TELECOM Digest OnLine - Sorted: Re: DSL Speed

Re: DSL Speed
Tue, 28 Jun 2005 21:10:54 -0600

Choreboy <> wrote:

>>> Between the CO and the customer, isn't voice service
>>> just bare wire?

> Neal McLain wrote:

>> Not necessarily. But let's clarify some terminology first.

>> I assume that:
>> - By "bare wire" you don't really mean "bare" (as in
>> uninsulated); you're simply implying that there's nothing in
>> the wirepair, other than copper conductors, that would affect
>> the transmission of signals.

> Oops, I was thinking "not coaxial" and "bare" popped into my head.

Actually, it is possible for a local loop to be "bare." It's called
"open wire" -- bare uninsulated conductors attached to glass or
ceramic insulators supported by crossarms. Phone companies used
open-wire lines for years. As recently as 2003, the Union Pacific RR
still had a few open-wire circuits in use.
(Figures 12-15).

>> Each RT is connected to a host CO, and from the point of view of the
>> customer, it's indistinguishable from the CO. POTS lines served
>> from the RT are switched at the CO; the RT simply relays signals
>> back and forth between the customer and the CO. Numbers are part of
>> the same NPA-NXX blocks as the host CO.

> Is it indistinguishable if the customer has a V.90 modem? I think
> I've read that an RT won't allow 56k dialups.

Some RTs allow V.90 modems; some don't. Apparently, yours does
(assuming that's the reason the farm can be so far from the CO and
still get DSL). A good tutorial on this issue is at .

>> Each [digital loop carrier] RT is connected to its host CO by one or
>> more digital circuits. Depending on the number of POTS lines
>> needed, the digital connection can be as simple as a single T1
>> implemented over two copper wirepairs, or it can be some multiplexed
>> combination of several T1s implemented over coax, fiber, or
>> microwave. See .

> Does an RT entail an extra A/D conversion?

Some do; some don't. The tutorial cited above describes the

>> Note that dialup-modem data signals carried in the 300-3000 Hz voice
>> passband are not demodulated at the CO or RT; instead, they are
>> sampled at 8000 sps just like voice or any other audio signal. This
>> fact imposes an absolute theoretical maximum dial-up data rate of
>> 64Kbps. As other contributors have noted, it's impossible to attain
>> even that rate in practice due to synchronization errors between the
>> user's modem and the sampling rate.

> A carrier vor V.90 must have some very precise modulation. It's
> amazing that an 8kHz sampling can capture it well enough to be
> useful.

Frequently it can't, which is why your modem downgrades to a slower

>> All of these noise sources collectively impair the ability of the loop
>> to carry DSL signals.

> Local loop cables (trunk cables?) seem to deteriorate. Phone men seem
> to look for available pairs when customers complain of noise. I
> wonder if voltage from nearby lightning strikes might cause pinhole
> damage to the insulation of twisted pairs, and over the years it gets
> hard to find a good pair.

Nearby lightning strikes likely would do a lot more than cause
"pinhole damage." But you're right about telco cables deteriorating
over time. Water intrusion can cause severe interference ("every time
it rains, I get static on my telephone!").

>> LOAD COILS. The frequency-dependent attenuation characteristics of
>> the loop (as described above) also affect voice band frequencies
>> (300-3000 Hz), resulting in rolloff of the higher frequencies of voice
>> signals. To solve this problem, telcos have traditionally installed
>> "load coils" at 6000-foot intervals on long (typically >18K feet)
>> loops. A load coil is a small inductor installed across the
>> conductors to cancel the affects of interconductor capacitance.
>> Although load coils reduce high-frequency rolloff within the voice
>> band, they cause severe attenuation above 4000 Hz. See
>> .

> At DSL frequencies I would have thought coil impedance would be too
> high to matter. I don't quite grasp it.

Any other reader want to tackle this question?

> Load coils might be one reason a particular phone sounds distorted at
> a particular location.

I doubt that, but I guess it's possible.

>> In areas where outside plant (OSP) is installed on utility poles,
>> telco drop terminals are called "aerial terminals" or "boots";
>> typically, a terminal is installed at each pole....

> That's what somebody pointed out to me as an inline amp. If I could
> remember who it was, I'd correct him!

Maybe that somebody was pointing to a cable TV amplifier. They look
somewhat like aerial terminals, except that cable amps are usually
aluminum rather than black. Drawings 8 and 9.

>> Each drop terminal has:

>> - Two cable ports for the distribution cable: input and output.
>> When a drop terminal is installed, these ports are often
>> sealed as protection against water intrusion. These seals
>> make it virtually impossible to gain access to the individual
>> wirepairs within the distribution cable.

