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Ramblin' Road says,

Introduction

You are familiar with the concept of two copper wires carrying your voice (for the last 120 years), and that CAT 5 wire carries computer signals on a office network. Who said copper can't do both? (With a bit of technology.)

Voice lines only carry a frequency range of about 400Hz to 4kHz. This is enough to recognize who's talking, and it's easier for the phone company to convert the analog signal to digital pulses that are sent to another office that converts them back to analog sound.

DSL uses two more frequency ranges. A uplink signal goes from your PC to a DSL modem to the phone company equipment, at about 30kHz to 110kHz. And a return (downlink) comes back to you in the range of 110kHz to 1.1 MHz. The DSL modem and the DSLAM (Photograph) (DSL Access Multiplexer) at the phone C.O., find a unused frequency band to communicate in. NOTE: there is some variance in different brands of equipment.

The different frequency ranges "don't interfere" with each other, but some sort of filtration is used to allow voice and data to co-exist on the copper pair. And to prevent interference from portable phones.

The phone system was designed to carry clear communication over long distances. DSL has a limit of about three miles because of "attenuation", a different form of resistance; because of "disturbers" (radio station noise); and because the devices that work for voice, stop the high frequencies. In physics a low frequency travels further (think of a piano low note that has sustain, and a high note that goes 'plink'). That's attenuation.

The lines between the C.O. and the user have to be tested and/or cleaned up. The distance problem is being solved by installing fiber lines to the neighborhood B-box (fiber has little resistance or attenuation).

Phone company lines meet the CAT 3 wiring specification (mostly) that is used for 10 MHz network connections. So, it is relatively easy to get 6MHz DSL signals (the European standard) over the wire. Since it is a analog signal, the upper limit is about 52 MHz on perfect wire and short distances.

In a few years, we will probably see wealthy subscriber areas being converted to fiber. This has a present limit of 3 THz (trillion). Any pure digital signal would save the phone companies a bundle, since the analog/digital converters are the most expensive item in the C.O. Of course, the voice lines would have be converted to digital phones (like the PCS types).

For perspective, I have a DSL line in my home working at 1.2 MHz. Full motion video in a "picture phone" conversation requires about 1.5 MHz. And a 6MHz signal means the phone co. will also offer "On-Demand" movies (you use your PC to select and download, what you want to watch, when you want to watch it).

Incidentally, ATT isn't just sitting around. They are also installing fiber to get more bandwidth. They put fiber to a half block away, and installed new coaxial line to my home. Now I get letters offering me one bill that includes, (1) 50 cable channels, (2) premium channels, (3) internet connection, (4) long distance service (5) local phone service. Gee, now I don't need the copper pair anymore!

Back to DSL. Since most people spend more time reading the download than requesting it, the DSLAM is searching for another request from another PC. As a multiplexer, it can handle many lines simultaneously. Each data request and response is interspaced with others. Currently, the DSLAM's were sold as being able to handle 5000 lines, however there are big problems when more than 3500 are connected. Now, demand has exceeded supply in several C.O.'s, and there is the added complication that the DSLAM no longer belongs to the phone co. As "foreign" equipment, it must be in a cage with limited access (and with rent to be paid).

The unexpected surge in PC sales and in ISP growth has caught both the phone co. and ATT by surprise. Neither are able to get enough equipment and personnel to connect people. Two weeks to two months is the normal wait for fast internet service.\

To add insult to injury, technicians go to the customer premises to install equipment, only to find out that the line wasn't programmed yet. This entails calling the various offices to find the omission, and requires consoling the customer while waiting the (usual) two hours for the back office work to be done. Not fun.

One more thing, there are about 600 DSL companies offering a connection in California. Some are as huge as COVAD or PAC BELL. Others are run out of a garage. Some have their equipment in a C.O. Some rent space on others equipment. None of them are perfect.

Even with the hassles, any fast internet access is better than dial-up.

To give you some background, try these explanations. (I don't want to get too technical yet.)

Hertz is a new name for 'cycles per second' this is a analog concept. It is abbreviated both as Htz and Hz. Baud originally referred to a change of states, e.g. high to low. It is usable (interchangeable) between analog and digital. But it falls apart technically when used for these high speeds. (Remember 300 baud modems?) Some times there are long strings of zeros, thus no "baud" rate.

Bits per second refers to '0' and '1' in any order or combination, which is digital. Bytes are sets of eight bits. Also a digital concept. (Yes, there are other sets available.) What adds to the confusion is that when you download, the "rate meter" is referring to kilobytes, a set of eight bits times 1000. (My DSL downloads read about 70 to 140 kilobytes, times eight, equals about 560k bits to 1.1 M bits per second.)

So I am referring to MHz, million cycles per second for DSL. And kHz thousand cycles per second, for analog modems.In both cases, this refers to the number of single bits (whether 1 or 0) per second. Not the baud rate.

A second problem is that we shift gears to refer to frequency bandwidth (such as 90.5 FM). This is analog, or a sine wave pattern.

DSL uses channels between the DSLAM (Photograph) and the DSL modem. The uplink channel sends a page request up to the net, and the downlink channel brings back large pages (with photos), so it's larger. When your PC requests a site page, the DSL modem finds a part of the frequency band of the uplink channel (that is unused) and communicates with the DSLAM. The request is so short that the frequency is quickly available again for some other PC to use for its own request. Looking for a part of the bandwidth is sort of like looking for a radio station frequency for your favorite music.The DSLAM may not use the same frequency for your next request, it doesn't matter at all.

