Author Topic: Contentious and rambling Analog multiplexing for connecting PABX to Central Office discussion  (Read 3939 times)

Alex G. Bell

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SxS (Strowger) switches are my favorite but it would not be practical for a primary PBX... it would be impossible to hook it up to the PSTN directly anyways
What makes you think so?  The dominant dial PBX technology in the Bell System and most everywhere else in the world from the 1920s through the 1980s, was some form of Strowger, before it was finally superseded by electronic systems, with some installations still in service into the 1990s. 

Many hobbyists have SXS dial PBX systems connected to the PSTN at this very moment, either by analog lines or by VoIP.  Some have crossbar as well or instead.  A few even have DID service.

bellsystem

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REALLY?

I plan to have a few touchtone phones (because I have to) but I by far prefer rotary to TT.

If only there were a way to have the touch-tone phones on a separate PBX that worked together with the SxS switch.

I know it used to be dominant for half a century. I wish there will still some today in service commercially.

How would you accomplish DID with Strowger switches directly connected to the PSTN? I'm not into crossbar much because it doesn't have the same meaning to me as Strowger. I like seeing the switches move up and down. Crossbar to me is just kind of boring...

If this is accomplishable, I just might go that route! But I'm not sure how you would set it up. A SxS system is literally just switches. What would the outside lines each connect to. And when a phone dials 9, wouldn't they all attempt to reach the same phone?

To reach 911, I'd want to have to dial 9911, not 911, which would work just fine, but I'd like to be able to reach 90 by just dialing 0 - at least on the main PBX. I believe I could do this using call forwarding... except that's not supported using SxS.

That's why I was thinking of using an electronic system as my root PBX, and have several nested PBX - including a nice big SxS one. I don't foresee any problems with this - but maybe someone who has an SxS switch can comment?

I just don't understand how you can get DID/DOD with an SxS setup...

Alex G. Bell

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REALLY?

...

I just don't understand how you can get DID/DOD with an SxS setup...
Yes. Hundreds of thousands, perhaps millions of Bell System PBX customers were served by SXS PBX plants.  DOD was a basic PBX feature starting in the 1920s, decades before DID was introduced in the late 1950s.

ANY SXS system consists of more than just switches.  Trunk circuits consisting of relays and transmission path coupling components are required between any two separate SXS exchanges.  Accordingly trunk circuits are required to interface a SXS PBX to the PSTN whether for DOD or DID.

Thousands of SXS PBX systems and millions of lines of SXS CO switching were converted to Touch Tone service starting in the early 1960s with the introduction of Touch Tone calling by adding Touch Tone converters which recognize dial pulses, store and outpulse the corresponding digits as a dial pulses at the slower DP rate.  Once an exchange is converted to TT operation rotary dial phones will continue to function on all lines.

There is much you will need to learn to understand how all this is possible and there is plenty of tutorial information available on line these days.  One basic point is that electronic technology does not make it possible to do things which were impossible with electro-mechanical technology because they were logically impossible.  This applies to such things as conflicting numbering plan objectives/requirements.

The smallest commercially mfd Bell System SXS PBX systems occupied a 24x36" footprint, standing 6' tall, with 12 connecting paths and 50 station lines.  It's possible to construct much smaller ones if fewer simultaneous calls are required.  In addition to the 5 levels used for station lines, the upper 5 levels 6-0 were available for accessing various trunk groups (attendant switchboard, local CO trunks, tie trunks to other PBXs, WATS lines).
« Last Edit: July 06, 2017, 08:39:21 PM by TelePlay »

bellsystem

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Wow, thanks!

Would you mind directing me to any diagrams or information about these systems online?

The thing is I would need * and # functionality on those TT phones (conference calls) so I would still need to get that to be able to work somehow.

Also, I would have a few key systems within the PBX.

Alex G. Bell

  • Guest
Wow, thanks!

Would you mind directing me to any diagrams or information about these systems online?

The thing is I would need * and # functionality on those TT phones (conference calls) so I would still need to get that to be able to work somehow.

Also, I would have a few key systems within the PBX.
Browse the TCI library at http://telephonecollectors.info/ & http://www.telephonecollectors.info/index.php/tci-library-inbox and use Google.

Don't get too far ahead of yourself with specific requirements.  When you are sufficiently up to speed you will understand what is feasible, what isn't and how to achieve the things which are.

But since you mentioned conference calls and since you are enamored of analog transmission, you should start off understanding its limitations:  To wit that conference calling is one function for which digital transmission is vastly superior and conventional 2-wire analog transmission is inherently deficient.  If you are diligent you will eventually understand why this is so.  The reasons are far beyond the time and space we have here.

bellsystem

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The Bell System introduced 3-way calling for analog systems (even though it required a digital switch).

Just to make myself clear - I have nothing against digital SWITCHING. The PSTN is now 100% digitally switched. It's just the TRANSMISSION I'm concerned about.

unbeldi

  • Guest
The Bell System introduced 3-way calling for analog systems (even though it required a digital switch).

Just to make myself clear - I have nothing against digital SWITCHING. The PSTN is now 100% digitally switched. It's just the TRANSMISSION I'm concerned about.

Just what sparks this concern ?   You express strong convictions, yet those concerns appear to be largely founded in ignorance, because you are asking questions one would expect researched and understood before raising concerns.

With the exception of the subscriber loop network, in countries like the US, transmission today is 100% digital too, and that started in the 1960s.


bellsystem

  • Guest
So far, analog has proven to be far more reliable in disasters and emergencies than digital.

The local loop is also critical as that is what allows for a common battery.

Victor Laszlo

  • Guest
Quote
"How would you accomplish DID with Strowger switches directly connected to the PSTN? I'm not into crossbar much because it doesn't have the same meaning to me as Strowger. I like seeing the switches move up and down.