> As I recall, a phone man appeared to have an aerial terminal open
> after I lost phone service one day. He said he'd made a mistake and
> would try to figure out how to reconnect me.

Those damn OPS records again!

>> - Several drop ports, one for each wirepair in the distribution
>> cable. These ports are usually implemented with screw
>> terminals or punchdown blocks.

> Across the street, a small trunk line (cable with lots of wire pairs)
> comes from the aerial terminal down a couple of feet to a fusebox on
> the utility pole. (I think the telco calls them something besides
> fuses.) The drop cables come out of that box.

Probably just a junction box.

>> These unterminated wirepairs act like tuned-stub filters. Since
>> they're unterminated, arriving signals are reflected back; these
>> reflected signals interfere with the primary signals. In the extreme
>> case -- when the reflected signal is 180 degrees out-of-phase with the
>> primary signal -- the primary signal is severely attenuated.

> Offhand, that sounds like a stub of 1/4 wavelength. Could the modems
> could mitigate the problem by the frequency they negotiate?

I believe the DSLAM can select both the downstream frequency (that it
uses) and the upstream frequency (that it tells the customer's DSL
modem to use). Perhaps some other reader can give us a more complete

>> DSL signals are modulated onto carriers in two bands:
>> Uplink (Modem to DSLAM) 30- 110 KHz
>> Downlink (DSLAM to Modem) 110-1100 KHz

> I wonder how they're modulated.

Can any other reader answer this question?

>> In a previous life, when I worked for a radio station, we sometimes
>> used phone patches for connections to remote locations. At each end,
>> we'd connect a "phone patch box" directly to the ring-and-tip of a
>> phone line. Then we'd dial up a connection with a conventional phone,
>> switch in resistors to keep the line open, and hang up the phones.
>> Voice quality wasn't as good as it would have been with a wideband
>> audio circuit, but it was certainly far better than it would have been
>> if we'd used the telephones themselves. More than adequate for a
>> sports or news report.

> I wonder if the phone patch box had adjustments to flatten the
> frequency response.

I don't think it did, but that was a long time ago.

>> Of course, making a direct electrical connection to a phone line was
>> illegal back in those days (late 50s, early 60s). But we were on good
>> terms with the phone guys, so they just looked the other way.

> Could you have gone to the federal penitentiary?

AT&T prohibited any direct electrical connections to its network, but
I don't think anybody ever went to jail for violating it -- AFAIK, all
AT&T could do was pull the plug.

> Was there a good reason for the law?

The story of how AT&T was forced to relax its no-connections rule, and
how subsequent events eventually led to AT&T's ultimate (if still
pending) demise, are perennial topics here on TD.

Tom Farley's "Telephone History Series" relates the story in detail at . The
direct-connection subplot starts on page 10 .

Mike Sandman's website has a history page (intermixed with pitches for
products he's selling) at . Mike
is a long-time sponsor of TD, so feel free to buy something from him.

>>> Think what would have happened if RG-59 hadn't been invented.
>>> Everybody would have used RG-6, which looks nearly the same but
>>> attenuates uhf much less. With better reception there would have
>>> been more uhf stations and less demand for cable.

>> As a former cable guy, I don't agree with that. Many UHF stations
>> depended on cable TV systems to distribute their signals throughout
>> their "specified zones" (which, back in the '60s and '70s, was a
>> 35-mile radius around the city of license). This was particularly
>> true in mountainous areas where cable T systems carried UHF signals
>> to specified-zone communities that were beyond the reach of their
>> transmitters.

> With a bow-tie antenna, a good UHF amp, a rotator, and RG-6U, we could
> receive so many channels that we weren't interested in cable.

Well, obviously you don't live in a place like Mahanoy City
Pennsylvania, Tuckerman Arkansas, or Astoria Oregon -- places where it
simply isn't possible to get any station -- UHF or VHF -- off the air.
Cable TV started in all three of those communities in 1948, and all
three still claim to have been first.

Neal McLain

[TELECOM Digest Editor's Note: And don't forget Independence, KS where
until cable came along (via Time Warner in the 1980's) our television
reception consisted of TWO channels; channel 6 and channel 9, but only
one of those two if you wanted a good picture. Most people had very
_high_ antennas on their house if they wanted television, and they
compromised by using a 'rotor' attached to their TV set to turn the
rooftop antenna one way or the other. If they could not afford the
rotor, then they left the antenna turned sort of in the middle and
lived with that. We got one station from Tulsa, Oklahoma (80 miles
almost straight south) and one station from Joplin, Missouri (90
miles more or less straight east.) Around here, 'big city' (as in
presence of television stations) means Wichita, KS which is 110 miles
northwest, or Topeka KS which is about 150 miles straight north, and
we could not get those stations very well at all in those days. PAT]

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