The physics classes in college teach mathematical reasons that high frequencies can't travel as far as low frequencies. I just remember how hard it is to shake fast, rather than slow. Attenuation notes that the energy level required for a high frequency is used up faster than a low frequency energy level.

By, the way, at this time, the DSL modem is a modem. DSL is using analog transmission. Unlike a 56k modem, the DSL modem uses higher frequencies.

Telephone transmission is limited to a bandwidth of 400 Hertz to 4,000 Hertz. The AD (analog to digital) converters at the C.O. sample the voice input in order to change it to a digital signal (laser pulses) for transmission over fiber to the next C.O. The signal is either repeated (not amplified) or run through a DA (digital to analog) converter and sent to the receiving phone set.

Yes, humans can speak and hear about 20 Hertz to 26,000 Hertz. But it's cheaper to only convert the lower frequencies. And, the human mind can sense the inflections and mannerisms that identify who is speaking. The AD and DA converters are the most expensive equipment in the phone system.

Back to DSL. There are a range of frequencies used. Generally, a bandwidth of 90kHz to 100kHz is used (in California) as the "uplink" band. The DSLAM (DSL Associated Multiplexer) in the C.O. will assign a frequency to a DSL modem (maybe several modems!) to listen for a packet which asks for a web site (e.g.; www.spam.org). Since a request is only a single packet, not much bandwidth is needed. Remember, a TCP/IP packet includes the destination, the routing servers, and the originators address.

The "downlink" bandwidth is from 100kHz to 1.1MHz. Since there are multiple packets required to send page one from a site (with text, flashing lights and dancing penguins), a larger bandwidth is required.

Uplink speed is 128kHz for basic DSL. Downlink speed ranges from 384kHz to 1.5MHz. Higher rates depend on conditions and cost per month that you are willing to pay. Maximum rates are about 6MHz for uplink and 24MHz for downlink (up to 54MHz in tests).

Compare this to the Internet 2, which is at 2.4THz (terra = trillion).

DSL can be transmitted digitally. As soon as people using the estimated 900 million analog phones in the world, convert to all digital, all the time.

It's possible that the homes (and businesses, later) will convert to digital in about three years. Someone will give users free digital phones for the home in order to accomplish the change (remember, cell phones before Nokia?). Businesses have PBX's to be converted, and they will, in order to get the extra features offered by a newer computerized PBX.

I think a lot of tech's will be busy for the next few years. (Then when TV broadcast is shut off in 2006, they'll be busier.)

DSL Problems

When DSL doesn't work, or works slowly. (Since when is 384k slow?) Unfortunately, lots of things could be wrong. The awful truth is that some ISP's limit the connection speed. However, here's some general info.

First, the telco could have "disturbers" on the line between the Central Office and your building. This can be equipment that works great for voice, but slows data transmission. There are devices that prevent DSL from working at all. EG: a ground loop, an MTU (multi test unit), or ISDN equipment.

Or, you could have two phone lines on one pair using DAML equipment, etc. Sometimes it's a nearby AM radio station tower. Or, the C.O. may be close, but the cable "goes around the mountain" and the total distance may be too long (max. distance is about 17,500 ft).

A "bridge tap" is where there is an extra pair of wires leading from the pair that holds your phone number. This happens because someone once needed a line, and your pair got used to another home. Later, when they moved, the pair became available again, but when the telco added your connection, the telco didn't remove the line from the cable to the other family's home. Those wires act like a capacitor and reduce the connection speed.

And, the problem could be with the field wire connections (since there isn't a direct pair of wires from you to the C.O.) they could be "dirty". The telco fixes the above problems. Second, the "build" may be faulty. This means that the setup programmed into the equipment could have slightly wrong data. This could be a fix by the ISP or the telco. (Depends upon their agreement as to responsibility.)

Ask them to run tests (they have special equipment) and to give you the distance, the downlink connection speed, the maximum downlink speed, the uplink speed, the margin, and the capacity. (Sorry about the fancy terms.) With this info (and you should insist on an explanation) you could get an idea as to the reason for slowness. As they explain what's wrong, ask them how they will fix the problems.

Another possible problem is that the wire in your building may be old and/or corroded. This will slow the speed. (I replaced the wire in my home.) Unfortunately, this is an expensive fix, and they may ask you to pay for it. Have them check the cable and "drop" connections first (should be free).

As to my DSL connection. My home is two miles from the C.O and I get 1.2 Meg speed. PacBell is installing fiber cable, so by next summer I will get 6 Meg as the basic speed, and I will get "on-demand" movie viewing at the same time that I do a videophone call on my PC. Hey!! See you on the Internet! My opinion of the web site speed tests is, they suck. Most are 30% different from test to test. I haven't found any accurate ones. Mostly I look at the download rate as I get some software from the web, the meter tells me the general speed. (It's in bytes, so multiply by eight to get BPS.)

If 384k is the best speed that you get then complain. Don't tell them your speed but say that your line is too slow. Hopefully, the conditions will be fixed so that you get better performance. Keep after them. The best part of DSL is the "fixed price/always on"; the higher speed is icing on the cake. Hope this helps you, Ramblin Rode

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