexible and relax early restrictions, this was all before VoIP came along.

So take the DMT - Dual Multi-Tone technology used by dial-up modems and qualify or "train" the line between the Central Telephone Office (the "CO") and your house (the "CPE" Customer Premise Equipment) and you can split up the available audio "tones" or audible "tones" and frequencies "beyond" those tones you can actually hear into [channels] each of which can carry a few bits of a byte simultaneously. This "spread spectrum" approach to dicing up a byte into bits spread across different tones means that a "good" line can carry a lot more traffic than a "bad" line.

Generally copper lines to your house get "noisy" and harder to use "full spectrum" for pushing and pulling bits the further away from the "CO" you get. And the "thinner" the copper in the lines the noiser they get "too". Mostly however lines are of a standard size. But heavier "larger diameter" lines will carry more traffic with less noise.

Copper is subject to corrosion over time, and injury from backhoes and other equipment in the wild, even squirrel bites and nibbling. So they need to be replaced after 10 or 20 years.

Also Copper is a "conductor" and it will pickup "cross talk" or "oersted induced currents" simply from being laid next to another wire with a varying current.. like a power cable or electric line. To make things worse laying either in the ground or running over head Copper lines act like very long "Antenna" and pickup all manner of Radio Frequency interference.. which tends to increase during the day or early evening when people are using RF generating equipment like radios, cars, microwave ovens ect.. the interference doesn't have to be "exactly" on the same frequency as the data traffic to cause interference harmonics and natural resonance circuits in the surrounding landscape can do the job nicely as well.

So while "clever" that basically the acoustic modem has evolved into a modem that uses not only audible tones but those far beyond the range of human hearing over the same lines.. Copper presents a constant maintenance problem. And a constant challenge when setting up a new subscriber to service.

The CO equipment has varied greatly over time, but basically its called a DSLAM or MSAN and its a DSP - "Digital Signal Processing" box with "line cards" that terminate the Copper pair coming from your house (the "CPE" customer premise equipment). These are pretty dumb and simply establish a "trained" circuit using ATM virtual circuits over virtual paths between your modem and the central office.

Once the virtual path is "Up" anything can connect to the interface of the virtual circuit and shout a login method to something on the other size. Very often this logon method is PPP Over Ethernet.. or PPPoE. PPPoE is a combined standard, it marries serial link PPP with the standard expectation that you will be connecting the PPP protocol interface over an Ethernet port. So these modems usually provide a dumb Ethernet interface that routes the PPP packets to ther other end.

The DSLAM or MSAN doesn't know anything about PPP but it does know what a serial link is, and it connects that to an Ethernet port which can carry the PPP to its backend access server.. the RASPPPoE server which can accept PPPoE login requests from a customer. If successful, or allowed by the RASPPPoE server, then the customer can begin sending TCP packets to route over the Internet. Part of PPP optionally also requests a DHCP or Dynamically assigned IP address as part of the logon process.. but its optional, if the customer has a known static IP address all they need do is login sucessfully and the are on the Internet.

Problems can crop up with the DSLAM or MSAN equipment because they are made by different manufacturers, they have Interoperability "bakeoffs" but that doesn't mean they all perform up to the same performance. CO equipment connected to by CPE equipment at the customer site may have "interop" issues and only connect as slower rates.. if at all. To say nothing of the [Type] of DSL that CO has decided the CPE is allowed to use.

Over time xDSL has evolved from ADSL to ADSL2 to ADSL2+ to VDSL to VDSL2 and so on.

ADSL is "asymmetric" or "lopsided" DSL.. it means the packet traffic path "down to the customer" is "larger" than the packet traffic path "up to the internet"

that is 1000 Kbs / 384 Kbps for example

1000 Kbs "down" to the customer

384 Kbps "up" to the internet

The reason for this [A] in DSL is because when it was decided to invent ADSL the assumption was customers would rather use [more] of that Multi-tone frequency range for [pulling] data from the Internet than [pushing] data to the Internet.. they weren't expected to run Internet servers.. they were expected to do things like use Internet Browsers or Watch Netflix.. [so more would be downloaded than uploaded].

