Author Topic: Contentious and rambling Analog multiplexing for connecting PABX to Central Office discussion  (Read 3941 times)

bellsystem

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Sound is not inherently digital. So as soon as sound is converted to digital, it is not longer an exact replica of that sound, like analog is UNTIL distortion gets introduced.
(Even this basic website says that the human voice is analog, if you don't believe me: http://www.diffen.com/difference/Analog_vs_Digital)

If, in a perfect world, analog systems were distortion-free and didn't suffer from distance degradation, analog would always sound better than digital.

Finally, here's something to read from HowStuffWorks: http://electronics.howstuffworks.com/digital-versus-analog.htm/printable

Here's a quote:
Some audiophiles argue that because analog recording methods are continuous, they are better at capturing a true representation of sound. Digital recordings can miss subtle nuances. But as digital recording processes improve, digital devices can use higher sampling rates with greater precision. Although the signal still isn't continuous, the high sampling rate can create a sound similar to the original source.

Before the 1970s, musicians recorded their performances on analog recording equipment. Microphones recording the sound generated an analog wave that other devices would then transfer directly to the proper media (usually magnetic tape). Assuming the recording artist used reliable equipment, the sound recorded was an accurate representation of the original sound.

Any time engineers have to convert a recording from one format to another, there's a chance that the quality will suffer.

Paragraph 1 from HowStuffWorks says that analog recording is continuous and can more accurately capture the true sound. It's basically the equivalent of infinite sampling rates, which is impossible. No matter how high the sampling rate for digital recording is, it will always miss something(s) every single second. The difference may be completely negligible, but it, from a technical standpoint, exists.
Paragraph 2 says if the equipment is reliable, then the recorded sound is an accurate representation.
Paragraph 3: The human voice is naturally analog. So when it is converted to digital, there is a chance quality will suffer.

The major drawbacks of analog in the PSTN are for long-distances. These "disadvantages" don't really exist if you call a telephone in the next room. Analog transmission travels at the speed of light, while digital transmissions are always subject to any delay introduced by the computers that process the signals. That's why VoIP is sometimes so choppy - often not all the packets even make it to their destination. Even with non-VoIP digital systems, it's not as instantaneous.

The article IS mostly about audio recording, not telephony, but the same principles apply.

Again, what is to all of you if I stick myself with "inferior" equipment. I'm not asking anyone else to use analog transmission mediums.

unbeldi

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Can Analog support Direct Inward Dialing? I know PRI can.


DID is not a technology that is bound to any particular method of transmission.  It is rather more of a methodology of routing and signaling of telephone calls.  It is essentially a trunking method to a private branch exchange and has been implemented on analog trunks as well as digital multiplexed circuits.  It is a method to minimize the cost of multiple communication channels w/r/t the actually anticipated need of multiple simultaneous channels during peak traffic hours.  During each call setup, the central office sends the destination address information via the trunk.  On analog circuits that can be DTMF or pulse dialing, on digital circuits it could be DTMF or any other digital message format, for example using RFC 4733 named telephony events as RTP payload.

While probably still available as a facility on 5ESS and DMS systems, you might be hard-pressed to find analog DID trunks at reasonable cost today. A fractional T1/PRI circuit was already cheaper some 10 years ago or so, IIRC, when you consider all costs involved.  It depends on the scale you are looking at, there is a cross-over point for cost vs capacity that has constantly been changing with resellers and time.

And for the "reliability purist" in you: Don't expect to power your equipment from the analog DID circuit either, it does not provide battery.
Of course, your PRI line will also go down if you don't take care of your own power on your end.
« Last Edit: July 06, 2017, 09:38:48 PM by TelePlay »

bellsystem

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Thanks,
I am well aware that phones on a PBX do not use the common battery from the central office. Their common battery is the PBX, and the PBX must have adequate backup power. But as long as the PBX has backup power/generators/etc., the lines from the Central Office will still work.

I'm not too concerned, but does there exist a case where analog DID trunks are cheaper than T1/PRI?

I'd also need DOD (Direct Outward Dialing), for the same extensions that DID is used on. Is that supported on analog trunks?

I did some research last night. Apparently, with analog trunks, Caller ID is not provided until after the first ring, whereas with PRI, it is provided instantly. This wouldn't be a big issue for me, seeing as how none of my phones will be able to display Caller ID anyways... but is this due to some inherent limitation of an analog trunk?

I looked up the 5ESS and DMS100. The descriptions all say they are TDM systems. Can they be configured to analog switching?
« Last Edit: June 29, 2017, 11:15:31 AM by bellsystem »

Alex G. Bell

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It is a method to minimize the cost of multiple communication channels w/r/t the actually anticipated need of multiple simultaneous channels during peak traffic hours.
Not exactly.  DID was introduced in the Bell System on SXS PBXs circa 1959 with DuPont in DE.  It's written up in an AIEE article authored by a BTL PBX developer. 

DID trunk groups supplanted ordinary PBX trunk groups, reducing the number of attendants required to complete or transfer calls which reached the wrong extension.  Calls to the LDN (listed directory number for the company) also complete over the same trunk group.  So the number of trunks required from the CO to the PBX is not changed by the introduction of DID.

