tony sayer wrote:
In article , Floyd L. Davidson
writes
tony sayer wrote:
In article , Floyd L. Davidson
writes
Have you ever verified what the CMRR actually is on such a
circuit? The perception that CMR just cancels out everything is
naive. Typically common mode rejection is *not* sufficient to
provide a functional dial loop on a line with 40+ volts of AC
voltage.
Yes but quite some time ago now. FWIW we don't or very rarely have long
lumps of overhead line anymore that carry baseband audio. For voiceband
circuits these days its digital end to end with a A/D and D/A convertor
at each end.
Most residential POTS service in the UK is fully digital from
end to end?
That certainly is not true in the US, and I've never heard anyone
in the UK say that it was there before either.
To clarify that..
Yes indeed! That is *not* end to end digital service, but
describes *exactly* what I've mentioned is done in the US.
The fact that it is the same in the UK merely says that
economics is primarily what drives that model.
BT the main operator in the UK has its exchanges linked together these
days by Fibre and has done so for many years now. They break the fibre
down to copper at the exchange, add in loop volts and line break etc for
older phones, and then route that to the subs premises. However the
distances aren't that great, around 3 to 4 miles or so in rural areas
and less in urban ones. Using a multipair but what also might make some
difference between UK and US practice is that this will invariably be
underground plant in their own ducts. An overhead multipair cable is
quite rare, and is likely to be of a lowish number of pairs and not that
long probably only from a distribution pole to a group of subs not that
far away, i.e. a few hundred meters.
That is not greatly different than typical US distribution systems.
Some places have more buried cable than others, some less.
The Cable co's in the UK, firms like Comcast and Bellcable etc are now
all rolled up into the ntl and Telewest outfit, soon to be renamed
again!. They use a similar practice but the distance issue is changed
again. They will have roadside cabinets instead of exchange office type
buildings, (though they will have a central switch building) that breaks
the fibre down using Nokia equipment's to copper to the subs premises.
However once again these all are underground ducted cables but the
distances there are more likely to be of a few hundreds of yards!.
We don't have this sort of arrangement over here;!...
http://www.annsgarden.com/poles/poles.htm
In fact I've never seen a phone cable tacked to a power pole carrying
more than 240 volts!.
And there we do have something that is distinctly different,
assuming again that you are accurately describing it. Cables
here are as shown in the above URL, and that would include poles
with high voltage distribution lines at the top. On the other
hand, if there is a choice the comm cables will be on poles
without the high voltage lines, and they are less commonly put
on poles that are for transmission between power stations,
though that is done sometimes too, particularly with fiber
(which is commonly owned by the power company rather than by a
telco leasing pole space).
And for that the cables are spece'd differently.
To have phone and comms cables with power lines at the sort of voltages
described on those poles just isn't done here!. The health and safety
police just don't allow!! apart from that there is an aversion to
overhead plant in the UK most all telecom power services is
underground.....
That is relatively common in the US, though it varies from place
to place. Here in Barrow, for example, that style is
ubiquitous; but we are sitting on permafrost and the cost of
trenching is astounding.
The only buried cables here that I can think of off hand are
between town and the area about 2 miles south of town where all
the communications satellite dishes are located. Both the power
and the communications cables (copper and fiber) are buried;
separately of course. What makes that unique here is the lack
of an junction boxes for the entire portion (about 1.5 miles)
that is buried. There is no other cable run that goes as far
without a junction.
Regardless, there are places in the US where cities have banned
aerial cables, and everything is underground.
And for phones its going much the same way, well over here at least.
BT have the 21CN nets which are data circuits which you run data or
audio or whatever you like over them..
I'm not familiar with the terminology. However there are of
course such circuits here too (ISDN, for example), but by far
the majority of POTS service is delivered as an analog line,
after being trunked to a remote unit with digital services.
Yes as above. However the market for bizz and residential telephony
services is rapidly changing in the UK . Mobile use is now very high and
climbing. In fact a lot of young people won't have a landline phone.
They see it that a mobile is a "must have" and for around 30 or so
pounds a month you can get around 500 minutes a month inclusive calls.
Very cheap rates are applied at evenings and weekends.
Same here; and my understanding is that we are *way* behind
some of the Asian countries in that respect. It is clearly the
direction the business will continue to go in the near future.
The extra monthly cost of a phone line can buy quite a bit more call
capacity, and the mobility issue a cheap and easy to use text service
landlines are finding it hard to compete!.
