DSL and filters – an hymenopteral explanation

If you look up “how DSL filters work”, you will find a bunch of scientific words like frequency, impedance, capacitance, inductance, and perhaps twisted pair. Twisted indeed. Your electrical engineering, or “EE”, friends (if you have any, which is unlikely) may be edified by such language, but most of us remain unimpressed. As I pondered how to explain the solution to last week’s DSL situation, I realized that a more accessible explanation of DSL and DSL filters would be extremely helpful. I think I have come up with a useful analogy. Like most analogies, it has its limits, but bear with me, if you will.

Think of the telephone wiring coming into your house as a pair of tubes, one carrying information in, and the other carrying information out. That isn’t how it really works, but go ahead and think of it that way.

Now think of old-fashioned telephone signals as a trail of ants marching in the tubes (see ant figure). Ants carry voices into your phone’s earpiece, which converts the ants to sound so you can hear your friends (or more likely, your Mom) talk. As you have no doubt figured out by now, the mouthpiece converts sound into other ants which march down the other tube, carrying the sound to the party with whom you are speaking.

I know that sounds crazy, but it’s not nearly as crazy as the way the EEs think it works.

Telephone wiring capable of carrying high-speed digital data over a voice line is called Digital Subscriber Loop or DSL. DSL was first demonstrated in the 1980s by Joseph W. Lechleitter, Bellcore employee and inventor of the DSL modem. The EEs think it works by adding high-frequency data signals to the relatively low-frequency voice signal.

In our analogy, data messages are carried by bees. (You may be tempted to think that the term “bit” might be derived from “bee,” but that is not even close.)

The DSL Modem (“modem” is short for a much longer word) converts the incoming bees into e-mail messages and YouTube videos. If you have enough bees, they can even be converted into Hulu TV programs. The modem also converts your personally generated content, such as your Facebook status and those cute photos you post, into outgoing bees, which fly through the tubes to the Internet.

OK, here is where the analogy begins to get technical. You may want to pause here and get a cup of coffee. I’ll wait.

Old fashioned telephone wiring acts like a large tube, a foot or more in diameter. Bell and his cohorts couldn’t make smaller tubes at the time, and the big fat tubes worked fine, since the ants were just crawling on the bottom of the tubes anyway.

But when the DSL data bees were introduced, there were a couple of problems. First, the bees made a buzzing sound in the telephone earpiece. (This part of the analogy is actually true.) Even worse, the bees tended to fly around freely and randomly in those big ol’ tubes, reducing the efficiency of the data transfer, and causing YouTube videos to freeze right at the good part (see ant & bee figure).

This problem was addressed by two inventions. (Hang on, we’re almost done.)

1. Tube manufacturers invented a new type of tubing called “CAT 5” (which has very little to do with the number 5, and even less to do with cats.) The CAT 5 tubes are smaller in diameter, more like, say, an inch in diameter (2.54 cm, for you metroids).

This still allows the voice ants to march down the tube unimpeded, but it constrains the data bees to fly directly to and from the modem, much more efficient than flittering around in a fat tube (see CAT 5 figure).

2. The DSL filter was also invented. In our analogy, the DSL filter can be thought of as a large flat plate, sized appropriately to cover the end of a tube, and having an ant-sized hole at the bottom of the plate. This allows the unrestricted passage of voice ants, but keeps the buzzy data bees out of the ear piece, and out of the fat tubes, too, if you put the filters in the right place (see DSL filter figure).

 


The Solution (finally)

The first time I hooked up the wiring, I had connected the old phone wire at the network interface, and dutifully installed a filter at the jack where the speaker phone connected, in the main living area (see original diagram.) That filter is not shown in the diagram because I had removed it, which was sort of a hint. The problem, of course, is that the one-foot tubes allowed the bees to flitter around aimlessly, with less than a third of them reaching their intended destination. So my daughter was getting less than 500 kilobees per second, rather than the 1500 she was paying for.

You may know that CAT 5 cable consists of four twisted pairs of wire. Well, only one pair is used to carry the ants and bees up to the office area, so here’s what I did. I installed a splitter in the voice port of the DSL filter, and used a phone repair wire (plug on one end, bare wires on the other) to connect the filtered signal through an unused pair and back down to the network box. At the network box, I connected this pair directly to the old steel cable leading up to the main living area, and, as the French say, “Voila!” You can see the final solution in this new diagram.

In the final use of this silly analogy, the filter in the office area forces the data bees to go to the DSL modem, while allowing the voice ants to go, not only to the cordless base station, but also back down the CAT 5, to the network box, through the fat tubes, and into the speaker phone.

Miscellanea

US Patent 5896443, 1999, describes a complete combined data network using DSL technology.

Hymenoptera is the class in which biologists categorize bees and ants. The word comes from the ancient Greek words for “membrane” and “wing.”

If it makes my analogy more believable to you, the ants and bees move at nearly the speed of light.

The form of digital telephony called Voice over Internet Protocol, or VoIP (usually pronounced “voip”) simply consists of teaching ants to ride on bees.

No hymenoptera were harmed during the research for this posting.

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