However DID is described in Harry Hershey's 1917 book.  In a direct control SXS CO environment it was always possible to integrate a customer premises dial PBX into the public exchange numbering plan.  However in those days there was no DDD, the majority of phones were manual, so as the Bell System converted to dial, first using common control Panel machine switching, they made no provision to pass stored customer dialed digits into a dial PBX even connected to the same Panel Office.  This was perpetuated in #1XB and #5XB.  Even as they used SXS in some medium size cities they chose not to offer DID service in those places. 

Finally when they implemented Line Link Pulsinng in #5XB, allowing a Dial Pulse Sender, which ordinarily sends digits out on trunks to other COs, to send them on lines to a customer PBX, DID became possible. 

So their choosing to not offer DID even in cities served by SXS CO equipment must have been a business decision more than a technical driven one.  Perhaps they choice that because they did not want to create a demand for service and pressure on themselves to modify the equipment serving larger cities to provide DID there too, although clearly once they decided that it was a service they could tariff and offer at a profit, they reversed that decision.

As an aside, they used DID for "official PBXs" (PBXs serving Bell System employees) well before they offered it to commercial customers.  This is true of many systems and service features they introduced to the public: they developed them and used them internally years before offering them to the public.
« Last Edit: June 29, 2017, 11:25:41 AM by Alex G. Bell »

unbeldi

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I think I kept my DID summary as short as possible.  When involved, attendants should certainly also be considered part of a communication channel.  Attendants and their jacks, plugs, cords, and keys were the forerunners of silicon chips.

bellsystem

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So, are there Central Office solutions (even if it requires me to build my own CO) that would allow analog PBX trunks that support both DID and DOD?

Alex G. Bell

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I think I kept my DID summary as short as possible.  When involved, attendants should certainly also be considered part of a communication channel.  Attendants and their jacks, plugs, cords, and keys were the forerunners of silicon chips.
The point is that DID did not reduce the number of channels required between the PSTN and the PBX.  Both the DID traffic and attendant traffic was usually carried over a DID trunk group of the same size as the previous incoming ringdown trunk group which appeared only at the attendant.

bellsystem

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I never said DID reduced the number of trunks needed. It just reduced the need for a PBX switchboard operator to manually transfer calls to their destination.

Alex G. Bell

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So, are there Central Office solutions (even if it requires me to build my own CO) that would allow analog PBX trunks that support both DID and DOD?
Trunk tables showing the available types for the widely deployed 701B SXS PBX list 2-way DID trunks allowing both DID and DOD service on a single trunk but I don't know whether any BOCs tariffed them. 

Pacific Bell told me that 2-way DID trunks were only available on T1 "because it is logically impossible".  Of course if it were logically impossible it would be impossible on a T1 also.  Essentially a 2-way DID/DOD trunk is technically no different from a 2-way loop signaling interoffice trunk, millions of which were deployed throughout the world.  The difference is that the exchange at one end is privately owned rather than both ends terminating at a Telco exchange. 

The fact that it was shown in the AT&T-published 701B trunk tables also demonstrates that it was not logically impossible at all.  (Just one of many examples of strongly held misinformation from my Pac Bell service rep.)  A T1 would have been cost prohibitive.

Alex G. Bell

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I never said DID reduced the number of trunks needed. It just reduced the need for a PBX switchboard operator to manually transfer calls to their destination.
And I never said you did.  I said unbeldi did.  You need to read more carefully before replying.

bellsystem

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Given that I can't use touchtone telephones with an SxS office, is there an electronic switching system that will allow for analog DID/DOD?

Offline AE_Collector

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As a moderator I cringed a few times reading this hoping that no hostilities would break out and they didn't. Great discussion everyone.

Terry

Alex G. Bell

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Given that I can't use touchtone telephones with an SxS office, is there an electronic switching system that will allow for analog DID/DOD?
Once again you are not reading carefully or not absorbing what people write.  I stated explicitly at least once, perhaps multiple times, that SXS PBXs were converted to TT service.

Please read more carefully and avoid repeating questions which have already been answered.

bellsystem

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Alex,
How can one use the # and * keys when connected to a SxS switch? Even if there were a converter at the end that converter tones to pulses, it can't convert the # and * into anything recognizable. Plus, the whole point of touchtone dialing was to speed up dialing. I'd much rather use a rotary phone than a pushbutton phone that is effectively pulse dialing (I'd rather use a rotary phone anyways though!).

Alex G. Bell

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Alex,
How can one use the # and * keys when connected to a SxS switch? Even if there were a converter at the end that converter tones to pulses, it can't convert the # and * into anything recognizable.
It depends on what you want to do with them, which you have not stated.  Or perhaps it was lost in the reams of verbiage.
Plus, the whole point of touchtone dialing was to speed up dialing. I'd much rather use a rotary phone than a pushbutton phone that is effectively pulse dialing (I'd rather use a rotary phone anyways though!).
In fact a TT/DP converter is FASTER than using a rotary dial because no time is lost winding up a dial (approx. 1 second per digit since human reaction time is a significant fraction of a second) and because "machine sent" DP minimizes the pauses between digits.

 ::) You can't have it both ways.  You rejected crossbar because you like to watch SXS switches.  I guess your mission will be to reinvent SXS switches which go directly to the required level without stepping through the lower ones in response to TT dial signals.   ;D