In Alaska the problem is population density, which isn't high enough
to make that economical. That is also true for much of the western
US.
BT 's 21st century programme is to do away with their circuit switched
networks and make it all IP based packet switched. A lot of people use
such services as Skype and other phone over the net services. The only
thing some people have a phone line in for is for broadband provision!..
However these can and are supplied over radio based nets as well as via
UMTS services.
However, none of that is relevant! Power line influence is, if
anything, *more* of a problem for digital services than it is
for old fashion POTS via an analog line.
Well it might be over there with all that overhead distribution but it
'taint here!. ADSL is very robust and I've never known a problem with
it. And as they use fibre a lot for phone lines between exchanges, not a
problem..
I'm sure that it is perhaps worse here, but yes it is important
there too.
The reason it is "more" of a problem for digital is a matter of
how much drama is associated with the effects. With an analog
circuit there is a smooth linear and continuous scale of
interference (which also happens to be reduced by the fact that
the "receiver" of significance is our ears, which don't hear
tones below 100 Hz well). With a digital circuit it is none of
that, and while it can operate normally, which is to say error
free, at a much lower SNR, the transition from error free to
total loss of all data is relatively quick. There is relatively
no "sort of" interfering!
Note that "phone lines" do not exist between exchanges. Message
traffic is *trunked* between exchanges! Lines are very distinct
from trunks. Either of them is a circuit though...
Fiber of course is not bothered by noise from power lines; but
on the other hand fiber itself is an *extremely* noisy media.
So much that it is generally unsuitable for analog transmission.
I don't have a great deal of confidence in someone who is
getting their information from "cable jointers" alongside the
road.
Not quite so. One of these guys was quite old and very experienced, and
could recall the days of lead covered cable where they had to do wiped
joints etc. So don't despise the benefit of experience. Those guys have
probably seem more cable close up than you'll ever likely too!.
Given another 10 or 20 years, they will no doubt catch up. :-)
Course they don't specify it, but some old BT "Poles and holes" staff do
know a lot more that you'd give 'em credit for!..
How would you judge whether that is true or not?
Lets be blunt: you don't know what you are talking about.
If that makes you feel better, and in someway superior, so be it..
you're welcome
It's not a matter of feeling this way or that. You don't have
enough background to even get the terminology correct, much less
have a perspective on the history.
One of the more interesting attributes those of us who started
working in the Alaskan telecommunications industry in the 60's or
earlier have is that we designed and implemented the entire
modern communications system as it exists today. What we started
with was essentially a non-existent infrastructure. Everything
that exists today, we built without any precedents to force
designs based on history. There was *no* privately owned
commercial long distance network! There were three different
military systems providing for various fighter bases and radar
installations, all of which were installed in the 1950's. Few
towns had telephone systems, and those that did often had only 1
or 2 long distance lines. We basically started with nothing,
and built a very modern system. (A few billion dollars worth of
oil being found over here in my back yard didn't hinder that
progression at all! ;-)
Hence I've worked on or seen equipment from the 1930s. J, K, L,
and O carrier all designed in the 1930's, and some of it
installed here during WWII, but most of it immediately post
war.
But not having that kind of background is *not* a problem, as
long as you don't try to pass yourself off as an expert. For
example, I have a little understanding power generation and
distribution, and could pontificate on that subject to a few
here electricians too... and end up in the same boat you are in
on this topic. I make an effort to *ask* them what my
experience means, rather than tell them what things appear, to
me, to be from my perspective. Several of them here are very
good at clarifying the technical details for me!
If you can't cite a valid source... please don't exaggerate what
you do know.
As above their use of multipair isn't that great. Its underground and
all in cable only ducts. They don't even run mains in there they source
that off street lightning cabs..
As above... their use of multipair cable *is* great. Virtually
every telephone line is delivered to the customer on that type
of cable.
Find some valid documentation and I'll accept your description
of the implementation as correct. I say that outside plant
multipair copper cable is virtually *all* shielded. The only
unshielded cable will be drop wire and customer premise house
wiring.
What do you define shielding as, just a wrap of aluminium foil with a
drain wire or a fully woven copper mesh?..
Shielding is shielding, whether it is aluminum foil or copper braid.