"If this is accomplishable, I just might go that route! But I'm not sure how you would set it up. A SxS system is literally just switches. What would the outside lines each connect to. And when a phone dials 9, wouldn't they all attempt to reach the same phone?"

An SxS system is definitely NOT "literally [sic] just switches."  It is a power plant, battery string, alarm panels, tone and ringing supply, Line & Cut-off relays, Group relays, Line-finders, Selectors, Connectors, and the interconnecting wires.  Also, depending upon the needs of the subscribers, in real life, or YOU, as the Master of your own Telephone Universe, there will be direct-dial CO trunks, DID trunks, attendant trunks, (you WILL need an attendant position, right? A cord board goes nicely with an old Strowger, or you can upgrade to a console). There are central-office grade pulse-to-tone converters, and such eclectica as digit-absorbing selectors, selector/connectors, and party-line connectors.  The systems can be as simple or as complex as you need.

Are you familiar with the various clubs and organizations who perpetuate the history and study of telephony? I suggest you join ATCA, TCI, and sign up on the CKTS.INFO website as a member of the world's largest, international, privately-owned telephone system, called "C*NET".

Many of us own and operate electromechanical switching systems. Some of these are PAX's, some are PBX's, some are key systems, and some are relics of central office equipment of Strowger, X-Bar, panel, and various iterations of electronic systems. Some are home-brew ("Franken-switch") assemblages, some are straight out of the box key systems, and some are just a POTS telephone connected vial a simple ATA.

It is simplicity itself to arrange to have DID trunks, with PSTN numbers, connected to a genuine step switch.

On another note, I worked as a cable maintenance splicer in Western PA for many years.  I can assure you that in terms of system reliability, a damaged, wet, paper insulated, lead sheathed cable serving 900 to 1800 subscribers, can never be put back into service as quickly as a damaged optical fiber of the same "pair" count.
« Last Edit: July 06, 2017, 08:53:20 PM by TelePlay »

bellsystem

  • Guest
Victor, I joined this mailing list just last week: http://lists.ckts.info/mailman/listinfo/voip
Is that what you mean by "signing up?" There is no other sign up. I can't register since I have nothing to connect to the network.

Victor Laszlo

  • Guest
Excellent. You will learn a lot from the members of that network.  There is expertise there that can never be duplicated.

Alex G. Bell

  • Guest
The Bell System introduced 3-way calling for analog systems (even though it required a digital switch).

Just to make myself clear - I have nothing against digital SWITCHING. The PSTN is now 100% digitally switched. It's just the TRANSMISSION I'm concerned about.
The Bell System introduced 3-way calling into CO switching systems with the No. 1 ESS, which was a stored program controlled (SPC) analog exchange using reed switches for the switching medium.  It was not a digital switch.

But the problem of providing usable conference facilities for multiple PSTN parties is very very different when the conferencing is performed at the CO than when it is performed on the customer's premises with additional voice frequency (VF) loss between each of the parties introduced by the loops between the CO and the premises where the conferencing is performed.

If you want to conference multiple parties on your premises that's a trivial matter.  PAX systems did that 100 years ago.  But this is lossy conferencing which is only satisfactory because there is no VF loop loss.  However I infer from your past statements that you want to conference multiple distant PSTN parties, not ones on your premises.

In many places, the Bell System stated about passive (analog) conferencing circuits located on the customer's premises with respect to conference calls which included more than one CO party: "transmission is not guaranteed", which is a way of saying that multiple distant CO parties may not be able to hear each other (due to the cumulative tandem circuit losses).

Offline Owain

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So far, analog has proven to be far more reliable in disasters and emergencies than digital.

The local loop is also critical as that is what allows for a common battery.

You can have VoIP with 'common battery' using Power over Ethernet. You just need an 8-wire 'local loop'.

bellsystem

  • Guest
Every VoIP phone I use has had PoE (no other cords connected). But when mains power goes out, so does the phone.

Also, backup for these is more intensive since these phones use more power (often have displays, etc.) and power is going to unnecessarily go to every computer in the building, even though the computers shouldn't be run off of backup/emergency power.

And there's no COMMON battery for VoIP. I'm talking about power supplied by the central office.

Dominic_ContempraPhones

  • Guest
BellSystem,

You're confused.  A PRI is the gold standard and offers the best sound quality between a PBX and Central Office, not analog.  Where did you get the impression that analog on the last mile, or space division analog switching is better in terms of sound?  It's absolutely not true.

Analog multiplexing.  Northern Telecom did have analog time division multiplexing in a PBX called SG-1, but it was inferior to digital time division multiplexing, SL-1.  The initial SL-1 telephones were analog to the PBX for voice, but digital for supervisory.  Time Compression Multiplexing is alternating half duplex mode of communication whereby each side transmits digitized signal at twice the speed only half the time.  The standard codec is lossy, but you can't tell.  You're using carbon microphones that are old.

I agree that VoIP is inferior in real world conditions, but it can work well in very small well managed and tightly controlled environment where the Ethernet network is voice only with excess capacity.  There are no queueing delays.

What are you trying to do?  Explain it, but keep it short.

ADID ... there are many ways of doing it -- DP, MF, DTMF, DISA ... COs like DMS are smart -- they can use DISA to do DID.

Rotary is supported on mine -- Option 11 with the proper variances which means 8 - 11 pps and a make/break ratio varying from x to y.  I have to look it up.

Our phones with screens and what have you will provide basic service during outages -- if you choose to use VoIP, there is PSTN fallback, but ... oh man, ... just explain you want.

It really depends on the PBX and CO you have, and what you're trying to do.  P