This ADSL became a [standard] its unbalanced to "favor" download capacity in the customers interest.

SDSL on the other hand, which was more akin to ISDN, was "symmetric" DSL and considered "Business class" because those customers "might" indeed want to run Internet Servers and want the capacity equally split. So it was priced appropriately for "Business class" users and is usally more expensive.

[Provisioning a Port] to the telco company means, hooking that Copper pair to a DSLAM that has chipsets and hardware for supporting only [one-type] of DSL.. ADSL or SDSL.. not both and can't be switched without a "Port change" (moving the copper pair to a different box) and changing your Bill for Services to charge you for the higher or lower cost of that service.. its a major pain.. so basically people buy ADSL or SDSL but not both and not both at the same time.

Within ADSL there have been ADSL, ADSL2, ADSL2+, VDSL, VDSL2 and so on.. these are all asymmetric services for customers (lopsided) on purpose. Generally ADSL has the longest "reach" or can "work" over the longest length of phone line Copper pair from the CO to the Customer but it offers the slowest service and is at the bare edge of possible. A closer connection using ADSL and not ADSL2 for example will run at ADSL speeds and be slighlty more reliable.

Each later generation ADSL2, ADSL2+ ect.. is higher speed, but shorter reach.. and harder to keep "Up".. when a line is "trained" between a Modem and CO line card it runs ATM packets all the time, if something happens to degrade this, (i.e if enough ATM packets are "garbled" in transmission and thus "uncorrectable") one or the other of the CO-to-CPE side will "disconnect" and often not notify the TCP or PPP link above it.. leaving those virtual connections in a state of undefined limbo.. and the customer wondering why the Internet has stopped working.

A clever customer may then manually try to "reboot" or reset the DSL modem hoping to "clear"  the lines of communication and re-establish a connection. But whatever external cause precipitated the first ATM disconnect is likely to persist and thus they will probably not be successful .. at least until much later when it will magically appear to "connect".

This can be very frustrating.

Some high-end modems can monitor the DSL ATM connection status and automatically "retrain" and re-establish, but likely will have the same amount of success as the "clever customer" and may fail to reconnect until after the conditions that lead to the first disconnect subside.

And worse ADSL and VDSL are incompatible.. later generations of VDSL modems [might] backwards detect and support ADSL with an [auto] feature but its rare and doesn't work well, often it fails.. and many new modems no longer support ADSL "at all".. so usually a truck-roll involves using a handheld gadget at your outside phone box (NID) to actually "measure" how far you are from the Central Office.. that tells them what is "possible" and they communicate how the [Port] for you should be "Provisioned" ADSL or VDSL for example and then by subcategory 2+, 2 or 1.

They usually try to start "high speed" with the maximum speed service they offer, (aim high) and then reel in their expecations after a site visit to measure what is really possible with your Copper line.

CO equipment also doesn't always work well with Customer CPE Modems, especially if they are known to have buggy software or were made with chips from a manufacturer other than the manufacture that made the chips in their DSLAM.

Tracking the chipsets between CO and CPE equipment is hard, made harder because even if you can get their manufacturer names.. often those companies have been bought out or changed names. xDSL tech is almost 30 years old, so there has been a lot of water under the bridge.

"Approved" modems by your CO provider (CenturyLink) are usually little more than "best guesses" that equipment they have seen work before.. may work again. They don't know if the most recent models from different vendors will work with their DSLAM or MSANs in a particular CO.. its usually a matter of trial and error. Modem makers also change their software and chipsets from time to time because of cost and parts availability.. so a particular modem replacement may have the same outside "shell" or cover on it.. but the internal circuit board may be a Version 1 or Version 2 or Version 3 and be totally different.

This ends Part One of my story regarding setting up DSL in 2016.

Later parts will cover Customer Premise wiring problems, Modem selection and diagnosis problems, and methods of overcoming really difficult Uptime problems.