Yes except that if we're talking like we were about currents circulating
in the "screen" of a multicore cable, then there is going to be quite a
bit of difference in practice between a heavily woven copper braid and
the light foil wrap where the connection to that is by a fairly thin
drain wire...
But that doesn't change the way the shielding functions. All it
does is change the effectiveness of that functionality, and
clearly copper braid is much more expensive... to a degree that
the difference is not worth the cost.
Once again if its circulating currents in a cable screen as per the
original discussion, then for a given diameter of cable there will be a
difference between the current that can flow in that a braided multicore
will carry more current than one with a ally wrap and small drain wire..
As I stated, it is not a difference in the functionality, merely
a matter of how effectively it is implemented. The two types of
shields accomplish *exactly* the same thing, but one does it
better... at a vastly higher cost.
Yes we sometimes do, but very rarely these days, it s getting to be a
very digital world over here. Analogue circuits are quite rare nowadays
and BT have been known to have to get guys out of retirement to work on
the few remaining ones!.
I'm finding that to be a little difficult to believe, given the
other statements you've made.
Nope. Sorry but the "phone engineer" as we know is a vanishing breed:!.
Your statement above is now something you have contradicted
yourself. The local loops are almost *all* analog. That is a
far shake from "quite rare nowadays".
I can't see how telecom systems engineers are vanishing either.
Their work is much more complex today than it was just 30 years
ago...
Give you an example. We installed a PABX system for a radio station
recently. Its all based on a PC and uses SIP phones. Most all the
outgoing calls are over a GSM gateway to other mobiles 70% of calls, and
inter office calls are via ADSL. Probably 15 % are carried over the ISDN
circuits connecting it to BT. It is now no longer a phone provider
service contract issue its an IT one now!..
So? I don't see what your point is?
All you have shown is that telecommunications has become so
huge, and so complex too, that it has grown to where it
manifests in dozens of ways, rather than only as a simple PABX
with a telephone operator to direct calls as needed.
There was a time when I worked in offices where the number of
circuits per technician was fewer than 50. Today it is rarely
fewer than several thousands.
Well they don't define what you are doing with that. Consider say 10
meters of Andrews LDF 4-50 cable connected to a transmitter with the
correct plug, what are they connecting that other end to?. Nothing or a
load partially connected?.
They detailed it precisely enough. The outer conductor is not
connected. It makes virtually *no* difference what you do with
the inner conductor. :-) The point is that depending on the
frequency and the length (not on what it is connected to) it
will (or not) act as a very good antenna.
Actually read that through again It isn't that wonderfully written for
what they want to convey. I've mailed that off to a few other people to
see what they "visualise" that cable to be doing in that description!..
If you are familiar with research, as opposed to design
engineering, it is written in the common vernacular. Why would
they, for example, bother mentioning the "load", because that
clearly (whatever it is) does not change the conclusion.
Please review any book on antennas! The statement made
describes the physical construction of more than one popular
variation of an antenna.
Yes I design and install aerials thanks.
Antenna's are what insects have
Are you a zoologist? If so, why are you working with
electronics? The term "aerial", as any decent dictionary should
tell you, refers to something relating to air. Cables that are
strung on poles are "aerial" cables... not something that
radiates like an antenna! Antennas are commonly connected to
radios, if you want the radio to be functional.
Regardless of how much fun we can have playing word games, if
you actually do design _antennas_ then you *should*, though not
necessarily will, have the background to understand the point
they made about physical length related to wavelength and its
functioning as an antenna.
I'll grant that in theory it would actually act as an antenna at
*any* length; their point was that as the length approaches zero
the radiation efficiency also approaches zero. It is an
"antenna", but has no value as a useful antenna. (It does have
value as a theoretical antenna though, because as the length
approaches zero it becomes more and more of a "point source",
which is a useful concept to understand too.)
Just over four decades in telecommunications.
I've been in TV transmission, Radio broadcasting, studio design and
maintenance, data comms, and two way radio......among others...
I did all of those things... but it was 40 years ago. Since
then I've done digital telephone switching, analog and digital
microwave system, fiber optic systems, analog and digital
satellite systems, analog and digital carrier systems, data
systems, computer administration and software engineering.
One of the benefits of the way it worked here in Alaska was that
we all go to do *everything*, if we wanted to.
Now what was the original argument again;?.....
Who can **** farther.
Ever have a biopsy on your prostate? :-)
--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)
10 metres audio cable going into PC = too long?
Linear Mode
